Bachus advises on gauging the health of pumps

All pumps should have suction and discharge-pressure gauges installed, according to Bachus, which has explained the reasons for this in a paper.

Most instrumentation is installed onto equipment fed by pumps mated to electric motors.

Sometimes a process engineer wants to know the fluid velocity, flow or pressure in a pipe.

A pump normally generates that velocity, flow and pressure; sometimes the pump also contributes to the temperature in a chemical process.

When the temperature, velocity, flow and pressure are not what they should be, this normally indicates that the pump is stressed and is heading for failure.

The industrial pump is a relatively simple device when compared with a V-8 engine or a refrigerator.

The shaft and impeller assembly is the only moving part and most 'pump problems' originate outside the pump.

Pump manufacturers use the term 'head' in feet for fluid force.

Maintenance people use the term pressure in PSI for fluid force.

The conversion factor 2.31 separates the two terms.

For water, head divided by 2.31 is pressure, and pressure multiplied by 2.31 is head.

If the liquid is not ambient water, then the liquid's specific gravity (SP GR) must be factored into the formula (head (ft) / 2.31) x SP GR = PSI (PSI x 2.31) / SP GR = head in feet.

For example, most communities have an elevated water tank that supplies water pressure to the neighbourhoods below the tank.

If the level of water in the tank is 200 feet above a kitchen faucet in one of the homes, the water pressure at the faucet is 200ft/2.31 = 86.6PSI.

There would be about 87PSI of water pressure at the kitchen faucet because the water in the supply tank is 200ft high.

If a pump is designed to develop 70ft of head, 70ft/2.31 = 30PSI.

There should be 30PSI of differential pressure across this pump.

This means if the suction pressure gauge reads zero PSI, the discharge gauge should read 30PSI (30PSI diff.).

If the suction gauge reads 55PSI, the discharge gauge should read 85PSI (30PSI diff.).

The pump is at its optimum design point at 30PSI of differential pressure.

As the pressures move away from 30PSI (observed on the gauges), the maintenance rises on that pump.

Process pumps need gauges installed otherwise the operator has no reference point to determine the system's health.

Most pumps don't have pressure gauges on the suction nozzle, so the operator has no reference point.

The instrumentation technician should install gauges on the pumps to prevent them from becoming high-maintenance items.

Pump gauges cost a fraction of the price of CMM programs or vibration analysers.

All pumps should have suction and discharge pressure gauges installed.

The gauges should be calibrated, clean and adequate for service.

Operators should monitor the differential pressure.

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