If the needle on a pressure gauge, downstream of a pump is jumping around there is probably air in the oil. Spray oil on the shaft seal of the pump as a test of air ingress past the seal. You will hear a change (quieting) of pump sound indicating momentary less aeration.

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From the book "The Practical Handbook of Machinery Lubrication":

A Canadian company engaged in the operation of several compressors experienced a problem where high temperatures caused the compressor oil to oxidize. The oxidized oil formed carbon deposits in the discharge pipes, which in turn created several discharge pipe hot spots. Conditions became such that a severe explosion occurred and a catastrophic failure resulted.

Compressor oil oxidation problems have been eliminated through the use of a diester-type synthetic compressor lubricant designed to resist high temperatures and deposit formation.

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Question: What are two ways a seal may change when in contact with an incompatible oil?

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"We have six 6-MW diesel generator sets to meet the power requirement of our cement plant. We are facing cavitation problems in the lubricating oil gear pumps. The damage occurs to both the housing and gear surfaces. This results in only 8,000 hours of service life, which is too low. What are the possible reasons for gear pump cavitation?"

Cavitation can have several root causes. Some relate to system and component design issues and others are more service related. Cavitation occurs when either air or vapor bubbles form in the suction line fluid and are subsequently emploded in the pump by the pressured oil. This leads to microjets of oil pounding and eroding adjacent surfaces. Below is a list of possible contributing causes of cavitation:

1. Tank design issues. Turbulence in the tank churns the air into the oil or simply doesn't allow air to be released from the oil. This can be caused by plunging oil returns, low oil level, a tank that is too small, lack of proper baffling, etc.

2. Suction-line leaks. Leaks between the tank and the pump can introduce air. Often this is associated with the shaft seal at the pump that allows air to leak in.

3. Suction-line restriction. Sometimes suction lines are too long, too narrow or they are simply plugged (for example, a plugged suction strainer).

4. Insufficient head. Depending on oil viscosity and suction line conditions, the pump must be located at a sufficiently low elevation to enable oil to flow readily from the tank to the inlet port of the pump.

5. Air release problems. As oils age and become contaminated, air release properties become impaired. This simply means that once air bubbles form, they stay locked into the matrix of the oil and don't detrain out of the oil in the reservoir. Moisture contamination and oxidation are known precursors to this problem, among many others. ASTM D3427 is a test for air release properties.

6. Water vapor. When hot oils become contaminated with water, superheated seam will form vapor bubbles in the oil.

7. High viscosity. When reservoir temperatures are too cold, during wintertime startup conditions for example, the viscosity may be too high to enable proper oil flow in the suction line and into the pump. Other causes of high oil viscosity can lead to the same problem.

Jim Fitch, Noria Corporation

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