Is air or water getting into your hydraulic fluid?

The hydraulic oil level in our molding machine reservoir gradually rises after we start the unit up. This causes oil to eventually blow out the breather cap when the clamp and injection cylinders retract. Several fixeddisplacement vane pumps are mounted o

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— Jim C.

Oil turning milky white generally means that water is entering the reservoir. There are three primary causes of water entering a reservoir:

  1. If the unit is located outside and not protected by a shelter, the water can enter the system externally through worn O-rings, the breather cap, or a worn gasket. This water can come from rainfall or if the hydraulic unit is washed down by personnel.
  2. Failure of a water-cooled heat exchanger can also introduce water. The internal tubes can degrade over time, allowing the water to mix with the oil. In this condition, the fluid level would continue rising until a steady flow of liquid exhausts out of the breather cap.
  3. When the unit is shut down, air in the reservoir cools down, which causes water to condense into liquid form. This water should be drained out the drain plug. A well-designed reservoir should be slightly tilted and fitted with a hand valve to allow the water to be drained out periodically. Another solution would be to seal the hydraulic fluid from the atmosphere, which often involves using a pressurized reservoir.

On-site analysis
A visit to this plant, discussion with personnel, and inspection of the equipment revealed that the oil was foaming. Foaming can be caused by the oil returning to the tank at too high a velocity. Cylinder speeds were measured and found to be no more than when the unit was first installed.

Foaming of the oil will occur whenever air enters a system. This only happens when the pressure in a line or component drops below atmospheric pressure — roughly 14.7 psia. When pressure drops below atmospheric pressure, a vacuum will be created.

Perhaps the most common source for air entering a system is through the pump suction line. A pin-hole leak in the suction line or loose fittings can cause this problem. Because the pumps on this particular system are mounted above fluid level, no external leakage from the suction hose or fittings would be visible when the pump is turned off.

Many larger pumps use O-rings on the inlet line connection to the pump. If the O-ring fails or becomes worn, air can enter the suction-chamber.

Another source of air entry on a fixed-displacement pump is through the shaft end seal. The pump case on a fixed-displacement pump is drained internally to the pump suction. When the pump is mounted above the reservoir fluid level, pressure in the suction line can drop below 14.7 psia. This vacuum will pull air in through the shaft seal. The shaft seal can fail because of misalignment, a bad coupling, or if the pump case pressure exceeds the rating of the seal.

Any of these air flow paths into the pump suction will cause a condition known as aeration. Any air that enters the pump gets compressed as it exits the pump. This compression of air will cause an erratic high pitched whining sound and eventually cause pump failure.

To troubleshoot air entering the pump through the shaft seal or suction line, squirt oil around the possible leak paths. If the pump momentarily quiets down, then the source of the leak has been found.

Another point at which air can enter the pump suction is when the fluid level drops to 2 in. above the pump suction line in the reservoir. A vortex will be formed, allowing air to flow into the suction pipe. This was obviously not the problem on this injection molding machine because the oil level was rising.

The problem on this system eventually was found to be a badly worn coupling on one of the pumps, which caused the shaft seal to fail.

This information was provided by C. A. (Al) Smiley, Jr., president, GPM Hydraulic Consulting, Inc., Social Circle, Ga. For information on GPM's interactive training CDs, textbooks, and other fluid power training resources, call (770) 464-0777 or visit

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