DISCHARGE HEAD D017
We have to know the discharge head of a centrifugal pump for several reasons:
- To select the correct size pump for a new application.
- To estimate stuffing box pressure in the event the discharge valve is shut with the pump running.
- To determine if an installed pump is running at its best efficiency point (BEP) Any time we go off the best efficiency point (BEP) the shaft is subject to radial side loading and potential problems with mechanical seals, packing, bearings and couplings.
- To be sure the pump will have enough capacity.
- To prevent cavitation problems if the head is too low.
There are two methods of learning the head:
- Look at the discharge pressure gauge that has been installed in the system, or use a chart recorder installed on the discharge side of the pump. Subtract this reading from the reading obtained from a gage or recorder installed on the suction side of the pump. Be aware that the gage is showing you the pressure only at the time you are looking at the gage. The pressure can vary with:
- Opening and closing valves in the system.
- Changes in tank pressures.
- Changes in tank level, especially if the tank is being filled from the bottom.
- Calculate the discharge and suction heads from losses in the piping and associated hardware in the lines.
Whenever you deal with a centrifugal pump be sure to remember that any time you change the capacity of the pump, the head will change also.