of 1000 psi is required. HP compressed air plantssupport functions which require high pressuresand high flow rates of compressed air by theaddition of HP storage flasks to the system. Anexample of such a system is one that provides airfor starting diesel and gas turbine engines.Reduction in pressure, if required, is doneby using specially designed pressure-reducingstations.Medium-Pressure AirMP air systems provide compressed air at anominal operating pressure of 151 psi to 1000 psi.These pressures are provided either by an MP aircompressor or by the HP air system supplying airthrough an air bank and pressure-reducingstations.Low-Pressure AirLP air systems provide compressed air at anominal operating pressure of 150 psi and below.The LP air system is supplied with LP air by LPair compressors or by the HP air system supplyingair through an air bank and pressure-reducingstations. LP air is the most extensive and variedair system used in the Navy,In addition to being used for variouspneumatic applications, LP and HP compressedair are used in the production of nitrogen.NITROGENFor all practical purposes, nitrogen isconsidered to be an inert gas. It is nonflammable,does not form explosive mixtures with air oroxygen, and does not cause rust or decay. Dueto these qualities, its use is preferred overcompressed air in many pneumatic systems,especially aircraft and missile systems, andwherever an inert gas blanket is required.Nitrogen is obtained by the fractionaldistillation of air. Oxygen/nitrogen-producingplants expand compressed air until its temperaturedecreases to –196°C (–320°F), the boiling pointof nitrogen at atmospheric pressure. The liquidnitrogen is then directed to a storage tank. Aliquid nitrogen pump pumps the low-pressureliquid nitrogen from the storage tank anddischarges it as a high-pressure (5000 psi) liquidto the vaporizer where it is converted to a gas at5000 psi. Oxygen/nitrogen-producing plants arelocated at many naval installations and onsubmarine tenders and aircraft carriers.CONTAMINATION CONTROLAs in hydraulic systems, fluid contaminationis also a leading cause of malfunctions inpneumatic systems. In addition to the solidparticles of foreign matter which find a way toenter the system, there is also the problem ofmoisture. Most systems are equipped with one ormore devices to remove this contamination. Theseinclude filters, water separators, air dehydrators,and chemical driers, which are discussed inchapter 9 of this manual. In addition, mostsystems contain drain valves at critical low pointsin the system. These valves are opened periodicallyto allow the escaping gas to purge a largepercentage of the contaminants, both solids andmoisture, from the system. In some systems thesevalves are opened and closed automatically, whilein others they must be operated manually.Complete purging is done by removing linesfrom various components throughout the systemand then attempting to pressurize the system,causing a high rate of airflow through the system.The airflow will cause the foreign matter to bedislodged and blown from the system.NOTE: If an excessive amount of foreignmatter, particularly oil, is blown from any onesystem, the lines and components should beremoved and cleaned or replaced.In addition to monitoring the devices installedto remove contamination, it is your responsibilityas a maintenance person or supervisor to controlthe contamination. You can do this by using thefollowing maintenance practices:1. Keep all tools and the work area in a clean,dirt-free condition.2. Cap or plug all lines and fittingsimmediately after disconnecting them.3. Replace all packing and gaskets duringassembly procedures.4. Connect all parts with care to avoidstripping metal slivers from threaded areas. Installand torque all fittings and lines according toapplicable technical instructions.5. Complete preventive maintenance asspecified by MRCs.Also, you must take care to ensure that theproper cylinders are connected to systems beingsupplied from cylinders.Cylinders for compressed air are paintedblack. Cylinders containing oil-pumped air have11-8
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