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DISTRIBUTION SYSTEMS The development of a distribution system is dependent upon a combination of factors, such as location and size of each service, time rate demand of larger services, and concurrence or demand factor of larger services. TYPES OF AIR DISTRIBUTION PIPING SYSTEMS The more common types of distribution systems or patterns (fig. 11-25) and their prime advantages are as follows: . Radial, one-way system-used for isolated or individual service or where special requirements dictate a single path. . Loop system-used for a closed route such as throughout a building. The two-directional flow capacity represents an economical way to provide constant pressure to all services and permits selective isolation when necessary. l Parallel system-used to provide dual service source to ensure at least one source will be available at all times. SIZING DISTRIBUTION SYSTEMS Compressed air distribution systems are sized mainly by calculating the friction loss to be expected from piping, fittings, and valves as well as various accessories you may install. Pipe diameters are determined from commercially available products, such as copper, stainless steel tubing, or steel piping. As contained pressure increases, the pipe wall thickness must increase and interior diameters decrease. This affects friction pressure loss; it should not exceed 15-percent pressure loss. When you are determining total friction loss for a distribution system, the total length of the system piping plus the equivalent length of each fitting, valve, or device is summed to produce an equivalent hydraulic length. The equivalent lengths of fittings, valves, and other devices can be determined from table 11-2. Friction loss in air hoses may be taken from table 11-3. LAYOUT DETAILS When installing compressed air systems, you must follow seven basic guidelines just as you must consider basic guidelines when installing any other type of piping or drainage system. Compressed air lines should be installed as level as practical with a slight pitch in the direction of airflow. This pitch is generally placed at 3 inches per 100 feet of piping. In cases when pipes must be pitched upward causing condensate to flow against the flow of air, the pitch upward must be 6 inches or greater per 100 feet, and the piping size should be increased one pipe diameter. The layout of the piping systems should always allow for the removal of dirt, water, oil, or other foreign material, which can accumulate over long periods of time. Because of this, pockets should be avoided and, where necessary, low points should be provided with driplegs. In addition to providing low points to drain foreign material from the system, the prevention of carry-over of this material into branch lines is necessary. Carryover into branch lines can be prevented by making connections from the top of the distribution mains. Piping must be placed with sufficient flexibility to prevent excessive strain or distortion caused by thermal expansion or sudden changes in pressure. By properly placing pipe supports,
Table 11-2.-Representative Equivalent Length in Pipe Diameters (L/D) of Various Valves and Fittings
Table 11-3.-Loss of Air Pressure in Hose Caused by Friction
as shown in table 11-4, movement of pipe can be accounted for. In addition, piping should be supported at all changes in direction and load concentrations, such as heavy valves. There are many other considerations in the layout of compressed air systems, which are beyond the scope of this manual. Refer to NAV-FAC DM 3-5, Compressed Air and Vacuum Systems, for further information. TEST PROCEDURES After installation, the compressed air system must undergo testing. Generally, all piping and pressurized components should be tested at 150 percent of maximum working pressure. When testing, use clean, dry air or nitrogen. The system should be held at test pressure without loss for at least 4 hours. |
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