The Cost of Gas Turbine Fouling
17 April 2007 11:45
All around the world, shockwaves from constantly high oil and gas prices and electricity rates are being felt in the budgets of public authorities, enterprises, gas turbine operators and households alike.
In a generating plant where electricity is generated by gas turbine the fuel cost represents a majority of the total costs. As an average the fuel cost represents about 60% of the electricity selling price. This is why operators around the world are looking for better filtration systems as a means for keeping fouling and fuel consumption at a minimum.
Axial compressor fouling
Axial compressor foulingis primarily caused by airborne particles of less than 3µ and the best correlation between fouling rate and particle size distribution is for particles less than 1µ in diameter. Very small particles will adhere to rotating compressor blades despite the very high centrifugal forces involved because the Van Der Waals attraction forces between the particle and the blade are stronger than the inertial forces generated by the very low mass of the particle.
Fouling changes the shape and smoothness of both rotating and stationary vanes and results in a reduction of both mass flow and pressure ratio generated by the compressor. The net result of fouling is a reduction of power output and an increase in heat-rate for a given combustor outlet temperature - conversely if an engine is not running at its temperature limit a fouled engine has to run hotter to produce a given output. The susceptibility of an engine to fouling depends on its size, pressure ratio per stage, airfoil geometry and cycle parameters and an Index of Susceptibility to Fouling (ISF) has been proposed based on parameters which correlates fairly well to field experience.
In general small high ratio per stage engines are more susceptible to fouling than large, low pressure ratio engines. The ratio of heat rate increase to output loss varies depending on the engine design but in general the heat-rate increases by about half of the percentage that the output power drops. A secondary effect of fouling is an increase in airfoil temperatures in the high-pressure turbine as fouling in the internal vane and blade cooling passages reduces heat-transfer effectiveness and ultimately reduces the life of the hot section.
The costs of fouling far exceed the cost of eliminating it but since capital costs tend to be weighed heavily in equipment purchase decisions most gas turbines are equipped with inadequate air filtration. The operator consequently ends up paying many times more in extra operating costs than what they saved in capital costs.
Recovering Compressor Performance
Most of the compressor performance loss due to fouling can be recovered with an offline/soak wash of the compressor provided it is done before the power has deteriorated too much – if left to go too far some of the blade deposits can be 'baked on' and can not be removed by washing. For engines in intermittent services this may be a viable operating strategy but it is not optimum for base load engines which must run for long periods or for applications where engine downtime results in a significant loss of revenue such as for CCGT and CHP plants as well as for GT's on offshore oil production platforms. This fact is becoming more and more important as cost of fuel and environmental requirements continues to increase.
Many operators of gas turbines equipped with high velocity air filters have become accustomed to frequent production interruptions for offline washing because of those filters use an electrostatic charge on relatively coarse fibres to get a presentable nameplate efficiency with an acceptable pressure drop but as this electrostatic charge dissipates in service the efficiency of the filter drops and the penetration increases with resultant increases in engine fouling rate. This filter efficiency drop when coarse synthetic fibres lose their charge is so significant that the latest European standard for testing filters, EN779:2002, stipulates that both initial (charged) efficiency and mature efficiency with the electrostatic charge removed must be measured and published so that the true in-service performance of the filter can be assessed.
Many operators are now looking at possibilities to operate their gas turbines for longer periods without shutdowns. Three years continuous duty is a target often mentioned. Keeping the engine clean by preventing small particles from fouling to enter the air inlet system is one important step to achieve this target. It means taking the gas turbine filtration systems from the now typical medium to high efficiency technology to the “clean room” technology or HEPA filtration. By going from the typical F8 grade to H12, penetration of sub micron particles is dramatically reduced. For example, penetration on 0,4 micron particle is reduced from typically 25% for a F8 filter to less than 0,5% in a H12 filter. A huge improvement which cuts fouling dramatically. With proper pre-filtration and sizing of the system, using LCC programme and other tools pressure losses and life of filters can be optimized to match over-all target.
LCC – GT Software Assesses Filter Efficiency
Camfil Farr Power Systems has developed an interactive Life Cycle Cost (LCC) program that allows operators to assess how different filtration solutions affect the total cost of operating a gas turbine.
Filter efficiency has a major impact on plant performance. The more dust, dirt and moisture that get through the filter of an air inlet system, causing fouling of the machinery it is meant to protect, the more performance suffer.
Plant efficiency is also directly affected by the pressure drop not only through the filter but also the complete inlet system with ducting and silencer - the greater pressure drop, the more efficiency suffers.
The LCC-GT software can be used to optimize an intake system for a specific gas turbine as they tend to differ in susceptibility to fouling. The variables used in the LCC - GT program include engine performance characteristics, airborne contaminant concentrations, filter efficiency, pressure drop, filter dust holding capacity and life, filter and fuel prices, the value of engine output, downtime costs for filter changes and off-line engine cleaning, labour cost for filter maintenance and filter disposal costs.
We can conclude:
The cost of clean air is not the cost of replacement filters - it is how much energy the filters use during their working life at the desired efficiency.
We have proven for customers around the world time after time - in lab tests and real operating conditions that it always pays to buy quality air filters with low pressure drop instead of low-cost products with poorly functioning filter media and/or insufficient filtration area.
Together with several end-users in the French gas turbine market we have made a lot of retrofits for existing generating plants. Gas turbines in France typically operate 4,500 hour from October 1 to April 1, due to gas tariffs and conditions governing the sale of electricity and prices to Electricité de France (EDF). The primary objective was to keep the compressor as clean as possible during the entire operating season, without the need for shutdowns for offline washing. An offline wash represents about six to seven hours of production losses. Each hour means – on a 45MW engine - a loss of 45 MW from the time the engine’s running speed is reduced, to the cleaning of the compressor and until the engine is started again.
In recent years 20 different installations have been retrofit with Camfil Farr intake filter system with gas turbines ranging from 5MW to 45MW. Common for these installations have been the customers need of:
- Improve compressor protection
- Reduce power and output losses
This has been met by changing to a two-stage filtration consisting of CamGT F7 as pre filter and CamGT F9 or H10 as final filter. By keeping the compressor clean no stops for off-line cleaning under operational season is needed compared to three stops earlier. Also the average pressure drop has been less giving more output and decrease in heat rate. As an example at one site with a 10MW gas turbine this has meant a gain of EUR 32000 and a pay-back of less than one operational season.
It has been proven every time with our LCC-GT software that it pays to perform a life cycle cost analysis so you can choose the right filter combination for the right air cleanliness. The result - lower energy consumption in gas turbines - is reflected right to the bottom line.
For more information on this company:
Camfil Farr - Offshore Filter Systems for Turbomachinery
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