In common engineering practice, a centrifugal pump (with standard check valve in discharge line) is used in solo / series / parallel operation in industries. While the pump is operating and power is cut off due to any trip or electrical problem and check valve does not close / fail, in that case fluid flow reversal shall occur. Under this situation, the pump acts like a hydraulic turbine and spins in opposite direction. Following are the failure mode effects either individual or in combination.
1. Detaching of impeller from shaft
If the impeller has threaded connection or with a Heli coil, reverse rotation may unscrew the pump impeller.
2. Failure of mechanical seal
Some seals (Single spring seals) are designed to run in one direction only. If power is restored while pump is rotating in reverse direction, it can cause hang up in multi-spring API 682 seals.
3. Resonance due to reverse flow with high head causing pump spinning faster than rated
Depending on the specific speed of the pump, the maximum speed in reverse rotation can be significantly higher than the normal running speed. If the 1st mode is close to rotor, then it can cause severe vibration during excursion to coast down with tapering hydraulic head. In case of horizontal pumps, gravity is resisting reverse rotation. So reverse speed will be less.
4. Mechanical Failure
While the pump spins opposite direction and the motor is energized accidentally, it can cause major damage to the pump and / or motor. In that case severe torque reversal shall occur and stresses can cause a shear failure of shaft particularly at notch / fillet area.
5. Damage of hydrodynamic bearings due to lubrication issues
Lubrication pumps are either individual motor driven or shaft connected. In either case, the reverse rotation may not supply adequate oil pressure to rotor sleeve bearings while it is reverse spinning at high speed. At lower speed during normal coast down, tenacious oil film is sufficient to prevent white metal damage but not for rotor spinning fast in turbine mode.
Protection methods to prevent reverse rotation of pump
1. Mechanical device
To prevent detaching of impeller from shaft keyed impellers are used or polygon shaped fitment are provided in pumps used in chemical industries. Some designs by vendor ensure shaft impeller set as self-locking and reverse rotation safe.
To prevent failure during reverse rotation of pump, multi-spring seals are used. Most pump multi spring design seals are bi-directional. As most seals used in hydrocarbon industry are dual pressurized, machinery engineer should ensure the seals are suitable for reverse pressure conditions.
Only way to prevent resonance is to stop the pump reverse rotation by ratchet & pawl mechanism shown in below figure. This device is commonly used in Vertical pump as they are more prone to fail during reverse rotation. For horizontal pumps, a horizontally mounted pawl is used to mesh with a fixed ratchet preventing reverse rotation. Gravitation action on the lumped mass urges the pawl into mesh with the ratchet, while centrifugal force on the lumped mass during forward, allowed rotation retracts the pawl away from the ratchet.
Simple ratchet and pawl arrangement require no lubrication and have no parts to wear during normal operation.
Anti-Reversing clutches are essential whenever a pump is operated against a high static head. The anti-reversing clutch will prevent reverse rotation of the pump in the event of non-return valve failure.
The clutch allows free rotation of the pump in one direction only and will engage and prevent rotation in the opposite direction. The clutch will disengage automatically and allow for normal rotation without the need for resetting or readjustment.
Fig 1 – Pawl & Ratchet Mechanism
2. Electronic protection for motor restarting while reverse rotating
To prevent motor getting restarted while spinning backwards a tachometer and some protection logic in the MCC in required that will not allow the motor to start unless it is zero RPM. One of famous machine monitoring vendor makes a reverse rotation monitor that could be used as an interlock to the motor].
Starting big motors with the machine running backward should trigger the motor protection relay operating as fast as possible. For large horizontal centrifugal pumps, fast acting motorized valve is used in discharge side with reverse rotation detection device is used.
3. Microcontroller based intelligent rotation shaft detection
The system features a failsafe reversal switch that constantly monitors a pump shaft to assure that it’s turning in the correct rotational direction.
Relay contacts in the unit can be used to electrically lock out the starting circuit of the pump to prevent accidental start-up during a condition of reverse rotation. The device is supplied with a custom-machined split collar wrap that is designed to clamp tightly around the pump shaft.
The system should be failsafe, a power failure would de-energize the relay. This means the starting circuit to the pump motor would be disabled. Upon restoration of power, the relay resets but would instantaneously switch out again if it
detected a shaft reversal, thus preventing the pump motor from starting during a reversal condition. It also displays the speed of rotation of the shaft in RPM in both side.
Fig 2 – Electronic protection for motor restarting while reverse rotating
Statutory warning – Even for minor maintenance / checking, always use proper Lock-Out-Tag-Out procedures.
All above are to be addressed by purchaser (Operations / Mechanical / Electrical /safety department) in RFQ document itself citing the purpose explicitly and shall be documented in motor and pump data sheets.