Saturday, March 30, 2019

PUMP CASE STUDY 1 - REVERSE ROTATION OF VERTICAL MULTI-STAGE BOILER FEED WATER PUMP

An interesting case came when one of the vertical multi-stage boiler feed water pumps was not developing the pressure and was having high abnormal sound from the start-up after a motor overhauling. The vibration data were collected and Direction of Rotation (DOR) was checked and found that the problem is with DOR.

How the pump can run in reverse?

There are two possible cases in which the pump can run in reverse.
  •      Backflow through pump: Usually, this happens after switching off the pump and the discharge and suction NRV/valve is passing. As the discharge head is high, water flows back through the pump making it run like a turbine. Now, the pump will be driving the motor. There is no problem in this case until the motor is started. Care should be taken so as not to start the motor while it is being driven by the pump. If started, it will add additional load causing for electrical damages in motor and/or mechanical damages in pump and shaft. But as long the rotation is not disturbed suddenly, there is no possibility of damage as the torque load on each component will be acting in the same direction.
  •     Wrong connection of motor terminals: This will create more problem than the previous case. When the driver/motor rotates in reverse direction, the torque load acts in reverse direction as compared to design, causing the failure of components like lock nuts, set screws, threaded joints and will even lead to high vibrations due to off-design flow through the pump. The common thing which can be noted when the pump is rotated in reverse direction for a short period is the loose impeller lock nut.

How to check the direction?

This is easy. Use a small plastic wire or lock tags and put it carefully on the rotating shaft surface. The direction of deflection can be noted to understand the DOR. Note that stroboscope is not a good tool for understanding the DOR. If the frequency setting is less than actual speed, you’ll find the correct DOR. If frequency setting is more than actual, you’ll find it running in the reverse direction. Or, the equipment can be turned off, and when the speed is coming down, you can understand the DOR.

Effects of reverse rotation

The pumps might contain several locking nuts/screws/bolts threaded to tighten from the natural DOR. Once it is operated in reverse, the forces on these nuts/screw/bolts will be reversed causing it to become loose. When the rotors come loose, it will lead to failure of equipment, sometimes jamming which will lead to shear failure of mechanical components or will overload and burn the windings of the motor.
In this case, the lock nut came loose and the pump seal and stage bushings got damaged due to excessive vibrations.

How to prevent failures associated to reverse rotation?
  1. Care to be taken in determining the direction of pumps. Following steps can be followed to prevent the failures associated with reverse rotation.
  2. If DOR is not marked on the pump casing, mark it during the assembly by understanding the arrangement of vanes. In some cases, by looking at the casing structure, one can determine DOR. Always cross check with the manual before marking.
  3. Check and ensure the driver DOR before coupling.
  4. Use Loctite thread lock compound (preferred 243 (blue), removable medium strength) on impeller lock nut thread to prevent it from becoming loose. 
  5. If the pump is being driven by backflow, bring the pump to rest before starting it again. Replace the passing valves as soon as possible to avoid the problems.

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