Petropedam Engineering Reference

Industrial Pump Selection Engineering Guide

A field-oriented engineering reference for industrial pump selection — from fluid behavior and corrosion to system curves, BEP, NPSH, operating regions, pump control, API requirements and practical selection decisions.

Original Petropedam training content · Engineering review against current pump standards · Updated September 2026

System-first selection BEP · POR · AOR screening NPSH margin API / HI engineering basis
01Define the serviceDuty cases · fluid · system
02Screen hydraulicsCurves · BEP · NPSH · POR/AOR
03Verify constructionMaterials · seals · API requirements
04Check the systemControl · parallel/series · lifecycle

What Is Pump Selection?

Pump selection is a system-engineering decision: the selected pump must satisfy the hydraulic duty, liquid properties, suction conditions, mechanical limits, materials, driver, control philosophy, reliability target and project standard.

⛭ PETROPEDAM ENGINEERING NOTE

Flow and head alone are not a complete pump specification. A technically acceptable pump must also be checked at minimum, normal and maximum operating conditions and against the actual system curve.

A pump is not selected from a catalog by flow rate alone. The operating point is created by the interaction of the pump characteristic and the system characteristic. Selection therefore begins with the process and liquid, not with a pump model.

An ideal pump-selection program requires all six areas emphasized in the original Petropedam training presentation:

1. Conditions of Service

Normal/minimum/maximum duty, temperature, pressure, operating hours, start/stop and upset conditions.

2. Pump Specification

Hydraulic, mechanical, materials, sealing, driver, testing and documentation requirements.

3. Procurement

Vendor qualification, bid evaluation, quality surveillance and certified performance data.

4. Installation

Foundation, alignment, suction/discharge piping, strain control and commissioning.

5. Operation

Operating envelope, minimum flow, control philosophy, monitoring and abnormal-condition response.

6. Maintenance

Inspection, spares, repair strategy, condition monitoring and lifecycle support.

Critical concept from the original presentation: mistakes in procurement, installation, operation and maintenance can often be corrected later. Errors in Conditions of Service or Pump Specification propagate through every later stage and are much harder to recover.

The Engineering Selection Process

Process & Liquid Conditions →System Characteristics →Pump Technology →Hydraulic Screening →Mechanical / Materials Review →Final Specification

Data Required for Pump Selection

A complete process datasheet is the foundation. Below is a comprehensive checklist of essential parameters.

⛭ PetroPedam Engineering Note

For chemically demanding services, Petropedam evaluates relevant major and trace constituents — including species such as halides and sulfides — together with concentration and temperature when they can influence material selection, corrosion, sealing or reliability.

ParameterWhy It MattersTypical Unit
Flow Rate (Normal / Min / Max)Defines pump sizing; must cover operating rangem³/h
Total Dynamic HeadSum of static, pressure, friction, and velocity headm
Suction PressureAffects NPSHA and suction-specific speedbar(a) or bar(g)
Discharge PressureRequired to overcome system backpressurebar(g)
Fluid TemperatureAffects vapor pressure, viscosity, material limits°C
Specific Gravity (SG) / Relative DensityConverts pressure to head; affects power—
Vapor PressureRequired for NPSH calculationbar(a)
ViscosityAffects pump performance; corrections needed for centrifugal pumpscSt
Rheological CharacteristicsFor non-Newtonian fluids; affects hydraulic design—
Specific Heat (Cp)Important when NPSHA is lowkJ/kg·K
pH (Acidity / Alkalinity)Critical for material selection—
Dissolved Gases / AerationAffects corrosion rate and pump performance%
Suspended SolidsCauses erosion-corrosion; affects material selection% or ppm
Allowable LeakageEnvironmental and safety requirements; affects seal selection—
NPSHAMust exceed NPSHR with adequate marginm
Operating Hours / Duty CycleAffects motor sizing, reliability, lifecycle costhrs/day
Applicable StandardAPI 610, API 674, ISO 5199, NFPA 20, ANSI, API 676—

Important: Trace constituents (halogens, halides, hydrogen compounds, sulfides) can have significant effects on pump materials even at very low concentrations. For example, fluorides affect high-silicon cast iron, and hydrogen sulfide affects various materials.

Fluid Properties — The Foundation of Pump Selection

Liquid properties can change pump type, curve correction, NPSH, materials, sealing, power and minimum-flow requirements. Use properties at the actual operating temperature — not only at ambient conditions.

Specific Gravity / Density

Head produced by a rotodynamic pump is approximately independent of density for a given speed and geometry, but pressure rise and shaft power change with density.

Vapor Pressure

Required for NPSHA. Vapor pressure must correspond to the actual liquid composition and worst credible temperature.

Viscosity

Viscosity can reduce rotodynamic flow, head and efficiency and change absorbed power. Correct water performance with a recognized method such as ANSI/HI 9.6.7 when applicable.

Rheology

Non-Newtonian fluids require a viscosity/rheology model over the expected shear-rate range; a single cSt value may be insufficient.

Temperature & Specific Heat

Affect viscosity, vapor pressure, material strength, sealing, thermal growth, temperature rise at low flow and NPSH.

Constituents & Solids

Major and trace constituents, gas content, particle size, hardness and concentration influence corrosion, erosion, plugging and material selection.

No universal “500 SSU” cutoff: the historical presentation used 500 SSU as a screening rule. Modern engineering should not reject every centrifugal pump above one viscosity value. Compare corrected rotodynamic performance against positive-displacement alternatives for the complete duty and lifecycle cost.

Critical Fluid Characteristics

  • pH / chemical composition: evaluate at concentration and temperature, including trace halides, sulfides and other aggressive species.
  • Suspended solids: define concentration, PSD, hardness, shape and settling tendency — not only a percentage.
  • Free / entrained gas: can reduce developed head, efficiency and stability and may cause gas locking. Distinguish it from gas that remains dissolved and from vapor bubbles caused by cavitation.
  • Allowable leakage / toxicity: may control the sealing concept and can justify dual seals, containment or sealless technology.
  • Product sensitivity: contamination, shear, crystallization, polymerization or temperature limits may govern technology selection.
MATERIALS FIRST · CORROSION AND MATERIAL CHOICE

First understand the liquid. Then select pump materials.

Before using head and flow to size the pump, identify what the actual liquid can do to the casing, impeller, shaft, seals and elastomers. The following lesson restores the corrosion mechanisms and microstructure examples from the original Petropedam training presentation.

01 · Identify liquid and duty environment02 · Identify likely damage mechanisms03 · Screen wetted materials and seals04 · Verify curves and hydraulics

Why pump service differs from a static coupon: a material compatible in a low-velocity test can fail when local flow, impingement, solids, vapor-bubble collapse or a crevice repeatedly damages its protective film. The company training material makes this distinction central to pump material selection.

How an electrochemical cell damages a pump

Anodic regions lose metal to the electrolyte; a reduction reaction consumes electrons at cathodic regions. For a sustained galvanic couple there must be an electrolyte, anodic and cathodic areas, and an electronic path. These areas can occur on different connected parts or on microscopic regions of one material. A small anodic area coupled to a large cathodic area can lose thickness quickly.

Electrochemical corrosion cell in pump serviceA small anodic region dissolves in a liquid electrolyte while a larger connected cathodic surface supports reduction; electrons travel through the metal.LIQUID ELECTROLYTESMALL ANODELARGE CATHODEElectron path through connected metalMetal dissolution → localized lossCathodic reduction
New teaching diagram based on the Petropedam training material; relative areas illustrate risk, not a quantitative corrosion-rate prediction.
01 · BROAD ATTACK

Uniform / chemical attack

General loss can arise from chemical or electrochemical interaction with a liquid. Check temperature, concentration, exposure time and each wetted material. A single corrosion-rate figure does not establish fitness for the entire pump.

Distributed surface loss
Uniform thinning Original schematic; it is not a photograph or a measured corrosion profile.
02 · CONNECTED METALS

Galvanic corrosion

Inspect dissimilar wetted metals and the electrical contact between them: casing, impeller, fasteners and repairs. The small-anode / large-cathode geometry is particularly unfavorable in a conducting liquid.

Casing: metal AFastener: metal BPossible local attackConductive liquid
Galvanic risk at a pump casing joint. Conceptual pump section: two electrically connected, wetted metals in a conducting liquid can form a galvanic couple. Which metal is anodic depends on the actual alloys and environment; this drawing does not diagnose a failed pump.
03 · CONCENTRATION CELLS

Pitting and crevice attack

Deposits, gasket gaps and stagnant pockets can create a different local chemistry and oxygen supply. Ask about chlorides, oxidants, shutdowns and crevice geometry; a shallow average loss may conceal deep pits.

IMPELLER BLADELocal pitCASING JOINTCreviceExposed surfaceRestricted gap
Two places to inspect on a pump. Conceptual cross sections show a local pit on a wetted impeller blade and attack beneath a gasket at a casing joint. Pit shape alone cannot establish its cause; inspect the component, liquid chemistry and operating history.
04 · MICROSTRUCTURE

Intergranular corrosion

In susceptible stainless steels, heating or welding can precipitate chromium-rich carbides at grain boundaries, leaving adjacent regions chromium-depleted and vulnerable in certain media. Check the grade, carbon level, fabrication and heat-treatment history.

SENSITIZED STAINLESS STEELCr-depleted zone
Chromium-depleted boundary Original mechanism sketch; a metallographic examination is needed to diagnose sensitization.
Real micrograph of intergranular attack in austenitic cold rolled stainless steel sheet
Actual intergranular attack in austenitic stainless steel sheet. A laboratory micrograph, not a pump component. Photo: Antkyr; original image; CC BY-SA 3.0. Original photograph displayed at reduced size.
05 · SELECTIVE REMOVAL

Dealloying / graphitic corrosion

The source calls this “graphitization”: selective attack of iron phases in gray cast iron leaves a weak graphite-rich skeleton even when the external shape appears intact. AMPP distinguishes the preferred term graphitic corrosion from metallurgical graphitization, the formation of graphite by carbide decomposition.

Iron dissolves; graphite network remains
Cast iron after selective loss Original schematic of graphitic corrosion in gray cast iron, not a diagnostic micrograph.
06 · STRESS + ENVIRONMENT

Stress-corrosion cracking (SCC)

A susceptible material under sustained tensile stress can crack in a particular environment, sometimes with little visible general thinning. Examine weld/residual stress, fluid chemistry, temperature and crack morphology; do not infer SCC from a surface crack alone.

Sustained tension + susceptible medium
SCC under sustained tension Teaching sketch; fracture examination is needed to identify SCC.
Real stress corrosion cracks at welded reinforcement collar
Actual cracking at a welded reinforcement collar, identified as SCC by its photographer; this is not a pump component. Photo: CEphoto, Uwe Aranas; original image; CC BY-SA 3.0. Original photograph displayed at reduced size.
07 · CYCLIC LOADING

Corrosion fatigue

Repeated shaft loading or pressure cycling in a corrosive environment can shorten fatigue life; a pit may become a crack-initiation site. Check load history, stresses, the medium and fracture features together.

Alternating stress + environment
Corrosion with repeated loading Teaching sketch; a crack requires fracture and service-history analysis.
08 · FLOW-ASSISTED ATTACK

Erosion–corrosion

Solids, high local velocity and impingement can strip corrosion products or passive films, exposing fresh metal. Consider impeller tips, bends, recirculation areas and startup/upset conditions.

A severely damaged cast-iron water-pump impeller with visibly missing metal along its outer edge and vanes
Actual water-pump impeller: extensive metal loss at the blade edges. The missing sections are visible without magnification. Its source does not establish seawater service or quantify the contribution of erosion, corrosion and other mechanisms. Photo: Dr. Báder Imre, University of Miskolc; original, CC BY-SA 3.0.
09 · LIVING ENVIRONMENTS

Microbiologically influenced corrosion

In wet or stagnant pump service, biofilm and deposits can change chemistry at the metal surface and contribute to localized attack. A pit or a green deposit does not prove a microbial cause: check deposits, water chemistry, microorganisms and the damage morphology together.

PUMP CASING: WET SIDEBiofilm + depositImpellerPossible local pit
Where to inspect in a seawater pump. Conceptual section: deposits and biofilm may accumulate in a sheltered wetted casing recess, with localized metal loss underneath. This diagram illustrates a possible pathway; it is not a photograph or a diagnosis of MIC. AMPP: evidence needed for MIC.
10 · BUBBLE COLLAPSE

Cavitation erosion

Vapor bubbles formed at low local pressure collapse after moving into higher-pressure regions and can damage impellers. Check NPSH margin, suction conditions and operation before attributing metal loss to cavitation.

Photograph of a centrifugal pump impeller with cavitation damage
Cavitation damage on an actual impeller. The missing metal is a visible consequence; suction and operating data are needed to assess the cause in any new case. Photo: Klausbärbel; original, CC BY-SA 3.0. Original photo, resized by the browser.
11 · CONTACT + MOTION

Fretting / fretting corrosion

Small oscillatory relative movement at a fitted shaft, bearing seat or fastener contact can remove surface material. Oxidation may accompany the wear, but micromotion itself does not require a liquid electrolyte. Examine contact fit, vibration and evidence of motion.

Contact / micromotion
Small relative motion at a contact Original schematic; fretting wear can also occur without an electrolyte.
Real fretting corrosion marks on the inner raceway of a ball bearing
Actual fretting damage on a bearing inner raceway; this does not identify damage in any Petropedam pump. Photo: Fabian Schwack and Felix Prigge; original image; CC BY-SA 4.0. Original photograph displayed at reduced size.
12 · STRAINED REGIONS

Local stress and strained-metal effects

The original teaching figures describe a “stress cell”: deformation or local residual stress may alter corrosion behavior at a notch or shaft shoulder. Stress alone does not prove that the site becomes anodic; evaluate film damage, material, liquid and loading. Distinguish this risk modifier from SCC and corrosion fatigue.

Shaft shoulder / local strain
Local strain is a risk modifier Risk-location schematic, not proof that every strained region acts as an anode.
13 · SELECTIVE LEACHING

Dezincification of susceptible brass

The presentation also mentions selective leaching in copper-alloy components. In susceptible brass, zinc can be removed preferentially, leaving a porous copper-rich region with reduced strength. Identify the exact impeller or fitting alloy before applying this example; bronze alloys are not all brass.

Zinc selectively removed
Brass dezincification Original schematic; verify alloy and attack by metallography and chemical analysis.

Two microstructure lessons worth keeping

Cast iron graphitization and stainless steel sensitizationLeft: iron phases dissolve next to retained graphite in gray cast iron. Right: chromium carbide at a grain boundary depletes chromium on either side of the boundary.GRAY CAST IRONSENSITIZED STAINLESS STEELIron phases dissolve;graphite-rich skeleton remains.Cr carbides form at grain boundary;nearby Cr-depleted zones may attack.
New explanatory illustration derived from the Petropedam training material, with stainless sensitization cross-checked against the British Stainless Steel Association. Neither picture alone identifies an alloy grade or proves suitability.
Critical distinction:

Chromium and iron are elements of the stainless alloy; merely having both does not create an inevitable corroding “Cr–Fe interface.” The issue illustrated in the original teaching figure is chromium depletion beside a grain boundary after carbide precipitation under susceptible thermal conditions. A separate microscopic example depicts a microscopic galvanic couple between phases in pearlitic iron. These are two different examples.

Each photograph and original teaching diagram is now directly below its corresponding mechanism. External photographs are attributed in their captions and do not establish the cause of damage in any specific customer pump.

From mechanisms to pump components: erosion and marine biofilms

What the impeller photograph shows. The damaged blade edges above are an actual pump component. The image shows extensive metal loss but does not prove which share was caused by flow, particles, corrosion or cavitation, and its service liquid was not identified as seawater. For a documented seawater-pump repair example, see this separately identified seawater-pump case; the case photos are not used here.

How microbiologically influenced corrosion can arise in seawater service

During shutdown, low-flow or stagnant operation, a biofilm may accumulate in sheltered wetted regions, such as a casing recess, beneath deposits or near a gasket. Organisms and their metabolites can change local chemistry and oxygen availability, increasing the possibility of localized attack. Biofilm or pits alone do not prove MIC: evaluate the fluid, deposits, microbial evidence and corrosion morphology together.

The pump-casing illustration in the microbiologically influenced corrosion entry above points out the sheltered recess to inspect. Its colored deposit is a conceptual example, not a real pump sample or proof of microbial attack.

CREVICE CORROSION · FASTENER EXAMPLE

Why can a bolt or washer develop a deep local pit?

The original figure shows a flange/fastener crevice next to a deposit. Both surfaces may be the same metal. When oxygen is consumed in a tight, wetted gap but replenished in the open seawater, they experience different local environments. This differential-aeration (oxygen concentration) cell can make the low-oxygen crevice more anodic and the accessible surface more cathodic. A small anodic area can lose metal locally even while most of the part looks sound.

Redrawn pump flange bolt showing an oxygen concentration cell and optional microbial biofilmCross-section of a bolted pump flange: seawater and dissolved oxygen reach the exposed metal surfaces while a narrow oxygen-depleted crevice under the washer and deposit becomes an anodic site and forms a pit. Optional microbes may colonize the deposit but this diagram alone does not diagnose MIC.SEAWATER · OXYGEN AVAILABLE LOW OXYGENunder washer / depositANODE: LOCAL PITCATHODE: EXPOSED METALGreen deposit may host biofilm; biology is possible, not established by appearance.
New diagram adapted from the bolt, dirt and oxygen-cell examples on the original teaching figure. The blue surface represents accessible cathodic metal, the orange gap an oxygen-limited anodic region, and the red feature a possible pit. Green biofilm is an optional contributor. It is not a photograph or proof of MIC.
1. Gap or deposit

Washer, gasket, rust or accumulated solids restrict water exchange at the metal surface.

2. Different local chemistry

Oxygen can be consumed faster than it is replaced in the gap. In susceptible alloys, chloride concentration and acidity may also change locally.

3. Local metal loss

A concentration cell and loss of passivity can support crevice attack. A surface pit can also start outside a crevice after local passive-film breakdown.

Pitting is a damage shape; concentration-cell corrosion is a mechanism. They can coincide, but they are not synonyms. The impeller-blade diagram above illustrates the shape but does not identify the underlying mechanism. For stainless steels, chloride is a common trigger for localized passive-film breakdown; a crevice adds restricted mass transport and oxygen replenishment.

What about bacteria?

Biofilms can change oxygen access, pH or chemical species and may contribute to localized corrosion. However, the bolt-and-washer drawing is an oxygen concentration cell without requiring microbes. Do not diagnose MIC from a pit or dark deposit alone; combine metallurgical inspection, liquid/deposit chemistry, operation history and targeted microbiological evidence.

Mechanism cross-checks: AMPP · concentration cells / crevices · BSSA · crevice corrosion · BSSA · pitting · Vadde et al. · marine biofilms.

Useful seawater-pump case for comparison

A documented seawater-pump repair case describes general corrosion in a cast-iron pump and galvanic attack at its wear rings; it does not claim MIC. The case photographs remain on the publisher's site and are linked here rather than reproduced.

Material availability, wetted construction and chemical duty

The training tables add a useful distinction: a material may be available in a pump construction without being compatible with a particular liquid. Availability, chemistry and mechanical loading must be checked separately.

Material availability by pump construction

A = Available. This source table records material availability by pump construction. A is not an A/B/C quality, suitability or corrosion-resistance grade. A dash means no entry was given in the source.

Material availability by pump construction — selected categories (A = available)
MaterialChemical (ANSI)Process (API)SlurryBetween bearings / multistage / high speed
Carbon steel—A——
Ductile ironA———
316 stainless steelAA—A
Alloy 20AA—A
CD4MCuA——A
HastelloyA———
Duplex alloysA———
TitaniumA——A
ZirconiumA———
Silicon ironA———
Chrome iron——A—
Rubber——A—
ThermoplasticA———
Thermoset plasticA———
GlassA———
CeramicA———
GraphiteA———

A means available in the historical source; a dash represents a blank original entry. These entries are not a current Petropedam product list, a certification statement or a chemical-compatibility rating. Specific grades and manufacturer availability must be confirmed. View original table.

Nonmetallic pump for corrosive service

Original nonmetallic centrifugal pump for corrosive service
The original section demonstrates nonmetallic wetted construction for corrosive service. Check the actual polymer or other nonmetal, reinforcement, temperature, pressure, permeation and allowable nozzle loads. Open original at full size.

Chrome-lined slurry pump for high-temperature erosion service

Original chrome-lined slurry pump for high-temperature erosion service
This source figure is captioned chrome-lined slurry construction for high-temperature erosion service. Retain this description rather than treating it as another polymer-lined pump. Verify the actual lining alloy and service limits. Open original at full size.

General material-duty screening matrix

This historical matrix groups several different grades and media. Its symbols reproduce the training source: × acceptable, ? borderline, * not acceptable. They are teaching categories, not approval for an actual chemical duty; concentration, temperature, impurities, grade, exposure and stress can change the result. In particular, the original row places alcohol with hydrocarbon examples, although alcohol is chemically a different class.

Original general chemical-service screening matrix
DutySteel / ductile iron316 / special alloysPlasticCommon rubbersGlass / ceramicPTFE / graphite / Viton
Mildly corrosive liquids××××××
Hydrocarbons (source examples: benzene and alcohol)××??××
Chlorinated hydrocarbons with trace HCl××**××
Alkali (e.g. NaOH)×××?*×
Acids (e.g. HCl)*?×?××
Physical stress: thermal shock / piping loads××*×*?

The grouped “316 / special alloys” column must not be read as a single alloy recommendation. View original matrix and symbols.

Include non-metallic wetted parts

Non-metallic pump components may include elastomers, plastics, ceramics, graphite and glass. Check seals, liners and other wetted nonmetals against the actual liquid, concentration, temperature, pressure and exposure duration. Possible outcomes include swelling or softening of an incompatible elastomer and environmental stress cracking of a susceptible polymer. Oxygen and ozone exposure can age rubber outside the wetted zone; this is a different exposure from attack by the pumped liquid. Inspect the actual component and confirm its compound or polymer grade before choosing a replacement.

The two photographs are independently documented examples of different nonmetallic damage mechanisms. Neither photograph shows a Petropedam product or establishes compatibility of a specific pump seal or liner.

Sealing-material selection references: Parker O-Ring Handbook · medium, temperature and swelling · Trelleborg · chemical compatibility. The cited photographs illustrate visible damage; they do not determine an allowable fluid or temperature for a Petropedam seal.

Selection handoff: the corrosion dossier

Liquid

Identity, concentration range, pH, temperature, chlorides/oxidants/sulfides, dissolved oxygen, solids and entrained gas.

Operating conditions

Normal/upset cases, stagnation, cleaning, flow velocity, pressure, cavitation risk, vibration and duty duration.

Construction

All wetted alloys and coatings, weld/heat-treatment records, metal couples, gaps, seal faces, elastomers and liner.

Evidence

Service-specific compatibility and corrosion/erosion data, material specifications, vendor experience and an engineer's review. Only then finalize pump type and hydraulic duty.

Source: Petropedam engineering training material. Cross-checks: AMPP · Forms of Corrosion · AMPP · Galvanic Corrosion · AMPP · Graphitic Corrosion Terminology · AMPP · Brass Dezincification · AMPP · Stress Effects · BSSA · Intergranular Corrosion. Engineering teaching material; no quantitative material grade or guaranteed service life is asserted.

From fluid and materials to pumping

What a pump does before we read its curves

The corrosion lesson identified which liquid and wetted materials need attention. Now follow that liquid through the pump and the whole installation. This is the transition made on the Petropedam training material.

A pump transfers mechanical energy from a driver to a liquid. In a centrifugal pump, liquid enters at the suction, passes through the impeller, and leaves the casing through the discharge. The added energy supports flow against differences in vessel pressure and elevation as well as losses in the pipework.

A centrifugal pump, from inlet to outletSuction inletImpeller eyeCasing / voluteDischarge outlet
New teaching drawing based on the pumping topics introduced on the Petropedam training material. Labels are conceptual and do not describe a particular Petropedam model.

Twelve terms in the original pumping overview

the original teaching figure lists the vocabulary used in the next chapters. The short explanations below let a first-time reader follow the system diagram; the later sections develop the equations and checks.

Energy and flow

1. Pump energy / head: energy added per unit weight of liquid, often expressed in metres. 2. Flow, Q: liquid volume per time. 3. Energy added: what the driver and pump deliver to overcome the system requirement.

System and suction

4. System characteristic: head required at each flow by the installation. 5. Hydraulic power: ρgQH delivered to the liquid (with Q in m³/s). 6. NPSHA: system-supplied absolute suction head above the liquid's vapor-pressure head.

Liquid and geometry

7. Dissolved gases: may come out of solution as pressure falls and affect suction behavior. 8. Specific speed: a unit-dependent index relating speed, flow and head to hydraulic type. 9. Suction specific speed: a separate suction-related index; record the unit convention before comparing values.

Operating behavior

10. Number of stages: impellers in series inside one pump. 11. Operating capacity: actual flow at the installed operating point, which may differ from normal or rated flow. 12. Stability: whether the pump and system can maintain a suitable operating state as conditions change.

Keep the concepts separate: pump head is the energy added by the pump; system head is the energy the installation requires at a given flow. NPSHA concerns the pressure margin at the suction and needs its own check.

System Characteristics & Operating Point

The pump does not choose the flow independently. The operating flow occurs where the pump curve intersects the system curve for the active piping, vessel pressures and control configuration.

The vessels and piping largely define the steady system requirement independently of the selected pump. For reciprocating or some rotary pumps, pulsation and acceleration can also change the instantaneous suction condition; evaluate inlet adequacy with the actual pump and duty.

Trace the liquid from vessel 1 to vessel 2

The source vessel sets pressure P1 and liquid level z1. The pump draws liquid through the suction pipe (point 2), adds head between its suction and discharge connections (points 2 and 3), and sends the flow through discharge piping to the receiving vessel at P2 and z2 (point 4). Pipe and equipment losses increase with flow; the energy balance therefore changes as Q changes.

Two vessels, pump and four energy reference pointsSOURCE VESSELDESTINATION VESSELP1P21: z14: z22: suction3: dischargePUMP
Redrawn from the original two-vessel system sketch. Points 1 and 4 are the vessel boundaries; 2 and 3 are at the pump. Pressure and elevation references must be defined consistently.

Read the source sketch's hydraulic gradient: energy falls along the suction and discharge pipework because of losses and rises across the pump by the head it adds. The suction pressure must also remain sufficiently above the liquid's vapor pressure; NPSHA is checked separately from the plotted total system head.

Hydraulic grade across the pumping system1: source2: suction3: discharge4: destinationPUMPPipe losses reduce the grade.The pump raises it before more discharge-line losses.
Conceptual redrawing of the energy profile under the piping sketch in Petropedam training material. Head falls along each pipe and rises through the pump; suction NPSHA is a separate absolute-pressure check at point 2.

From the physical route to the operating point

For this illustrative turbulent-flow case, required head is approximately Hsystem(Q) = Hstatic+pressure + hloss(Q), with hloss often approximated as KQ² over a limited operating range. The intersection with the vendor's pump H–Q curve gives the installed operating point; changing vessel levels, pressures or a valve changes the system curve.

System characteristic and operating pointHead HFlow rate QSystem curvePump curveOperating pointStatic + pressure head at Q = 0Flow at intersection
Conceptual curves without measured axes or a certified duty point. The pump and system curves must be built for the actual installation.

Source: Petropedam engineering training material; Hydraulic Institute: pump/system operating point and NPSHA terminology.

Static / pressure component

Elevation difference and vessel-pressure difference can create a head requirement that is largely independent of flow.

Friction / equipment component

Piping, fittings, valves, filters and exchangers create flow-dependent losses. For turbulent piping, the friction component often varies approximately with Q² over a limited range.

Normal, rated, minimum and maximum are different concepts. Rated flow is the specified design point. It should not automatically be treated as the maximum flow or as “normal + 5%.” Define every required operating case explicitly.

Reference Datums & Pressure Basis

  • Use a consistent elevation datum for static head calculations.
  • Pressure differences used for TDH may be based on gauge or absolute pressure provided both are consistent; NPSH calculations require absolute pressure.
  • For vertical pumps, define the applicable first-stage impeller datum for NPSH.
  • Calculate the system curve for each meaningful valve/equipment configuration, not only one “normal” condition.
PUMPING ENERGY AND SUCTION

From pump energy to suction conditions

With the two vessels and their connecting pipework in view, follow the original lecture sequence: pump energy, flow definitions, system head, changing system conditions, NPSHA, cavitation, gas and the transition to pump classification.

Pump energy and energy at suction

The pump adds energy to move liquid from suction to discharge. That energy must overcome the difference in boundary pressure and elevation plus losses in suction piping, discharge piping, valves and equipment. At a steady operating point the pump's developed head equals the system's required head at the same flow; this is why discharge pressure alone does not state the complete pump head.

The original figure's “net energy at suction” leads into NPSHA: compare absolute total suction head with the pumped liquid's vapor-pressure head at its actual temperature, using the specified pump datum. This is a suction condition to check separately from total system head.

Rated, normal and minimum flow

Rated flow

The flow specified as the rated design duty for the selected pump. It is a contractual/design point, not automatically the highest flow the system will see.

Normal flow

The flow expected for most operating time. State whether the process has more than one normal case.

Minimum flow

The lowest required operating flow; it must be checked against the pump's allowable and continuous minimum-flow limits.

What the original figure says: it treats rated flow as generally the maximum and proposes a 5% margin over normal flow to avoid oversizing. Project check: neither assumption is universal. Define the normal, rated, minimum and maximum cases from the process specification, then check the manufacturer's operating limits. A 5% design margin can be considered only where justified for that service.

What makes up system head?

The original figure breaks the system requirement into pressure difference, liquid-level/elevation difference and friction losses. At equal boundary velocities: Hsystem(Q) ≈ ΔP/(ρg) + Δz + hfriction and equipment(Q). Include a velocity-head difference if the chosen boundaries require it. Use one elevation datum and the same pressure basis at both ends.

Pressure + elevation componentFlow-dependent lossesHead required by the installation = boundary difference + pipe/equipment losses
Original redrawing of the three system-head contributions. The fixed boundary-pressure and elevation component combines with flow-dependent piping loss.

The training material sketches hf proportional to Q for laminar flow and hf approximately proportional to Q² for turbulent flow. These are teaching approximations under specified pipe and fluid conditions; calculate friction with the actual Reynolds number, roughness, fittings and equipment. Its reference-datum examples are the horizontal shaft centerline, a vertical pump's first-stage impeller, and a can pump's suction-nozzle centerline. For procurement, use the datum required by the applicable standard and the manufacturer's drawing; do not substitute a generic datum for NPSH.

The system curve can move

the original teaching figure shows a family of system curves rather than a single fixed line. As source or destination levels change, a pressurized receiving vessel fills and its gas pressure varies, or valve resistance changes, the required head changes. Draw credible minimum and maximum cases. Their intersections with the same pump curve define a range of possible operating flows.

Higher system headLower system headFlow rate QExamples: tank level, vessel pressure or valve state changes
Conceptual reconstruction of changing system conditions: the upper and lower system curves bracket changing levels, vessel pressures or hydraulic resistance. The source figure uses a filling gas tank as an example.

Build NPSHA from the suction system

The figure's five terms can be written for a source vessel with gauge pressure Ps1,g as: NPSHA = (Ps1,g + Patm − Pv)/(ρg) + (z1 − zdatum) − hf,1→2 − ha. Here Pv is absolute vapor pressure at pumping temperature, hf,1→2 is suction-line loss, and ha is an acceleration-head allowance only where pulsating or transient suction flow requires it. If source pressure is already absolute, do not add atmospheric pressure again.

Absolute surfacepressure headLiquid-leveldifferenceSuction-linelossVapor-pressureheadAcceleration headif applicable+−−−NPSHA at the specified suction datumUse absolute pressure; evaluate the worst credible level, pressure, temperature, flow and transient.
Redrawn five-term NPSHA balance. The acceleration term is service-dependent; it does not automatically apply to steady centrifugal-pump operation.

The figure notes that two pressure terms cancel when a source liquid is at saturation: specifically, source-surface absolute pressure equals liquid vapor pressure if the source is in equilibrium. That cancellation does not make the remaining suction-loss or elevation terms disappear. Check the worst credible combination of source level/pressure, temperature, flow and transients. The original figure instructs calculating suction friction at the maximum flow rather than normal flow; this is an important high-flow check, but another combination may produce the lowest NPSHA.

NPSHA, NPSHR and cavitation

As suction pressure falls, a liquid can form vapor cavities; when those cavities move into higher pressure, collapse may cause noise, vibration, loss of head and eventual impeller or casing damage. NPSHA belongs to the system. NPSHR is a manufacturer-supplied pump characteristic for specified flow, speed and liquid. NPSH3 identifies the tested 3% head-drop criterion; equality with it does not mean cavitation-free service. Apply an appropriate project-specific margin and assess the full operating range.

The figure describes NPSHR for a centrifugal pump as a head and discusses NPIP for reciprocating and some rotary pumps. Use the governing pump standard and vendor terminology for each technology. Its shorthand that NPSHR is independent of specific gravity does not establish universal independence from liquid properties or service; confirm the vendor basis and any required corrections.

Dissolved gas and free gas

The figure raises three effects: gas can lower pump performance; gas bubbles are distinct from vapor cavities that collapse; and a gas phase can sometimes cushion a pressure pulse, as the figure suggests may reduce damage. This possible cushioning is not a design method or cavitation safeguard. The distinction is useful, but the figure calls gas “incompressible,” which is incorrect. Gas is compressible. Gas dissolved at high pressure can come out of solution when suction pressure drops, creating a free-gas fraction. Free gas can reduce centrifugal-pump head, disrupt priming or cause gas binding. It should never be treated as a reliable way to prevent cavitation or erosion.

Gas dissolved in liquidFree gas after pressure dropVapor cavitation is a separate phenomenon; verify the liquid, temperature and inlet conditions.
Teaching diagram: dissolved gas can form a free-gas phase when pressure falls. Gas ingestion, vapor cavitation and their effects require separate assessment.

Next chapter: pump classification

The next section introduces the classification of rotodynamic and positive-displacement pumps. Continue with the pump-classification chapter after the hydraulic and suction checks below.

Source: Petropedam engineering training material. Technical cross-checks: Hydraulic Institute system curves, HI definitions and NPSHA, and HI NPSH margin guidance. Statements from the original lecture that would mislead when applied universally are retained and qualified alongside the corrected engineering explanation.

Pump classification

Pump Classification — Choose the Technology from the Service

The original presentation separates kinetic/rotodynamic and displacement pumps. That framework remains useful, but no single variable such as pressure or viscosity should decide the technology by itself.

🔵 Rotodynamic Pumps

Continuous energy transfer to the liquid

CentrifugalMixed FlowAxial FlowRegenerative / special types

Cover a very broad range of flows and heads. Particularly attractive for continuous transfer where a smooth flow and efficient hydraulic match can be achieved.

🟠 Positive-Displacement Pumps

Displace a defined volume per cycle/revolution

Rotary: screw, gear, vane, lobe…Reciprocating: plunger, piston, diaphragm…

Strong candidates for viscous fluids, high differential pressure, controlled-volume or metering-like duties and services where a PD characteristic is advantageous.

Technology Screening Matrix

Service factorRotodynamic tendencyPositive-displacement tendency
High flow / moderate headOften strongTechnology-specific
Very high differential pressure at modest flowMay require multistageOften strong
High viscosityRequires viscous correctionOften advantageous
Precise flow regulation / meteringNeeds control systemOften advantageous
Abrasive solidsSlurry-specific hydraulics/materialsDepends strongly on PD type
Shear-sensitive liquidEvaluate speed/impellerEvaluate PD technology
No universal priority order. The historical presentation uses Centrifugal → Reciprocating → Rotary as a selection preference when multiple options exist. For a current engineering guide, rank feasible technologies by hydraulic fit, suction performance, operating range, fluid properties, control, reliability, maintainability, energy and lifecycle cost.

Calculating Total Dynamic Head (TDH)

TDH is the specific mechanical energy that the pump must add to the liquid, expressed as head of the pumped liquid.

General form: Hpump = (P2 − P1)/(ρg) + (z2 − z1) + (V2² − V1²)/(2g) + hL

For pressures in bar and specific gravity SG, a convenient approximation for pressure-head difference is:

ΔHpressure (m) ≈ 10.197 × ΔP(bar) / SG

Static / Elevation Head

Difference between the selected suction and discharge reference elevations.

Pressure Head

Difference in source/destination pressure converted to head of the pumped liquid.

Friction & Equipment Losses

Pipe, fittings, control valves, filters, exchangers and other equipment at the specified flow.

Velocity Head

Include when suction and discharge reference velocities differ materially.

Acceleration head is not a universal fixed allowance. The original training material used a legacy rule-of-thumb for some suction lines. For reciprocating pumps, long suction piping, rapid transients or fast valve actions, calculate/model acceleration and transient suction pressure explicitly.

Understanding Centrifugal Pump Performance Curves

A complete selection reviews head, efficiency, NPSHR and absorbed power together. The diagrams below use the same illustrative SOH2 80-50-250 dataset so the relationships are easy to compare; final engineering must use the certified performance curve for the selected pump.

Read four performance curves at the same flow

Before comparing different specific speeds, examine a typical pump characteristic with head, shaft input power, efficiency and NPSHR (NPSH3) against flow. Shutoff is at zero flow. BEP is the maximum of the efficiency curve. Head typically falls across the illustrated range, shaft power rises in this example, and the NPSHR curve rises on its measured portion. The y scales for the four quantities are independent; use the separate charts below for actual numerical comparisons.

Original typical centrifugal pump performance chart showing head, efficiency, power, NPSHR, shutoff and BEP against flow
Original teaching chart of a typical centrifugal pump performance characteristic. Head, efficiency, shaft power and NPSH3 use separate vertical scales. The curves are illustrative and do not establish the duty or operating limits of a specific pump.
2. Efficiency & BEP
Illustrative operating envelope0204060801000255075100Flow Q (m³/h)Efficiency η (%)Efficiency η–QBEPη = 82% @ 70 m³/h
BEP is the maximum-efficiency point for this example curve. The shaded band is illustrative only — formal POR/AOR limits are pump-specific and must not be inferred from a universal percentage.
3. NPSHR vs Flow
02040608010002468Flow Q (m³/h)NPSHR (m)NPSHRRated-point NPSHR3.2 m
NPSHR often rises toward high flow. Selection must compare the manufacturer-supplied NPSHR with the calculated NPSHA and the required application-specific margin.
4. Absorbed Power vs Flow
02040608010006.2512.518.7525Flow Q (m³/h)Power P (kW)Absorbed powerRated point13.5 kW estimated shaft input
Motor sizing must use the actual shaft-power curve, service factor/project rules and the maximum absorbed power over the specified operating envelope — not only the power at one duty point.
Flow (m³/h)Head (m)Efficiency (%)Power (kW)NPSHR (m)Interpretation
078——1.2Shutoff reference
3070708.22.2Low-flow point
60657913.53.2Example rated point
70638214.73.8BEP in example
85587817.44.8High-flow point
100507218.96.2Runout-side example
Do not infer a formal POR/AOR from this example dataset. The certified pump curve, applicable standard and manufacturer limits govern the actual operating region.
Rated-point power checkFor water at Q = 60 m³/h and H = 65 m: hydraulic power ≈ 10.63 kW. With η = 79%, estimated shaft input ≈ 10.63 / 0.79 = 13.46 kW ≈ 13.5 kW.

Best Efficiency Point (BEP)

BEP is the flow at which efficiency is maximum for a specified speed and impeller geometry. It is a useful hydraulic reference, not a universal acceptance band by itself.

⛭ PETROPEDAM ENGINEERING NOTE

Petropedam ranks selections by proximity to BEP together with POR/AOR, NPSH margin, power, impeller trim, materials and mechanical suitability. A pump is not accepted only because its duty point is “near BEP.”

Low-flow side of BEP

Potential concerns include suction/discharge recirculation, radial thrust, internal heating, vibration, seal/bearing loading and unstable operation.

High-flow side of BEP

Potential concerns include rising NPSHR, higher velocity, erosion, driver overload, runout and inlet choking depending on pump design.

Engineering rule: use the manufacturer- or standard-defined operating region for the actual pump. Fixed percentages such as 70–120% or 80–110% of BEP may be useful as rough screening examples in some contexts, but they are not universal POR limits.

Engineering Master Guide

Specific Speed & Suction Specific Speed

Similarity parameters explain hydraulic geometry and suction behavior, but their numerical values depend on the definition and unit system. Never compare values from different conventions without conversion.

Specific Speed, Ns

Ns ∝ N√Q / H3/4

Evaluated at a defined reference point, specific speed helps relate radial-, mixed- and axial-flow geometry, curve shape and practical stage selection.

Suction Specific Speed, Nss / S

S ∝ N√Q / NPSHR3/4

Used as an indicator of suction geometry and inlet-recirculation sensitivity. It is not a stand-alone pass/fail criterion.

Radial-flow tendency

Generally associated with higher head per unit flow and lower specific speed.

Mixed-flow tendency

Intermediate hydraulic geometry; often selected where both head and flow are significant.

Axial-flow tendency

Generally associated with high flow, low head per stage and higher specific speed.

Historical Nss limits in the original presentation

The original training material discusses historical suction-specific-speed screening values of 11,000–12,000 (US convention), and 15,000–17,000 for particular designs and operating ranges. Preserve these values only as historical screening guidance. Current selection should evaluate the actual NPSH margin, inlet geometry, recirculation behavior, service severity and manufacturer experience rather than impose one universal Nss cutoff.

IMPELLER GEOMETRY AND PERFORMANCE

How specific speed changes impeller geometry and pump curves

The original profile figure shows six sectional impeller silhouettes across eight labeled Ns values. The next figure compares three characteristic curve groups, followed by a chart of BEP efficiency across pump sizes.

Define the number and follow the impeller profiles

US customary specific speed: Ns = n × √Q(BEP) / [H(BEP,stage)]0.75. Here n is rpm, Q is total pump flow at BEP in US gpm, and H is head per stage at BEP in ft. The original figure uses the maximum-diameter impeller. The 500–15,000 values below are specific speed Ns, not suction-specific speed Nss; they are indexed figures, not measured duties, efficiencies or limits for a product.

Original Petropedam engineering figure showing impeller passage geometry versus US specific speed from 500 to 15000
Original impeller-profile figure supplied by Petropedam. Six sectional silhouettes span the eight specific-speed labels; the drawings illustrate radial, mixed and axial geometry.
Reading the eight values in the lecture figure
Ns (US)Illustrative impeller tendencyWhat to expect from the geometry
500 · 1,000Narrow radial passagesRelatively high head per stage and lower flow.
2,000 · 3,000 · 4,000Radial geometry opening toward mixed flowIncreasing flow in relation to head; do not infer an efficiency percentage.
5,000Transition toward mixed flow in the source illustrationA broader exit passage than the low-Ns sketches.
10,000 · 15,000Mixed-to-axial and propeller formsRelatively high flow and lower head per stage.

The profiles and groupings are qualitative; these eight values are points on a continuous index, not universal boundaries. To convert the same definition to the Hydraulic Institute's metric convention (rpm, m³/s, m head per stage), divide the US value by approximately 51.6. Check whether another source uses total flow or flow per impeller eye before comparing double-suction designs.

Worked example · calculate all eight Ns cases

To calculate a duty point consistent with each specific speed, fix speed and head per stage and solve the defining equation for flow: QBEP = [Ns × HBEP,stage0.75 / n]². For this arithmetic illustration alone, let n = 1,800 rpm and HBEP,stage = 100 ft. The same head and speed are hypothetical and are not taken from a Petropedam test.

Calculated Q at fixed 1,800 rpm and 100 ft per stage
Ns (US)5001,0002,0003,0004,0005,00010,00015,000
QBEP (US gpm)773091,2352,7784,9387,71630,86469,444

Important: Ns plus this chosen speed and BEP head determines one calculated BEP flow. It cannot determine the whole head–flow, power or efficiency curves. The three drawings below reproduce the qualitative trends in the lecture; project curves require a manufacturer curve or test.

Head, shaft power and efficiency change with Ns

The source diagrams plot three different quantities against flow for low, intermediate and high Ns. Head starts at shutoff (zero flow), efficiency rises to a maximum at BEP and falls away, and shaft input power changes differently for each hydraulic design. The three lines are separately scaled teaching curves; their crossings have no numeric meaning.

Original Petropedam performance figure with three groups of head, power and efficiency curves at different US specific speeds
Original characteristic-curve figure supplied by Petropedam. Each panel compares head, shaft power and efficiency against flow; ordinate values are schematic.
How to read the three source curve groups
Ns band in lectureHead–flow curveShaft-power trendEfficiency
500–1,000Often flatter near low flow.Generally rises toward higher flow in the sketch.Peaks at BEP for the particular design.
2,000–5,000Typically more drooping as Ns rises.May level toward high flow; verify any peak.Peak and usable region depend on actual hydraulics.
10,000–15,000Steeper decline with flow in the sketch; greater shutoff-to-BEP head ratio is possible.Can decline as flow increases; maximum load may occur at low flow.Efficiency still has its own BEP.

What the comparison changes in selection

The original says that shutoff head at higher Ns may reach about 2–3 times the BEP head. Treat this as a possibility shown in the teaching material, not a guaranteed ratio. Check the selected pump's certified Q–H curve and the system's shutoff pressure rating. Low-Ns sketches show a positive shaft-power slope; high-Ns sketches show a negative slope. A design can have maximum shaft power inside its operating range and be called non-overloading only when its full power curve and motor rating support that claim.

The lecture also warns that chasing a non-overloading power curve by selecting an oversized pump may reduce efficiency. Evaluate the actual BEP, the specified operating range, motor load and power peak before accepting a design. At the same Ns, different impeller geometry, trim, speed and liquid can yield different curves.

Attainable BEP efficiency depends on both size and Ns

The efficiency chart plots BEP efficiency (%) against Ns for separate curves labeled by BEP capacity (US gpm). This is a comparison between differently sized pumps at their own best-efficiency points. It is not an efficiency-versus-operating-flow curve for one selected pump. The historical source also distinguishes single/double suction pump efficiency (solid curves) from wet-pit bowl efficiency (dashed curves); the original chart below retains both sets of curves.

Original historical pump efficiency versus specific speed chart, with solid centrifugal-pump curves and dashed wet-pit bowl curves
Original historical teaching chart (Fig. 6.5) of efficiency versus US customary specific speed, with flow families in US gpm. Solid lines depict single- and double-suction centrifugal pumps; dashed lines depict wet-pit pump bowl efficiency. Read it qualitatively; it is not a guaranteed efficiency for any model.

Read the figure qualitatively: larger BEP flow generally offers higher attainable efficiency, while specific speed has an optimum region for each configuration. The current Hydraulic Institute HI 20.3 uses pump type, BEP flow, specific speed, design features, roughness and internal clearances to estimate normally attainable BEP efficiency. This historical plot must not replace that procedure or a certified efficiency curve. For a small pump, a wet-pit bowl, a slurry pump or a double-suction API pump, use the appropriate type and flow convention.

Original source: Petropedam engineering training material. Cross-check: Hydraulic Institute, Pump Principles (specific speed convention, conversion and qualitative curve trends); Hydraulic Institute, Pump Curves (head, efficiency and shaft input power).

Engineering Master Guide

BEP, POR, AOR & Allowable Flow Range

The operating region is a reliability envelope. A pump can produce the requested head and still be an unacceptable selection if it operates outside its allowable region.

Current reference: ANSI/HI 9.6.3-2024 addresses operating regions for rotodynamic pumps. POR and AOR are not universal fixed percentages for every pump.

Low-flow riskAORPOR — around BEPAORHigh-flow risk
▲ BEP reference

Minimum-flow constraints

Thermal rise, suction/discharge recirculation, radial/axial loads, vibration, seal/bearing limits, minimum stable flow and process requirements may govern.

Maximum-flow constraints

NPSH, power, runout, inlet choking, erosion/velocity, vibration and hydraulic stability may govern.

Minimum-flow recycle is engineered, not assumed. Set the recycle flow from the actual continuous/intermittent operating limits and service requirements. The original PowerPoint correctly emphasized continuous vs intermittent operation, but its historical numerical examples are retained with their applicability context in the detailed operating-limits chapter.

NPSH — Suction Performance & Cavitation Margin

NPSHA is a system characteristic. NPSHR is a manufacturer-supplied pump characteristic; NPSH3 is the tested NPSH associated with a 3% head drop under the applicable test definition.

Current guidance: ANSI/HI 9.6.1-2024 is the current Hydraulic Institute guideline for NPSH margin. Its 2024 update explicitly distinguishes NPSHA, NPSHR and NPSH3 and uses manufacturer-supplied NPSHR as the margin reference.

NPSHA — available

Calculate from absolute suction pressure/head, velocity and elevation terms minus vapor-pressure head and suction losses, using the correct datum and worst credible operating condition.

NPSHR — required

Use the manufacturer-supplied value for the selected pump, speed, impeller and operating point. Do not extrapolate casually across large changes in speed or geometry.

NPSH margin = NPSHA − NPSHR     |     NPSH ratio = NPSHA / NPSHR

What Cavitation Means

Cavitation occurs when local pressure falls sufficiently for vapor cavities to form and then collapse as they move into higher-pressure regions. Effects can include noise, vibration, erosion, head loss and instability. Erosion or noise may begin before the conventional 3% head-drop point.

What Reduces NPSHA?

Higher temperature

Usually raises vapor pressure and therefore reduces available suction margin.

Higher suction loss

Long/small piping, fittings, strainers and high flow reduce suction pressure at the pump.

Lower source pressure/level

Vacuum, altitude or low vessel level can reduce NPSHA.

Transient effects

Acceleration head, pulsation, rapid valve changes and long lines can temporarily reduce suction pressure.

Do not apply a universal 15%, 1 m or 50% margin rule. Those values appeared in the historical presentation as legacy guidance. Determine margin from service severity, operating range, liquid properties, energy level, transient behavior, applicable standard and manufacturer guidance.
Reciprocating and rotary inlet terminology

Positive-displacement pump inlet systems are strongly affected by pulsation and acceleration. Depending on the applicable standard and pump technology, inlet adequacy may be expressed using NPSH- or pressure-based terminology such as NPIP. Always use the terminology and method required by the governing pump standard/vendor.

How the 3% head-drop test is performed

At a fixed flow and rotational speed, the test pump is operated at several progressively lower values of available suction head. The head initially remains almost constant, then decreases as cavitation affects the passages. The NPSHA at a 3% reduction in the reference head is reported as NPSH3; for a multistage pump, this criterion applies to the first-stage head. The original training material calls this value NPSHR. Here, NPSH3 identifies the test criterion explicitly, while NPSHR remains the manufacturer-specified requirement used for selection.

Insufficient suction head can cause both cavitation erosion and loss of performance. These consequences need separate assessment: preserving most of the measured head does not establish that the impeller is free of damaging cavitation.

Original chart of pump head against available NPSH, showing breakdown, erosion and incipient cavitation
Original head-versus-NPSHA figure from the supplied engineering presentation. Read the head-loss criterion separately from the marked cavitation conditions. Open original at full size.

Cavitation effects do not all reach their limits at the same NPSH

The following original figure follows erosion, noise and pressure pulsations as NPSHA decreases. It shows why a single head-drop measurement cannot describe all cavitation effects. The severity may change non-monotonically; a quieter condition alone does not prove that erosion has disappeared.

Original plot of erosion, noise and pressure pulsations as available NPSH decreases
Historical cavitation-effect chart retained with its original regions, legend and source credit. The plotted behavior illustrates the mechanism; it is not a quantitative damage forecast for every pump. Open original at full size.

Two limitations of interpreting the head-drop test

The presentation identifies two limitations: the dependence of head breakdown on the vapor-to-liquid specific-volume ratio, vg/vf, at the inlet, and the relationship between the measured head loss and cavitation severity. Liquid temperature and thermodynamic properties can therefore change how a water-test result relates to the actual service.

The original reduction chart compares hydrocarbon liquids and high-temperature water, with temperature in °F and head reduction per stage. The next figure compares cavitation tests in different liquids at constant speed and capacity. Together they illustrate a thermodynamic effect, rather than an instruction to subtract a generic correction from every published NPSH curve. Apply a liquid-specific correction only with the manufacturer's validated method and the applicable project requirements.

Original thermodynamic NPSH reduction chart for hydrocarbon liquids and high-temperature water
Original historical thermodynamic reduction chart. Preserve the plotted units, liquid families and temperature basis when reading it. Open original at full size.
Original constant-speed constant-capacity cavitation test comparison between cold deaerated water and another liquid
Original comparative cavitation-test figure, including the critical-NPSH and liquid-dependent shift shown in the presentation. Open original at full size.

Why erosion can occur before a 3% head loss

The presentation describes research available in 1984–1986: depending on impeller material, hydraulic design and energy level, cavitation causing a 3% head loss—and even cavitation producing a smaller loss—could cause erosion. A smaller head-drop criterion was consequently proposed as an alternative definition of the required suction head.

Further observations showed that erosion could occur with available NPSH above the 3% head-drop value. Merely redefining the test around a smaller percentage did not establish an erosion-free condition. In the presentation's design discussion, suction specific speed brings related inlet-design characteristics into a common index. It supplements assessment of cavitation and recirculation; it cannot replace a service-specific NPSH margin.

Suction specific speed: definition, units and impeller-eye flow

Suction specific speed is a similarity index describing inlet hydraulic characteristics independently of physical pump size. The source uses the symbol S and bases its charts on the 3% head-drop criterion. To make the US unit convention explicit, the equation below uses Nss.

Nss = n × √Qeye,BEP / (NPSH3BEP)0.75
  • n: rotational speed in rpm.
  • Qeye,BEP: flow per impeller eye at BEP, in US gpm. Use total flow for single suction and half the total flow for double suction.
  • NPSH3BEP: the defined 3% head-drop NPSH at BEP, in ft, for the maximum-diameter impeller.

These numerical indices depend on the unit convention; Nss is not a flow rate even though the source associates its values with US gpm. Specific speed Ns uses head per stage, while suction specific speed Nss uses NPSH3. Keep the two indices separate.

Higher suction specific speed and the operating range

The source highlights two design tendencies as suction specific speed increases: a greater separation between incipient-cavitation NPSH and the 3% head-drop NPSH, and a greater tendency toward inlet recirculation. Noise, vibration and impeller erosion can become more significant, especially when the required flow range extends farther from the design condition. These tendencies must be checked against the actual inlet design and operating curves.

Original low and high suction-specific-speed curves comparing incipient cavitation and 3 percent head drop
Original comparison of low- and high-suction-specific-speed hydraulics. The incipient-cavitation and 3% head-drop lines describe different criteria. Open original at full size.

The presentation reports historical screening values of 11,000–12,000 in the US convention. It then discusses research indicating that 15,000–17,000 could be used for particular designs and operating ranges. Both ranges are retained here as part of the engineering lesson. They are neither universal current-standard limits nor blanket approval for a selected pump.

The next curve family shows how suction behavior changes across percent rated flow at different suction-specific-speed values. Review the full range rather than only the rated point. The following relationship chart compares specific speed and suction specific speed for different impeller forms; the plotted experience applies to its source population and should not be treated as a general acceptance boundary.

Original required NPSH versus percent rated flow for several suction-specific-speed families
Original NPSH curve families and source stability discussion. Use these curves to understand flow-range sensitivity, not as substitute curves for a selected model. Open original at full size.
Original relationship between specific speed and suction specific speed for radial mixed and axial impeller forms
Original comparison of Ns and Nss for different centrifugal-pump impeller forms. Keep the source definitions and unit convention when comparing designs. Open original at full size.

Definition cross-check: Hydraulic Institute — Pump Principles. Project selection also requires the applicable NPSH-margin guidance and the manufacturer's service-specific curves.

System corrections during starting and fast control changes

For normal steady centrifugal-pump flow, the presentation uses the actual operating flow for pipe-friction calculations rather than the pulsation peak-flow method associated with reciprocating pumps. Starting, stopping or rapidly changing flow still requires transient assessment.

Long suction piping and fast-acting control valves can make acceleration head important to instantaneous NPSHA. The source describes transient inlet disturbances, cavitation and pressure shock during starting. Shorter, appropriately designed suction piping and a controlled start or valve sequence may reduce these disturbances; use a pump-specific procedure rather than a universal valve instruction.

The presentation gives 1.5 ft (approximately 0.46 m, rounded to 0.5 m in the source) as an acceleration-head allowance for suction lines shorter than 70 pipe diameters, with further analysis for longer lines. Preserve these as historical preliminary assumptions. Actual acceleration head depends on flow acceleration, line geometry and controls, not length ratio alone.

Choosing the suction margin for the actual system

The source calls for worst-credible NPSHA, including liquid properties, gas, acceleration and system-induced transient reductions. Margin above the required value is intended to account for performance deterioration and cavitation erosion as well as transient conditions.

Its historical recommendations are approximately 15% of NPSHR, with a minimum of 3 ft or about 1 m, potentially increasing to 50% of NPSHR for severe transient/high-energy cases. These examples are preserved here alongside the earlier NPSH explanation. Use the applicable margin guideline and manufacturer requirements for final selection; these figures are not universal acceptance rules.

Head per stage, efficiency and suction

Choosing the number of pump stages

Stage count is a hydraulic selection decision: match the required head without sacrificing suction performance or attainable efficiency.

The source gives two historical US-specific-speed screening rules: Ns above 500, and above 1,000 when energy consumption is a major concern. These are design heuristics from the teaching material, not universal requirements for all centrifugal pumps. Evaluate the candidate's actual efficiency, duty range and construction.

At fixed flow and speed, increasing head per stage reduces Ns. The presentation considers two ways of increasing Ns or preventing it from becoming too low:

  1. Increase rotational speed. This can raise specific speed, but the resulting suction requirement can also increase. Since the system cannot always supply additional NPSHA, acceptable NPSHR and the actual suction margin constrain this approach. Recalculate the indices and review the manufacturer's curves rather than assuming that speed alone improves the selection.
  2. Reduce the head developed by each stage. Distribute the required total head over several stages in a multistage pump, or over pumps operating in series. The source describes this as reducing head per eye; use head per stage consistently in the Ns equation. Verify first-stage suction performance and downstream pressure containment.

The target is a pump arrangement with suitable efficiency and acceptable suction performance. The original selection map below relates head and flow to candidate arrangements; it illustrates the trade-off between energy consumption and suction requirements rather than specifying a guaranteed selection.

Original head-flow pump selection map for energy conservation and acceptable suction requirements
Original historical pump-selection map, retaining the source head/flow scales and arrangement labels. Confirm the selected arrangement using current manufacturer data. Open original at full size.
Pump and system interaction

Operating capacity and hydraulic stability

The actual duty belongs to the combined installation, not to the pump curve alone.

Operating capacity when suction performance changes

The operating flow is determined by the intersection of the pump head curve and the system head curve. Available suction head must satisfy the manufacturer's requirement with the appropriate service margin throughout the expected range.

If NPSHA falls below the required value, cavitation can reduce the head actually developed. The operating point still depends on the intersection of the system curve and the degraded pump curve; it no longer necessarily lies on the unaffected catalog curve. The original figure shows both the pump/system intersection and the available/required NPSH relationship.

Original pump and system interaction diagram with head curves and available and required NPSH
Original pump/system interaction figure. Compare the changed head curve with the suction-head curves at the same flow. Open original at full size.

A distinct, stable intersection

The presentation describes a clear, unique intersection as the desired operating condition. A continuously rising system curve intersecting a continuously falling pump curve is a common way to achieve it. Stability comes from the interaction of the two curves, rather than from the pump in isolation.

Original four-panel illustration of stable and unstable pump and system curve combinations
Original stable and unstable pump/system combinations. Read each complete pair of curves; the shape of the pump curve alone is insufficient. Open original at full size.

Head separation and controllability

Across the intended operating range, the relationship between pump head and system head must provide usable control authority. At the steady operating point the heads are equal; away from it, their relative slopes and separation determine how an adjustment in resistance or speed changes flow.

A drooping pump characteristic can form a stable system when paired with a suitable system curve. Conversely, multiple intersections or a poorly defined intersection require additional review of branch stability, valve behavior, operating transitions and the control strategy. This is why the original lesson asks the engineer to assess the complete installation.

Engineering training · centrifugal constructions

How Centrifugal Pump Construction Changes the Selection

The same flow and head can be produced by different mechanical arrangements. Petropedam's original training material illustrates horizontal and vertical shafts, single and multistage hydraulics, separate or close-coupled drivers, and different seal and bearing layouts. A hydraulic duty alone does not choose among them.

01 / OVERHUNG

Overhung impeller (OH)

An impeller outside the bearing span gives a comparatively compact arrangement. Evaluate shaft deflection, seal environment, driver coupling, piping loads and temperature for the selected configuration. Horizontal or vertical orientation does not by itself establish API compliance.

02 / BETWEEN BEARINGS

Impeller between bearings (BB)

Bearings support the rotor on both sides of the impeller or stages. Consider casing split, suction arrangement, axial thrust, seal access and the specific hydraulic duty. A double-suction design can reduce inlet velocity for a given total flow, but the actual NPSH characteristic must still be verified.

03 / VERTICALLY SUSPENDED

Vertical suspended construction (VS)

A suspended wet end can serve sumps, pits and deep liquid levels. Verify setting depth, minimum submergence, suction flow pattern, column losses, bearing lubrication and installation clearance. A submersible motor is a different architecture; do not identify every immersed pump as an API VS type.

Conceptual pump construction comparisonThree original simplified diagrams show an overhung rotor, a rotor between two bearings and a vertical suspended rotor. They illustrate construction and are not drawings of a particular product.Overhung: support on one sideBetween bearings: two supportsVertical: immersed wet end
Original teaching schematic. Bearings and impellers are simplified; classification and dimensions must be verified from each manufacturer's drawing.
What to request with a quotation

Arrangement drawing, certified duty curves, NPSH data, seal plan, materials, rotor and bearing design, minimum-flow limit, allowable nozzle loads, installation envelope and the standard specified in the purchase documents.

Engineering basis: Petropedam training material; Hydraulic Institute pump-type definitions. The figures above are conceptual, not certified Petropedam product drawings.

Reading the original chemical-process pump constructions

The following source figures show classifications, impeller sections and driver arrangements. Read the suction path, impeller, shaft, casing and driver relationship in each drawing. These historical illustrations teach construction; their embedded ANSI references and supplier credits do not establish current product certification.

Sealed, seal-less and special designs

Centrifugal pump types for chemical process — selected source categories
Source groupConstruction
Sealed
  • Horizontal, single stage
  • Vertical-in-line
  • Slurry
  • Between bearings, single stage
  • Multistage
  • Vertical multistage
  • High speed, single stage
Seal-less
  • Vertical sump
  • Vertical cantilever
  • Kestner
  • Magnetic drive
  • Canned motor
Special designs
  • Induced vortex
  • Regenerative turbine
Source-based classification adapted to the displayed scope. View unmodified original source chart.

The chart's sealed/seal-less grouping belongs to its original terminology. In selection, identify the actual shaft-sealing or containment arrangement rather than inferring zero leakage from a category name.

Open-impeller chemical-process pump

Original sectional drawing of an ANSI chemical-process pump with open impeller
Original open-impeller section. Follow the inlet into the impeller and inspect the impeller/casing clearance, shaft support and sealing region. Open original at full size.

Closed-impeller chemical-process pump

Original sectional drawing of a closed-impeller ANSI chemical-process pump
Original closed-impeller section. Compare the shrouded passages with the preceding open-impeller drawing; verify actual clearances, solids passage and service requirements. Open original at full size.

Construction and dimensional reference

Original chemical-process pump sectional and dimensional reference drawing
Original source section and dimensional reference. Enlarge to read the embedded detail. Use current manufacturer arrangement drawings for dimensions and installation requirements. Open original at full size.

Vertical inline motor arrangements

Original diagram of vertical inline motor arrangements VC VM and VB
Original vertical inline arrangement comparison, retaining the source VC, VM and VB labels. Compare shaft support and driver connections; do not translate these labels automatically into API pump classes. Open original at full size.

OH3, OH4 and OH5: identify the shaft and coupling arrangement

All three types are vertical inline, single-stage overhung arrangements. The distinction is how the pump shaft connects to the motor and which bearings support it. Compare the cutaways below with the preceding source arrangement chart.

OH3 — separate pump bearings and a flexible coupling
PumpWorks PWI-BB OH3 vertical inline cutaway showing bearing housing and motor coupling
OH3: a separate bearing housing supports the pump shaft; a flexible coupling connects it to the driver. Manufacturer reference: PumpWorks PWI-BB OH3.
OH4 — rigidly coupled
PumpWorks PWI OH4 vertical inline cutaway showing rigid coupling
OH4: separate pump and motor shafts are joined by a rigid coupling, with shaft support provided through the motor bearings. Manufacturer reference: PumpWorks PWI OH4.
OH5 — close coupled
Sulzer OHVc OH5 cutaway showing the impeller mounted directly on the motor shaft
OH5: the impeller is mounted directly on the motor shaft. The motor is outside this wet-end cutaway. Manufacturer reference: Sulzer OHV/OHVL brochure, OHVc design variant.

These credited manufacturer illustrations explain construction and are not Petropedam product photographs. The original historical drawings remain available: rigid coupling, external inline view, and separately coupled VB section. Verify the specified API class and compliance against the actual supplied design.

For a quotation, request the specific arrangement drawing, current dimensions, duty curves, seal/containment details, allowable nozzle loads and installation/maintenance clearances.

Slurry liners, wear plates and construction

These original sections show how the wetted construction changes for solids handling. Follow the impeller, casing liner, wear surfaces, shaft and bearings rather than identifying a material from its colour.

Rubber-lined slurry pump

Rubber-lined slurry pump
The original rubber-lined slurry section shows the liner arrangement around the impeller and casing. Open original at full size.

Hard-metal slurry pump with replaceable wear plates

Hard-metal slurry pump with replaceable wear plates
The original hard-metal section identifies replaceable wear plates. Compare their position with the rubber-lined section. Open original at full size.

Hard-metal slurry pump with a solid casing liner

Hard-metal slurry pump with a solid casing liner
The original cutaway photograph shows the solid casing liner and assembled shaft support. Open original at full size.

circulating pump

circulating pump
The original circulating pump illustrates a different casing and flow-path arrangement; it is not an OH3/OH4/OH5 classification example. Open original at full size.

Between-bearings and multistage casing arrangements

Single-stage, radially split, double-suction volute pump

Single-stage radially split double-suction volute pump sectional drawing
This original double-suction section is single-stage. Two suction paths feed the impeller; it should not be described as a multistage pump. Open original at full size.

Multistage configurations and thrust balance

The source table compares horizontal and vertical casing arrangements with tandem or opposed impellers. U means usual; A means available; a dash is the source table’s unmarked entry, not a universal prohibition. This is a historical configuration comparison, not a current availability guarantee.

Multistage pump configurations — original comparison
ImpellersThrust balanceHorizontal segmentHorizontal axial splitHorizontal barrelVertical segmentVertical can
TandemNone————U
TandemIndividualU———A
TandemBalancing deviceUAUU—
OpposedOpposed impellersAUA——

View the original configuration table.

Segmental casing: multiple potential leakage points

Segmental casing: multiple potential leakage points
The original diagram marks joints between segments and casing ends. More joints create more sealing interfaces to inspect; actual leakage depends on design, assembly and service. Open original at full size.

Axially split multistage pump with opposed impellers

Axially split multistage pump with opposed impellers
The original section shows opposed impeller groups used to reduce hydraulic axial thrust. Residual thrust and bearing duty still require verification across the operating range. Open original at full size.

Double-casing barrel pump with a radially split inner element

Double-casing barrel pump with a radially split inner element
The original barrel section shows an inner stage assembly enclosed by an outer pressure casing. Retain the source distinction between the outer barrel and the radially split inner element. Open original at full size.

Axial-thrust balancing devices

Axial-thrust balancing devices
The original drawing compares (a) balancing disc, (b) balancing drum, (c) stepped drum, and (d) combined disc/drum. Follow the axial and radial clearances and the return-to-suction passage where labelled. Balancing leakage and clearance condition affect performance and thrust; selection requires the manufacturer’s design data. Open original at full size.

Rotor stiffness and maximum running speed

Two pumps with the same hydraulic duty can have different rotor behaviour. The original guidance chart relates a shaft-stiffness parameter to maximum running speed for between-bearings rotors. Read the shaft span, mean diameter and rotor weight together; a long, slender rotor is more sensitive to imbalance, initial straightness and internal-clearance wear.

Original rotor stiffness guidance chart versus maximum operating speed
The chart retains its slender-shaft region, recommended wet-running design line and lower upper-limit line for dry-running capability. The horizontal axis is speed in thousands of rpm. Its numerical limits are historical guidance for the illustrated rotor model, not universal pump acceptance limits. Open original at full size.
K = √(W L³ / Dm⁴)

In the source’s N/mm convention, W is rotor weight in newtons, L is the bearing span in millimetres, and Dm is the mean shaft diameter in millimetres. The source also prints factors of 3.132 for kg/mm and 0.418 for lb/in. Use the matching weight/mass and length convention; K is the source’s index, not a dimensionless number or stiffness in N/m. This screening chart does not replace a rotor-dynamic analysis, critical-speed separation review or manufacturer evaluation.

Vertical multistage pumps: trace the stages and thrust path

Standard vertical multistage construction

Original standard vertical multistage pump cutaway with numbered component callouts
The original cutaway retains the component callouts, driver, shaft and repeated hydraulic stages. Follow the shaft downward and the liquid through the stages; the figure’s numbers are preserved without inventing a missing component legend. Open original at full size.

Vertical double-casing can pump

A can surrounds the submerged hydraulic assembly and connects it to the suction arrangement. The thrust load has to be carried by a specified bearing, either in the pump assembly or in the motor. The original examples below distinguish these two load paths.

Sulzer SJD-API vertical can pump cutaway with numbered stages and shaft details
Clearer modern construction reference: Sulzer SJD-API brochure. Numbered features: 1 first-stage impeller; 2 subsequent impellers; 3 column bearings; 4 discharge head; 5 shaft sealing; 6 spacer coupling; 7 suction can. This is a credited manufacturer illustration, not a Petropedam product photograph or an exact reproduction of the historical bearing arrangement.
Thrust-bearing arrangements in the original can-pump examples
Impeller arrangementLoad carriedThrust support
Individually balanced impellersResidual hydraulic axial thrust and rotor weightSeparate pump thrust bearing
Unbalanced impellersHydraulic axial thrust and rotor weightMotor thrust bearing
View the original section with separate thrust bearing
Original vertical double-casing can pump with individually balanced impellers
Original section: residual hydraulic thrust and rotor weight are supported by a separate thrust bearing. Retained labels include cooling, vent, oil sight glass, oil drain, can drain, suction and discharge. Open original at full size.
Original vertical can pump with thrust supported by motor bearing
Original contrasting arrangement: hydraulic axial thrust and rotor weight are carried by the motor thrust bearing. Compare the load path with the preceding separate-bearing example. Open original at full size.

High-speed single-stage pumps with integral gearing

The gearbox allows the impeller to run at a different speed from the driver. Read the hydraulic stage and transmission together: bearing loads, seal conditions, lubrication, gear cooling and suction requirements are part of the selection, alongside flow and head.

Barske-type high-speed pump

Original high-speed single-stage Barske-type pump cutaway
The original Barske-type construction is retained. Trace the gear train, high-speed shaft and pump hydraulic stage; it is a distinct impeller construction, not another view of the Francis-type pump. Open original at full size.

Francis-type high-speed pump

Original Francis-type high-speed pump section with labelled gearbox components
The source identifies the gear housing, thrust bearing, high-speed pinion gear, low-speed pinion gear, enclosed Francis-vane impeller, mechanical seals, lube-oil pump and gear-housing cooling fan. These labels are also provided as readable text. Open original at full size.

Historical seal-less wet-pit and dry-pit arrangements

In these source diagrams, “seal-less” describes arrangements without a conventional liquid shaft seal at the illustrated pump location. Leakage is directed back into the tank or return path, and a vapour seal is shown above the liquid. This terminology must not be confused with a hermetically contained magnetic-drive or canned-motor pump.

Wet-pit single-stage arrangement

Original single-stage wet-pit seal-less pump with submerged lineshaft bearings
The figure locates the vapour seal, discharge, suction tank, lineshaft bearings, liquid level and leakage from the pump. The bearing shown below the sump level needs the specified lubrication and liquid conditions. Open original at full size.

Dry-pit single-stage arrangement

Original single-stage dry-pit seal-less arrangement with leakage return
The original dry-pit diagram retains the liquid-below-vapour-seal requirement, leakage-return path and discharge direction. Dry-pit installation does not establish dry-running capability; assess the actual lubrication, drainage and containment design. Open original at full size.

Multistage wet-pit arrangement

Original multistage wet-pit seal-less arrangement showing shaft flow and leakage
The source detail shows the vapour seal, pump discharge, tank, leakage path, shaft and hydraulic stages below the drawing. Identify the pressure boundaries and returned leakage before choosing this arrangement for a fluid. Open original at full size.

From shaft-seal arrangements to hermetic containment

Single-stage vertical wet-pit sump pump

Original single-stage vertical wet-pit sump pump section
The original sump construction shows the driver above the pit and the hydraulic stage below. Compare the shaft span, support arrangement, discharge path and required liquid level with the preceding wet-pit diagrams. Open original at full size.

Magnetic-drive centrifugal pump

A magnetic coupling transfers torque through a stationary containment shell. The inner pump shaft is separated from the external driver; fluid compatibility, internal bearing lubrication, circulation and protection against overheating remain part of selection.

Original hermetically sealed magnetic-drive pump section and sleeve-bearing detail
The original drawing retains the magnetic-drive section and the zero-leak sleeve-bearing detail. Identify the containment boundary and internal support; it differs from the historical sump leakage-return arrangements. Open original at full size.

Canned-motor centrifugal pump

The pump and motor rotor form an integrated assembly, with a stationary can separating the wetted rotor from the stator. The illustrated CNP design uses a partial liquid flow to cool the motor and lubricate bearings, returning it through the hollow shaft toward the impeller suction.

HERMETIC CNP canned-motor pump cutaway showing internal cooling and bearing-lubrication flow
Clear vector-derived illustration from HERMETIC CNP product information. Red arrows show this design’s internal circulation. This is a manufacturer teaching reference; cooling routes and permissible fluid conditions vary by design.
View the original canned-motor pump illustration
Original canned-motor pump for corrosive service
Original corrosive-service canned-motor section from the training material; retained for comparison with the modern CNP example. Open original at full size.

Induced-vortex or torque-flow pump

Original induced-vortex or torque-flow pump sectional drawing
The original section shows the recessed impeller and open passage. Compare the hydraulic layout and solids passage with conventional impeller arrangements; the actual particle limit and performance come from the selected design. Open original at full size.
Engineering training · rotary displacement

When a Rotary Positive-Displacement Pump Is the Better Candidate

The original presentation compares internal and external gears, screws, lobes and vane designs. All move liquid by changing the volume of enclosed spaces, but their clearances, allowable solids, shear characteristics and maintenance needs differ.

GEAR

Gear pumps

Often considered for clean, lubricating liquids and controlled flows. Check viscosity and temperature at startup and operation, internal slip, allowable differential pressure, wear and compatibility of clearances with the fluid.

SCREW

Screw pumps

May provide comparatively smooth delivery in suitable viscous-liquid services. Verify the specific screw design, suction capability, allowable contamination and the manufacturer's operating envelope.

LOBE

Lobe pumps

Can be considered where fluid handling, cleanability or reduced product damage is important. Suitability for solids, abrasives or sanitation depends on the actual construction, speed and material selection.

VANE

Vane pumps

Check liquid lubrication, particle size, vane and liner wear, vapor pressure and the limits of the selected arrangement. Do not infer broad solids tolerance from the rotary principle alone.

Positive displacement flow principleOriginal diagram shows liquid entering an expanding cavity, carried through a pump and discharged when the cavity volume contracts. It is a teaching representation, not a specific pump cross-section.INLETMOVING CAVITIESDISCHARGE
Original conceptual schematic. Flow pulsation, internal slip and solids tolerance differ by technology and must be checked for the specified pump.
Five essential checks before selection

Specify minimum/normal/maximum viscosity at operating temperature; allowable particles and gas; required differential pressure across all cases; suction pressure and start-up conditions; and independent overpressure protection. Positive-displacement pumps require protection against a blocked discharge. Never assume a catalog's maximum flow and maximum pressure occur simultaneously.

Engineering basis: Petropedam training material. For petroleum, chemical and gas services, consult the contractual API standards and publication status for the specific pump technology.

Rotary pump selection: liquid first, mechanism second

Original rotary pump selection process flowchart
The source sequence is rotary pump → type group based on liquid → specific type based on performance requirements → system/rating corrections based on the selected type. It continues the existing rotary-pump checks with a structured selection route. Open original at full size.

Three liquid-based mechanism groups

Original rotary groups and liquid characteristics
GroupMechanism support / force transmissionLiquid characteristics
1Internal mechanism support and force transmissionMedium viscosity; Newtonian; good lubricity; free of solids and entrained gas
2External mechanism support and force transmissionMedium viscosity; Newtonian; poor lubricity; nonabrasive solids; entrained gas in some designs
3Elastomer element in the mechanismMedium to high viscosity; non-Newtonian; abrasive solids

These group numbers belong to the source classification. Their broad liquid descriptions are a screening route; they do not establish solids tolerance or allowable gas for every pump in a group. View original group table.

Select a group from the liquid properties

Original liquid-property decision chart for rotary pump groups
The original chart sends non-Newtonian liquids to Group 3. For Newtonian liquids, entrained gas leads to Group 2; nonabrasive solids lead to Group 2 and abrasive solids to Group 3. With neither gas nor solids, low viscosity/lubricity leads to Group 2 and high viscosity/lubricity to Group 1. Confirm the actual design after this historical screening step. Open original at full size.

Map the group to the source pump types

Original sealed and seal-less rotary pump type classification
Original sealed types: Group 1 — sliding vane, internal gear, external gear, three screw, two screw; Group 2 — lobe, external gear, two screw; Group 3 — flexible vane and helical rotor (single screw). Original seal-less types: Group 1 — magnetic-drive sliding vane or external gear; Group 2 — peristaltic flexible tube. Some technologies appear in more than one group because the mechanism arrangement differs. Open original at full size.

Typical flow pulsation values in the training source

The source expresses pulsation as a percentage of mean flow. These historical typical values compare delivery patterns; a listed zero is idealized and does not guarantee zero installed-system pulsation. Actual waveform and pulsation depend on geometry, speed, fluid, pressure, piping and dampener design.

Original typical flow pulsations
Pump type / arrangementFlow pulsation, % of mean
Any with inlet dampener0
Screw: two and three screw0
Gear: helical0
Vane5
Lobe: two lobe25
Lobe: three lobe15
Helical rotor3
Peristaltic: two roller, planar cam100
Peristaltic: three roller50

View original pulsation table. Compare the required delivery smoothness with the process and system response; retain the independent overpressure protection already described above.

Rotary performance capabilities and construction details

The following original sections show how each rotary mechanism transports liquid and how bearing, shaft-seal and timing arrangements differ. Read them alongside the liquid-based selection workflow above.

Typical performance capabilities by rotary type

Rotary pumps for chemical processing: typical performance capabilities in the training source
ContainmentSource groupTypeFlow (gal/min)Differential pressure (lb/in²)Viscosity (SSU)Temperature (°F)
Sealed1Sliding vane1,000125500,000450
Sealed1Internal gear1,0002501,000,000650
Sealed1External gear2,0003001,000,000650
Sealed1Triple screw3,0003,00050,000200
Sealed1Twin screw4,0002,0001,000,000300
Sealed2Lobe4004501,000,000500
Sealed2External gear2,0003001,000,000650
Sealed2Twin screw10,0002,5001,000,000850
Sealed3Flexible vane10030100,000180
Sealed3Helical rotor1,2503001,000,000200
Sealless1Magnetic-drive sliding vane6015N.D.N.D.
Sealless1Magnetic-drive external gear6010035,000N.D.
Sealless2Peristaltic300220—180

Values and units are retained from the training table. N.D. is the source notation for data not given; a dash represents a blank source cell. The source group numbers refer to the preceding historical classification. These figures are teaching comparisons, not Petropedam product ratings or simultaneous operating limits. SSU is a viscosity measure expressed in Saybolt Universal seconds; conversion to other viscosity units requires the measurement conditions.

View the original performance-capability table
Original rotary-pump performance-capability table with flow, differential pressure, viscosity and temperature
Original numerical table, retained unchanged for comparison with the readable transcription. Open original at full size.

Sliding-vane and gear pumping mechanisms

Sliding-vane pump

Vanes move radially in rotor slots as the rotor turns in an eccentric housing. The spaces between adjacent vanes expand at the inlet and contract toward the outlet.

Sliding-vane pump original construction drawing
Original internal, vane-in-rotor pump drawing. Follow the vane motion and changing chamber volume. Open original at full size.
Internal gear pump

Liquid fills the spaces exposed as the gears separate at the inlet, travels around the casing, and is displaced when the teeth re-engage at the outlet. The three original views show this sequence.

Internal gear pump original construction drawing
Original internal-gear pumping sequence: liquid inlet, passage of liquid and liquid outlet. Open original at full size.
External gear pump

Two externally toothed gears transport liquid in the spaces between the teeth and casing. Liquid travels around the outside of the gears; the meshing zone separates suction from discharge.

External gear pump original construction drawing
Original external-gear pump transverse section and flow-direction arrows. Open original at full size.
External gear pump with wetted bearings

This section adds the shaft-support and sealing arrangement to the preceding pumping principle. Bearings in the pumped liquid make liquid lubrication and contamination important selection inputs.

External gear pump with wetted bearings original construction drawing
Original simple external-gear pump: bearings in the pumped liquid and one shaft seal, as identified in the source. Open original at full size.
Externally timed external gear pump

External timing and bearing support provide a different arrangement for liquids with poor lubricity. Compare the external support and shaft seals with the wetted-bearing construction.

Externally timed external gear pump original construction drawing
Original externally timed external-gear pump for liquids with poor lubricity: four shaft seals and external bearings, as identified in the source. Open original at full size.

Screw pumps: flow paths, timing and rotor support

Double-end screw-pump construction

The original schematic shows suction at both ends and discharge in the middle. Follow the arrows from the two suction regions marked Pₛ toward the central discharge marked Pᴅ.

Double-end screw-pump construction original section
Original double-end screw construction showing the two inlet flow paths and central outlet. Open original at full size.
Internally timed twin-screw pump

The source section shows the internally timed arrangement. Compare the rotor relationship and internal support with the externally timed example that follows.

Internally timed twin-screw pump original section
Original internally timed twin-screw pump section. Open original at full size.
Externally timed twin-screw pump

The timing mechanism is outside the pumping chamber, with external bearings. Liquid lubricity and the location of bearings and seals distinguish this construction from the preceding example.

Externally timed twin-screw pump original section
Original externally timed, external-bearing twin-screw pump section. Open original at full size.
Three-screw pump

The original longitudinal section shows three intermeshing screws and the inlet and outlet arrows. Their rotation moves enclosed liquid volumes along the screw passages.

Three-screw pump original section
Original three-screw pump longitudinal section; retain the inlet and outlet arrangement shown. Open original at full size.

Rotary lobe pumps: rotor view and bearing arrangement

The rotor-face photograph and longitudinal section show complementary views of a lobe pump. Liquid travels around the outside of the lobes; external timing gears maintain their relative position. The bearing and sealing arrangement must be considered with the required differential pressure and liquid properties.

Original overhung rotary-lobe pump rotor-face photograph
Original overhung rotary-lobe pump photograph, showing the two pumping rotors and casing. Open original at full size.
Original between-bearings rotary-lobe pump section
Original between-bearings rotary-lobe pump longitudinal section. Compare the shaft support and timing mechanism with the overhung construction. Open original at full size.

Further reading: Hydraulic Institute pump-type definitions; Pump School rotary selection guidance. The original construction illustrations and numerical table remain the engineering training source.

Flexible vane, helical rotor and peristaltic constructions

Flexible vane: operation, performance and section

The original sequence shows the vane deformation, the flow–pressure curves at three rotational speeds, and the pump cross-section.

Flexible vane pump operation — original Figure 7.31
Original flexible vane operation drawing. Follow the inlet and outlet arrows and deformation of the vanes around the rotor. Open original figure at full size. View supplied source image.
Typical flexible vane pump performance curve — original Figure 7.32
Original flexible vane curves at 500, 870 and 1,160 rpm, with the normal operating range marked. Axes are flow in gpm and total head in psi. This is a source teaching curve, not a Petropedam pump rating. Open original figure at full size. View supplied source image.
Flexible vane pump section — original Figure 7.33
Original flexible vane pump section, retaining the shaft, sealing and pumping chamber details. Open original figure at full size. View supplied source image.

Helical rotor / progressing cavity: construction and materials

The closed-throat and open-throat sections show the rotor/stator assembly, drive connection and inlet arrangements. The material table is reproduced below in readable form, followed by the supplied original.

Helical rotor pump section — original Figure 7.34
Original helical rotor pump longitudinal section. Open original figure at full size. View supplied source image.
Rotor and stator materials for various services — original Table 7.4
PartMaterialService
RotorNitrided steelAbrasive, noncorrosive
RotorChrome plated 316Corrosive, with or without abrasives
RotorMonel or high nickel-molybdenum alloysHighly corrosive
StatorNatural rubberGeneral
StatorNitrile rubberOil, fats, effluent
StatorCast urethaneAbrasive, aqueous slurries
StatorHypalonaMineral acids, oxidizing chemicals
StatorVitonaAliphatic and aromatic hydrocarbons, high temperature

a Registered trademark of DuPont, as noted in the original table. These service descriptions are retained from the training source; verify compatibility for the actual liquid composition and temperature.

Rotor and stator materials for various services — original Table 7.4
Original rotor and stator material table, retained for comparison with the HTML transcription. Open original figure at full size. View supplied source image.
Open throat helical rotor pump — original Figure 7.35
Original open throat helical rotor pump longitudinal section. Open original figure at full size. View supplied source image.

Peristaltic pumps: hose compression and delivery

The original illustrations compare flexible-tube, three-roller and sliding-shoe arrangements. The performance drawing shows the qualitative change of delivery as suction pressure decreases and discharge pressure increases.

Flexible tube and three roller peristaltic pumps — original Figures 7.36 and 7.38
Original flexible tube pump and three roller peristaltic pump drawings. The hose is compressed by the moving rollers. Open original figure at full size. View supplied source image.
Sliding shoe peristaltic pump and qualitative performance variation — original Figures 7.39 and 7.37
Original sliding shoe peristaltic pump photograph and performance variation drawing. The performance plot has no numerical scale; it must not be used to infer a specific flow or pressure rating. Open original figure at full size. View supplied source image.
Engineering training · reciprocating displacement

Inside a Reciprocating Pump: From Stroke to System Behavior

The original teaching figures show a piston or plunger moving through suction and discharge strokes. Check valves control direction. The cyclic motion creates flow pulsation, which makes suction and discharge system design central to reliable operation.

Reciprocating pump operating principle — original Figure 8.1
Original suction and discharge stroke drawings. In the suction stroke, the inlet valve is open and the discharge valve is closed; their roles reverse during discharge. Open original figure at full size. View supplied source image.
SUCTION

Protect inlet conditions

Check minimum inlet pressure, acceleration effects, suction-line losses, available inlet energy and fluid vapor pressure through the entire operating range. For viscous liquid or entrained gas, obtain project-specific vendor guidance.

PULSATION

Evaluate piping response

Check cyclic pressure, pulsation-control equipment, valve behavior, piping forces and vibration. A simplified average flow does not capture instantaneous line conditions.

PROTECTION

Prevent overpressure

Provide discharge pressure protection appropriate to the system. Check seals/packing, valve maintenance, driver torque, corrosion and abrasive wear against each duty case.

Practical comparison

A reciprocating pump can be attractive for high differential pressure at a suitable flow, but the final decision must include suction conditions, liquid cleanliness, speed, pulsation limits, maintainability and total system cost. Pressure alone is not sufficient.

Engineering basis: Petropedam training material; API Standard 674 publication plan. The edition required by the purchase specification governs.

Reciprocating selection and classification

Selection from seals, solids and pressure

The original decision tree starts with whether seals are allowed, then checks abrasive solids and pressure. It uses 2,000 psig for the nonabrasive branch and 1,500 psig for the abrasive-solids branch. The starred options indicate designs for solids handling.

Reciprocating pump selection — original Figure 8.18
Original reciprocating selection tree. It distinguishes diaphragm, piston and plunger options, including mobile separator and solids-handling designs. These historical screening thresholds do not replace the vendor operating envelope. Open original figure at full size. View supplied source image.

Power and controlled-volume pump classes

The power-pump branch identifies horizontal or vertical orientation, single- or double-acting operation, piston or plunger elements, and simplex, duplex or multiplex arrangements. The controlled-volume branch includes plunger, piston and diaphragm designs, mechanical or hydraulic diaphragm coupling, and manual or automatic control.

Classes of reciprocating pumps — original Figure 8.20
Original classification of reciprocating pumps, retaining both the power and controlled-volume branches and their connections. Open original figure at full size. View supplied source image.

Double-acting operation

Both sides of the piston participate in pumping. In the illustrated outward stroke, discharge occurs from the outer end while suction occurs at the inner end; the valve roles reverse on the inward stroke.

Double acting reciprocating pump — original Figure 8.2
Original double acting reciprocating pump drawing, including the suction and discharge valves and displacement volume. Open original figure at full size. View supplied source image.

Metering, diaphragm and piston pump construction

Metering pump motion and stroke length

Original training figure: Metering pump operating principle
Original metering pump operating-principle drawing, showing the drive motion, reciprocating pumping element and liquid-valve arrangement. Open original figure at full size. View supplied source image.
Original training figure: Metering pump at 100 percent stroke length
Original drawing at 100% stroke length (h), with h max marked at the adjustment and pumping mechanism. This is the depicted full-stroke condition, not a universal flow calibration. Open original figure at full size. View supplied source image.

Diaphragm metering pump and drive arrangement

Original training figure: Diaphragm metering pump with freely moving double diaphragms
Original AREX diaphragm metering pump type CS with freely-moving double diaphragms, as captioned in the supplied training image. Open original figure at full size. View supplied source image.
Original training figure: Pump cam and gearing with mechanically coupled diaphragm pump head
Original pump cam and gearing with mechanically coupled diaphragm pump head. Follow the cam-to-diaphragm motion and inlet/outlet valve arrangement. Open original figure at full size. View supplied source image.
Original training figure: Diaphragm pump head with freely moving plastic diaphragm
Original diaphragm pump head with freely-moving plastic diaphragm. The drawing distinguishes pumped liquid, hydraulic fluid under variable pressure and hydraulic fluid under atmospheric pressure. Open original figure at full size. View supplied source image.
Numbered parts in the supplied diaphragm head
NumberPart
1Double diaphragm
2Perforated disc
3Control pin
4Tappet
5Replenishing valve
6Air release valve
7Spring
8Plunger
9Pressure limiting valve

Eccentric crosshead and plunger construction

Original training figure: Eccentric crosshead gear with plunger pump head
Original AREX eccentric crosshead gear with plunger pump head. The two pumping heads and central drive arrangement remain as shown in the source. Open original figure at full size. View supplied source image.

Source performance envelope

Original training figure: AREX performance summary
Original AREX performance summary with pressure in bar and flow rate in l/h on logarithmic axes. J, C and CS are the source curve labels. Retained as a training reference; these envelopes are not Petropedam product ratings and the upper flow and pressure limits must not be combined as a single duty point. Open original figure at full size. View supplied source image.

Power pump frame loading

Original training figure: Power pump frame loading
Original Figure 8.22 compares single-acting discharge-stroke-out, single-acting discharge-stroke-in and double-acting arrangements. The arrows identify tension and compression loading in the frame. Open original figure at full size. View supplied source image.

Duplex diaphragm and side-pot piston arrangements

Original training figure: Duplex diaphragm pump
Original Figure 8.26: duplex diaphragm pump, retaining the pump intake, air inlet, air exhaust and pump discharge labels. Open original figure at full size. View supplied source image.
Original training figure: Side-pot type piston pump
Original Figure 8.28: side-pot type piston pump, retaining the photograph and longitudinal section together. Open original figure at full size. View supplied source image.
Engineering training · curve interpretation

What Changes When the Duty Point Moves Away from BEP?

The training teaching figures distinguish low-flow, preferred and high-flow behavior. These are risks to evaluate, not universal alarm thresholds: the manufacturer's POR, AOR, minimum flow and suction requirements govern for the specific pump.

LEFT OF BEP

Low flow

Investigate temperature rise, suction or discharge recirculation, radial loading, seal cooling and minimum continuous stable flow. A bypass can preserve pump flow, but its required capacity must be calculated.

NEAR BEP

Preferred hydraulic zone

Review the manufacturer's stated POR and all normal operating cases. BEP proximity by itself does not validate suction margin, material choice or mechanical suitability.

RIGHT OF BEP

High flow

Check required NPSH, power, motor loading and any inlet or discharge limitation. Where conditions vary, verify the full system curve and actual pump operating points.

Engineering basis: Petropedam training material; ANSI/HI 9.6.3 operating regions and ANSI/HI 9.6.1 NPSH margin.

Practical Pump Selection Workflow

A robust selection is a sequence of feasibility checks followed by engineering ranking — not a single lookup by Q and H.

1. Duty cases →2. Liquid properties →3. System curve →4. Pump technology →5. Hydraulic fit →6. NPSH →7. POR/AOR →8. Materials & seals →9. Driver →10. Standard →11. Lifecycle review

Feasibility gate

Reject candidates that cannot meet duty, NPSH, operating-region, pressure, temperature, material or mechanical requirements.

Hydraulic ranking

Compare BEP proximity, efficiency, NPSHR, impeller trim, curve stability, power and operating envelope.

System ranking

Compare control method, parallel/series behavior, minimum flow, transients and plant operating philosophy.

Lifecycle ranking

Compare energy, maintainability, spares, seal strategy, reliability, inspection/testing and project risk.

⛭ PETROPEDAM ENGINEERING NOTE

When several pumps are hydraulically feasible, Petropedam should present the best selection plus clearly separated alternatives. Do not combine different pump models or speeds into one synthetic performance curve.

API 610Rotodynamic process pumps

API 610 — Centrifugal Pumps for Petroleum, Petrochemical & Natural Gas Industries

Standards status — verified September 2026: API's official Standards Plan lists API 610, 13th Edition, dated June 29, 2026. A project purchase specification may still invoke Edition 12 or another contractual edition; the project document governs.

API 610 establishes minimum requirements for critical process pump design, materials, mechanical integrity, inspection, testing and documentation. It should not be interpreted as making complete pumps universally interchangeable between manufacturers.

OH — Overhung

API 610 includes multiple overhung configurations (OH1 through OH6). Exact construction and driver arrangement depend on the type.

BB — Between Bearings

BB1 through BB5 cover single- and multistage between-bearings configurations, including axially/radially split and barrel-style designs.

VS — Vertically Suspended

VS1 through VS7 cover vertical suspended configurations with different bowl, casing and discharge arrangements.

Stage count: select from required head per stage, hydraulic geometry, efficiency, NPSH, pressure containment, rotor dynamics, axial thrust, maintainability and mechanical limits. Do not impose one universal minimum specific-speed value as a pass/fail criterion.

When API 610 Is Appropriate

Typically considered for demanding petroleum, petrochemical and natural-gas process services where reliability, testing, documentation and consequences of failure justify the standard. For less critical industrial services, other standards/specifications may be more economical if the project permits.

⛭ PETROPEDAM ENGINEERING NOTE

Apply the contractual edition and project supplements. “Current edition” and “project-required edition” are not always the same.

API 674Reciprocating positive-displacement pumps

API 674 — Reciprocating Positive-Displacement Pumps

Publication status — verified September 2026: API's published catalog identifies API 674, 3rd Edition (2010). API's Standards Plan lists Edition 4 in development/editing. Do not label Edition 4 as published until API issues it.

API 674 addresses reciprocating positive-displacement pumps for petroleum, chemical and gas-industry services. Reciprocating-pump selection is a system-dynamics problem as much as a pump-rating problem.

Pulsation & Vibration

Evaluate suction/discharge pulsation, acoustic interaction and piping vibration. Pulsation-control devices may be required.

Acceleration Head / Inlet

Valve dynamics and cyclic flow can reduce instantaneous inlet pressure. Size suction piping and evaluate acceleration effects.

Relief & Pressure Protection

A PD pump can continue developing pressure against a blocked discharge. Provide appropriate pressure-relief protection.

Valves, Packing & Materials

Fluid cleanliness, solids, corrosion, lubricity and pressure affect valve, packing/diaphragm and material selection.

Do not select a reciprocating pump because of pressure alone. Flow, speed, pulsation, NPSH/NPIP, solids, valve life, maintainability and driver behavior must also be checked.
Flow sharing and energy sharing

Pumps in Parallel & Series

Choose the arrangement by reading the combined pump curve against the system curve, then check every operating combination.

Why consider parallel pumps?

The presentation identifies four reasons to share the flow between two or more pumps:

  • The required flow is too large for a suitable single pump.
  • Available NPSH is low and flow sharing may permit more suitable inlet hydraulics.
  • The service includes sustained, wide changes in required flow.
  • The driver required for one pump would be too large.

Each branch shares the common suction and discharge headers and develops approximately the same differential head, allowing for branch losses. At a given common head, add the individual flows: Qtotal = QA + QB. The resulting operating point is the intersection of the combined characteristic and the system curve; two pumps do not automatically produce twice the single-pump operating flow.

Dissimilar pumps can share flow poorly

The source advises against parallel operation of dissimilar pumps in regions where one or both characteristics are flat. One unit can remain at zero contribution until the common head falls sufficiently for it to deliver. This makes the usable curve range, individual shutoff heads and check-valve behavior important.

In the original example, pump B contributes no flow until pump A reaches approximately 35% of its BEP flow. With both pumps running, B's individual contribution is approximately 130% of its own BEP flow, where greater NPSHR and power demand can be expected for the illustrated pump. These percentages describe this example; they are not operating limits for every parallel installation.

Original parallel-pump piping and head-flow diagram for dissimilar pumps A and B
Original parallel-operation figure, retaining the individual and combined curves and BEP labels. Read each pump contribution at the common head. Open original at full size.

Why consider pumps in series?

The presentation lists five reasons to share the energy rise between pumps:

  • The required energy rise is too high for one suitable pump.
  • The available NPSH is low and a suitable first pump or booster arrangement is needed.
  • The required system head varies widely.
  • The initial pressure is high, requiring a carefully chosen pressure-rated arrangement.
  • The driver needed for a single unit would be too large.

Series pumping does not by itself solve inadequate suction conditions at the first pump. The first unit must meet its suction requirements; each downstream unit must withstand its higher inlet and discharge pressures.

Series operation depends on the system curve

Without intermediate leakage, branching or withdrawal, the same flow passes through every series pump. At that flow, add the developed heads: Htotal = HA + HB.

The original figure compares a flat, high-static-head system with a steep, friction-dominated system. In the flat-system example, one pump cannot overcome the static head, so no forward flow is obtained until the full series arrangement supplies enough head. This is a condition of the illustrated head levels, not a consequence of every flat system curve. In the steep-system example, adding a pump raises the total head and produces an increased operating flow.

Original series-pump piping and combined head-flow curves for flat and steep system curves
Original series-operation comparison. The actual flow comes from the relevant system-curve intersection, not from summing flows. Open original at full size.

Parallel: check every combination

  • Verify individual BEP/POR/AOR, minimum flow, NPSH and driver load.
  • Review check valves, start/stop transitions and flow sharing.
  • Check common-header losses and suction adequacy with the maximum number of pumps running.

Series: check pressure containment

  • Give particular attention to sealing and downstream inlet pressure.
  • Verify casing MAWP, seal-chamber limits and transients.
  • Review start/stop sequencing and permitted operation with one unit unavailable.

For the operating-point and stability explanation, see Duty and hydraulic stability.

Matching performance to the process

Adjusting Pump Performance and Controlling Capacity

Changing the pump characteristic and changing system resistance move the duty point in different ways.

Impeller diameter and blade setting

The source describes reducing the outer impeller diameter in radial/Francis-type and mixed-flow designs, and changing the blade angle in adjustable axial-flow designs. Diameter adjustment is generally used to match a rated duty rather than as a continuous operating control.

As specific speed increases, the range over which performance can be adjusted by diameter alone generally becomes more restricted. The source also notes that trimming can increase NPSHR in some designs, with this concern becoming more pronounced at higher specific speed. Treat that statement as a design-dependent effect: use the selected pump's actual trim and suction curves.

Diameter scaling at constant speed

Q₂/Q₁ ≈ D₂/D₁   |   H₂/H₁ ≈ (D₂/D₁)²   |   P₂/P₁ ≈ (D₂/D₁)³

These are the preliminary diameter relations shown in the presentation. Its comparison indicates that the calculated trimmed performance can be lower than the actual result and gives an empirical correction chart. Both original figures are retained below. The correction belongs to its historical method and design range; it is not a universal correction for all impellers.

Original head-flow and system-curve comparison when changing impeller diameter
Original comparison of performance at different impeller diameters and the resulting operating point. Open original at full size.
Original empirical impeller-diameter correction chart
Original historical diameter-correction chart. Read its axes and diameter-ratio basis before using the source method. Open original at full size.
D
Use verified trim curves for the final selection.

Trimming changes hydraulic geometry, so similarity is approximate. Confirm head, efficiency, absorbed power, NPSHR and the permitted diameter range using manufacturer curves or test data.

Throttling changes system resistance

A discharge control valve increases resistance and moves the intersection along the pump curve toward lower flow. The original example shows reduced pump input power as flow falls, but the valve still dissipates generated head as an additional energy loss. A reduction in power is not universal across radial, mixed and axial designs; review the actual power curve.

Original flow-control diagram showing throttling resistance and pump power
Original throttling figure. Compare the new system resistance and operating point with the pump head and power characteristics. Open original at full size.

Bypass changes process flow while preserving pump flow

With the bypass closed, the greatest flow enters the downstream process in the source arrangement. Opening the bypass diverts more flow through the return path, reducing process delivery while allowing greater total flow through the pump. With substantial bypass opening, very little of the pumped flow may reach the process.

For the illustrated characteristic, greater bypass opening lowers pump head and increases pump flow and power demand. The main benefit is that reducing process demand need not force the pump into damaging low-flow operation. Review the real characteristic: increasing power is not a universal response for every hydraulic design. Also check recycle heating, suction conditions and minimum-flow protection.

Qpump = Qprocess + Qbypass
Original bypass-flow control diagram including pump curves and return piping
Original bypass arrangement and characteristic curves. Distinguish total pump flow from useful process flow. Open original at full size.

Three performance-adjustment routes in the source

  • Impeller diameter: generally a fixed adjustment to match the rated duty.
  • Rotational speed: shifts the pump characteristic during operation.
  • Available NPSH: the source describes a specialized submergence-control approach for saturated liquid.

In that specialized submergence-control discussion, liquid level in a vessel influences the available suction head and hence the delivered flow. The presentation calls the method uncommon and cites applications with NPSH below 6 ft (approximately 1.83 m). This is historical application guidance, not a general instruction to use cavitation as flow control. Ordinary installations must preserve the required suction margin; this approach requires validated pump and process design.

Speed scaling at unchanged impeller diameter

Q₂/Q₁ ≈ N₂/N₁   |   H₂/H₁ ≈ (N₂/N₁)²   |   P₂/P₁ ≈ (N₂/N₁)³

The source describes speed change as the more common performance-adjustment route. It gives N₂/N₁ ≥ 0.5 as the range in which its speed-scaling estimates have acceptable accuracy without an additional correction. Retain that value as the presentation's approximation range, not as a universal allowable minimum speed or an accuracy guarantee.

Apply the relationships to corresponding points on the pump curves. Find the actual new operating point from the unchanged or modified system curve. Verify efficiency, suction performance, driver/VFD limits, cooling and mechanical operating restrictions.

Original head-flow and power curves showing variable-speed operation
Original variable-speed diagram, retaining the constant-speed and variable-speed power comparison and the system-curve intersections. Open original at full size.
Why variable speed can reduce control losses

Throttling creates additional resistance. Speed control changes the generated head instead. The energy benefit depends on the static-head fraction and the operating profile; assess the actual system rather than assuming a fixed saving.

Cross-checks: Hydraulic Institute — Pump Curves; KSB — Impeller Trimming. Original diagrams and historical examples: Petropedam engineering training material.

Speed control, BEP and suction requirement

The presentation highlights two benefits of speed adjustment compared with throttling: the duty can remain closer to the appropriate BEP, and the pump can generate energy closer to the process requirement. It particularly emphasizes potential savings when the required flow varies substantially. These benefits depend on the system curve and duty profile; a high static-head fraction can move the operating point away from the scaled BEP.

For suction performance, the source gives the following historical speed approximation:

NPSHR₂/NPSHR₁ ≈ (N₂/N₁)1.7

Retain the exponent 1.7 as the presentation's empirical approximation, not as a universal NPSHR law. Recheck the manufacturer's suction curves at the new speed and corresponding flow, and apply the service-specific margin.

Flow range and reliability

Operating Limits and the Effects of Varying Flow

Different mechanisms govern different parts of the flow range. Establish continuous and intermittent limits for the actual pump, liquid and operating conditions.

As a centrifugal pump operates across different flows, inlet, outlet, thermal and mechanical effects can each restrict the allowable range. The source emphasizes pump design, liquid properties and rotor energy as the factors determining how strongly each effect matters. The following original overview places these mechanisms on a common flow axis; the marked regions are teaching examples rather than universal alarm settings.

Original flow-range overview showing inlet outlet rotor-load and thermal constraints
Original overview of varying-flow effects on centrifugal-pump behavior. Read each limiting mechanism against the selected pump data. Open original at full size.

Inlet choking and discharge recirculation

Inlet choking: at high flow, limited passage area between the impeller blades and increased inlet velocity can cause the required suction head to rise sharply. The source associates choking with approximately 105% of BEP flow for high suction specific speed through 130% for low suction specific speed. These are historical comparison values, not a universal allowable-flow band.

Discharge recirculation: secondary and reverse flow at the impeller outlet can wear the shroud fillet, contribute to shroud fracture and produce fluctuating radial and axial thrust, described in the presentation as rotor shuttle. The source places onset around 70% of BEP flow for a low-efficiency design, while the illustrated better designs can place it beyond BEP. Onset and severity require pump-specific assessment.

Suction recirculation and rotor loading

Suction recirculation: reverse flow around the impeller-eye region can produce surging, vibration, cavitation-like noise and impeller erosion. Its relationship with suction specific speed is important. The source compares onset around 50% of BEP flow at low suction specific speed with onset beyond BEP at high suction specific speed. These source values describe design sensitivity rather than a single minimum-flow rule.

Rotor loads: steady and fluctuating hydraulic forces depend on both hydraulic and mechanical design. Rotor deflection may damage seals and disturb shaft/bearing clearances. The source illustrates a rotor-load-limited low-flow region extending from the start of the curve to roughly 50% of BEP flow; the actual limit must come from the chosen design.

Temperature rise at low flow

Energy losses in the pump heat the liquid. As flow decreases, the same heat input can act on less liquid, increasing temperature rise and vaporization risk. Heated liquid may remain liquid in a higher-pressure region but vaporize when internal leakage returns it toward the lower-pressure suction region, reducing developed head.

The original presentation reports the following temperature-rise expression:

ΔTtotal = H / (778 × η × Cp)

Its legend calls H leakage head, η decimal efficiency and Cp specific heat. The factor 778 indicates a US energy-unit conversion. The supplied expression is retained as source notation; its head and heat-accounting basis must be established before calculation. Do not insert SI units into this expression or treat all developed hydraulic head as heat.

For an explicitly defined heat balance, an engineering estimate is ΔT = Pheat / (ṁ × cp), with heat transferred to the liquid in W, liquid mass flow in kg/s and specific heat in J/(kg·K). Identify the relevant losses and recirculation path. For shutoff or a closed recycle volume, use a transient heat balance and the manufacturer's minimum-flow method.

Original thermal minimum-flow chart comparing water at high NPSH with hydrocarbon service at low NPSH
Original temperature-rise and thermal minimum-flow comparison. The source contrasts a few percent of BEP flow for water with high NPSH against a higher thermal limit for hydrocarbon service with low NPSH; these are illustrative conditions. Open original at full size.

Continuous and intermittent allowable flow

The source distinguishes continuous and intermittent operation. A brief permitted excursion is not approval for sustained operation. The original wear-rate figure shows why operation away from the favorable region can shorten service life, with the trend depending on energy level.

Original pump-wear rate versus flow chart showing its dependence on energy level
Original general effect of operating flow on pump wear. The teaching trend is not a life prediction for a particular product. Open original at full size.

Choosing a minimum-flow bypass

A bypass is commonly used to prevent the pump from entering the lower intermittent operating region. If the continuous and intermittent minimum-flow limits are close, or if extended operation below the continuous limit would otherwise occur, the source calls for sizing the bypass to preserve the minimum continuous flow.

Defining the range is difficult because the mechanisms overlap and off-BEP behavior varies between designs. A limiting phenomenon for one pump may not govern another. Establish the governing thermal, hydraulic and mechanical limits rather than copying a percentage.

Historical flow-limit guidance retained from the source

Source guidanceHow to use it in selection
Maximum intermittent flow should remain short of the sudden steep rise in NPSHR; maximum continuous flow is separated by about 5% of BEP flow.The 5% is a flow separation, not a 5% NPSH margin. Confirm actual continuous and intermittent boundaries.
Minimum continuous flow is related to suction-recirculation onset; the source gives 60% of that flow for hydrocarbon service.The 60% refers to the stated recirculation-related flow, not automatically 60% of BEP. Confirm fluid-specific acceptability.
The one- and two-stage example identified as 100 hp per stage has the same continuous and intermittent minimum flow.Retain as a source example; it does not establish an all-pump power threshold.

Which mechanism sets the minimum?

  • Rotor loads and temperature rise commonly govern the intermittent minimum in the source discussion.
  • If the thermal limit extends beyond the otherwise allowable continuous minimum, the thermal limit becomes the governing continuous minimum.
  • In high-speed single- or multistage designs, rotor loads can govern both continuous and intermittent minimum flow.
  • The required head rise for stable operation, motor power in high-specific-speed designs, and the need to handle liquid containing gas bubbles can also determine the required bypass flow.

Combine these constraints with the manufacturer's permitted operating duration, POR/AOR, suction margin, seal cooling and process requirements. The historical percentages above preserve the teaching material; they do not replace manufacturer operating limits.

Further off-design checks

Three radial-load mechanisms

  • Steady radial thrust: nonuniform pressure around a single volute during off-design operation, asymmetric pressure distribution in a double volute, or an eccentric diffuser can create a net radial force.
  • Fluctuating radial loads: secondary flow and pressure pulsations at the impeller outlet can generate time-varying forces.
  • Low-frequency rotating forces: diffuser stall can create a rotating hydraulic disturbance.

Viscosity changes the hydraulic performance

The source identifies greater frictional losses, thicker boundary layers that change effective passage geometry, reduced developed head and efficiency, and increased power demand. It notes stronger efficiency penalties at low specific speed. The original figure compares performance at different viscosities; use a recognized viscosity-correction method and verified liquid properties at operating temperature.

Original viscosity comparison chart for centrifugal pump head efficiency and performance
Original liquid-viscosity teaching chart. Read the head and efficiency changes at the stated viscosity and flow; it is not a certified curve for a current product. Open original at full size.

The presentation states a historical screening ceiling of 500 SSU for centrifugal pumping. This value is retained as source guidance, not a universal prohibition. SSU is temperature-dependent efflux time and is not numerically interchangeable with cSt. Technology choice requires viscosity at the actual temperature, hydraulic corrections, efficiency and lifecycle assessment.

Entrained gas changes the head curve

The source chart compares 1–6% gas by volume with gas-free liquid and shows changing developed head and minimum-capacity behavior. The effect depends on hydraulic design, gas fraction at suction conditions, pressure and liquid properties; the plotted percentages are test comparisons rather than generic gas-handling ratings. Distinguish free/entrained gas from dissolved gas and cavitation vapor.

Original gas-free and one-to-six-percent entrained-gas head versus capacity curves
Original entrained-gas performance comparison. Confirm the gas-volume basis and suction conditions before applying the trend to another installation. Open original at full size.

Cross-check: Hydraulic Institute — NPSH and operating regions. The applicable operating-region guidance and manufacturer limits govern the final selection. Original diagrams and historical examples: Petropedam training material.

From guide to selection

Have a real duty point?

Use the Pump Selection Software to screen Petropedam candidates, then verify NPSH, operating region, power and construction against the project requirements.

Open Pump Selection Software

Petropedam Project References

Selected company project references supplied by Petropedam, spanning Oil & Gas, Refinery, Petrochemical, Mining and Water services.

Company-supplied reference dataProject counts and commercial reference details in this section are Petropedam company records and should be maintained by the company as those records are updated.
140+
Total Projects
45+
API 610 Projects
30+
API 674 Projects
20+
NFPA 20 Fire Pumps
12+
Mud / Slurry Pumps
10+
Gear Pumps

Selected Project Highlights

YearProject No.CustomerPump ModelLiquidFlow (m³/h)Head (m)Power (kW)Standard
1401OPS-P-ME-403039Jondi ShapourKND1450-175*2 (BB1)Heavy Crude Oil1441.3347.52000 API 610
Under FabG26S1-PRC-VCNT-PWW22Gohar MesMHG4 (BB4, 6 Stage)Raw water248580800 API 610
14011208-00-RE-POR-313APIDECTriplex Plunger TH 100Ammonia to Urea1.4-5.6112437 API 674
1400RTP-7700496287-RTTehran RefineryScrew Pump W7T.3Z85Vacuum Residue163—75 API 676
1401110052/01-77MMTEBB1 CS250-600/4Process Water109591.2450 API 610
1400 41-106/1996Mobin PetrochemicalMHP70H-3 (VS4)Sea Water1622530 API 610
140140-90-0052400020/P08Abadan RefineryReciprocating 1500 (545)Fuel Sludge18.2—15 API 674
98Potable PumpTana EnergyWarman PGr300Potable Water1100118560 DIN
9494/1/5878FRW ORGFire Fighting PumpWater & Foam—20014 HP NFPA

Full project reference list and current company totals are available from Petropedam upon request.

Pump Selection Checklist

Use this checklist before a pump is released for quotation or final specification.

Engineering Master Guide

Standards, References & Engineering Basis

The original Petropedam PowerPoint provides the training framework; edition-specific rules and legacy rules-of-thumb have been reviewed against current published information.

  • API 610, 13th Edition — 29 June 2026 — Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries (API Standards Plan).
  • ANSI/HI 9.6.1-2024 — Rotodynamic Pumps Guideline for NPSH Margin.
  • ANSI/HI 9.6.3-2024 — Rotodynamic Pumps — Guideline for Operating Region(s).
  • ANSI/HI 9.6.7-2021 — Rotodynamic Pumps — Guideline for Effects of Liquid Viscosity on Performance.
  • ANSI/HI 9.6.6-2022 — Rotodynamic Pumps for Pump Piping.
  • ANSI/HI 9.8-2024 — Rotodynamic Pumps for Pump Intake Design.
  • API 674, 3rd Edition (2010) — published reciprocating positive-displacement pump edition identified in API's catalog; Edition 4 is listed by API as in development/editing.
  • API 676, 4th Edition — 2 February 2022 — Positive Displacement Pumps — Rotary.

Engineering disclaimer: This guide supports education and preliminary selection. Final pump selection must be verified against the certified pump curve, contractual project specification/edition, actual fluid data, site conditions, materials, mechanical limits, applicable standard and vendor documentation.

Frequently Asked Questions

You need flow rate (normal, min, max), total head, suction and discharge pressures, fluid properties (temperature, SG, viscosity, vapor pressure), NPSHA, operating hours, and applicable standard (API 610, 674, ISO, NFPA, ANSI).
Define duty, calculate head and NPSH, select pump type (OH1, OH2, BB, VS), review Q-H curve, check BEP and power, select materials, driver, and standard.
Head = static head + pressure head + friction head + velocity head. Formula: TDH = (P_disch – P_suct) × 10.197 / SG + (Z_disch – Z_suct) + h_friction.
Net Positive Suction Head Available — the energy at the pump suction above vapor pressure. Depends on suction pressure, liquid level, losses, and vapor pressure.
Net Positive Suction Head Required — the suction-head value specified by the manufacturer for a defined performance criterion (commonly NPSH3, corresponding to a 3% head drop); operation at NPSH3 does not guarantee freedom from cavitation. Apply the required project margin.
Best Efficiency Point — the flow at which the pump operates at its highest efficiency. Operating near BEP maximizes reliability and minimizes energy consumption. BEP is at the peak of the efficiency curve.
Oversizing leads to operation away from BEP, throttling losses, higher vibration, reduced bearing/seal life, and increased lifecycle costs.
Higher viscosity can reduce flow, head and efficiency and change absorbed power. Apply Hydraulic Institute viscous-performance correction guidance and compare rotodynamic and positive-displacement options for the actual duty instead of using a single viscosity cutoff.
Reciprocating pumps are used for low-flow, high-pressure applications, especially for metering, injection, and viscous or shear-sensitive fluids.
API 610 is the standard for centrifugal pumps for petroleum, petrochemical, and natural gas industries. Specifies design, materials, testing, and documentation.
API 674 is the standard for reciprocating positive displacement pumps for petroleum and chemical services. Covers piston, plunger, and diaphragm pumps.
Cavitation is the formation and collapse of vapor bubbles in the pump. It causes erosion, noise, vibration, performance loss, and eventual failure of impeller and casing.
Dissolved gas and free or entrained gas must be distinguished. Free gas can reduce pump head, efficiency and hydraulic stability, while vapor bubbles formed when local pressure falls below vapor pressure are associated with cavitation.
Suction specific speed is a hydraulic similarity parameter calculated from speed, flow and an explicitly defined NPSH reference. Its numerical value depends on units and convention. It helps screen suction behavior but no single value establishes cavitation-free operation or universal acceptability. Verify the actual pump curve, operating region and project-specific NPSH margin.
Continuous operation is the normal operating range where the pump can run indefinitely. Intermittent operation is for short periods only. Minimum flow limits differ for each; continuous limits are more conservative.

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System Analysis

Hydraulic system analysis, including system-curve development and, where required by the service, transient or surge evaluation.

Material Selection

Corrosion analysis and material optimization for fluid compatibility, with full trace element evaluation.

Sealing Solutions

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Lifecycle Support

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PETROPEDAM TRAINING MATERIAL

Complete original engineering presentation

All 166 pages of Petropedam’s original training presentation are reproduced in their original order and colors. The source is in Persian. Tap or click an image to inspect the original figures and wording at full resolution. The English engineering guide above provides the reading path and technical context.

These original pages document the company’s teaching material. Equations, examples and legacy numerical rules must be checked against current standards and the selected pump’s certified data before use in a project.

Original pages 1–12
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This guide has been developed by PetroPedam Engineering based on engineering principles, industry standards, the original Petropedam training material, and company-supplied project experience.

PETROPEDAM — Petro Pump Eng. Design & Mfg. Co. | Shamsabad Industrial Estate, Tehran

This guide is for educational and preliminary selection purposes. Final engineering selection must be validated by a qualified engineer for the specific application.

Contact: info@petropedam.com | +98-21-88086883

Written by: Amir Almasi — CEO, Petropedam Engineering Company