First, the net positive suction head available (NPSHa) could have decreased because the steam drum is vented or the drum pressure is lower than normal. In this scenario the pressure on the liquid surface in the drum is reduced.
This means that the only NPSHa to the pump is the static elevation of the liquid above the pump minus any frictional losses. This is insufficient, causing cavitation at the pump suction.
Second, the system resistance—which consists of the static elevation pumped plus the resistance of pipe fittings, valves etc.—could have decreased. The pump will increase flow until its developed head intersects with the new system resistance curve. As net positive suction head required (NPSHr) rises with flow there may not be enough NPSHa to satisfy NPSHr.
Third, the BWCPs are used to help force-cool the boiler in the event of a tube leak. As the pressure and temperature in the boiler are reduced to repair, the tube leak the BWCPs are left running to force additional flow through the boiler. This results in a drop in NPSHa, which may fall below NPSHr.
If cavitation occurs it will likely cause damage to the main and reverse thrust bearing from loss of hydraulic lift in the impeller as a consequence of vapor entering the suction. This can cause the rotating assembly to shuttle between bearings faces causing damage. It will also likely damage the impeller, which sounds like marbles pumped through the impeller and can cause pitting on the surface as the bubbles collapse. More commonly, damage is attributed to increased vibration resulting in increased wear on the radial and thrust bearings.
Other Considerations for Older Motors
The following are not specific to cycling plant operation, but are key for an asset management strategy for an aging BWCP.
Changes to System Resistance
When the power plant was designed, a system resistance was calculated based on the designed static head plus the frictional loses due to pipework, valves, orifices, etc. The frictional losses vary with the square of the flow. The pump will operate where the pump curve and system curve intersect. The pump curve changes if the speed of the motor or the impeller diameter is changed, which is uncommon.
Changes to the system such as modified valve arrangements, changes to pipework configuration or diameter, degradation of drum baffles, etc., affect system resistance, which dictates where the pump operates along its curve.
This can result in the pump operating away from its designed duty point, which can cause increased vibration. This will result in accelerated damage to the radial and thrust bearings. If the system resistance has increased, the pump will run back on the pump curve, i.e. lower flow, higher head. The increased differential pressure across the impeller can cause increased thrust on the rotating assembly accelerating bearing wear.
Discharge Valve Inspection
The BWCP either has a single discharge or a double discharge from the pump case. In both cases the discharge pipework has an isolation valve to isolate the BWCP from the boiler. If the discharge valves are not maintained, they can partially close when unintended. If the discharge valve is partially closed with the pump in operation, it will force the pump to operate back on the pump curve with a large differential pressure across the pump. The increased differential pressure across the impeller can cause increased thrust on the rotating assembly accelerating bearing wear.
In the case of double discharge pump cases, if one discharge valve has not been maintained and floats to a partially closed position, this affects the alignment of the impeller within the pump case. The impeller and rotating assembly will locate away from the discharge with the partially closed valve. The off-center alignment causes uneven wear on the radial bearing, which will increase the UMP on the rotor.