require small amounts of air at lower pressures, whichMAINTENANCE OF LP AND HP AIRis supplied through pressure-reducing stations.DEHYDRATORSAlways use caution with HP systems! When HP airenters suddenly into pockets or dead ends, the airtemperature in the confined space increasesdramatically. If there is any combustible material in thespace and the air temperature increases to the ignitionpoint of the material, an explosion may occur.Explosions of this type may set up shock waves thattravel through the compressed air system. This travelmay cause explosions at remote points. Even a smallamount of oil residue or a small cotton thread may besufficient to cause ignition.Follow the scheduled maintenance of LP or HP airdehydrators according to the PMS requirements. Thefollowing is a sample LP air dehydrator maintenanceschedule and is for general information only:1. Daily:- Check applicable power on lights, flowmeterreadings, cooling water temperatures, heatertemperatures, and outlet air temperatures for properoperation.Some common pressure requirements for HPsystems may be as follows:Torpedo workshopAmmunition depotWind tunnelsTesting laboratories600 to 3,000 psig100, 750, 1,500, 2,000,and 4,500 psigOver 3,000 psigUp to 6,000 psig- Note any dehydrator filter element pressuredrops for element replacement.- Periodically blow down and clean the conden-ser water strainer.Figures 6-5 and 6-6 show a maintenance index page(MIP) for one design and make of an HP air compressorsystem. This will give you an idea of the differences inplanned maintenance requirements between the LP air(see figs. 6-2, 6-3, and 6-4 for comparison) and HP airsystems. It is very important that the PMS you are usingare the correct ones.- Blow down type I and type III dehydrators.Dump the valve if more than 1/2 pint of water drainsout; the automatic feature is not working.- Check the purge pressure and the free move-ment of the purge flowmeter float of type II and type IIIdehydrators.2. Weekly:Blow down the inlet separator,prefilter, and trap dump valve of dehydrators by openingthe manual drain valve.3. Monthly:You should inspect air flasks, receivers, separators,and piping for damage or external corrosion once every6 months. Enter the inspection date and results in theMaintenance and Material Management (3-M) Systemsby submitting a work request for any discrepanciesfound. You must document completion of all inspectionresults through PMS.- Check the outlet air moisture content. The dewpoint should be below -40°F at 80 lb/in2 for both type II2and type III dehydrators, and below 40°F at 80 lb/in fortype I dehydrators. An excessive dew point indicates amalfunction.AIR DRYERS OR DEHYDRATORS- Check the inlet and outlet filters and the purgefilter for the type II and type III dehydrators. Replacethe filter elements if necessary.4. Quarterly:- Clean the condenser water tubes.The Navy uses three types of air dehydrators fordrying LP air. These are the refrigeration (type I),desiccant (type II), and combination of bothrefrigeration and desiccant (type III). HP air applicationuses only the type III. The Navy is replacing the varioustypes of desiccant used in the fleet with activatedalumina beads in 1/8-inch diameter spheres. This typeof desiccant is also intended to reduce dust problemsproduced by the other various types. The dust causesclogged filters and other component malfunctions.- Disassemble and clean the inlet and interstageseparators and purge the solenoid valves.5. Annually:- Remove and calibrate the pressure gauges.Adjust them to give maximum error 2 lb/in2 (1 percentof full scale).- Disassemble the desiccant chambers. Cleanthe assembly of dust, oil, and dirt. Replace the desiccant6-5
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