INTRODUCTION TO GAS TURBINEPrepared by: Muhammad Ridhwan Abdul Rasid, Mechanical Trainee Date: 2nd April 2010

1. OPERATING PRINCIPLE

A gas turbine is an internal combustion engine. From all points of view, it can be considered a self-sufficient system: in fact, it takes and compresses atmosphere air in its own compressor, increase the energetic power of the air in its combustion chamber and converts this into useful mechanical energy during the expansion process that takes place in the turbine section. The resulting mechanical energy is transmitted via a coupling to a driven machine, which produces power useful for the industrial process in which the gas turbine is applied.

Figure 1: Force at the blade

The thermodynamic cycle of a gas turbine is known as the Brayton cycle. Figure 1 illustrates a diagram of a gas turbine. This diagram is useful to understand the meaning of the thermodynamic cycle more easily.

Figure 2: Brayton Cycle

There are intended for the process industry services as:

i. Compressor driveThe power from the gas turbine will run the shaft which connected to compressor. The shaft will directly rotate the impeller of the compressor.

Figure 3: Mechanical Drive

ii. Generator drive.Same as compressor drive, but the power shaft will be connected to generator. The shaft will rotate the altenator to produce electricity for another users.

Figure 4: Generator Drive

2. NUMBER OF SHAFT

Differences between single and double shaft of the gas turbine gas are based on the drive machine. Basically gas turbine has single or double shafts, except jet turbine which can be up to three shafts. The reasons of difference in number of shaft are:

i. Generator drive needs a shaft to be rotated at constant speed. Therefore a single shaft gas turbine is used that rotate a constant speed. Meanwhile to avoid surge or stall problems on its internal axial compressor.

Figure 5: Single Shaft Gas Turbine

ii. If we need to drive compressor (mechanical drive), we need a two shaft gas turbine because the HP rotor (axial compressor) continue at constant speed, while the LP rotor (power shaft) can change its speed in the range 60% - 105% of its speed.

Figure 6: Double Shaft Gas Turbine

To control speed of HP and LP rotor in a two shaft gas turbine, the turbine nees a variable nozzle. This is because to divert the maximum power of the HP rotor to the LP rotor at required power.

Figure 7: Variable Area Nozzle Open

3. MAIN SECTION IN GAS TURBINE

A gas turbine is composed of three main sections, described in the following paragraphs.

CompressorThe compressor is an axial-flow type. The axial-flow design produces high air flows, necessary to obtain high value of useful power with reduced dimensions. A compressor consists of a series of stages of rotating blades, which increase air speed in term of kinetic energy into higher pressure.The number of compression stages is related to the structure of the gas turbine and above all, to the pressure ration to be obtained.The compressor serves also to supply a source of air needed to cool the walls of nozzles, buckets and turbine disks, which are reached via channels inside the gas turbine, and via external connecting piping. Additionally, the compressor supplies sealing air to bearing labyrinth seals.

Combustion sectionAir enters each combustion chamber in the opposites direction to the hot inner gas path. Initially, the combustion process is ignited by one or more spark plugs. Once ignited, combustion continues unaided, as long as fuel and combustion air supply conditions are maintained. The hot gas path from the combustion system to the turbine inlet passes through transition pieces which

transform the flows of gas from the single combustion chambers into a continuous annular stream matching the first stage nozzle ring inlet.

Turbine sectionThe turbine section comprises a certain number of stages, each one of them consisting of one stator stage and one rotor stage. In the stator stage, high temperature and high pressure gases delivered by the transition piece are accelerated and directed towards a rotor stage of buckets mounted on a disk connected with the power shaft.The rotor stage completes the energy conversion, as kinetic energy is transformed into energy that drives the shaft, thus generating the power required to drive the compressor or alternator (generator).

Figure 8: Main Sections of Gas Turbine

4. PERFORMANCE

i. Influence of External Factors

A gas turbine uses atmospheric air; therefore its performance is greatly affected by all factors that influence the weight flow rate of air delivered to the compressor. Below are the external factors which affect the performance of the gas turbine:

TemperatureAs the compressor inlet temperature increases, the specific compression work increases, while the weight flow rate of the air decreases (because of a decrease in specific weight). Consequently, the turbine efficiency and useful work (power) decrease.

PressureIf the atmospheric pressure decreases in comparison with the ISO reference pressure, the weight flow rate of air decreases (because of a reduction in its specific weight). Useful power is reduced proportionally to the weight flow rate of gas.

Relative humidityRelative humidity influences the specific weight of compressor inlet air. In fact, humid air is less dense than dry air. So if the relative humidity increases, the power output decreases and heat rate (HR) increases.

ii. Influence of Internal Factors

Added to the three ‘external’ factors described in the preceding paragraph, there are other factors which notably affect the performance of a gas turbine. These may be called ‘internal’ factors, because they are related to the auxiliary systems of the gas turbine.

They are the following:

Pressure losses in the compressor inlet sectionPressure losses are caused by the gas turbine inlet system, composed of an air filter, a silencer, an elbow, pipe section variations, etc., installed upstream of the compressor suction flange. When air flows through this system, it is subjected to friction, which reduces its pressure and specific weight. These losses cause a reduction in useful power and an increase heat rate, as mentioned previously due the case of the influence exerted by ambient pressure.

Pressure losses in the turbine exhaust systemThese are caused by the gas turbine exhaust system, composed of one or more silencers, an elbow, a recovery boiler (in case of combined cycles or cogeneration), diverters, diffusers, etc., through which exhaust gases are expelled into the atmosphere.

Exhaust gases flowing through this system are subjected to friction losses, which increases the value of back pressure as opposed to the value of external, atmospheric pressure. these losses reduce the amount of turbine expansion, which results in reduced useful power and increased heat rate.

Fuel typeBest performance is achieved if natural gas rather than diesel oil is used. This behavior is due to higher heating value of product originated by the combustion of natural gas, as the latter has a higher content of water vapour, resulting from a higher ratio between hydrogen and carbon, which is typical of methane, the main component of natural gas.

Air extraction from the axial compressorIn some gas turbine applications, it may necessary to extract compressed air from the compressor discharge. The extraction of air will influence output power and heat rate, taking into consideration also ambient temperature.

iii. Compressor stall

To develop high pressure ratios, multi-stage axial compressors are employed. If each stage operates at the same speed, it is possible that early stages could be overloaded and later stages would operate inefficiently at some compressor speeds. The worst case scenario could cause compressor stall which is similar to the stall on an aircraft wing. We can say that some blades are like mini airfoils. This is also true of compressors. The air flowing around the blades may be at too high an angle of attack for certain speeds and could separate resulting in compressor stall.

Figure 9: Stall Effect

The figures showing the aerofoil’s lift force reduce and the drag force increased due to large turbulent wake. The air flow creates pressure different between top and beneath the airfoil which cause lift force or rotation of turbine blade.

iv. Surge

If the mass flow through the compressor is reduced beyond a certain point, the directions of the velocities relative to the blades are so different from the blade angles that the flow breaks down completely. The compressor is then said to surge. The surge effect will cause extreme vibration to the turbine, damage the blade and noisy.