HPU Pump Motors Are Continuously Running

PROBLEM DEFINITION
The Hydraulic Power Unit (HPU) pump motors are continuously running at excessive current levels.

DETAIL
Many Hydraulic Power Unit (HPU) pump motors have been observed to be running with excessive current while the turbine is at steady-state operating conditions. This condition may also be accompanied by an inability to hold normal operating pressure of 1600 psi with one pump running and/or unusually high-pressure swings during valve testing or other transient operation conditions. High fluid temperatures with normal cooling water flow through the coolers may also be observed.

Pump flow and motor current are directly related, except in cases where the pump is worn, so if a high motor current condition persists on pumps that are known to be in good condition, excessive flow should be suspected. Long-term operation in this condition can cause rapid pump wear and, if uncorrected, eventually lead to pump or motor failure.

Recent investigations have shown that improperly set relief valves, which should begin to relieve at 2000 PSIG of pump discharge pressure, have been a frequent cause of excessive flow conditions. Many portions of the hydraulic system can be responsible for leakage, but the HPU relief valves, which are located just downstream of the pump discharge, are capable of passing far more flow than most other portions of the system. Valves found to be improperly set were typically either bench set at lower than normal operating temperatures or were set in place with both pumping loops in operation.

It is possible to ascertain whether a relief valve is improperly set by adjusting the HPU manifold pressure to 1600 to 1700 PSIG and checking the temperature of the vertical pipe below the relief valve. If this pipe, which carries the relief flow to the reservoir, is approximately the same temperature as the discharge line from the pump, an improper relief valve setting should be suspected. Whether or not this test is conclusive the possibility can be eliminated by following the procedure given below.

SOLUTION
Prior to setting the HPU relief valves, the following initial conditions should be obtained:
A. Only the pumping loop being worked on should be running as the system check valves do not seal positively at low differential pressures. (The other pumping loop should be left in standby, not locked out.)
B. The hydraulic fluid should be at normal operating temperatures of 110 to 120
degrees F.
C. Ensure that fluid accumulators are properly charged per the manufacturer’s instructions.
D. The turbine should be reset, if possible, to approximate normal system flow.

Be sure to have an MD&A Turbine and Generator Controls Division representative on hand to guide the calibration.  To properly set a relief valve proceed as follows: (Repeat for both pumping loops.)

1. Raise the pump discharge pressure to 2000 to 2050 PSIG with the compensator on the fluid pump. (Raise the relief valve setting with CW adjustments as required. Adjust the compensator in 50 to 100 PSIG steps allowing the pressure to stabilize after each increase. Do not exceed 2050 PSIG.)
2. Reduce the relief valve setting (CCW adjustment) until the pump discharge pressure is 2000 PSIG and lock the adjusting stem with the lock nut.
3. Reduce the pump discharge pressure with the compensator adjustment until the HPU manifold pressure is 1600 PSIG and lock the compensator adjustment in place.

For more information, please contact MD&A Turbine and Generator Controls Division at 970-224-2223.