SUBJECT: What do we mean by pump
When we talk about automobiles and discuss efficiency, we mean how
many miles per gallon, or liters per 100 kilometers. When we discuss
centrifugal pumps we are comparing the amount of work or power we get
out of the pump to the amount of power we are putting into the pump.
As an example:
do we measure the horsepower or kilowatts coming out of the
pump? All we have to do is multiply the pump head by the
pump's capacity, and then use a simple conversion number. Let's take
Flow = 300 gallons per minute of fresh water as measured coming
from the pump discharge.
Head = 160 feet. We measured it at the discharge side of the pump
and corrected it for the fact that the gage was two feet above the
pump center line. Look at the following diagram where we have
calculated the discharge head from the formula shown on the right
hand side of the illustration. If there were any positive head on the
suction side of the pump that head would have to be subtracted. A
negative suction head would be added to the discharge head.
The centrifugal pump
pumps the difference between the suction and the discharge heads.
There are three kinds of discharge head:
- Static head. The height we are
pumping to, or the height to the discharge piping outlet that is
filling the tank from the top. Note: that if you are filling the
tank from the bottom, the static head will be constantly
- Pressure head. If we are pumping
to a pressurized vessel (like a boiler) we must convert the
pressure units (psi. or Kg.) to head units (feet or meters).
- System or dynamic head. Caused by
friction in the pipes, fittings, and system components. We get
this number by making the calculations from published charts ( non
included in this paper, but available in the chart section of this
Suction head is measured the
- If the liquid level is above the pump center line, that level
is a positive suction head. If the pump is lifting a liquid level
from below its center line, it is a negative suction head.
- If the pump is pumping liquid from a pressurized vessel, you
must convert this pressure to a positive suction head. A vacuum in
the tank would be converted to a negative suction head.
- Friction in the pipes, fittings, and associated hardware is a
negative suction head.
- Negative suction heads are added to the pump discharge head,
positive suctions heads are subtracted from the pump discharge
is the formula for measuring the horsepower out of the
Remember that we are using the actual horsepower or kilowatts
going into the pump and not the horsepower or kilowatts required by
the electric motor. Most motors run some where near 85%
An 85% efficient motor turning a 76% efficient pump, gives you a
real efficiency of 0 .85 x 0.76 = 0.65 or 65% efficient.
A survey of popular pump brands demonstrates that pump
efficiencies range from 15% to over 90%. The question then arises,
"Is this very wide range due to poor selection, poor design, or some
other variable which would interfere with good performance?" The best
available evidence suggests that pump efficiency is directly related
to " the specific speed number " with efficiencies dropping
dramatically below a number of 1000 . Testing also shows that smaller
capacity pumps exhibit lower efficiencies than higher capacity
Now that we have learned that pump efficiency is closely related
to the shape of the impeller, and
the choice of impeller shape is usually dictated by the operating
conditions, you should be aware of various conditions that decrease
the efficiency of your pump.
- Packing generates approximately six times as much heat as a
balanced mechanical seal.
- Wear rings and impeller clearances are critical. Anything that
causes these tolerances to open will cause internal recirculation
that is wasting power as the fluid is returned to the suction of
the pump. If the wear ring is rubbing, the generated heat is
- A bypass line installed from the discharge side of the pump to
the suction piping. The heat generated from this recirculation
can, in some cases, cause pump cavitation as it heats the incoming
- A double volute design pump restricts the discharge passage
lowering the overall efficiency.
- Running the pump with a throttled discharge valve.
- Eroded or corroded internal pump passages will cause fluid
- Any restrictions in the pump or piping passages such as
product build up, a foreign object, or a stuck check valve.
- Over lubricated or over loaded bearings.
- Rubbing is a major cause. It can be caused by:
- Misalignment between the pump and driver.
- Pipe strain.
- Impeller imbalance.
- A bent shaft.
- A close fitting bushing.
- Loose hardware.
- A protruding gasket rubbing against the mechanical
- Cavitation. (5
- Harmonic vibration.
- Improper assembly of the bearings, seal, wear rings,
packing, lip seals etc..
- Thermal expansion of various components in high temperature
applications. The impeller can hit the volute, the wear rings
can come into physical contact etc.
- Solids rubbing against the rotating components, especially
- Operating too far off of the best efficiency point of the
- Water hammer and pressure surges.
- Operating at a critical speed.
- Dynamic, non o-ring elastomers that cannot flex and roll,
but must slide, eventually fretting the shaft or sleeve.
- A build up of product on the inside of the stuffing box
rubbing against the mechanical seal.
- Grease or lip seals rubbing the shaft next to the
- Over tightening packing or improper seal installation.
- Vortex pumps can lower efficiency by as much as 50%.
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