February 2009, Vol. 21, No.2
Operating Centrifugal Pumps Safely
Following electric motors, pumps are the most widely used rotating equipment in the world. The two most common pump designs are centrifugal and positive displacement.
Examples of positive-displacement pumps in our industry are air-operated diaphragm pumps, plunger pumps, rotary-lobe pumps, progressing-cavity pumps, rotary-screw pumps, and peristaltic pumps. While commonly used for return and waste activated sludge, high-solids, and shear-sensitive services, positive-displacement pumps are in the minority when compared to the existing population of centrifugal pumps.
The popularity of centrifugal pumps can be traced primarily to their ability to operate along a curve (inversely proportional flows and pressures). Centrifugal pumps also have the unique ability to operate against a closed discharge valve without causing serious damage to people or property — a feature that makes them inherently safer than positive-displacement pumps.
If an operator accidentally closes the discharge isolation valve on a centrifugal pump, it will cavitate and eventually boil the liquid out of the pump, if not shut down in time. A cavitation-damaged impeller, destroyed mechanical seal or, in the worst case, broken shaft is generally the extent of the damage. Maximum pressure capability of the pump is limited to the shutoff (deadhead) pressure indicated on the performance curve.
On the other hand, should an operator accidentally close the discharge valve on a positive-displacement pump, something will fail in a catastrophic manner unless the pump is protected by a properly sized and maintained pressure-relief system.
Even though centrifugal pumps have that one very important safety feature, proper care still must be taken to operate and maintain them safely.
First and foremost, it is critical that maintenance personnel use a lockout–tagout procedure to ensure that electrical power cannot be applied to the pump during maintenance procedures. It is not in an operator’s best interest for a pump to be energized while various parts of his body are in close proximity to motors, couplings, drives, impellers, and shafts.
Second, when a pump is initially placed into operation or restarted after inspection or repair, verify the proper rotation of the electric motor with the pump disconnected from the motor. Motors transmit power to pumps through flexible couplings, V-belt drives, and similar devices. In the event of a close-coupled pump and motor — where the motor shaft is the pump shaft — extreme care should be taken to prevent injury and equipment damage during rotation checks.
Why is it critical to verify rotation with the motor disconnected? Pump impellers can be loosened or backed off during reverse rotation. Loosened or backed-off impellers disrupt the tolerances inside the pump, which can damage components and cause injury to persons in the area. For example, the impeller may impact the wear plate, thus locking up the pump and motor shaft.
Never assume a pump is rotating correctly simply because it was previously observed to do so. During normal electrical power-line maintenance or damage repair, it is common for power company technicians to reverse phases when reattaching electrical connections. Reversing incoming power phases in a three-phase system causes the motor to run backwards. This can cause an impeller to back off or, at the least, dramatically affect the pump’s ability to do its job.
Third, operators should never attempt to operate or maintain a centrifugal pump without adequate training, complete support documentation, appropriate tools, proper diagnostic equipment, and the necessary personal protection equipment.
At a minimum, diagnostic equipment should include pressure–vacuum gauges, a voltmeter, an amp meter, and a tachometer. Without gauges, it is almost impossible to distinguish suction cavitation from discharge cavitation or cavitation in general from mechanical noise generated by a faulty bearing or rubbing impeller.
The following personal protective equipment should be considered mandatory when working on or near pumps: properly selected gloves, safety glasses, and hearing protection.
Fourth, in the event a centrifugal pump becomes excessively hot during operation, the technician should never attempt to open the pump until the pump has ample time to cool — preferably overnight. Life-threatening steam burns produced by extremely hot pumps are, unfortunately, all too common.
Further, maintenance personnel should never attempt to cool a hot pump by spraying it with cool water. Hot pump casings have been known to explode when sprayed with cool water. A general guideline to follow is that if it is possible to hold one’s hand on a surface for more than 2 seconds, the surface temperature is less than 140°F (60°C). Since the boiling point of water is 212°F (100°C), a pump temperature of 140°F (60°C) is an indication that the pump can be opened safely.
Finally, keep in mind that local professional distributors of centrifugal pumps are an important operator resource. Products purchased through a local authorized distributor should include specialized training and operational support. Successful operators form an alliance with their local equipment distributors, utilizing them for pump sizing, training, failure analysis, parts supply, equipment replacement, and operational advice. Distribution firms often provide these services at no additional cost to their valued customers.
Centrifugal pumps are designed to be operated and maintained safely. Shortcutting or ignoring procedures from the manufacturer’s operations and maintenance manual is a recipe for trouble. If there are questions about the equipment, contact the authorized distributor or manufacturer.
Ben Humphries is a municipal sales manager at Delta Process Equipment Inc. (Ruston, La.).