Figure 12-10. Effect of c.eaning on power output.
blades reducing the flow area. This ash must be removed and in many cases this means that the hot section blades and nozzles must be washed every 100-120 hours. This is done by bringing the turbine down and running it on turning gear till the turbine blade temperatures are around 200 0F ((93.3 0C)， in most cases this is reached in about six to eight hours. The turbine hot expander section is then blasted by steam and most of the ash is removed. The turbine is then brought up to speed after the turbine blade section is dried. This whole process takes about 20 hours.
Compressor Washing
Compressor washing is also a very important part of turbine operations. Two approaches to compressor cleaning are abrasion and solvent cleaning.The use of abrasive cleaning has diminished due to erosion problems， liquid washing is primarily used. The new high-pressure compressor are very susceptible to dirt on the blades that not only can lead to a reduction in performance but can also lead to compressor surge. Washing efficacy is site-specific due to the different environmental conditions at each plant. There are many excellent techniques and systems for water washing. Operators must often determine the best approach for their gas turbines. This includeswhat solvents if any should beused， and the frequencies of wash. Many operators have found that water wash without any solvent is as effective as with the use of solvents. This is a complex technical-economical problem also depending on the service that the gas turbines are in and the plant surroundings. However the use of nondemineralized water could result in more harm than good.
Water washing (with or without detergents) cleans by water impact and by removing the water-soluble salts. The effect of water cleaning is usually not very effective after the first few stages. It is most important that the manufacturer"srecommendations be followed with respect to water wash quality， detergent/water ratio， and other operating procedures. Water washing using a water-soap mixture is an efficient method of cleaning. This cleaning is most effectivewhen carried out in several steps， which involve the application of a soap andwater solution， followed by several rinse cycles. Each rinse cycle involves theacceleration of the machine to approximately 50 percent of the starting speed， after which the machine is allowed to coast to a stop. A soaking period follows during which the soapy water solution may work on dissolving the salt.
A fraction of airborne salt always passes through the filter. The method recommended for determining whether or not the foulants have a substantial salt base is to soap wash the turbine and collect the water from all drainage ports available. Dissolved salts in the water can then be analyzed.
Online washing is being widely used as a means to control fouling by keeping the problem from developing. Techniques and wash systems have evolved to a point where this can be done effectively and safely. Washingcan be accomplished by using water， water-based solvents， petroleum-basedsolvents， or surfactants. The solvents work by dissolving the contaminants while surfactants work by chemically reacting with the foulants. Water-basedsolvents are effective against salt， but fare poorly against oily deposits. Petro-leum-based solvents do not effectively remove salty deposits. With solvents， there is a chance of foulants being re-deposited in the latter compressor stages.
Evenwith good filtration， salt can collect in the compressor section.During the collection process of both salt and other foulants， an equilibriumcondition is quicklyreached， after which re-ingestion of large particles occurs. This re-ingestion has to be prevented by the removal of salt from the compressor prior to saturation. The rate at which saturation occurs ishighly dependent on filter quality. In general， salts can safely pass through the turbine when gas and metal temperatures are less than 1000 0F. Aggres-sive attacks will occur if the temperatures are much higher. During cleaning， the actual instantaneous rates of salt passage are very high together with greatly increased particle size.
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