Function

The function of lubrication is to interpose a film of lubricant such as grease or oil between the moving surfaces in a bearing. Lubrication reduces friction, minimises wear, provides cooling and excludes water and contaminants from bearing components. The protection of rotating heavy machinery depends greatly on the effective operation and supervision of lubricating oil systems and bearings.

9.1.1 Oil Properties

Oxidation Stability

Oxidation stability is the property of oil resistant to oxidation.

When oil oxidises it‟s lubricating and cooling properties significantly reduces, placing the bearings at risk. Oxidation will take place due to the affect of heat when in the presence of water and air. As oil oxidises it becomes darker in colour and forms sludge which causes corrosion of the oil pipe-work and bearings. Oxidising agents or inhibitors, can be added to the oil to reduce the oxidising affect and increase the oil life.

Demulsibility

Demulsibility is the property of the oil to separate rapidly from water. Water contamination not only contributes to oxidation but also leads to the oil emulsifying. When oil emulsifies with water it appearance changes to a white milky colour and loses it‟s lubricating properties. Considerable precautions must be taken to prevent the contamination with water or remove the water before emulsification can occur.

Water may enter an oil system through the atmosphere, coolers, or through turbine glands.

Rust Prevention

Corrosion can occur on any metal surface in contact with the oil. Oxidation of oil results in the formation of oil acids which attacks the bearing and lube pipe-work metal surfaces.

Generally, rust or corrosion inhibitors are added to the oil to provide greater protect.

Viscosity

Viscosity is one of the most important properties of an oil as a lubricant. It is the ability of the oil to flow into spaces or gaps between rotating bearing components, without shearing or breaking down. High viscosity oil is thick oil, which does not flow easily and is used for heavily loaded or high-speed bearings. High viscosity oil also produces greater heat due to the higher frictional forces generated by the oil shearing and requires greater cooling to maintain normal operating temperatures. Low viscosity oil would be used for lightly loaded low speed bearings or in cold climatic conditions. Care is required with low viscosity oil as lubricating properties can be lost under high temperatures causing loss of the oil film and metal to metal bearing contact and subsequent failure.

The viscosity of the oil is greatly influenced by the oil‟s operating temperature. The viscosity of the oil is greatly reduced with increasing temperatures and increased when cold. For these reasons oil temperatures are critical. Hot oil causing low viscosity can lead to loss of lubrication, while similarly, cold oil causing high viscosity can also lead to loss of lubrication under cold climatic conditions or when first placing the turbine in-service. This is the reason behind oil pre-heating systems, such as electric heaters, to maintain oil within a defined operating temperature in order to maintain the correct oil viscosity.

Nominal Turbine Oil Operating Temperatures Normal Operation 38 – 45 Deg Celsius Turning Gear 25 – 35 Deg Celsius Turning Gear Permissive 25

High Limit 48

9.1.2 Causes of Oil Deterioration

High Temperature

Oil is subject to high temperatures due to the heat developed from the loaded bearings, internal bearing friction and in the case of steam driven turbines, heat transfer along the shaft.

The heat must be removed by oil coolers to maintain the oil within a pre-defined range ( usually 40 – 45 degC ) in order to maintain correct viscosity and to minimise oxidation which accelerates at high temperatures. The rate of oil deterioration from excessive temperature is doubled for each 10 degrees Celsius rise.

Water

Water adversely reacts with the oil to aid oxidation and cause emulsification which breaks down the oil‟s lubricating properties. The presence of water increases the mechanical wear of contact bearing components by displacing the lubricant from the bearing surfaces. Additionally, and generally during out of service conditions when oil temperatures are low the water can combine with impurities in the oil to form sludge which can settle in the oil tank or eliminate or prevent totally all air from being drawn into the oil system. Air is usually drawn in along the shaft at the bearings and via the oil tank breathers as the oil tank is maintained under a slight vacuum to prevent oil leakage along the shaft and to remove oil potentially explosive oil vapour's from the oil tank.

Contaminants

Foreign material can enter the lube oil system as dirt or dust from the atmosphere, sludge from oxidation or from debris remaining after maintenance. This matter can be highly abrasive and when carried with the oil to the bearings can

cause unnecessary bearing wear or damage leading to bearing failure. In-line lube oil strainers / filters and oil centrifuges / purification units are utilised to remove entrained contaminants.

9.1.3 Establishment of Oil Film

Oil lubricated bearings rely on the physical separation of the two bearing surfaces by a thin film or wedge of oil. In order to establish and maintain this oil film the following conditions must be established.

1) There must be relative motion between the two bearing surfaces to build up sufficient pressure within the oil to prevent the film breaking down.

2) There must be an uninterrupted supply of oil available to the bearing.

3) The bearing surfaces must not be parallel and need a narrow angle between them. This is to enable the oil to be shaped into a thin wedge tapering off in the direction of the motion.

Oil Film Dynamics

Refer Figure 48

1) With the shaft at rest the journal lies in the bottom of the bearing. The weight of the shaft tends to squeeze the oil out of the bearing so that metal to metal contact occurs.

2) As the shaft commences to rotate the first action of the journal is to climb up the bearing wall until it begins to slip and some metal to metal contact occurs.

3) As the shaft continues to increase in speed the oil is

1) Shaft at rest 2) Shaft as rotation commences Oil

Line of Contact Line of Contact

3) Increasing shaft speed 4 ) Shaft at full speed

Minimum oil film

Minimum oil film ( oil wedge established ) (film being established )

Figure 48: Establishing oil film

9.2 Components of a Turbine Lubricating Oil System