Saturday, March 14, 2015

The Gas Turbine


The gas turbine comes under the heading of high speed rotating machinery (i.e., Dynamic versus reciprocating operation and in excess of 3000 R.P.M.).
Gas turbines must operate at high speeds for proper efficiency and economy. While speed is essential in gas turbine operation, it is also a danger to the machinery. In the event of trouble the high speed limits the amount of time available to safely shut the machine down. Human reaction time, from receipt of information to starting action is normally in excess of one second. Human reaction is not able to start the shutdown of a unit within a time frame necessary to prevent major damage.

Let us look more closely at this speed problem by comparing a reciprocating engine’s speed versus a gas turbine’s speed. A typical recip engine that does 900 R.P.M. ( or 15 revolutions a second) is 15 times slower than a typical gas turbine doing 13,500 R.P.M. ( or 225 revolutions a second). Now you have 1/15 of the time necessary to react to a problem occurring. Because of this speed-time problem high speed machinery controls are highly automated, particularly with regard to the safety systems.

Another factor of concern with high speed rotating machinery is the problem of vibration and its effects on the machinery. The faster the speed of a machine, the more critical it is to have the rotating elements balanced properly, also the less vibration the machine can take without

Coberra® 6000 DLE

damage. The limits of allowable vibration in a high speed machine are much less than in a low speed machine. The high speed has also caused manufacturers to consider natural frequencies, criticals, and torsional criticals versus rotational speeds as a problem area.

While the gas turbine has some drawbacks; it is still a very efficient, economical, and reliable piece of machinery. All of the work done in gas turbines is done dynamically in a continuous process, without valves or a split cycle,( i.e. recip. Engine intake, compression, power exhaust).

Because of this continuous process, the gas turbine gets a maximum horsepower per unit weight much greater than the reciprocating engine.

Dry Low Emission variant of RB211
The design of gas turbines is to use the fluid molecules as the only contact in or rubbing parts in
the machine. The gases flowing through the machine and the oil in the bearings are the only contacting mediums with the metal parts of the machine.

This design creates a high degree of mechanical reliability in theat mechanical wear is severely curtailed.
Having now characterized the equipment, let us consider how it works and how it is similar to what you are familiar with. I am sure you have all heard of a turbocharged automobile engine.
In this system air is compressed by the compressor of the turbocharger for delivery to the cylinders. After using the air in the combustion process, the exhaust gas is ducted to the turbine section of the turbocharger. By replacing the recip engine with a fire box, where continuous burning takes place, we turn the system into a gas turbine.

DLE Variation of the RB211 Gas Turbine

Now we have an engine which compresses air by dynamic mechanical energy. The compressed air enters the combuster where fuel is added and burned, thus increasing the energy level of the combustion products above that of compressed air only. The hot gases expand through the turbine where some of the hot gas energy is converted to mechanical work. If the design is reasonable, the mechanical work produced by the turbine will be great enough to drive the air compressor and leave a surplus of energy available at the shaft to drive some kind of load such as a gas compressor or an electric generator.

In some cases, the exhaust gases are run through a heat exchanger and used to drive other machinery, also to heat liquids or gases for certain processes.

The idea to remember is the gas turbine can be broken into components to make it easier to understand.
GG Turbine
Gas Generator Anatomy
Non-DLE arrangement
VIGVs IP Compressor Bearings HP Compressor
Combustor GG Turbine

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