The steam produced from the recovery boiler at this pulp mill is used to generate electricity with a back-pressure steam turbine directly coupled to an air-cooled generator. The generator capacity is 20 MW—it normally produces close to 18 MW, which is about 60 percent of the plant demand.
The power supply from the utility grid to the mill was interrupted. Mill personnel activated their load shedding procedure. This prevented a full blackout, and the mill was able to continue operations at a reduced rate.
While resynchronizing the turbine-generator to the utility grid, the generator tripped on overvoltage due to overcurrent in the excitation system. The generator trip resulted in a total mill blackout. The mill was without power for about 45 minutes, until the utility grid power was restored.
The turbine’s emergency DC (direct current) lubricating oil pump failed to start automatically when the mill lost all AC (alternating current) power. The power loss caused the turbine to lose lubrication oil for approximately two minutes—until an operator manually started the DC lube oil pump.
The turbine shutdown procedure was initiated, but efforts to engage the turning gear failed. Operators allowed the turbine-generator unit to continue to rotate with no load in an effort to prevent bowing of the rotor. Approximately four hours later, the turbine tripped on high vibration.
The subsequent investigation determined that a defective keyed switch in the emergency DC lube oil pump start-up circuit prevented the unit from automatically starting.
Upon dismantle, it was learned that the turbine rotor journals, thrust bearing, extension shaft and seals experienced a combination of axial and radial rubbing due to the loss of lube oil.
The on-site electrical test of the generator confirmed that the field windings had shorted, probably due to the overcurrent incident during the synchronization effort. Visual inspection revealed evidence of overheating of the field windings at the end turns. The generator rotor will require a full rewind.
As is often the case, several factors contributed to the outcome of this event:
- The defective keyed switch in the emergency DC lube oil pump motor control circuit prevented the DC lube oil pump from automatically starting upon total loss of AC power. This defeats the purpose of an automatic, backup emergency system.
- Neither the auxiliary AC nor the emergency DC lube oil pumps were being tested for functionality. Regular, functional testing of emergency DC lube oil pumps would have identified this problem, allowing for corrective action. Testing should include a check to determine whether the pumps start at the appropriate pressure upon loss of the AC systems.
- Operators allowed the unit to continue operating; the DC lube oil pumps are meant to only provide the minimum bearing lubrication needed for a controlled coast-down, not continued operation. This emphasizes the need for training and retraining on emergency operating procedures.