Joining pipe

Piping refers to the overall network of pipes, fittings, flanges, valves, and other components that comprise a conduit system to convey liquids. Whether a piping system is used to convey fluids from one point to another or to process and condition the fluid, piping connections serve an important role in the operation of the system.

10/01/2001


Sections:
Threaded
Welded
Soldered
Compression
Crimped or grooved
Cemented
Sidebars:
Types of pipe joints

Piping refers to the overall network of pipes, fittings, flanges, valves, and other components that comprise a conduit system to convey liquids. Whether a piping system is used to convey fluids from one point to another or to process and condition the fluid, piping connections serve an important role in the operation of the system.

Pipes can conduct fluid under gravity, vacuum, low-pressure, or high pressure in a process, pneumatic, or hydraulic system. Depending on the application, pipe connections can take a variety of forms.

Pipe and fittings are produced in a wide range of materials including; ductile or cast iron, malleable iron, brass, copper, cast steel, plastic, and fiberglass.

Pipe joints can be permanent or mechanically joined, allowing disconnection. Permanent joints involve welding or crimping metal pipe and cementing plastic pipe. Joints that can be taken apart include threaded, flanged, and coupled designs. Flanges can be loose, threaded, or welded to pipe ends and are used on metal and plastic pipe.

Threaded


Threading is one of the most popular and least expensive methods of joining steel pipe. The tapered male fitting is forced into the tapered female fitting. Yielding metal creates a seal. Threads other than taper pipe threads can be used for piping connections where tightness of the joint depends on a seal weld or seating surface other than the threads. While threaded joints can be dismantled, it is preferable to use unions. Threaded pipe is available in sizes from 1/16-24-in. OD.

Welded


Welded joints are commonly used with steel pipe because these joints are stronger and less prone to leakage than threaded and flanged joints. Also, this method does not add weight to the piping system as flanges do or require a pipe wall thick enough to be threaded.

Pipe up to 2-in. size is generally socket-fitting, fillet welded. Larger pipe, 3-36 in., is usually butt welded. The most common joint is the circumferential butt joint. During the welding procedure, to avoid entrance of welding material into the pipe, backing rings may be used.

Soldered


A soldered joint is a rigid, pressure-type joint made with a filler metal that, when heated to its melting point, is drawn into the annular space between pipe and fitting by capillary action. This type of joint is generally limited to pipe up to 8 in. because of the difficulty of applying heat evenly to the joint.

Brazing is similar to soldering except higher heat is required for the filler metal. It is used where higher pressure ratings are required.

Compression


Compression couplings usually can be used with all types of pipe and do not require any pipe preparation. They consist of an inner elastomeric gasket and an outer metallic sleeve with integral bolts for compressing the gasket. They are available for pipe up to 144-in. OD

Crimped or grooved


The use of mechanical joints in piping systems is becoming popular. A crimped joint is designed to join light-wall steel and copper pipe up to 2-in. OD. The grooved joint is designed for joining any type of pipe, metallic or nonmetallic, that is capable of being cut or roll grooved, up to 42-in. OD.

Cemented


Solvent joining can be used on vinyl pipe, ABS, PVC, and CPVC, available up to 12-in. OD. Polyolefin, PE, PP, PB, and PVDF pipes require heat fusion and are available up to 6-in. OD. Fiberglass pipe requires an adhesive, resin impregnated fabric, or threads to make a joint. It is readily available in sizes from 1/2-144-in. OD.

Pl ant Engineering magazine extends its appreciation to McGraw-Hill for providing the 7 th Edition of Piping Handbook (ISBN 0-07-047106-1) as a reference in the preparation of this article.



Types of pipe joints

Permanent

Welded

Soldered

Crimped or grooved

Cemented

Detachable

Threaded

Compression



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