Gas Turbine Main Auxiliaries

The gas turbine system must not be intended as limited exclusively to the flange-to-flange part, because it is actually composed of a series of accessory equipment and systems without which it would never be able to work.

Figure 1: Typical Main Auxiliaries

The auxiliaries systems illustrated above are the following:

A. Inlet systemB. Exhaust systemC. Lube and control oil systemD. Fuel systemE. Cooling and sealing air systemF. BaseplatesG. Starting systemH. Accessory gearI. Load gearJ. CouplingsK. Electric systemL. Sound-proof enclosureM. Ventilation systemN. Oil cooling systemO. Control system

Therefore, correct sizing of each of the auxiliaries systems are fundamental to ensure good operating results of the ‘gas turbine assembly’ in term of:

Reliability Capability to fulfill all operating functions of gas turbine Protection against external sources of disturbance and failure Economical operation

1. Inlet SystemThe inlet system of a gas turbine has the function to convey combustion air to the axial compressor inlet section, therefore:

The required degree of filtration for correct operation of the compressor and the turbine under the existing environmental conditions of installation

Compliance with noise limits in proximity of the system and at a distance from it as agreed upon in the contract.

Compliance with the contractually agreed upon power and fuel consumption values Regularities of air flow to the compressor inlet section, and consequent regular fluid

dynamic operation of the latter. Levels of reliability and availability not lower than in the rest of the plant.

The gas turbine inlet system consists of the following main components parts:

i. Inlet filterii. Duct

iii. Silenceriv. Elbowv. Inlet plenum

vi. Accessories

Figure 2: Typical Inlet System

2. Exhaust SystemThe exhaust system has the function of discharging into the atmosphere the exhaust gases originating from the expansion process that takes place in the gas turbine. This system must be designed and built in a way to meet the following requirements:

Compliance with the contractual noise levels in proximity to the system and at a distance from it

Compliance with contractual power and fuel consumption level Compliance with contractual personnel safety requirements of heat insulation Gas flow even, optimal speed distribution from the exhaust flange to release into the

atmosphere, so as to reduce turbulence, leakage and abnormal stress on structures to a minimum.

To prevent conditions of self-stress from developing during thermal transients. Reliability and availability levels not lower than those of the rest of the plant.

The exhaust system of a gas turbine consist of the following main elements

Diffuser or exhaust plenum Transition ducts Exhaust silencer Stack Expansion joints Thermal and acoustic insulation

Service ladders and walkways Support frames

Figure 3: Straight Up Exhaust Main Components Parts

3. Lube and Control Oil SystemThis system has the task of supplying oil to the gas turbine bearings, to the driven machine, to integrated auxiliaries (accessory gear, starting motor with torque converter, couplings, etc.), to the control oil and to the hydraulic oil systems.The oil supplied must meet all purity (filtration), pressure and temperature requirements necessary for the different systems it is fed to.

4. Fuel SystemOne of the peculiar characteristic which have distinguished gas turbine in the course of their history is their flexibility in the use of different types of fuel, as specially heavy duty gas turbines can burn a large variety of fuels. Futhermore, combinations of these fuels can be used in the same turbine, with dual fuel applications, thus increasing its flexibility and availability of use.The most commonly used fuels are the following:

i. Natural gas (mixture of methane, ethane and hydrocarbon with progressively growing molecular weight, with the addition of other components such as CO, H2, etc.)

ii. Mixtures of propane and butane, also called LPGiii. Refinery gases with high hydrogen content.iv. Coal-derived gas and other gas with a low heating value.v. Conventional liquid fuels, such as gas oil, crude oil and residual oil.vi. Less conventional liquid fuels (kerosene)vii. Unconventional liquid fuels such as naphtha and gasolinesviii. Liquid fuels such as process residualsix. Methanol.

5. Cooling and Sealing Air SystemThe need for cooling a number of internal parts of the gas turbine is to prevent them from deteriorating at high temperatures. Sealing air in necessary in order to prevent lube oil from leaking from labyrinth seals adjacent to bearing.

Figure 4: Cooling and Sealing Air simplified diagram

The diagram above shows a typical representation of the two systems along with the respective ‘targets’, which are:

i. Cooling hot parts of the gas turbine (buckets and nozzles)ii. Cooling the exhaust casing frames and housing

iii. Sealing air for bearing labyrinth sealsiv. Antisurge system

Depending on machine model, air is bled from one or more compressor stages and sent to the various service points via calibrated orifices in the quantity needed. It must remembered that the air flow rates must be optimized to the minimum necessary in order not to affect the gas turbine efficiency.

6. BaseplatesThe gas turbine baseplate holds the machine and some of its auxiliaries, the number of which depends on the configuration. It is built of structural steelwork made by welding longitudinal and transversal metal plates, open or tubular, schematically illustrated in the two typical sectional views that follow. The plate material is carbon steel, unless otherwise prescribed.

Figure 5: Typical Baseplates Made of Structural Steelwork

7. Starting SystemThe main purpose of the starting system is to accelerate the gas turbine up to self-sustaining speed. In this respect, it must remembered that a gas turbine reaches a self-sustaining condition when the energy available in the combustion chamber is at least equal to the sum of energy required by compression and mechanical losses in the gas turbine and the driven machine.

After the torque during start up phases up to self-sustaining speed, the starting motor is disconnected and the turbine accelerates by using its combustion energy alone.

8. Geari. Accessory Gear

The accessory gear has the following function:

To connect the starting means with the turbine shaft for start up. To drive a number of user elements (pump, etc.) mounted on shafts connected

with one another via gears, which receive input energy from the axial compressor shaft of the gas turbine in running condition, via an accessory coupling.

Figure 6: Accessory Gear

ii. Load GearThe load gear has the function of adapting the gas turbine speed value to that of the driven machine in all cases in which it is impossible for them to be equal. This need

derives from the fact that, in many cases the useful power produced by the driving machine (gas turbine) requires a higher or lower speed than the optimum operating speed range of the driven machine.

Figure 7: Load Gear Operating Diagram

9. Electric SystemThe electric system on a gas turbine refers to the equipment related to the machine and its auxiliaries, including the relative junction boxes. It serves the following purposes:

i. To connect wires which carry signals coming from the instrumentation on the machine body and from the apparatuses inside the enclosure (lighting, fire fighting system, dangerous atmosphere, etc.) to the junction boxes located at the baseplate border.

ii. To connect power cables for the various user elements (electric motors, etc.) to their power supply sources.

When designing and selecting cable paths, several factors must be taken into consideration, of which the most important are:

i. Temperatures – shielding systems are used in the most critical cases to reduce the effect of irradiation on instrumentation and cables.

ii. Equipment vibration – provide fixed cabling system, must not provoke the detachment of conduits or cable trays.

iii. Cable transit areas on the machine – the instruments installed in the machine interior must cross the machine casings or piping’s walls without provoking hot gas leaks.

10. Oil Cooling SystemThe gas turbine oil heats up as it flows through the bearings and on account of irradiation from hot casings and from piping. Therefore, it is necessary to remove heat on a continuous basis, in order to limit its temperature and obtain correct lubrication and adequate cooling of bearings and shafts.

The oil cooling process can take place in the following ways:

i. With water/oil coolers in an open-loop systemii. With water/oil with water a closed-loop water systemiii. With oil/water coolers.