A high-pressure injection system that needs less water to clean gas turbines than conventional methods can reduce equipment maintenance costs for aircraft, offshore platforms, and power plants. Gas Turbine Efficiency (GTE) in Jarfalla, Sweden, has developed a high-pressure injection system that cleans turbines using atomized droplets and needs 90 percent less liquid than previous methods. With this technique, the operators of offshore oil platforms, power plants, refineries, and aircraft in several countries are reducing the purchase costs of new fluids, the disposal costs of spent cleaning fluids, and maintenance downtime. In creating their washing system, designers considered the differences in cleaning aviation and stationary engines. The turbine-washing system is available in mobile versions for aircraft engines and permanently installed versions, for the off-line cleaning of stationary turbines. GTE also designed two models to serve the very small and very large turbines. The GTE 30 A services the small turbines, ranging from 0.5 to 10 megawatts, that are used in industrial, power-generation, marine, and test-cell applications as well as turboprop aircraft, turbofan craft, and helicopters.
THE AIR AROUND us contains a variety of particles both natural and manmade. In gas turbines, these atmospheric "extras"-pollen, dust, oil vapor, and industrial pollutants- can build up on internal compressor surfaces, interfering with airflow through the engine. Such deposits can then lower compressor efficiency and raise filel consumption, operating temperatures, and emissions. According to Holger Lukas, a consulting engineer in Schenectady, N.Y., and former chair of ASME's International Gas Turbine Institute, this can reduce an industrial gas turbine's power output by 5 to 10 percent.
The most widely practiced method to keep turbines clean involves fluids-typica lly waterborne solvents or surfactants, or deionized water alone. The liquids must be applied in large quantities, however, and can prove costly. Gas Turbine Efficiency (GTE) in Jarfalla, Sweden, has developed a high-pressure injection system that cleans turbines using atomized droplets and needs 90 percent less liquid than previous methods. With this technique, the operators of offshore oil platforms, power plants, refineries, and aircraft in several countries are reducing the purchase costs of new fluids, the disposal costs of spent cleaning fluids, and maintenance downtime.
The need for a new way of keeping gas turbines depositfi ·ee has been evident for a few decades. A popular technique years ago was the use of solids, in which a dry pow- der, such as crushed pecan shells in Texas or crushed sea shells in Sweden, was injected into a running turbine to remove contaminants by erosion. This method was gradually abandoned in the 1970s, Lukas said, "when high-efficiency cooling of turbine blades and vanes was used to accommodate higher inlet temperatures. The cooling passages within the blades would become clogged by the solids." Running an engine- particularly an aircraft engine-during cleaning was impractical and dangerous, and the solids could roughen and erode blade sUlfaces. Also, dry-powder cleaning had a marginal effect on oil-based deposits.
Removal with fluids became more widespread because these agents eliminated erosion problems and were more effective on oil-based deposits. These liquid cleansers, however, come with their own limitations as well. For example, solvents are ineffective against inorganic deposits such as salts and corrosion residue, have a hard time removing insects, and risk being baked in as corrosive salts. "In fact," Lukas said, "if you wash a turbine exposed to a saline atmosphere, you actl,lally wash the salt into the turbine section."
The process also usually requires large quantities of solvent- 400 to 1,000 liters for a typical 40-megawatt turbine. Spent solvents are costly to dispose of, due to environmental regulations, and · complicated to transport because many are flammable and hazardous. In addition, spill ed so lve nts ca n damage paintwork and tires.
For these reasons, surfac tants are now di splacing some solve nts in tu rbine clea ning beca use th ey reac t chemica lly w ith p article de pos its, co nverting di r t removed from the fo rward part of th e engine into a dry powder th at is ca rri ed away with t h e air fl ow. Only small amounts of surfac tants in an atomize d form are applied, whi ch redu ces the amount of cleaning fluid.
After first studying this range of conventional methods in 1987, GTE president Peter Asplund, a power- plan t-system engineer and ASME membe r, so u gh t d evelop a b etter cleaning me thod. H e sp ent nine yea rs developing a cost-competitive sys tem th at would offe r high cleaning effi ciency and reduce the environmental impact of the cleaning process by us- ing just 10 percent of the fluid volume required by estab lished techniques. Key to this new method was a higher degree of atomization-forming much smaller droplets than oth er methods. Ultimately, the droplets' size and speed will determine whether the clea ning fluid will be peripherally disp erse d by the centrifu gal force of the spray or whether it can follow the airflow through the engine as desired.
The Swedish mechanical engineer experimented with Bethree diffe rent ways of achieving small droplets for a given stationfluid fl ow. Two involved the use o f low-pressure flow, ranging from 4 to 6 bars, either with or without an air mixture. Although these methods produ ced the needed droplet size, both low-pressure methods had to use 20 to 30 nozzles in two separate manifolds to deliver the fluid flow required to clean turbine surfaces. Asplund found that such a design would entail high nozzle-installation costs, increase the risk of nozzle blockage, extend overhaul letintervals, place space and weight demands on the turbine, Onand be diffic ult to retrofit due to its space requirement.
Conversely, when Asplund used higher pressure with- atout air to deliver fluid, fewer nozzles were required, in stallation costs were lowered, the risk of nozzle blockag~ was lessened, retrofit was simplified, and man-hours dur ing overhaul were saved. In ad ditio n, he adj usted not only droplet size but also the spray angle of his turbine cleaning system to improve performance
Because increases in a liquid's temperature improve the quality of the spray pattern, increasing the spray angle and decreasing the droplet size, GTE designers incorpo rated heaters in their turbine-washing systems to keep ~h e temperature to 60°C. Asplund stayed with water as the primary fluid, combined with surfac tan t made by RMC in Londo n, because higher- viscosity liquids provide narrower spray angles.
The Difference In Engines
In crea ting their washing system, designers took into account the differences in cleaning avia tion and sta ti onary engines. Cleaning aircraft engines, fo r example, involves injecting 300 to 400 Iiters of cold wa ter from a mobile cleaning sys tem into the en- gine by hose. This volume can ca use undu e st ress on fan blades, compressor blades, and th e e n g in e starting sys tem. The large amounts of water in this process make it less eco nomical to use special washing flu ids, bu t water alone is not too effective on oil-based deposits. Furthermore, water cannot be used below freezing tempera tures and often causes excessive spillage around the aircraft. In addition, the centrifugal fo rce of th e spray separates the water toward the p eriphery, limiting effectiveness.
The stationary gas turbines, used to generate electricity and heat for marine vessels and the compressor drives in processing plants, are often equipped with permanently installed washing systems consisting of an array of nozzles that deliver atomized solid particles or fluid droplets. Be cause they are much larger than aviation engines, station ary gas turbines require fa r larger amounts of waterborne surfac tan ts or solvents, raising the cost of water use as well as of the transportation and disposal of the grea ter volumes of spent fluid.
Stationary turbines are cleaned when they are off- line or when they are op era ting at base load or p art load.. While off-line cleaning is considered to be more effi - cient, it incurs the cost of shutting the turbine down, let ting it cool, and preparing the turbine for cleaning. On line cleaning at part load requires new cleaning liquids formulated to withstand temperatures up to 400°C in at omized form. However: because th ese liquids are typ cally used at high flow rates, special care must be taken to minimize the load on compressor blades.
GTE designers therefore tested th eir cleaning system on both aviation and stationary turbines, ranging fro m 100 kilowatts to 50 megawatts, between 1987 and 1996. Tur- bine models included a Rolls Royce Avon tu rbine from the Swedish utility Vattenfall, the F404/RM 12 aviation engine in the Swedish air force's Gripen fighter aircra ft, and an RR Proteus marine en~i ne used in the Swedish missile frigate Norrkoping. Demonstrations showed that the GTE small-droplet cleaning technique removed dirt from the entire engine interior with as little' as 10 percent of the fluid used in other techniques. In addition, loads on compressor and turbine blades were lowered due to a reduced flow rate, surface erosion was minimized, and maintenance tinle was cut.
Based on these successful tests, Asplund and his colleagues designed a series of mobile and stationary cleaning systems to suit different turbine sizes and applications. These are equipped with four to 10 nozzles in each manifold, depending on the installation.
The GTE 80 I, GTE 160 I, and GTE 400 I turbinecleaning units serve industrial, power-generation, marine, and test-cell turbines with capacities up to 10, 45, and 250 megawatts, respectively. All the units have sheetmetal covers that are powder-coated with zinc primer for corrosion resistance, but the large-scale GTE 400 I is built on a tubular steel chassis that is also coated with zinc powder.
All the GTE models have twin stainless-steel tanks that heat and hold the washing liquid. A GTE 80 I tank holds 40 liters, a GTE 160 I tank holds 80 liters, and a GTE 400 I tank holds 200 liters; these tanks are heated by 1.5-, 3-, and 6-kilowatt heaters, respectively. The GTE 80 I and GTE 160 I use quick connectors at their outlets, while the GTE 400 I outlet relies on Parker's A-LOK 12-millimeter connector. All three outlets are equipped with adjustable pressure-relief valves.
Tanks in all three models are equipped with visual level indicators, and their heaters are equipped with a low- level switch-off. Each GTE tank carries a temperature indicator with a thermostat, and all incorporate automatic shutoffs that are activated when any tank runs out of fluid.
Electrical pumps send the cleaning fluid through the GTE series in stationary-turbine applications, although gasoline-powered motors are used in aviation applications to provide greater flexibility.
GTE also designed two models to serve the very small and very large turbines. The GTE 30 A services the small turbines, ranging from 0.5 to 10 megawatts, that are used in industrial, power-generation, marine, and test-cell applications as well as turboprop aircraft, turbofan craft, and helicopters. The large-scale GTE 300-600A mobile unit cleans turbines that generate more than 250 megawatts; it weighs 500 kilograms dry, and draws fluid from twin 150- or 300-liter stainless-steel tanks.
The company has already delivered 100 turbine-cleaning systems, most of which are used on ABB turbine sets. Applications include offshore oil and gas platforms in Finland, Turkey, Indonesia, Australia, and the North Sea; European Gas Turbine Frame 6B turbines operated by Fellside Heat & Power in Great Britain; and Frame 9 turbines used by IVO Peterborough in Great Britain.
Elsewhere, Porvoo Energy in Finland cleans its solar turbines with GTE machines, and Volvo Aero Turbines in Sweden uses them to clean VR T 600 engines. Aerospace end users include Saab Aircraft's Saab 2000 and Sa ab 340 engines, the Hercules military transports of the Swedish air force, and F-18 Hornets and Hawk planes serving in the Finnish air force. For industrial processing applications, N este Oil in Finland uses the GTE system to clean General Electric Frame 6B and Frame 6FA turbines as well as a Fiat TG20 turbine.
Coating Reduces Fouling
COMPRESSOR WASHING IS not the only way to clean gas turbines: Operators are also reducing fouling by applying metallicceramic coatings. For example, the SermeTel metallic-ceramic coatings developed by Sermatech in Limerick, Pa., were originally designed to combat turbine-part erosion, a role they continue to fulfill. However, because the interval between washing coated blades is at least double that of uncoated blades, SermeTel can also slow fouling and help coated blades last longer.
The SermeTel base coats consist of a water-based slurry containing aluminum powder pig ment particles dispersed in an acidic solution of metallic phosphates and dichromates. The slurry is sprayed onto turbine components and thermally cured at 650°F for 30 minutes, producing a well-bonded, glasslike inorganic polymer matrix with aluminum pigmentation. The coating is then blasted with aluminum oxide, or another abrasive media, until it becomes electrically conductive and galvanically sacrificial to the metal surface below.
"Galvanically sacrificial" means that even if the coating is damaged in service to leave the turbine component exposed to the environment, the coating will corrode, sacrificing itself to protect the bare metal. In many Sermatech coating systems, this sacrificial coating layer is in turn sealed with a topcoat. These topcoats create an aerodynamically smooth surface that resists corrosion. Some SermeTel systems incorporate fluorocarbon resins to create a nonstick surface. The smooth surfaces provided by these resins slow fouling and make the turbine components easier to clean with water than a bare metal surface.