Hydraulic pump

Pumps with a Case Drain line, Usually Variable Volume types, route all their bypass oil into the Case but have no path from the Case back to the Inlet or Suction side of the pump like Fixed Volume designs such as Gear and Vane types.

Case flow is nominally around 4-6% of rated pump flow at maximum operating pressure and most recommend to replace the pump when that flow reaches or excedes 10%. That conserves energy and can often make rebuild of the pump economical. If you wait for the pump to fail it can send a lot of contamination into the system and may require a complete disassembly to clean the circuit.

I specify a Flow Meter in the Case Drain line that has a Limit Switch set for 6-7% pump flow that sends a signal to the control circuit to notify the need for a pump change. That often can allow the pump to be changed at the next shutdown and not in the middle of a production run.