Page 95 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

inch (or kilograms per square centimeter, in the metric system). The
pump manufacturer denotes pressure in feet of head (or meters of
head). The pump operator needs a pump that generates 20 psi. The
manufacturer offers a model that generates 46 fi of head.
To understand pumps and analyze their problems, its necessary to
dominate the formula that changes feet of head (H) into psi. This is
explained in Chapter 2, but here is a brief review:
The formula is:

H (Head in feet) x sp. gr.
Pressure in psi =
2.31

And in the other direction:

Head in Feet = p si x 2.31
SP. gr.
If the liquid is water, the specific gravity is 1.00. We see that two factors
separate ‘psi’ from ‘head in feet’. First is the 2.31 conversion factor, and
second, the specific gravity.
The pump companies develop their curves using head in feet (H),
because when they make a new pump, they don’t know the ultimate
service of the pump (they don’t know the liquid that the pump will be
pumping), but they do know how many feet of elevation the pump can
raise that liquid. This is why it’s necessary to spec$ pumps in feet of
head and not in psi. Let’s begin by exploring the H-Q curve of the
pump, using feet of head.

H-Q
~ ~~~~~~
The matrix of the pump curve graph is the same as the mathematical
‘x-y’ graph. On the horizontal line, the flow is shown normally in
gallons per minute or cubic meters per second. The vertical line shows
the head in feet or meters. See Figure 7-1.
By definition, the pump is a machine designed to add energy to a liquid
with the purpose of elevating it or moving it through a pipe. The pump
can elevate a liquid in a vertical tube up to a point where the weight of
the liquid and gravity will permit no more elevation. The energy
contained in the liquid’s weight is the same as the energy produced by
the pump. This point on the pump curve would be the ‘shut-off head’.
Shut-off head is the point of maximum elevation at zero flow. It’s seen
in Figure 7-2.