FACTORS TO BE CONSIDERED AT THE DESIGN STAGE OF A POWER PLANT

The electrical engineer should take full account of the site location and environmental conditions that a gas turbine generator will need to endure. These conditions can seriously affect the electrical power output that will be achievable from the machine. The starting point when considering the possible output is the ISO rating.

This is the declared rating of the machine for the following conditions:-

• Sea level elevation.
• 15⁰C (59⁰F) ambient temperature.
• Basic engine, no losses for inlet or exhaust systems, no losses for gearbox and mechanical transmission.
• Clean engine, as delivered from the factory.

The gas turbine manufacturer provides a standardized mechanical output power versus ambient temperature characteristic. (Some manufacturers also give the electrical output power versus ambient temperature characteristic. Therefore care must be exercised to be sure exactly which data are to be given and used.)

The following derating factors should be used in the estimation of the continuous site rating for the complete machine:

• ISO to a higher site ambient temperature, typically 0.5 to 0.8% per ◦C.
• Altitude, usually not necessary for most oil industry plants since they are near sea level.
• Dirty engine losses and the ageing of the gas turbine, assume 5%.
• Fuel composition and heating value losses, discuss with the manufacturer.
• Silencer, filter and ducting losses, assume 2 to 5%.
• Gearbox loss, typically 1 to 2%.
• Generator electromechanical inefficiency, typically 2 to 4%.
• Auxiliary loads connected to the generator, typically 1 to 5%.

Dirty engine losses

Consideration should be given to the fact that engines become contaminated with the combustion deposits, the lubrication oil becomes less efficient, blades erode and lose their thermodynamic efficiency and air filters become less efficient due to the presence of filtered particles. These effects combine to reduce the output of the machine. A rule-of-thumb figure for derating a gas turbine for dirty engine operation is 5%. This depends upon the type of fuel, the type of engine, the environment and how long the engine operates between clean-up maintenance periods. Individual manufacturers can advise suitable data for their engines operating in particular conditions. Dirty engine conditions should be considered, otherwise embarrassment will follow later once the machine is in regular service.

Fuel composition and heating value losses

The chemical composition and quality of the fuel will to some extent influence the power output. However, it is usually the case that more or less fuel has to be supplied by the fuel control valve for a given throughput of combustion air. Hence it is usually possible to obtain the declared normal rating from the machine, but attention has to be given to the supply of the fuel. In extreme cases the profile of the fuel control valve may require modification so that adequate feedback control is maintained over the full range of power output. The appropriate derating factor is usually 100%, i.e. no derating.

Silencer, filter and ducting losses

The amount of silencing and filtering of the inlet combustion air depends upon the site environment and the operational considerations.

Site environmental conditions may be particularly bad, e.g. deserts where sand storms are frequent; offshore where rain storms are frequent and long lasting. The more filtering that is required, the more will be the pressure lost across the filters, both during clean and dirty operation. This pressure drop causes a loss of power output from the machine.

The amount of inlet and exhaust noise silencing will depend upon, the location of machine with respect to people in say offices or control rooms, how many machines will be in one group since this affects the maintenance staff and total noise level permitted by international or national standards. The effects of a silencer are similar to a filter since the silencing elements cause a pressure drop.

With offshore platforms it is not always practical to locate the main generators in a good place regarding the position and routing of the inlet and exhaust ducting. Long runs of ducting are sometimes unavoid- able. It is then necessary to allow a derating factor for the pressure drop that will occur. The manufacturer should be consulted for advice on this aspect. For a typical offshore or onshore situation with a reason- able degree of silencing a rule-of-thumb derating factor would be 98%. In a poor location assume 95%.