SUBJECT: Some more things you should know about centrifugal pumps 11-4

The limitations of a magnetic drive pump

When do you switch from anti-friction ball and roller bearings to hydrodynamic (sleeve) bearings in a centrifugal pump?

Increasing the impeller speed increases the efficiency of centrifugal pumps.

If the wear ring clearance is too large:

Pumps are normally throttled with a discharge valve, but in rare cases it can be done with a suction valve.

Because an overhung impeller does not require the extension of a shaft into the impeller suction eye, single stage impellers are preferred for pumps handling suspended matter such as sewage.

Electric motors are sized considering the specific gravity of the liquid being pumped. If a low specific gravity pump is tested with water, or any higher specific gravity fluid, the increase in motor amperage could burn out the motor.

Do not hydrostatically test a high temperature pump with water. Water trapped in small recesses and gaskets will flash to steam in high temperature applications, expand and then break something.

There are several ways to prime a centrifugal pump with a suction lift:

Pumps with variable speed drives have several potential problems:

You cannot vent a running pump. Centrifugal force throws the liquid to the outside of the volute leaving the air at the eye of the impeller.

Operating off the BEP can break the pump shaft because the force is always in the same direction while the shaft is turning. This has the affect of flexing the shaft twice per revolution. In many cases you can easily exceed the endurance limit of the shaft material.

Slurry pumps have some features that make them different than chemical pumps.

If you need a pump with high head, low capacity features:

The optimum control valve location is within five feet (1,5 meters) of the pump discharge to prevent too much surging of fluid in the system when the discharge is throttled.

Here is the proper way to vent a centrifugal pump after it has been installed, or the system has been opened. I am assuming the pump is empty of liquid and both the suction and discharge valves are shut.

If you are using a high speed pump (greater than electric motor speeds) there are some additional things to consider:

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