Pumps & Systems, December 2008
Editor's Note: This is part one of a two part series about the 'commandments' of increasing pump reliability and life. To read the second part, click here.
In Mel Brook's movie, History of the World, Part 1, Moses comes down from Mount Sinai carrying three tablets. "People of Israel", Moses exclaims, "I bring you these 15 [dropping one tablet]. . . 10 Commandments!"
Only a handful of people know the untold commandments. In the next two articles, we will review two of these and the reasoning behind them.
Commandment 11: Operation at BEP
The majority of the failure mechanisms that significantly reduce the reliability and life of a centrifugal pump are caused by operation away from the pump best efficiency point (BEP). To achieve best-in-class life, it is essential to either operate close to the pump BEP (which is always the preferred method) or to provide provisions that anticipate the various effects of off-BEP operation and attempt to mitigate these consequences.
The pump BEP is the operating condition at which the angle of the fluid entering the impeller is parallel to the impeller blade. The farther that one operates away from the BEP, the greater dissimilarity between the flow incidence angles and the inlet vane geometry. Similarly, as the pump operation becomes farther removed from BEP, the possibility of serious problems and the severity of existing problems will almost always increase.
The most detrimental effects of operation away from the best efficiency point are experienced due to:
- Suction recirculation
- Discharge recirculation
- Larger radial loads
- Opening of wear ring clearances
Each of these problems will induce other problems that negatively impact reliability and performance, such as separation-cavitation, axial shuttling or increased shaft deflection.
Suction recirculation most often occurs when off-BEP operation is coupled with large impeller eye areas. The stalled area on the pressure side of the inlet vane, caused by the mismatch between the fluid angle and the blade angle, results in the formation of fluid swirl. The result is a separation of the fluid from the vane that has enough room (via the large inlet area) to recirculate.
When severe, the fluid circulates out of the impeller eye, interfering with normal suction flow. This fluid swirl can cause localized pressure drops, which reduce the total head generated and cause the suction pressure to fall below the fluid vapor pressure. The effect of this pressure drop is known as separation/cavitation.
The consequences of suction recirculation are increased rotor vibration from fluid instability and impeller damage due to cavitation. The design emphases to significantly reduce or eliminate these issues are:
- Reduce the impeller eye diameter to the extent allowed by suction (NPSHa) conditions
- Operate the pump as close to its BEP as possible to minimize separation effects
- Provide state-of-the-art air foil shapes to the impeller inlet vanes to be more permissive of off-peak operation
1. Reducing the Impeller Eye Diameter
Limiting the suction specific speed (NSS) of the impeller can reduce the impeller eye diameter. Suction specific speed (NSS) is a dimensionless parameter that helps define impeller geometry. When designing an impeller, lower values for suction specific speed lead to smaller eye inlet areas and less susceptibility to inlet fluid recirculation.
Empirical data has shown that the failure frequency of pump components significantly increases with an NSS > 11,000 based on a 3 percent ΔH. This analysis is based on J. L. Hallam's refinery industry study of pumps handling hydrocarbons.[af1] For water applications, a maximum NSS of 9,500 should be used to achieve reliable operation.
2. Operation Near the BEP