Gear Pump Seizing

Gear Pump Seizing is needed. A polystyrene (PS) sheet line was constructed using a two-stage, vented extruder and a gear pump. On several occasions the bearings on a gear pump would seize. In both cases, the gear pump was identified as the root cause of the failure, and claims were placed against the manufacturer.

After the third pump seized, a full evaluation of the line was performed. The analysis indicated that the first-stage metering section of the extruder screw was not controlling the specific rate of the process as designed. Instead the specific rate was about 90% of the flow due just to rotation; i.e., historically known as the drag flow rate. Here the rate was controlled by a poorly designed solids conveying section. During typical operation, the extruder would operate with the first-stage metering channel at essentially zero pressure, and the second stage metering section would use only two diameters of filled length to supply an inlet pressure of 6 MPa to the pump. On very rare occasions, the solids conveying section would deliver a large amount of material such that first stage metering section was operating at 120% of the specific rotation rate.

This event would completely fill the second-stage metering section such that a very high and nearly instantaneous discharge pressure would occur. This pressure was estimated at about 60 MPa. This high pressure coupled with a low pump discharge pressure of 15 MPa created enough differential force to deflect the rotors and overload the gear pump bearings, leading to the seizing of the pump.

For a normal control scheme, the controller for the screw speed would have decreased the screw speed to match the 6 MPa inlet pump pressure set point. But because the pressure surge happened extremely fast, the controller could not respond fast enough when the high pressure surge occurred, causing the catastrophic failure of the pump.
The solids conveying section of the screw was modified through screw design and process temperatures such that the first-stage metering channel was always operating full and under pressure at a specific rate of 110% of the calculated rate due just to screw rotation. The pump never seized again.