Gas Turbine Fundamentals: How Lubrication Can Drive Turbine Productivity

Last week, one of our customers asked about which oil to use in a GE Frame 6 Gas turbine. Answering the question was easy enough, but the inquiry did get me thinking about what aspects of lubrication are critical to understand to ensure gas turbine operators can maximize the productivity of their equipment.

So, this week’s tip will cover the basics of gas turbine lubrication, highlighting key areas that operators must consider when developing a lubrication program.

Basic components of a gas turbine

A simple gas turbine is composed of a compressor, a combustor, and an exhaust turbine. During operation, air enters the compressor at ambient temperature and is compressed to a higher pressure and temperature. Upon leaving the compressor, the air enters the combustor. where fuel is injected and combustion occurs. During combustions, the gases in the chamber rapidly expand, moving toward the exhaust turbine. It is the turbine section of the gas turbine where energy is converted into work as is used to drive the compressor or the generator or the propulsion drive to which it is attached.

Gas turbine designs can vary, but they all have the same basic components. A key difference can be the number of shafts – there are single shaft and multiple shaft turbine designs.

In a single shaft turbine, all components operate at one speed, but in a multiple shaft turbine the components operate at different speeds. As shown at left, the multi-shaft turbine allows the high pressure rotor to turn the compressor while the low pressure rotor to operate at variable speeds turning the load.

The components of a gas turbine that are lubricated include the bearings and seals. The bearings include journal and thrust type bearings, which support the operation of the compressor and turbines. Labyrinth-type oil seals are mounted on either side of the journal bearings and these must be lubricated to prevent seal damage from journal rotation.

Gas turbine lubrication

The lubrication system for a gas turbine contains a tank, pumps, coolers, filters, valves and various control and protection devices. Shown below, it furnishes normal lubrication protection while also removing heat from the load of the gas turbine. 

Typical gas turbines operate at a rotation speed of approximately 3600 rotations per minute (RPM), with exhaust temperatures often in excess of 625°C and at pressure ratios to atmospheric exceeding 18.7:1. The bearings may see operating temperatures near 120°C. This means the oil, in addition to having the correct viscosity, must also contain additives to address oxidation, varnish formation, foam stability, filterability for long oil life, multi bearing metal compatibility and finally protection against wear.

A twist in gas turbine lubrication is the development of the combined-cycle turbine system. In this system, the exhaust from a gas turbine is extracted and used to produce steam for a steam turbine/generator combination. The schematic for this provided below.

In the combined cycle arrangement, there is often only one “shared” lubrication system, so the oil must be able to lubricate both steam and gas turbine bearings. This means that in addition to the qualities mentioned above, the oil must have the ability to remove and shed water.

Hopefully, you found this week’s tip helpful. It was meant to be a basic overview, but if you have any further questions, don’t hesitate to leave a note in the section below!

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