Overheating Die-Caster: Cooler or Internal Leakage?
A die-cast machine has a 25 gpm pressure-compensated pump, a relief valve set at 2,200 psi, a 100 gal reservoir, and an air-blast oil cooler rated 25 hp at a 40 deg F oil-to-air temperature difference. After two hours the oil reaches 170 deg F with 95 deg F ambient. The cooler inlet is 170 deg F and outlet is 150 deg F. Pump case drain measured at 2,000 psi is 4.2 gpm; the pump specification allows 1.5 gpm maximum. The relief valve body is 190 deg F, and system pressure fluctuates between 1,900 and 2,100 psi even when the machine is idle. The pump compensator is set at 2,100 psi. The machine cycles slowly and the operator has been leaving it running during breaks. The question is whether the cooler is undersized or the system has an internal leakage problem.
1. Calculate the heat from the 4.2 gpm case drain at 2,000 psi. If the relief is also passing 8 gpm at 2,200 psi, what is the total loss, and is the cooler obviously undersized?
Read the worked answer
Case-drain heat = 4.2 gpm x 2,000 psi / 1714 = 4.9 hp. If the relief is passing 8 gpm at 2,200 psi, that is 8 x 2,200 / 1714 = 10.3 hp. Together that is 15.2 hp, or about 38,700 BTU/hr, before pump inefficiency. The cooler is rated 25 hp at a 40 deg F oil-to-air difference, so its nameplate rating is larger than this calculated loss; therefore the cooler is not obviously undersized. But the actual approach is 170 - 95 = 75 deg F, so the cooler should have even more capacity unless airflow is blocked or the fins are fouled. The case drain is already more than 2.5 times the allowable 1.5 gpm, so internal leakage is a confirmed problem, not a theory.
2. How would you determine whether the relief valve or the pump is the main heat source? Give specific measurements.
Read the worked answer
Do not condemn the cooler first. Measure relief outlet flow and temperature; a hot relief outlet pipe with continuous flow means the relief is passing. Cap or plug the relief safely and use a test valve to load the pump; if case drain stays near 4.2 gpm at low pressure, the pump is worn. Measure case drain at 500, 1,000, and 2,000 psi: a worn pump's case drain rises steeply with pressure, while a good pump stays nearly flat. Check cooler air flow, fan rotation, and fin blockage. A pump flow test with a loading valve and flow meter separates pump delivery from relief loss.
3. Recommend the repair and settings, and state the permanent fix to prevent repeat overheating.
Read the worked answer
Rebuild or replace the pump if case drain exceeds 1.5 gpm at 2,000 psi. Clean or replace the relief valve and verify it seats; set the compensator at 2,100 psi and the relief at 2,200 psi. Clean the cooler and verify fan and airflow; if oil temperature still exceeds 140-150 deg F, add cooler capacity or an unload valve so the pump idles at low pressure during breaks. Check oil viscosity and suction strainer, and repair the root cause before adding a larger cooler, or the extra cooling will only hide the leakage.