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TECHNICAL INFO CAVITATION – A DANGEROUS ENEMY TO PUMPSBack
CAVITATION – A DANGEROUS ENEMY TO PUMPS
Author:
Robert J. Hart, P.E.
Robert J. Hart Enterprises, LLC

The term “Net Positive Suction Head – Required” applies to all pumps. It is a term that has been – and continues to be – misunderstood by many of those selecting pumps and designing the piping systems in which they are installed. As a result, a number of pumps operate in varying degrees of cavitation, an underlying cause of the high maintenance costs often associated with them. Cavitation, at its worst, sounds like loose gravel passing through the pump. Damage to seals, bearings and impellers will usually be experienced well before the noise of cavitation can be detected by the human ear.
 

The values of NPSH-Required, published in most manufacturers’ Generalized Performance Curves, are values that place the pump in controlled, but heavy cavitation. The NPSH-Required is defined as the NPSH applied to the pump at a given flow rate which causes sufficient cavitation to reduce the Total Dynamic Head (TDH) by 3%. This is an established pump industry standard procedure used to indirectly measure, at a reasonable cost, the suction side pressure loss inside a pump before mechanical action increases the liquid pressure. The measurement is taken while pumping water with a minimum of inlet stream turbulence (i.e. no close connected, double elbows), no entrained gas, and frequently, with water that has most dissolved gas removed.

 

These factors will increase the NPSH-Required values obtained during the test. While this may appear to be an idealistic system, and is not realistic for actual operating conditions, it is the only method that can provide reproducible test results. The user must apply a margin between the liquid in let total pressure and its vapor pressure greater than this pressure loss to prevent vapor formation at the impeller inlet.

 

Even though Figure 1 illustrates the typical vendors’ published non-cavitating Total Dynamic Head curves (head vs. Flow) and the NPSH-Required curve (NPSH-Required vs. Flow) on the same graph, it must be understood the NPSH-R values applied to the pump will reduced the Total Dynamic Head developed by the pump by 3% at any flow rate. While the hydraulic loss due to reduced TDH is typically not significant, the resulting shock on the equipment can reduce the mechanical life of the seal, bearings, and impellers.



Figure 1 Published generalized performance curve compared to TDH with NPSH-A/NPSH-R = 1.0

 

NPSH-Available:

Manufacturers expect the user to supply an NPSH-Available which exceeds the NPSH-Required value that is published. The margin between the NPSH-Required becomes a commercial decision and should be properly evaluated by the person selecting the pump and developing the piping system. Systems which are pumping liquids at their equilibrium condition, i.e. ready to flash with an increase in temperature or a decrease in pressure, require special consideration when pump is being selected and the system is being designed. The NPSH-Available in such systems normally can be significantly increas3d only by:


- Increasing the supply tank elevation above the pump;
- Lowering the pump relative to the supply vessel;
- Providing a booster pump;
- Cooling the liquid to reduce the vapor pressure characteristic


PUMP APPLICATION CONSIDERATIONS:

As a guideline, the NPSH-Available should exceed the NPSH-Required by a minimum of 5 feet, or be equal to 1.35 times the NPSH-Required, whichever is the greater value. As an example, for an NPSH-Required of 20 feet, the NPSH-Available should be a minimum of 27 feet. This is considered the minimum acceptable margin, and even then, some degree of mechanical and erosion damage can be experienced. Increasing the margin will improve the reliability and acceptable operating range of the pump. In order to totally eliminate cavitation, which affects the Total Dynamic Head, the Net Positive Suction Head-Available must be from two to five tomes the NPSH-Required, depending on the operating flow rate relative to the Best Efficiency Point flow rate of the pump design, as Figure 2 illustrates.


Figure 2 Mechanical damage requires little elapse time (hours to weeks). Erosion damage requires longer elapse time (days to months)


Pumps that operate with NPSH-A/NPSH-R ratio above 1.35, but below the 2X to 5X parameter, can have what is considered “acceptable” (but not optimal) seal and bearing life; however, they may be vulnerable to erosion damage to the impeller, which will require more frequent impeller replacement than would otherwise be experienced had the cavitation been totally eliminated.


 
Anyone developing pumping systems designed with these minimum recommended margins should consider testing the equipment to be delivered to confirm that it meets the published data. The following information should be considered when ordering the pumps to decide if tests should be conducted:


- Most manufacturers do not hydraulically test pumps unless required to do sop by the purchaser

- The first test5 of a specific pump may result in higher NPSH-Required and Total Dynamic Head values than indicated by the published data due to casting variations of the casing and impeller and the manufacturer’s documentation to test procedures. To correct this condition, the manufacturer may have to grind the controlling surfaces of the impeller and case to reduce the NPSH-Required and reduce the impeller diameter to reduce the TDH values within acceptable tolerances. After such modifications are made, the pump is retested to confirm the result of the rework.

 

- The testing procedures followed by most manufacturers (per hydraulic Institute Standards) typically yield the minimum information, which is not considered adequate for most critical services, especially as it relates to NPSH-Required testing. Recommended test procedures, the critique of proposed test procedures and/or witnessing of critical equipment tests can be supplied on requests.

 

In today’s business environments, plant investment must reap the maximum return. It is advisable that those making decisions regarding new pumping systems or troubleshooting existing systems take heed of this information. Assistance on these topics can be supplied on request.

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