Machinery's Handbook, 31st Edition
Metal Pipe 2763 Pipe Size (NPS) system. ISO has designated metric equivalent pipes for customary US pipe sizes. As with US pipe, metric pipe dimensions often differ from the nominal size designations. Sizes of ductile iron pipe follow the Ductile Iron Pipe Sizes (DIPS) standard. These sizes and dimensions can be found in Table 8. Flanged pipes are normally furnished with a maximum length of 19’ - 6”. The minimum length varies with pipe size. The minimum class thickness for ductile iron flanged pipe to be threaded is Special Thickness Class 53 for sizes up through 54 and pressure class 350 for 60 and 64 sizes. Greater pipe wall thick nesses are available. Pressure Ratings: When working with ductile iron pipe applications, the designer should choose the appropriate class of pipe to carry the anticipated working pressure. When working with other types of metal pipe, wall thickness must be selected or calcu lated based on working pressure. ANSI/ASME Standard B31.1-1998: “Power Piping” details a method of calculating the minimum wall thickness for pipes. The maximum working pressure for a pipe is often provided by the manufacturer, and can be calculated using US standard units in the following equation: p max 2 S 0.875 t A – ( ) D 2 Y 0.875 t A – ( ) – = ------------------------ where S is the allowable tensile stress in psi, t is the wall thickness in inches, D is the outer diameter in inches, A is an allowance for corrosion and threading, and Y is a temperature factor given in Table 9. A factor of safety should be used when selecting allowable stress. Allowance A is added to compensate for corrosion, erosion, threads, and bends, which thin the walls of the pipe. Allowance A = 0.05 for threaded steel up to 3/8” OD and for unthreaded steel up to ½ OD. A = 0.065 for unthreaded steel larger than 3/8” OD. For threaded steel larger than 3/8” OD, A is equal to the thread depth. Wall thickness based on pressure requirement can be calculated using the following equation and Table 9. This is for straight pipe, and wall thickness tolerances should be taken into account. t pD o A + = where D o is the outside diameter of the pipe, p is internal pressure, Y is the temperature coefficient given in Table 9, A is the allowance discussed above, and S is the maximum allowable stress in the material. S should be modified to account for welds and joints ac - cording to the standards, and a factor of safety should be used. The temperature at which material properties are given must be considered. Many hy- draulic systems run at a temperature of approximately 100°F (38°C). A factor of safety of 3 to 4 should be used when calculating pressure ratings. If the tube is carrying liquid, the pressure rating must be derated to account for the pressure spikes that occur during valve closures. Derating for pressure spikes (water hammer) is often 20 percent. Derating to ac- count for fittings is usually also done, with 25 percent additional reduction a typical value. Connections: There are many types of connections for metal pipe, the most common of which are threaded, flanged, welded, and soldered. Threaded connections on pipes normally use tapered threads per the standard for either American National Pipe Thread (NPT) or British Standard Pipe Thread (BSP). Metric pipe uses ISO standard pipe threads. These threads are detailed in PIPE AND HOSE THREADS starting on page 2089. Sealing is accomplished through thread interlocking and the use of a sealing compound or PTFE tape. The compound or tape lubricates the threads and ensures that spiral leak paths through the threads are blocked. Tape must be used in just the right amount to be effective. Too much tape can cause the joint to fail. Tape should not be used on bull plugs on heat exchangers. Tape is available in several 2 S pY + ( ) ------------
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