PLANT

Go with the flow

Knowing how to maintain pumps and their components while dealing with malfunctions effectively goes a long way to improving pump performance, increasing reliability and saving some money.


October 6, 2010
by Steve Gahbauer

A cutaway view a Viking vane pump, used for liquid transfer applications from chemicals to liquefied gases.

Photo: Viking Pump

Knowing how to maintain pumps and their components while dealing with malfunctions effectively goes a long way to improving pump performance, increasing reliability and saving some money.

Here are some nuggets of pump wisdom gleaned from presentations at a Fluid Prime Movers seminar, organized by the Society of Tribologists and Lubrication Engineers.

Pump failure modes. When a centrifugal pump does not reach design flow rate or design head, or operates for a short period but then loses prime, it probably has insufficient net positive suction head or entrained air.

The likely cause of a pump exhibiting similar symptoms but also showing decreased power consumption with no discharge or flow while running, is a greater than anticipated system head. Other causes include the direction of rotation is reversed, the pump speed is too low, or the impeller is too small. Also check for a plugged impeller, suction line or casing. Improper priming could be the cause of decreased power consumption.

There could be damaged bushings, thrust bearings or impeller rubbing when a pump runs with excessive noise from the wet end and exhibits an increase in power consumption. For noise in magnetic drive pumps, check for rubbing of the inner magnet against the shell. Excessive noise could also be caused by abnormal fluid rotation due to complex suction piping, which should always be straight.

There are several possible causes for increased power consumption and excessive noise from the power end, including: bearing contamination appearing on the raceways as scoring, scratching or rusting; brinelling of bearing caused by incorrectly applied forces during assembly, identifiable by indentation on the raceway; thrust overload on bearings; misalignment; bearing out of square with the centreline; or a bent shaft.

Pump selection criteria. When selecting positive displacement pumps, look at the following criteria:

• capacity (constant or variable)

• viscosity (high or low)

• pressure (discharge and suction)

• solids and abrasives (size, hardness and amount)

• air and/or gas (volume, entrained, or slugs)

• shearing properties (shear thinning, Newtonian, etc.)

• temperature (high, low, ambient)

• whether a variable speed drive pump is necessary for total process control.

Also consider pump material (soft, hard, coated, corrosion resistant); proper sealing (packing, seal type, magnetic drive); lubrication (product, grease, oil, oil mist or external system); relief valves in the system; and instrumentation for monitoring what’s going on inside the pump (pressure, speed, temperature, vibration).