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DECONTAMINATION
Flushing - 14018_174

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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METHODS There are four basic methods used to decontaminate aircraft hydraulic systems. The methods are recircu- lation cleaning, flushing, purging, and purifying. Recirculation Cleaning Recirculation cleaning is a decontamination process in which the system to be cleaned is powered from a clean external power source. The system is cycled so it produces a maximum interchange of fluid between the powered system and the SE used to power it. When decontaminating a system, the contaminated fluid is circulated through the hydraulic filters in the aircraft system and in the portable hydraulic test stands. Decontamination that uses the recirculation cleaning method is a filtration process. It can remove only that foreign matter that is retained by the filter elements normally found in the equipment. A key factor in recirculation cleaning is the use of high-efficiency, 3-micron (absolute) filter elements. Absolute filter elements have no fluid bypass when the filter clogs. The filters have a large dirt-holding capacity in the portable test stands used for this purpose. In a single fluid pass, these filters remove all particulate matter larger than 3 microns, and a high percentage of the other particles down to submicron size. Recirculation cleaning is effective in removing hard particulate matter from hydraulic fluid that is otherwise serviceable. It must be recognized that the filters are not capable of removing water, other foreign fluids, or dissolved solids. Therefore, recirculation cleaning is limited to decontamination of systems found to have a particulate level in excess of Navy Standard Class 5, whose fluid is considered otherwise acceptable. For specific procedures on recirculation cleaning, you should refer to the applicable MIM. Use recirculation cleaning to remove excessive particulate matter that results from normal component wear, limited component failure, or external sources. Clean the system by powering it with an external portable hydraulic test stand. Operate the aircraft systems so maximum interchange of fluid is produced between the aircraft and the test stand. View A of figure 4-6 shows a flow diagram for recirculation cleaning. Figure 4-6.-Fluid flow during decontamination. 4-15







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