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  Combined Cycle Power Plant

Traditionally, the vast majority of the world's electricity supply has been produced by raising steam through burning of fossil fuels such as coal, oil and gas and using this steam to drive a turbine generator to produce electricity.

This method currently accounts for roughly half of the world's electricity supply with the remainder provided by nuclear, renewable and other fossil fuel-fired supplies, such as diesel engine and gas turbine based power plants.

Recent deregulation and privatisation of electricity generation industries throughout the world has also led to a new breed of producer, ranging from private independent power producers, to larger integrated energy suppliers.

Many of the 'new' producers have looked to the gas turbine as a source of electrical power since it offers many advantages over the traditional fossil fuel fired power plant, namely relatively low equipment cost and lead time, high fuel to energy efficiency and low emissions. As a result, such equipment has been and continues to be developed by the manufacturers to meet customer demand for larger, more efficient machines.

At about the same time, many of the world's boiler manufacturers were working in conjunction with the turbine manufacturers to develop boilers to recover waste heat from the turbine exhaust gases. This heat could then be used to raise steam that might then be used to drive a steam turbine generator, further increasing fuel-to-energy efficiency to over 50%. This made a highly efficient power generation plant when compared to conventional coal fired stations with energy efficiencies of 3538%.

With a gas turbine based power plant becoming a viable alternative to provide base load electricity, power generation companies began placing orders for such equipment, based principally on their benefits over conventional fuel-fired power plants.

Advanced technologies have been developed or introduced from the aircraft industry to allow such demands to be met. Though it is clear that there are distinct advantages to using a gas turbine based plant, such equipment is not without its problems. All of the leading equipment manufacturers have suffered from various failures as technological boundaries are pushed to meet new demands and these have led the insurance industry to be wary of recently developed and 'improved' machines.

Despite all the benefits provided by the use of gas turbines and the increasing normalisation of this leading edge technology achieved through numerous technical developments, the machinery breakdown exposure and ensuing business interruption impact are still causing significant concerns to both electricity generators and underwriters.

Gas Turbines

The process is a combination of Brayton Cycle of the Gas Turbine and Rankine Cycle of the Steam Turbine. The Gas Turbines are typically consists of operation of gas turbines that are fired by natural gas/naphtha. The gas turbines convert thermal energy of the fuel air mixture to mechanical energy to drive a generator, which in turn generates electricity. A diverting damper usually leads the hot combustible gases from the gas turbine after expansion in the turbine to the HRSG or boiler.

The Gas Turbine system consists of a combustion chamber where the fuel air mixture is burned. The inlet air is filtered and admitted through the air filter banks. The fuel is fed by the fuel system and atomised using air. Atomising air is supplied from auxiliary air compressors. Supplying purge air does the removal of superfluous fuel from the lines downstream of the fuel control valves.

The next important system in the Gas Turbine is the lubricating oil system. Since the Gas Turbine operates at very high speeds and high temperatures it is very important to have an efficient lubrication system. In many plants, the lube oil is stored in a reservoir and is pumped by means of a main shaft driven pump or auxiliary emergency pump. The lube oil is sent to bearing header, gears, generator and hydraulic supply system. The lubricant is cooled down using heat exchangers and then filtered. The lube oil system is fitted with a trip system whereby in case of reduction in oil pressure, fuel stop valves are operated to stop supply of fuel to gas turbines.

Starting system of the gas turbine provides necessary cranking and turning power for start up of the turbine. It is necessary that the GT is operated at about 100 to 200 RPM prior to star up and also during the cooling period after unit shut down.

Other systems forming part of the gas turbine includes the hydraulic supply system, cooling air system, compressor water wash system and leak detection system.

Heat Recovery Steam Generator

The function of the HRSG is to utilise the waste heat of the gas turbine exhaust gases for generation of high-pressure steam. In the absence of HRSG, the exhaust gases would be discharged to the atmosphere and thermal energy which otherwise could have been utilised is wasted. By using HRSG, this energy is harnessed and used to generate additional electricity which increases the unit efficiency. The water for steam generation is supplied from the DM Plant. This high pressure and high temperature steam is then used to drive the steam turbine. Exhaust steam from the turbine is then condensed in the water-cooled condenser and again fed to the boiler. Thus the cycle is a closed cycle.

Steam Turbine

The steam turbine converts the thermal energy of steam into mechanical energy, which in turn, is used to drive the Generator and produce electrical energy. Steam Turbine consists of two sections viz., high pressure and low pressure. HP steam is supplied from the common main steam header from all the three HRSG through two stop and control valves and LP steam is supplied from the common LP main steam header through one stop and control valve. Turning gear arrangement is provided to keep the steam turbine rotor always turning during the shut down period. The Steam Turbine is also provided with various systems like lube oil system, hydraulic oil system and various other control systems.

Balance of Plant

Power evacuation is done using transformers. These transformers step up the voltage from 11KV to 220 KV for transmission to the grid. Other auxiliaries used by the plant involve Raw Water System, which supplies water to the Water Pre Treatment Plant. In the pre treatment plant the water is softened prior to sending it to the condensor cooling. In the Demineralised Water Plant or DM plant, the water is treated for removing dissolved contaminants such as salts and silica and send to feed water supply to the HRSG. In the pre-treatment plants chemicals are used to improve the water condition. Apart from the above, the plant uses cooling towers for removal of heat from cooling water, heating, ventilating and air conditioning system and material handling system for loading unloading and storage for maintenance.

Combustion Chamber

Within the combustion chamber the compressed air is mixed with vaporised fuel and the mixture is burned. This creates products of combustion that are at a higher temperature than the compressed air and is used to do more work than the energy used in compressing the air.

Turbine

The hot, high pressure products of combustion are passed to the turbine where they are allowed to expand through several rows of alternate stationary vanes and rotating blades. Each vane/blade set is known as a turbine stage, and as the mixture accelerates past each stage, the kinetic energy within the expanding gas is converted into rotational energy using the rotor blades.

Fuels

Several different types of fuel may be used to fire gas turbines. Natural gas is the most commonly used fuel though there are facilities that use other gases such as synthesis gas (syngas), a combination of carbon monoxide and hydrogen formed by gasification of residues, or other solid organic materials or liquids such as light fuel oil. Some gas turbine installations that are designed to burn syngas feature an integrated fuel gasification system. These generally use turbine exhaust gases to raise steam which may then be used within the fuel gasification plant, and are generally found within refineries or other facilities with large amounts of organic waste materials. It should be noted that turbines are specifically designed for the fuels they are to burn and as such a gas turbine that is designed to fire natural gas will not operate effectively with a syngas fuel stream and vice-versa. This is primarily due to differences in the calorific values of the fuels, meaning that different quantities of each fuel would be required to achieve the same output. Gas turbines designed to burn more than one fuel are normally optimised for the main fuel, with a trade-off in performance on the reserve fuel supply. This allows for optimum reliability in critical applications with questionable or problematic fuel supplies and many designs allow for on-load change-over of the fuel supply.

Risk Management

Operation

The modern plants are operated with most modern DCS systems which send out various alarms in case of equipment mal-function and also auto shut down in case of emergency. However, optimum shaft vibration monitoring is critical. Correct operation of these machines is an important issue. Many problems have been caused through poor procedures leading to failure in communications and important steps of operations being skipped. Cooling air valves have been left in the incorrect position after maintenance activities, lubrication systems have been left out of service prior to start up and pre-start checks have been carried out incorrectly. All of these issues may in themselves only seem minor failings, but it only takes moments of running a high speed turbine with little or no lubrication to cause extensive damage to bearing surfaces and overheating of equipment components.

Maintenance

Use of correct components and properly designed spare parts are of equal importance during maintenance activities and manufacturer's guidelines should be properly adhered to. Improperly controlled maintenance activities can also lead to problems. Bolts can be left untightened, tools can be left in the machine and auxiliary systems may not be recommissioned correctly. This should be monitored properly. Debris or foreign objects left following maintenance activities, or loose components should be clear and not get drawn into the turbine causing damages. Gas turbine internals are relatively fragile. Cleanliness is critical to prevent damage to machine blading, burners and other internal areas, as one small foreign particle can have disastrous consequences.

Quality Assurance

Quality assurance of component parts and materials is extremely important as gas turbines operate at high speed with high operating temperatures and pressures, and low tolerances between blades and veins. Failure of a relatively minor component within the machine can cause extensive damage.

Fuels

Use of proper fuel is very critical. Fuel quality is of importance as rogue chemicals can cause deposits, erosion or corrosion of machine internals leading to long term damage. Fuel pulsations as a result of varying fuel quality or irregular supply systems can cause vibration in combustion systems and turbine areas leading to mechanical damage that is exacerbated as it is exposed to high temperatures and further operation. Extremely fine particles in fuel can have an effect similar to a sand blaster on turbine blading or may become embedded within or stuck to blade surfaces leading to a build up of material and subsequent machine imbalances.

Important

The information set out in this document constitutes a set of general guidelines and should not be construed or relied upon as specialist advice. Independent legal advice should always be sought. Therefore Risktechnik accepts no responsibility towards any person relying upon these Risk Management Guides nor any liability whatsoever for the accuracy of data supplied by another party or the consequences of reliance upon it.

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