Greater Internal Pressures than Discharge Pressure 1

[CoreyG]

I am currently analyzing some test data of a centrifugal pump. In this test the volute was tapped in four places along the developing flow path and pressure gauges were inserted. A diagram of the volute is attached. The pump was tested from 0 flow to wide-open, taking pressure readings at Suction, Discharge and at all tapped ports.
Tests results showed that at lower flows pressure develops along the flow path and is greatest at the discharge, as expected. At higher flows the results showed pressure developing from suction to the third internal port but losing pressure by the forth port & discharge. So at high flows the pump is developing a higher internal pressure than seen at the discharge.
Has anyone ever come across this issue or know of some literature that my shine some light on this phenomenon? Thanks

 

[Compositepro]

Think about how a centrifugal pump works. The impeller does not directly create any pressure - it gives the fluid momentum. Pressure is created when this moving fluid encounters a resistance to flow. If the discharge of the pump is completely unrestricted the outlet pressure is about zero. This is also when the pump will have maximum flow so the pressure drop from start to finish of the volute will be greatest. At deadhead, or zero flow, the highest pressure is at the discharge because the volute is doing what it is designed to do - direct the flow so that momentum of fluid leaving the impeller can be converted into static pressure with good efficiency.

 

[Artisi]

probably discharge recirculation or leakage back to the inlet.