NPSHr and NPSHa are very close to each other
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I happened to see that the condensate pump in a certain project has an NPSHr of 1.8 m, which is quite close to an NPSHa of 2.3 m. The pump has already been purchased, and the condensate tank along with its foundation have been prepared. I’m worried about cavitation; testing is about to begin. What should I do?
A difference of 0.5m is still quite safe. Generally, NPSHa = NPSHr + 0.5 or NPSHa = 1.3 × NPSHr
One is the pressure drop, and the other is the excess pressure; if they are close to each other, there should be no major problems
Regarding the margin between NPSHr and NPSHa, different standards have slightly varying requirements: the standards for Class I and Class II pumps require a margin of not less than 0.5 m, while the petrochemical industry standards require a margin of not less than 0.6 m. According to engineering practice, for ordinary small pumps, a head of not less than 0.6 m is required; for larger pumps and important pumps, a higher head is needed to prevent cavitation, usually around 1.0 m. For the situation described to the original poster, normal operation should be fine, but if it’s any smaller, there will be risks.
Theoretically, a surplus of +0.5 is sufficient; during the manufacturing phase it is important to ensure that the product undergoes factory testing by the pump manufacturer under conditions where the value does not exceed 1.8 m, and it is also necessary to avoid operating at excessive flow rates during use. Your pipeline installation must also comply with the standards.
When selecting the NPSHa value, it is usually calculated based on the lowest liquid level. When the liquid level is high, the NPSHa value will be higher, which provides greater safety; however, there may be a risk of cavitation in the end. Under normal circumstances, cavitation does not occur
Pay close attention during use; ensure the liquid level in the condensate tank is maintained at an appropriate level, set a low alarm value, and also establish a high alarm value for the pump’s flow rate.