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Solution to March's rebuilt HST problem
The potash mine conveyer lift problem on the original hydraulic pump suffered severe cavitation or aeration damage. This problem should not occur if the intake of the pump is maintained at approximately 250 psi or higher. The passages in the pump are designed for the maximum pump flow with minimum pressure drop, thereby keeping any implosion damage from cavitation or aeration to a minimum.
If the shuttle relief is located 600 ft below the pump, the weight of the column of oil being pushed up to the pump causes about 240 psi. Add this to the 250 psi pressure needed at the pump inlet, thus requiring the shuttle relief to be set at about 490 psi.
Unfortunately, the 490-psi backpressure at the motor would reduce the torque output of the motor.
When the calculations were made, the torque output of the motor was based on the maximum pump pressure setting with a 200-hp diesel engine. The charge pump backpressure needed to be considered when making this calculation. We found that if the shuttle relief was lowered to 250 psi, reading it at the motor, the system would lift the normal potash load. However, the inlet gauge at the pump only showed about 50 psi. This pressure was not enough to minimize the cavitation damage experience by the original pump.
We also wondered why the low charge pump pressure did not cause stroking and response problems. On further examination, we found a 125-psi check valve was installed just before the charge pump pressure filter, providing about 175 pi for control pressure.
The mine manager had another diesel engine rated at 245 hp. With the larger engine, we were able to increase the main pressure and have the pump inlet pressure at 250 psi. Not only did this solve the problem, they were able to lift more tonnage each shift.
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