Working Under Pressure
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ENGINE YEARBOOK 2002 Working under pressure Jim Whalen of Progressive Technologies discusses why ultra high pressure cleaning technology has found favour in the aero-engine overhaul fraternity, and where its characteristics and economics make it superior as compared with conventional processes.
oday’s aircraft engines employ and can produce hazardous waste that is many different advanced technolo- difficult and expensive to dispose of. Tgies to enhance performance and Progressive Technologies of Grand Rapids, reduce the costs of ownership. Often, this Michigan, specialises in the production of presents challenges to the organisations ultra-high pressure (UHP) waterjet responsible for maintenance and over- cleaning equipment for the aerospace haul. One example of this is the removal industry (UHP commonly refers to and reapplication of high technology pressures above 25,000 psi). The company coatings. These may have been applied to also builds robotic thermal spray coating improve fan and compressor efficiencies, equipment, pneumatic abrasive blast to reduce surface fretting or to provide a cleaning and shot peening systems. This thermal barrier that improves the high combined experience significantly Many repair processes require the temperature capabilities of combustors enhances the functionality, robustness and and turbine components. level of process control found in UHP coatings to be removed for base Many repair processes require the waterjet cleaning equipment. material inspection,restoration coatings to be removed for base material Technology background and coating replacement. inspection, restoration and coating replacement. The task of removing these The heart of any UHP waterjet cleaning coatings is quite challenging and, machine, the intensifier pump, was traditionally, mechanical and chemical invented and first used in the industrial methods have been used. Mechanical environment during the early 1970s. methods usually involve machining, Wood processing and furniture grinding or pneumatic abrasive blasting. manufacturers utilised the waterjet Although such methods are fairly efficient process to debark trees and to cut in material removal, they tend to lower complex shapes in fibreboard. As a result the life cycle of the component as they of these early practices, many other often remove parent material along with applications of the UHP waterjet cutting the coating. Chemical methods are more process were developed. Paper products, precise in removing only the coating but plastics, and even food have since been these techniques are usually very slow processed using waterjet technology.
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The mid-1980s witnessed the introduction Fan jet nozzles, in contrast, are larger in of abrasive additives into the high- size, 0.014” to 0.024” (0.35 to 0.61 mm) pressure waterjet stream. This new in diameter. The fan jet disperses the process, called abrasive waterjet cutting, water stream into a fan shaped pattern. was subsequently used to cut hard Fan jet nozzles are very effective on materials such as steel, super alloys, hard coatings that need to be eroded composites and stone. It also significantly from the substrate rather than peeled increased the demand for waterjet from the surface. The fan jet creates a technology which has since become uniform wide pattern of high velocity widely used and accepted in water droplets that hammer the coating, manufacturing processes. It was not until eroding it from the base material. the late 1980s that the potential benefits Typically, both round and fan jet of using UHP waterjet cleaning nozzles are rotated at speeds of between technology were realised. As demand to 500 to 1500rpm to help spread the remove thermal sprayed coatings from nozzle’s energy evenly over the work aircraft engine components increased, the piece being cleaned. cost of installing and operating a UHP There are several different techniques waterjet cleaning facility became easier to for nozzle movement. Typically, justify. articulated or gantry robots are utilised to move the rotating waterjet nozzle Process and equipment over the work piece. Uniform, UHP waterjet cleaning equipment used controlled nozzle positioning is critical to remove thermal sprayed coatings is to controlling the UHP waterjet process. complex and capital intensive. Systems Nozzle angle and traverse speed over range in price from $400,000 to over the work piece are very important in $1,000,000. In general, a UHP waterjet assuring that coatings are removed and cleaning system consists of five major that the base metal is left intact. sub-systems Processing components in a UHP 1) Intensifier pump waterjet system produces a work 2) Waterjet nozzle and drive environment that is hot, noisy and high 3) Process cabinet in humidity. However, process cabinets can efficiently contain and reduce these 4) Nozzle/part manipulator effects to acceptable levels. A mist 5) Waste removal and water filtration extractor removes mist and humidity The intensifier pump produces between and the cabinet interior can be designed one to three gallons of water per minute to absorb most of the acoustic energy. (3.8 to 11.4 litres/min) at pressures up With the correct engineering approach, to 55,000 psi (370Mpa). The water is UHP waterjet systems can easily be delivered to the waterjet nozzle via integrated into any manufacturing stainless steel high-pressure tubing or a factory environment. special flexible hose. There are many The final component of a waterjet different configurations of waterjet cleaning system is the water filtration nozzles depending on the coating to be system. This system is critical to the removed and the geometric access performance of the process since it not limitations of the component. only removes the solid waste created, There are two basic types of nozzles used but it also controls the quality of the to remove coatings from aero-engine inlet water to the pump. Most UHP components - the round jet and fan jet. waterjet cleaning systems produce no Round jet nozzles create a coherent focused water discharge and therefore, no waste stream of water of between 0.005” to water is created. 0.012” (0.13 to 0.3mm) diameter. Round jet The dirty water filter has replaceable nozzles are most effective on soft coatings filter media to remove large coating such as rubber, insulation and abradable particles from the process water. linings. These soft coatings spall off the Typically, several levels of filtration are substrate when the round waterjet pierces used to remove coating particles down to the coating and disperses at the bond line 3 microns in diameter. The water is then between the coating and the substrate, passed through the clean water filter that peeling the coating from the part. further cleans the water down to 0.2
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microns which controls biological growth Although influenced by the process and the accumulation of dissolved metals. application and use, waterjet systems The water is then stored for use by the will cost $20 to $40 per pump hour to UHP intensifier pump. It should be noted operate, inclusive of energy and that poor water quality from the consumable parts. filtration system leads to premature wear UHP waterjet cleaning systems also have of intensifier seals and waterjet nozzles, limitations with regard to the types of thereby increasing operating costs. coatings that can be removed. It is not practical to use the UHP waterjet Benefits process to remove coatings that have For years, the removal of thermal been applied with the high velocity sprayed coatings from aero-engine oxygen fuel (HVOF) thermal spray components has been a very difficult process or brazed honeycomb. Even and costly task for overhaul and with these limitations, many users have maintenance facilities. Traditional repair justified the use of the UHP waterjet techniques have been partially process over mechanical or chemical responsible for increasing turn times removal methods. Most see a return on and, in some instances, increasing their capital investment within nine to component scrap rates . However, the 12 months after system installation. use of the UHP waterjet cleaning process has greatly simplified the task of Common applications removing thermal sprayed coatings and The first application of UHP waterjet have realised the following benefits: cleaning was the removal of thermal The selective removal of coatings barrier coatings from aero-engine without the removal of base metal combustors. These coatings consist of a Faster coating removal rates as ceramic topcoat with an oxidation compared with the chemical resistance metallic bond coat. removal processes Traditionally, this coating required Less occurrences of base metal abrasive grit blasting to remove the removal as compared with ceramic topcoat and a chemical bath to conventional mechanical removal remove the metallic bond coat - a very methods (turning, grinding, time consuming and expensive process. abrasive blast cleaning) Today, the UHP waterjet process has broad application across all modules of Environmental friendliness aero-engines. Abradable coatings are Most waterjet cleaning systems can be cost frequently removed from fan and justified simply because of their compressor cases at engine overhaul. productivity gains as compared with Whilst these coatings are relatively soft, chemical or mechanical removal processes. they are difficult to remove mechanically. The additional benefits of reducing base The UHP waterjet process removes them material damage can result in significant very quickly, improving productivity and savings as a result of reducing the reducing part damage as compared with purchase of new parts. The environmental competing mechanical machining or friendliness of the process is also a benefit scraping methods. that is of increasing importance as disposal Combustors can be cleaned with the regulations and workplace environment waterjet process. This remains the most rules become more demanding. difficult UHP waterjet cleaning application since the combustor’s high operating Limitations temperatures cause the metallic bond coat All manufacturing processes have to diffuse into the base material, making it limitations. The most significant very difficult to remove. The competing limitation of UHP waterjet cleaning is chemical process is time consuming and related to cost. As previously mentioned, with the introduction of more stringent these systems are complex and very environmental regulations, the resulting capital intensive, with starting prices in costs of chemical stripping become the region of $400,000. In addition to the prohibitive. capital costs, UHP waterjet equipment Another challenging application is the has moderate to high operating costs. removal of wear or restorative coatings.
110 Engine Yearbook 2002 ENGINE YEARBOOK 2002
Coatings such as tungsten carbide, a process that continues to receive quite chrome carbide, nickel aluminide, and a lot of research and development inconel are commonly used to protect funding from the equipment suppliers components from wear, or to restore a and even the end users. Advances will worn features. Wear resistant coatings are be made in equipment component hard and require aggressive UHP waterjet design to reduce maintenance costs. cleaning, nozzle and work piece velocity Also, process research will evaluate the control is critical to ensure base metal benefits of even higher water pressure protection. Restorative coatings are also (above 60,000 psi, 410 Mpa), possibly relatively hard but are often thicker. increasing the cleaning rate or reducing Nevertheless, aggressive parameter the detrimental effects on base materials. settings are also required to effectively As aero-engine component designs remove these coatings. become more complex, waterjet The final class of engine components application engineers will continue to processed are rotating parts. The number develop smaller, innovative nozzles to of applications that process compressor remove coatings from confined areas. rotors, rotating seals and shafts is Equipment manufacturers will also increasing. Wear coating on seal teeth, begin to design and build smaller and abradable coatings of stator rub lands compact UHP waterjet cleaning cells are now being removed with the waterjet with a lower cost of ownership. In process. general, technology will continue to advance towards lower capital and Future advancements operating costs while new methods of UHP waterjet cleaning has become a process application will be developed mature process over the last decade. It is for a wider array of components.
Engine Yearbook 2002