head for the pump in terms of simple lift. This
is the static head, which is subdivided into
discharge head and suction head. In this
drawing, the suction head is negative, so this
pump would have to be self-priming to lift
water up to the pump.
NOTE: Pumps really don’t suck fluid up
into them. Positive-displacement pumps
create a partial vacuum in their pumping
chamber, which allows atmospheric pressure
to push the water up into the pump to fill the
partial vacuum.
Figure 19-7 shows a configuration with a
positive suction head. This means that a non-
self-priming pump can be used, such as a cen-
trifugal pump.
Note that the total static head is the dis-
charge head combined with the suction head.
If the suction head is negative, this increases
the total effective lift height and you add the
negative suction head to the discharge head
to get total static head. If the suction head is
positive, this decreases the total effective lift
height and you subtract the suction head from
the discharge head to get total static head.
You can also see that the head is measured to
the top of the fluid being pumped from or
into. This is because the column of fluid
above the inlet or outlet has a static head (a
pressure) that must be overcome to increase
the fluid level in the upper container. (If the
container didn’t empty, the static head wouldChapter 19:Selecting and Sizing Pumps
Figure 19-7. Static
head with positive
suction head