Reverse Motor Operation
 Most industrial pumps are driven by 3-phase motors. The rotation direction of a 3-phase motor may be reversed by switching the leads between any two of the three phases. It is not possible to determine the direction of rotation by visually examining the wiring. The most common method is to wire the motor and operate it uncoupled from the pump to determine correct rotation. Once the correct wiring is determined, the leads should be marked. In the case of a close coupled or submersible pump, where the rotating components are not readily visible, a phase rotation meter may be used to verify proper wiring (Fig 3).
Reverse operation of a pump that has an impeller that is threaded onto the pump shaft, or threaded shaft sections, can severely damage a pump. Upon start-up, the moment of inertia of the impeller, coupled with the fluid resistance, will cause the threaded parts to decouple, driving the impeller into the pump housing. The usual end result is a bent shaft, bent or broken impellers, broken housings, and an expensive repair. Threaded impellers and shafts sections should be avoided to prevent the possibility of this type of damage. However, there are pumps that, because of design requirements, utilize threaded shafts and impellers. Therefore it is a good practice to treat every pump as if reverse rotation might be damaging, by verifying proper rotation prior to coupling the motor to the pump.
If a centrifugal pump is operated with reverse rotation it will still pump; however, performance will be degraded. Typically the head will be 50-60% of normal. If a pump has been out of service and restarts with significantly reduced head and amperage, reverse rotation should be a prime suspect.
Pump Backflow
Whenever a centrifugal pump discharges to higher elevation or into a pressurized system, there is a risk of backflow through the pump as soon as the pump driver is de-energized or uncoupled. Fluid flow will always occur from a high pressure to a low pressure zone.
Unless there are operating protective devices in place, the impeller will act as a turbine and accelerate the rotor assembly in reverse rotation.
Non-reversing Ratchets
 On deep well pumps, allowing the pumps to run in reverse could result in a run dry condition for the line shaft bearings as the column drains through the pump casing. Installation of a non-reversing ratchet on the motor will prevent reverse rotation of the motor in applications where there is risk of damage to the equipment (fig 4).
Back-flow Prevention
 If the pump takes suction from an open pit or sump, it is possible to back-flow the contents of a higher volume system, that is at a higher pressure or elevation, into the sump, causing an overflow. This is especially serious when handling environmentally sensitive material. Vertical sump pump installations should always be equipped with back flow prevention check valves (fig 5). These valves are safety devices and should not be relied on solely for back-flow prevention. Rags and other sump debris can and do easily become trapped in these valves, preventing them from closing properly. Sump pump operating procedures should include positively actuated valves to ensure a positive shutdown of backflow in the piping system.
Start up of Centrifugal pumps
Most pump manufacturers recommend starting centrifugal pumps against a closed or slightly open discharge valve. Centrifugal pumps should never be started against a reverse rotation backflow condition. Current inrush associated with decelerating the rotor and accelerating it under open-valve conditions will likely exceed the electrical rating. Radial loads may also well exceed the design limits for shaft deflection when starting under a reverse flow condition due to the cumulative forces of fluid deceleration and rotor acceleration.
In Closing
For one particular pump, a few seconds of reverse rotation may be no cause for concern. For another design, in a different system, there can be a real risk of damage to equipment, the environment, and personnel. It should not be left to the operator to determine when reverse rotation is an issue and when it is not. Ideally system design and operating protocols for every pump system should prevent or minimize the risk reverse of rotation.
Acknowledgements
Figs 1 & 2 courtesy of John Crane, Inc. http://www.johncrane.com
Fig 3 courtesy of Extech Instruments Inc. http://www.extech.com
Fig 4 courtesy of US motors division of Emerson Electric Co. http://www.usmotors.com
Fig 5 courtesy of the Metraflex Company. http://www.metraflex.com
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