Targeted maintenance of BWCPs will help reliability and longevity.
by Stephen Jones
August 22, 2018

Impeller Wear Ring

The impeller wear ring is set with a predetermined diametrical clearance to the pump case or diffuser wear ring to control the pressure drop across the surface and the amount of fluid recirculated. The distance from the pump case to motor case mating face and end of the impeller (known as the “A” dimension) is controlled to ensure no axial contact. In scenarios where the plant is quickly changing load or fully starting and stopping, the motor is typically at a different temperature than the pump case and rotating assembly. Depending on that temperature differential, the impeller and pump case will grow or shrink at different rates. This reduces the wear ring clearance and can cause contact.

In severe situations, binding of the rotating assembly to the static wear ring can occur. Frequent load changes will cause accelerated degradation to the wear rings. This increases the clearance and reduces efficiency but more severely shortens the life of the wear rings and can cause major damage to the impeller and rotating assembly. Excessive contact of the impeller-to-pump case can bend the rotor, damage the radial bearings and damage the stator windings due to debris being circulated through the motor.

Pump Case

The pump case on a BWCP is welded into the pipework and typically made from a thick-walled carbon steel casting. Frequent thermal cycling on the pump case due to changing operating loads subjects the pump case to thermal stresses as the pump case tries to expand freely yet is constrained by the pipework and connection to the motor. Frequent thermal stress cycling can present problems with fatigue crack development at critical regions such as thicknesses, nozzle regions and welds. Operators should inspect pump cases for the development and propagation of cracking. If the pump case welds are not part of a maintenance program associated with “high energy pipework” they should be included in the periodic inspection of the pump case.

Thermal stresses on pump caseImage 2. Thermal stresses on pump case from fluid or metal differential temperatures

Image 2 shows the highest stressed areas of a representative pump case and are the areas that should be inspected first for fatigue cracking.

Image 3 shows cracking that was discovered in a BWCP pump case. In this case the plant was unable to return the BWCP back into operation and had to order a new pump case that had a considerable lead time. In addition, they were required to inspect the other pump cases on short notice resulting in unplanned outages.

Cracks in a BWCP pump caseImage 3. Cracks in a BWCP pump case

Electrical Motor

The electrical motor is under the most stress during a startup. The in-rush current of a typical BWCP is approximately four-and-a-half to five times the full load current. The majority of BWCPs are wet stator units (WSU), which means the rotor and stator are submerged in water. WSUs use a cross-linked polyethylene (XLPE) insulating material to insulate the copper wire from the water in the stator. This current causes the copper wire to heat up, creating a short-term negative effect on the insulation. Every time a motor is started, it reduces the lifetime of the insulation, leading to increased risk of an insulation failure.

When the stator is initially wound, the cable in the slots are wedged together to keep them packed tightly. With normal operation of the electrical motor, the cable in the slots bunch together and pull to the outside of the stator from the magnetic forces in the motor. This can loosen cable in the stator slots allowing additional cable movement.

During startup of the motor, the cable in the end turn phases exerts magnetic forces on cables in adjacent end turn phases causing the end turns to move and vibrate. The outer coils are pushed toward the outside of the stator shell and the inner coils are pulled toward the rotor. The end turns flex with the changing magnetic fields induced by incoming current.

This shows failure to ground due to mechanical fretting and insulation wear.Image 4. This shows failure to ground due to mechanical fretting and insulation wear.

Over time with high numbers of starts to the motor, this end turn cable movement, combined with the increased movement in the slot, can result in the cable rubbing on the stator end plate. If this continues, the cable insulation wears down until an insulation failure occurs (as shown in Image 4). On some occasions, the insulation failure causes an electrical arc, which damages the stator end plate and stator laminations. This can mean a full or partial restack is necessary, along with a stator rewind resulting in extended motor downtime and significant repair costs.

UMP

UMP occurs when the centerline of the rotor does not line up with the centerline of the stator. Upon startup, the electric motor acts like a magnet, pulling the rotor approximately in the direction of the smallest air gap. This can cause a static or dynamic eccentricity in the rotor motion. As mentioned previously, under extreme conditions with sufficient bearing wear there could be rotor-to-stator contact.

Insufficient NPSHa

During fluctuating plant load (MW) or unplanned shutdowns some have observed that the BWCP can experience cavitation. This can be caused by a few different scenarios.

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