Factors Affacting Lubrication

Circulation and Heating in the Presence of Air

Heat is generated within the bearings by friction and heat conduction along the shaft. Oil is broken into droplets while it is flowing. This allows greater exposure to air. During operation, oxidation (combination of the oil molecules with oxygen) occurs. Fine metal particles resulting, from wear or contamination and water act as a catalyst (enhance the rate) to oxidation. The viscosity of oil increases with oxidation. Insoluble oxidation products such as varnish and sludge may settle out on governor components, in bearings, heat exchangers, and strainers. Their accumulation will interfere with governor operation and oil flow to the bearings.

Contamination

Water is the most prevalent contamination in turbine lubrication systems. Three common sources of water include:

1. Leaking turbine and pump seals
2. Condensation of humid air
3. Water leaks in heat exchangers

Emulsion will form when the oil is mixed with water. The emulsion will separate quickly when the oil is new and clean. The water will settle in the reservoir where it can be removed by purification equipment. Oxidation or contamination of the oil will increase the tendency of the oil to emulsify. Emulsions can mix with insoluble oxidation products and dirt to form sludges. Water can combine with air to form red and black rust, which is similar in appearance to pipe scale. Particles of rust have the following effects:

• Act as catalysts that increase the rate of oil oxidation.
• Scratch the journals and cause excessive wear.
• Get entrained into the small clearances of the governing system. This will cause sluggish operation and, in extreme cases, disasters (due to slow operation of the governing valve).

Oil can become contaminated by air to form “bubbly” oil. This oil is compressible and can cause sponginess in hydraulic controls. It may reduce the load-carrying capability of oil films. Entrained air increases the rate of oxidation. An excessive amount of air can generate foaming in the reservoir or bearing housings.