Understanding Pipe Fittings
Pipe fittings are components used to join pipe sections together with other fluid control products like valves and pumps to create pipelines. The common connotation for the term fittings is associated with the ones used for metal and plastic pipes which carry fluids. There are also other forms of malleable iron pipe fitting that can be used to connect pipes for handrails and other architectural elements, where providing a leak-proof connection is not a requirement. Pipe fittings may be welded or threaded, mechanically joined, or chemically adhered, to name the most common mechanisms, depending on the material of the pipe.
There is some inconsistency in terminology surrounding the terms pipe, tube, and tubing. Therefore, the term Carbon Steel Pipe Fitting will sometimes be mentioned in the context of tubing as well as pipe. While similar in shape to tube fittings, pipe fittings are seldom joined by methods such as soldering. Some methods overlap, such as the use of compression fittings, but where these are commonplace for connecting tubes or tubing, their use in pipe connections is rarer. It suffices to say that while there are general distinctions, the common usage of terms can differ from supplier-to-supplier, although they represent the same items.
In this article, the concentration will be on discussing typical fittings and connection methods associated with rigid pipe and piping, with a limited presentation of the fittings that are associated with flexible tubes, tubing, or hose.
To learn more about the varieties of pipe, consult our related guide to pipe and piping.
Pipe Fittings Explained: Fitting Materials and Manufacturing Processes
Cast and malleable iron
Fittings for cast iron pipe fall under hubless and bell-and-spigot styles. Hubless designs rely on elastomeric couplers that are secured to the outer diameters of the pipe or fitting by clamps, usually a stainless steel band clamp that compresses the elastomeric material and forms a seal. These hubless or no hub designs are sometimes referred to as rubber pipe couplings or rubber plumbing couplings and are especially popular for transitioning from one material to another—from copper to cast iron, for instance. Bell-and-spigot, or sometimes, hub-and-spigot, fittings are joined today primarily with elastomeric gaskets that fit inside the bell and accommodate the insertion of the plain pipe end or fitting. Older systems before the 1950s were caulked using a combination of molten lead and a fibrous material such as oakum. Cast iron pipe is sometimes joined with bolted flanges, or in some cases, mechanical compression connections. Flanged joints employed in underground applications can subject the pipe to settlement stresses unless the pipe is adequately supported.
While there are both malleable iron pipe fittings and ductile iron pipe fittings available, the improved mechanical properties and lower cost of ductile iron is causing a shift towards greater use of that material.
Fittings for steel (aka, “black pipe”) and galvanized pipe as found in residential and commercial plumbing work are generally cast and referred to as “malleable iron fittings.» They can be galvanized. Although standards list threaded fittings up to fairly large diameters, these generally are not used today as the threading of large-diameter pipe is considered needlessly difficult.
Steel and steel alloys
Galvanized malleable iron pipe fittings are often extruded or drawn over a mandrel from welded or seamless pipe. In smaller sizes they are often threaded to match threads on the ends of pipe. As sizes and pressures increase, they are often welded in place by either butt-weld or socket-weld methods. Socket-weld fittings, usually forged, are restricted to smaller pipe diameters (up to NPS 4, but usually NPS 2 or smaller) and are available in 3000, 6000, and 9000 class pressure ratings, corresponding to Schedule 40, 80, and 160 pipe. Socket fittings are welded into place with fillet welds, which makes them weaker than butt- welded fittings, but still preferable to threaded fittings for high-end work. The need for an expansion gap in the fitting precludes their use in high-pressure food applications.
Flanges are also used, with the resulting flanged sections of pipe connected via bolts. The use of flanges makes breaking the pipeline feasible so as to enable replacement of valves, etc. Most pipeline equipment such as pumps and compressors are also connected via flanges for this same reason.
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