STEAM TURBINES

Small horizontal turbines are commonly fitted with ring oiled bearings and some small machines are fitted with sight-feed lubricators, but all large sets and many small units are equipped with pressure circulating systems.

Satisfactory turbine circulating oil must not only provide adequate adn dependable lubrication, it must also serve as a coolant and protect against rust, the turbine parts with which it comes into contact. Very often it is the hydraulic medium in governors and other control gears. Also, the reduction gearing of geared turbine is generally lubricated from the same system, although separate lubrication is provided in some cases.

The deteriorating influences which a turbine oil has to withstand are well known. Air and moisture are inevitably present in the lubrication system and intimate mixing of the oil with air and water vapour occurs in bearing housings, gear cases and other parts of the systems. At operating temperatures this subjects the oil to oxidizing conditions which may lead to the formation of the acids, sludge and permanent emulsions. Moreover, the oil is constantly in contact with catalysts such as copper, iron oxides and brass. Even in small quantities, such substance can greatly increase the rate at which oil oxidation takes place. These conditions may be aggravated by other factors such as local overheating or by contamination of the oil.

With the development through years, turbines are becoming increasingly compact in design. The use of steam at higher pressures and temperatures have resulted in demands on the lubricant becoming progressively more severe. Under modern conditions, oils which have not been specially prepared are found to deteriorate rapidly and in some cases necessitate an emergency shutdown; overhaul of the system; and a change of oil. 

These conditions and demands have been met by the development of special turbine oils whose high stability, ready separation from water, and rust -resisting properties have been proved over long periods of service. However, even the best oil is not invulnerable and it is worth considering some of the factors which may have great effect on the life of a charge of oil.

FACTORS PROMOTING OIL DETERIORATION

A. INEFFICIENT COOLERS

Coolers may become inefficient either through sludge deposits or through scale or dirt coating the water side of the cooling surfaces. This will give rise to general increase in the temperature of the oil and thereby hasten oxidation.

B. IMPURITIES IN THE SYSTEM

Foreign matter in the system is obviously objectionable, because it may interfere with the circulation of oil or cause mechanical trouble. Even in very small quantity it will tend,