Page 110 - Know and Understand Centrifugal Pumps

The System Curve

change. An example of this would be closed systems like water in
the radiator of your car. Another example would be a swimming
pool re-circulating filter pump. The vessel being drained (the pool)
is the same level as the vessel being filled (the pool). If there is a
difference in elevation across the system, this difference is recorded
in feet and called Hs.
2. Hp - the pressure head, or the change in pressure across the system.
It is expressed in feet of head. The Hp also may, or may not exist in
every system. If there is no pressure change across the system, then
forget about it. An example of this would be a recirculated closed
loop. Another example would be if both the suction and discharge
vessels have the same pressure. Think of a pump draining a vented
atmospheric tank, and filling a vented atmospheric vessel. The
atmospheric pressure would be the same on both vessels, thus no
Hp. If Hp is present, then note the pressure change and employ it
in the following formula. Sometimes, it is necessary to use a pump
to drain a tank at one pressure (like atmospheric pressure), while
filling a tank that might be closed and pressurized. Think of a boiler
feed water pump where the pump takes boiler water from the
deaerator (DA) tank at one pressure, and pumps into the boiler at a
different pressure. This is a classic example of Hp. The formula is:

Apsi x 2.31
Hp =
SP. gr.

where: Apsi = boiler pressure - DA tank pressure
3. Hv - the velocity head, or the energy lost into the system due to the
velocity of the liquid moving through the pipes. The formula is:

where: V = velocity of the fluid moving through the pipe
measured in feet per second, and g = the acceleration of gravity,
32.16 ft/sec2

Hv 15 normally an inqnificant figure, like a fraction of a foot of head or fraction of a
psi, which can’t be seen on a standard pressure gauge. But you can’t forget about it
because it is needed to calculate the friction head. If the Hv converts to a pressure I
that can be observed on a standard pressure gauge, like 6 or 10 psi, the problem is the
inadequate pipe diameter.