A hydraulic system that overheated at idle. The pump was a 22 cc/rev variable displacement piston pump, running at 1500 RPM. The system pressure was 140 bar during work. At idle (no work), the system sat at 140 bar with no flow going anywhere. The oil temperature climbed to 75°C after 2 hours. The customer thought the cooler was undersized. It was — but the real issue was the relief valve setting. The pump was a pressure-compensated design, and it should have destroked at idle. It wasn’t destroking. This is about relief valve function in a pressure-compensated circuit.

The pressure-compensated pump

A pressure-compensated (variable displacement) pump reduces its flow when the system reaches the set pressure. The compensator senses system pressure and strokes the pump down to maintain pressure. At idle (no actuators moving), the pump should provide just enough flow to maintain pressure — about 1-2 L/min of leakage. The horsepower consumed is: P × Q / 600. At 140 bar and 2 L/min: 140 × 2 / 600 = 0.47 kW. That’s nothing. The oil shouldn’t heat up.

But this pump was consuming 5.5 kW at idle. Why? The compensator was set wrong. The pump was still delivering 12 L/min at 140 bar. 140 × 12 / 600 = 2.8 kW. Plus the relief valve was cracking at 160 bar, bypassing 5 L/min over it: 160 × 5 / 600 = 1.3 kW. Total waste: 4 kW. All of it turned to heat in the oil. The cooler (rated 3 kW) couldn’t keep up.

What I changed

1. Adjusted the compensator setting. The compensator was set to 180 bar (above the relief valve at 160 bar). The pump didn’t start destroking until 180 bar. Between 140 and 180 bar, the pump delivered full flow. I set the compensator to 145 bar (below the relief at 160). Now the pump destrokes at 145 bar. At idle, it delivers 1-2 L/min. The idle heat dropped from 4 kW to 0.5 kW. The oil ran at 45°C instead of 75°C.

2. Set the relief valve 20% above working pressure. The relief valve should only open in an emergency — not during normal operation. I set it to 160 bar (140 × 1.15). It only opens if an actuator reaches end-of-stroke and pressure spikes. During normal work, it stays closed. No flow over the relief valve means no heat from the relief valve.

3. Added a load-sense circuit. For machines with multiple actuators, I add a load-sense (LS) pump. The pump only supplies the pressure needed by the active actuator. If a clamp needs 50 bar and a press needs 140 bar, the pump supplies 140 + 20 bar LS margin. The unused pressure isn’t wasted over a relief valve. This drops idle heat by another 50%.

The heat load calculation

To size a hydraulic cooler, calculate the heat load. The main heat sources:

  • Pump waste: (P × Q_pump – P × Q_used) / 600 kW. At 140 bar, pump 22 L/min, used 15 L/min: (140 × 22 – 140 × 15) / 600 = 1.63 kW.
  • Relief valve waste: P_relief × Q_relief / 600. At 160 bar, 5 L/min over relief: 1.33 kW.
  • Pressure drop across valves: ΔP × Q / 600. At 10 bar drop, 15 L/min: 0.25 kW.
  • Motor/pump inefficiency: about 10% of hydraulic power. At 3.5 kW output: 0.35 kW.

Total heat: 3.56 kW. The cooler must dissipate this. A 5 kW cooler keeps the oil at 45°C. A 3 kW cooler runs at 65°C. Size the cooler 20% above calculated heat for summer ambient.

The valve settings I enforce: compensator 5% above working pressure, relief valve 15% above compensator. The hot oil wasn’t a cooler problem — the pump was delivering full flow against a relief valve. Set the compensator correctly and 4 kW of waste heat disappears. Always calculate the heat load before sizing a cooler.