Pump Shutt-off head

I have witnessed a factory pump test, and for 0 flow the discharge is briefly turned off, and pressure and motor readings are made quickly to be able to start some flow again. Power input is voltage times current time other factors as needed, efficiency is work out/ work in and therefore equal to 0.

Not sure I understand your last question. In a system curve you are plotting system pressure to flow rate, not pump pressure. I am not surprised suction head is lower than discharge head.

The head developed by a pump at any point on the performance curve is the discharge head minus the suction head. This assumes a fixed rotational speed of the pump and unchanging liquid properties, viscosity and specific gravity (only for centrifugal pumps). Of course if the suction pressure is too low and the pump cavitates you have in effect a change in liquid properties.

Put a centrifugal pump on the test stand and run it at a constant speed and in the lower range of its flow (you do this by closing down a valve on the discharge side). Then close a valve on the suction side a little; but not so much that the pump will cavitate. (you'll know it's cavitating by the onset of very noticeable new noise) Now watch both the discharge and the suction pressures and you will see that for every unit the suction pressure drops the discharge pressure drops the same amount. This relationship isn't perfect in the real world because there are a number of second order effects like the slight changes in induction motor speed with load. But the point is that actual heads don't count. It's the head increase across the pump and the weight of liquid per unit time that is pumped through a centrifugal pump that constitute its hydraulic power output. The input power is what ever the drive motor produces. In common pump industry practice commercial test stands for pumps are usually equipped with calibrated induction motors where the motor manufacturer has certified speed, voltage and current measurements throughout the power range of the motor.

A typical centrifugal pump test run will be on water at room temperature at the rated speed of the pump. 5 or 6 condition points are run at flow rates varying from 0 (shutoff) to 125% of the flow at the maximum efficiency point for the pump under test. Readings of discharge and suction pressure, flow (capacity in the vernacular of the pump industry), motor voltage and current are taken. Results are calculated and plotted typically as head, power and efficiency all vs. capacity in units of volume per unit time. Head in linear feet or meters is convenient practice for this type of pump since it is a direct measurement of the energy content of the flow and is directly related to the way the pump converts energy.

Ed Weldon

The net effect of a pump is to increase system pressure. That is why discharge head is more than suction head. Suction head is no more than the pressure available at the inlet of a pump to move the fluid through the pump. Power is still measured electrically, there is work being done to heat the water, turn the shaft, and make the noises it will make when you forget to open the discharge valve.

Why not first check out a few answers readily avalible and the ask what you are still missingor cant easily find?

http://en.wikipedia.org/wiki/Hydraulic_head

• hw does the vendor calculate the pump shut- off head? During a pump performance test the vendor does not calculate the shutoff head, he measures it.
• At shut-off head, flow rate is 0 GPM, & efficiency is also 0, hence hw does once calculate power input to the pump? One does not calculate the power input to the pump, one measures it. In theory hydraulic power is zero as Q is zero, but in reality one has to overcome many losses - internal recirculation losses, disc friction, seal losses, bearing losses etc.
• Also when we plot our system curve, the head indicated at 0 flow rate is the suction static head which is far below than the shut-off head, why is it so?? This one confuses me because it is not so. If you close the discharge valve of the pump, then the head measured at the discharge will be the suction head plus the differential head (in this case shut off head) of the pump as per the curve.