Causes of Gear Pump Cavitation

Noria Corporation
Tags: gear lubrication, contamination control, oil oxidation, viscosity

"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, while others are more service related. Cavitation occurs when either air or vapor bubbles form in the suction-line fluid and are subsequently imploded in the pump by the pressurized 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 simply plugged (e.g., 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.

6. Water vapor. When hot oils become contaminated with water, super-heated steam 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.

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