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Unknown pump performance increase

Q: I have conducted an annual firewater pump test and I am puzzled with the pump curve result that I have obtained. The pump is from the centrifugal pump type and utilizing sea water as the fluid service. I am supposed to compare the pump curve result with the previous year result and I have found out that the pump performance has improved significantly compared to the previous year. This should be good news, but the problem is no improvement work has been done for the pumps for the past two years and theoretically the pump should either maintain its last performance or continue to degrade. I have used a different test procedure from the previous test, in which the main difference is the location of the water discharge. Does the location of the water outlet affect the pump discharge pressure? Will the upstream pressure drop affect the pump discharge pressure? The discharge pressure that I am referring to is the pressure obtained from the pressure gauge located as soon as the pump discharges and not at the end of the pipeline.

A: When the location of the water discharge is changed in a pumping system, that changes the system, which will, in turn, change the point on the performance curve at which the pump will operate. The comment was made that the pump performance improved over the previous year, but we don’t know how it changed. We will assume it means that the flow rate has increased. This would be perfectly normal if the discharge pressure that the pump is required to produce is lower than it was on the previous test.

The rule to remember is that a centrifugal pump is a slave to the system, and will only do what the system demands. Therefore if the system requires a lower output pressure from the pump, then the pump operation will move further out on its curve and deliver a higher flow rate.

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