Pipe Fittings
Roy A. Parisher, Robert A. Rhea, in Pipe Drafting and Design (Third Edition), 2012
Screwed and Socket-Weld Fittings
Screwed and socket-weld fittings perform the same basic functions as butt-weld fittings. Like butt-weld fittings, elbows, tees, and reducers are manufactured for screwed and socket-weld applications. There are, however, a few differences that must be examined. Screwed and socket-weld fittings are normally reserved for installations where the nominal pipe size is 3″ and smaller. Screwed and socket-weld fittings are also available in cast iron, malleable iron, or forged steel. Typically, forged steel fittings are used on high pressure and temperature lines. However, low pressure and temperature lines, such as air, water, or condensate, are fabricated using either cast or malleable iron fittings.
Pipe lines containing high pressure and temperature commodities, which are subject to substantial amounts of movement and vibration, mandate fittings made of forged steel. For these reasons, forged steel screwed and socket-weld fittings are manufactured in two pressure classes—3000# and 6000#. The sizing charts, shown in Appendix A, provide the dimensional measurements for 3000# and 6000# screwed and socket-weld fittings. Figures 3.54 and 3.55 display a portion of the screwed and socket-weld fitting dimensioning charts found in Appendix A.
Most screwed fittings are manufactured with internal, or female, threads as defined by the American Standard and API thread guidelines. As shown in Figure 3.56, of particular concern to the pipe designer is the amount of pipe length lost during the assembly of screwed fitting configurations. When screwed fittings and threaded pipe are assembled, a certain amount of pipe length is lost as a result of the internal and external, or male, thread connecting process. Each time a threaded connection, or engagement, is made, the configuration gets shorter. The length of this engagement varies depending upon the nominal pipe size and pound rating of the fitting. The procedure to determine the actual center-to-center length of a threaded configuration is to subtract the total length of all the thread engagements from the total unassembled length of pipe and fittings. The unassembled length can be thought of as all the pieces, both fittings and pipe, being laid out end to end. From this unassembled length, the total of all the thread engagements is then subtracted to determine the total assembled length. The formula below applies the values shown in Figure 3.56 to calculate the assembled length.
Some fittings, such as plugs and swages, however, are manufactured with external threads and their assembled lengths are treated differently, as will be explained later.
The socket-weld fitting has become the fitting of choice for many fabricators because it offers greater strength at each point of connection. Even though screwed fittings can be seal-welded if necessary, strength of the fitting is decreased when the threads are cut during the manufacturing process. Socket-weld fittings can be easily fitted and welded without the need of special clamps or tack-welds, which are often required to hold a fitting in place before the final weld is made. Like screwed fitting configurations, during the assembly of socket-weld configurations, there is pipe length loss. This lost length is equal to the depth of the socket, or socket depth, and must be accounted for when calculating overall lengths of pipe runs. However, there is a slight difference from screwed pipe assemblies. On socket-weld connections, a gap is factored into each socket-weld connection. Figure 3.57 provides a sectional view of two socket-weld elbows and the connecting pipe. Notice two socket depths must be subtracted from the total unassembled length of the two elbows and the piece of pipe, then ⅝″ is added back to account for the two gaps, before an assembled configuration length can be determined. If a formula were applied to calculate the assembled length using the values shown in Figure 3.57, it would look like
Fittings
Like butt-weld fittings, screwed and socket-weld fittings are used to make similar routings in the piping system, but only in smaller pipe sizes. Screwed and socket-weld fittings differ in size and shape, but they achieve the same purpose as butt-weld fittings. However, there are some differences. Ninety degree elbows are not available as long-radius or short-radius, and their center-to-end dimension must be found on a dimensioning chart, as no formula is available for calculating their radius length. Figure 3.58 provides examples of some screwed and socket-weld fittings.
Screwed and socket-weld fittings are represented differently on drawings than their butt-weld counterparts. For example, screwed and socket-weld elbows are drawn with square corners, using short hash marks to represent the connection points of the fitting and its mating pipe. Some engineering companies even draw short ears on the hash marks to indicate a difference between screwed and socket-weld symbols (see Figure 3.59).
There are, however, some fittings that are unique to the screwed and socket-weld family of fittings. These fittings do not lend themselves to butt-weld applications and are manufactured solely for use in screwed and socket-weld configurations. A brief discussion of those is as follows.
Unions
The union, shown in Figure 3.60, is a fitting placed within a piping configuration that will allow the assembly to be disassembled for inspection, repair, or replacement. Manufactured for screwed and socket-weld applications, the union is represented on drawings as shown in Figure 3.61. Unions should be positioned in locations that will facilitate the easy removal of critical pieces of equipment. Figure 3.62 shows how unions are placed in a configuration to allow easy removal of a valve.
Plug
The plug, like a cap, is designed to seal the end of a run of pipe. Plugs are manufactured for screwed fittings with male threads and are screwed into the end of a pipe to create a seal. Figure 3.63 shows the drawing symbols for the plug.
Coupling
Although this fitting is used in butt-welding applications as a branch connection, its primary use is to connect lengths of screwed and socket-weld pipe together. Some clients may stipulate, however, that all socket-weld pipe must be connected with a butt-weld rather than a coupling.