Page 26 - Know and Understand Centrifugal Pumps

Basic Pump Principles

The concept of Head versus Pressure causes confusion between maintenance people
and the pump manufacturer. The maintenance technician reads his gauges recording
pressure in psi, and the pump manufacturer uses the term head. The term head is the
constant for the manufacturer. A pump that generates 90 feet of head can elevate
water, gasoline, caustic soda, and any liquid to a height of 90 feet. The manufacturer
doesn't know the ultimate service of the pump when he manufactures it. He only
knows that his pump will develop 90 feet of head. The psi reading is a function of the
conversion factor 2.31 and also the specific gravity. This is why you cannot specify a
pump by the psi. If the maintenance engineer or mechanic wants to have an
intelligent conversation with the pump manufacturer, he must understand and use
the concept of 'head: This is also the reason that too many pumps are sold without
adequate gauges. It's somewhat like selling a car without a dashboard. There's more
information on this in Chapters 7 and 8.

Given the following information:
sp. gr. of water = 1.0
sp. gr. of gasoline = 0.70

sp. gr. of concentrated sulfuric acid = 2.00
sp.gr. of sea water = 1.03
A pump capable of generating 125 feet of head would provide the
following pressures:
Pressure = (Head ft. x sp.gr.) / 2.31

Water:

Gasoline: P= 1'25 O O 7 = 37.8 psig
2.31

Conc. Sulfuric Acid: P = 1*25 2.0 = 108.2 psig
2.31

Sea Water

This pump (Figure 1-6) is raising the liquid from the level in the
suction vessel to the level in the discharge vessel. This distance is called
the Total Head.