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

The rise in total generation capacity of renewable energy sources such as wind and solar is resulting in traditional base load coal generation plants being more frequently used to follow fluctuating electricity demand, also called peak load operating. This change in operation has an effect on the power plant equipment, since machines are now subjected to operating conditions they were not originally designed for.

The term base load is used to mean a plant operating at a stable electrical output for a sustained period of time. This is typically at or near the peak efficiency of the plant. What this means for the boiler water circulating pump (BWCP) in the plant is there are minimal thermal cycles and, hence, minimal cyclic thermal stresses. The equipment can be operated at a stable, constant condition rather than frequently being started and stopped or ramped up and down the pump curves.

In contrast, the terms peak following or peak load mean operation that fluctuates up or down to match the short-term electrical grid power demand.

This plant cycling means the BWCPs experience increased thermal cycling and either starts and stops or fluctuating operation along the pump curve as the plant flexes to provide a changing electrical demand. The fluctuation of boiler evaporation rate and, in some cases, boiler pressure has an effect on the operation of the BWCP.

The BWCP is typically used in forced circulation drum boilers to provide the necessary head (pressure) to overcome the frictional losses encountered in the boiler tubes. This allows the water circulating through the boiler to be turned into steam to generate electrical power in the turbine-generator set.

Typical BWCPImage 1. Typical BWCP (Images courtesy of Hayward Tyler)

The BWCP is a glandless, sealless combined motor and pump. There is no dynamic mechanical seal between the motor and pump, and they share a common shaft. The motor and pump are designed for the same system pressure, but the motor is maintained at a cooler temperature using a thermal barrier and heat exchanger. A BWCP typically uses a wet stator design. A typical BWCP is shown in Image 1. The combination of a fluid-filled electric motor and pump makes this a unique component when considering all the areas affected by changing plant operation.

Thrust Bearing

A typical BWCP is supported axially using a hydrodynamic thrust bearing. The bearing uses a water wedge formed between the thrust disc, which is keyed to the shaft, and tilting thrust pads. The thrust bearing accommodates the weight of the rotor and additional thrust loads from the pump including a suitable design safety factor. When the motor starts up, the hydraulic lift from the impeller lifts the rotating assembly upwards to establish a film of water between the thrust bearing surfaces during stable operation.

When the BWCP is shut off, the rotor loses the hydraulic lift and returns down to the reverse thrust bearing. Initially during startup there is insufficient fluid film between the nonmetallic bearing and the thrust pads, so the friction between the pads can cause a small amount of material wear. This reduces the thickness of the bearing material and begins to open up the distance the rotor can travel (rotor end float). This means the rotor is traveling farther every time the motor is de-energized, resulting in a larger thrust.

Long-term cycling can result in damage to the thrust bearing that requires accelerated maintenance or—in some cases—catastrophic failure of the thrust bearing requiring extensive repairs. Thrust inspections are recommended every three years. Users should then develop an asset management strategy based on the as-found condition of the thrust bearing components. Monitoring pump performance and critical clearances, as found during inspections, allows users to better understand the wear associated with their plant running regime and adjust maintenance cycles (as far as reasonably practical) to maximize the life of the BWCP.

Radial Bearings

Similarly to the thrust, the radial bearings in the BWCP use hydrodynamic bearings. The radial bearings are at the thrust end and pump end. Accurate alignment of the radial bearing is controlled by manufacturing tolerances of the stator housing and motor case. The radial bearings are sized to absorb loads from unbalanced magnetic pull (UMP) on the rotor and hydraulic fluctuations in the pump creating unbalanced loads and safe rotor dynamic characteristics. When the BWCP starts there is insufficient fluid film between the journal sleeve and the bearing surface. This creates wear on the pads resulting in lost material and larger clearances. The bearing wear allows the rotor to deviate from the center axis of the stator, which increases the UMP, in turn increasing bearing wear. This allows the rotor to deviate from alignment with the stator, then the damage propagates due to worn radial bearings. If the bearing clearances become excessive under extreme conditions, during startup the UMP could cause rotor-to-stator contact due to bending in the rotor, which could damage the stator and rotor lamination packs, resulting in the need for restacking.