PUMP HEAD

The term “head” is used almost exclusively in the centrifugal pumping industry to express pressure. All pump curves are calibrated to read “feet or meter of head” as a measure of pressure rise. Similarly, suction pressures, and often friction losses are also expressed as feet or meter of head, not psi or kg/cm2.

The concept of head is derived from the fact that a column of liquid will exert a local pressure proportional to the depth of that liquid. For example, the pressure of a column of water increases 0.433 psi for every foot of depth or 0.099878 kg/cm2 for every meter of depth. In other words, at a depth of ten feet, the pressure is 4.33 psi higher than at the surface; at 100 feet, 43.3 psi higher; at 1000 feet, 433 psi higher, etc.

The depth, or distance in feet, can therefore be used as a measure of pressure. For water, the equivalent pressures are:

1 foot of head = 0.433 psi (for water at 60oF and 1.0 specific gravity)
or
1 psi = 2.31 feet of head (for water at 60oF and 1.0 specific gravity)

Similarly, visualize a centrifugal pump connected to vertical pipe on its discharge. The discharge pressure from the pump would push the liquid up the column to a level where the pressure from the height would equal discharge pressure. This height would be the feet or meter of head noted by the pump manufacturer as “total head’ across the pump.

One reason the centrifugal pump industry has settled on head, or feet or meter, as a measure of pressure rise is that a pump will develop the same head regardless of the fluid’s specific gravity. A pump that develops a column of water (SG =1) 1000 feet high will also develop a column of hydrocarbon (SG = 0.7) 1000 feet high.

Of course, the actual pressure, in psi, would be quite different between water and hydrocarbon. The pressure developed in a pump, and the pressure at the bottom of a column of liquid are both proportional to specific gravity. To convert from feet to psi (and vice versa) use the following equation:

Pressure (psi) = feet x SG x 0.433