Keeping your turbines up to speed

June 24, 2009
by Steve Gahbauer

Turbines need plenty of attention. You’ll keep them running efficiently by understanding the types, systems, maintenance issues and lubrication needs, and applying the best troubleshooting techniques.

Turbines are grouped as steam, conventional and aero derivative gas, gas and steam-combined cycle units, water and wind. The distinction is important when choosing lubricants.

An F-class gas turbine being assembled at GE Energy’s Greenville, SC plant.

Steam turbines are traditionally high-power units with an efficiency range of 25% to 40 per cent. Conventional gas turbines are also high-power machines, but of a more compact size with a power range of 35 per cent. Aero derivative gas turbines have a very high power-to-weight ratio, tolerate extreme high thermal stress and can be lubricated with synthetic-based aviation oils. Gas and steam-combined cycle units have a 50% to 60% efficiency rate. Water turbines have a low power-to-weight ratio, but high mechanical strength, an efficiency of 90% or more, and can be lubricated with higher viscosity oils. Wind turbines are gear units producing electricity from wind power.

Steam turbines are the heart of coal-fired and nuclear power generating plants. The steam progressively expands through sets of blades and a generator converts the mechanical energy into electrical energy.

Conventional industrial gas turbines have three major components: a compressor, combustion chamber and turbine. The compressor feeds high-pressure air into the combustion chamber where fuel is injected and burned. High-pressure, high-temperature gases react against the turbine blades causing the shaft to turn, which drives the compressor and the machine; hence there are efficiency losses. The power output ranges from five to 300 megawatts with a firing temperature of up to 1,450 degrees C.

Aero derivative gas turbines are based on technology used in aviation jet engines. The power output is lower—up to 50 megawatts—and they can be lubricated by synthetic polyol ester aviation turbine oils.

Combined cycle turbines can be single or multiple shaft models, the latter being the most common design. Oil reservoirs are normally separate for each turbine.

Water turbines, also called hydro turbines, work by converting the kinetic energy of flowing water into mechanical rotary energy that drives a generator.