SUBJECT: What do we mean by pump efficiency? 6-1

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:

How 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 an example:

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 changing.
• 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 web site).

Suction head is measured the same way.

• 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 head.

Here is the formula for measuring the horsepower out of the pump:

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% efficient.

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 designs.

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.

These would include:

• 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 consuming power.
• 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 liquid.
• 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 turbulence.
• 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 seal.
  • Cavitation. (5 kinds)
  • 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 the seal.
  • Operating too far off of the best efficiency point of the pump.
  • 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 bearings.
  • Over tightening packing or improper seal installation.
• Vortex pumps can lower efficiency by as much as 50%.

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