The only way to combat fouling is to wash it away. Washing is done by spraying a washing liquid from a set of nozzles installed upstream of the compressor inlet. The liquid follows the air stream into the compressor, where the mechanical movements and chemical act of the washing liquid releases the deposits. in terms of gas turbine efficiency improvement, what washing does is to restore engine efficiency that would otherwise be lost by fouling. Washing can be done off-line and on-line, where off-line washing is defineci by no fuel is being burnt during washing while with on-line washing, the engine is firing fuel. The washing mechanism of the two methods is very different. For off-line washing, the engine shaft is cranked by the starter motor. This turns the rotor relatively slowly, typically up to 1000 rpm, resulting in low air velocities through the engine. This allows the washing liquid to move around in the compressor and slowly find its way towards the exhaust end. The washing resul! is usually very good. Sometimes up to 100% cleaning effect can be achieved. On-line washing, however, is very different. On-line washing can never be as complete as off-line for the following four reasons. First, with the rotor running at high speed, centrifugal forces will push the liquid out to the periphery where it can do no cleaning job. Second, the air velocity is very high, resulting in a short retention time for the liquid through the compressor. Third, at running conditions turbulence is very strong, resulting in liquid being lost to the duct walls before entering the compressor. Fourth, compression results in a temperature increase inside the compressor, and liquid may start to boil off about halfway through the compressor. High-pressure compressor wash system nozzle interconnection (blue hose) On-line washing has been criticized recently for causing erosion damage to compressor blades. Erosion comes from the energy transferred at droplet impact. The way around this is to wash with droplets having a smaller diameter. The effect of a droplet size change is dramatic. The droplet volume is a function of radius cubed, i.e. a droplet of halt the size has only one eighth of the volume and thereby only one eighth of the mass and impact force. MAKALE/ ARTICLE The efficiency of gas turbines has steadily improved as new technologies and materials are used To improve on-line washing, a high-pressure washing system has been developed. in the high-pressure system the nozzles are installed closer to the compressor and point towards the compressor inlet face. Liquid is atomized at around 70 bar, resulting in a high-velocity spray. A number of useful features go with high-pressure on-line washing. The liquid penetrates into the core air stream and relatively little liquid is lost to the walls. Further, the spray droplets will have alması the same velocity as the air entering the compressor and are therefore more likely to penetrate deep into the compressor. This is known as a high droplet-to-air velocity ratio. The high-pressure system, alsa referred to as a 'direct injection system', results in fewer nozzles to install as the liquid capacity per nozzle is higher compared to a low-pressure system. This alsa reduces installation and maintenance costs. The second photograph in this article shows the rear of an installation on a Frame 5 gas turbine. Only a few nozzles are required and the installation is simplified by the blue hydraulic flex hose interconnecting the nozzles. The third photograph in this article shows another view of a highpressure compressor wash system in action. Table 2 summarizes the pros and cons of low-pressure and highpressure washing systems. Table 2. Compressor washing technologies Low-pressure system - off-line and on-line washing • typically up to 9 bar atomizing pressure • many nozzles required (low capacity per nozzle) • small nozzle orifice may result in clogging ! •• risk for liquid streaking (low liquid exit velocity from nozzle) separate set of nozzles for on-line and off-line 1 • small risk tor nozzle wear • low pump power High-pressure system - off-line and on-line washing • typically 70 bar atomizing pressure • low installation and maintenance cost (few nozzles to install) • less liquid is used • good on-line liquid penetration (high liquid-to-air velocity ratio) • small risk of erosion damage (less than 100 micron droplet size) • easily retrofittable • same nozzles for off-line and on-line ENERJi & KOJENERASYON DÜNYAS I 91
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