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A pipe fitting is defined as a part used in a piping system, for changing direction, branching or for change of pipe diameter, and which is mechanically joined to the system. There are many different types of fittings and they are the same in all sizes and schedules as the pipe.
Buttweld FittingsScheduled 40, 80 are defined in the ASME B16.9 standards. Light-weight corrosion resistant fittings are made to MSS SP43.A piping system using buttweld fittings has many inherent advantages over other forms.
- Welding a fitting to the pipe means it is permanently leak-proof
- The continuous metal structure formed between pipe and fitting adds strength to the system
- Smooth inner surface and gradual directional changes reduce pressure losses and turbulence and minimize the action of corrosion and erosion
- A welded system utilizes a minimum of space
The ends of all buttweld fittings are bevelled, exceeding wall thickness 4 mm for austenitic stainless steel, or 5 mm for ferritic stainless steel. The shape of the bevel depending upon the actual wall thickness. This bevelled ends are needed to be able to make a "Butt weld".
ASME B16.25 covers the preparation of buttwelding ends of piping components to be joined into a piping system by welding. It includes requirements for welding bevels, for external and internal shaping of heavy-wall components, and for preparation of internal ends (including dimensions and dimensional tolerances). These weld edge preparation requirements are also incorporated into the ASME standards (e.g., B16.9, B16.5, B16.34).
Material and Performance
The most common materials used in fittings produced is carbon steel, stainless steel, cast iron, aluminum, copper, glass, rubber, the various types of plastics, etc..
In addition, fittings, like pipes, for specific purposes sometimes internally equipped with layers of materials of a completely different quality as the fitting themselves, which are "lined fittings".
The material of a fitting is basically set during the choice of the pipe, in most cases, a fitting is of the same material as the pipe.
ASTM Material for Fittings
- A234 = This specification covers wrought carbon steel and alloy steel fittings of seamless and welded construction.
- A420 = Standard specification for piping fittings of wrought carbon steel and alloy steel for low-temperature service.
- A403 = Standard specification for wrought austenitic stainless steel piping fittings.
Threaded joints probably represent the oldest method of joining piping systems. Like Socket Weld fittings, threaded fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
The dimensional standards for taper pipe threads are given in ASME B1.20.1. That document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters.
- Threaded piping is commonly used in low-cost, noncritical applications such as domestic water, fire protection, and industrial cooling water systems.
- Threaded fittings are normally made of cast gray or malleable iron, cast brass or bronze, or forged alloy and carbon steel.
- They are available in three pressure ratings: Class 2000, 3000 and 6000.
All fittings on this page are described, are provided with NPT thread, ASME B1.20.1. The American National Pipe Thread Tapered, is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal. NPT has a tapered male and female thread which seals with Teflon® tape or jointing compound.
Advantages and Disadvantages of Threaded Fittings
- Installation productivity is moderately high, and specialized installation skill requirements are not extensive.
- Leakage integrity is good for low-pressure, low-temperature installations where vibration is not encountered.
- Rapid temperature changes may lead to leaks due to differential thermal expansion between the pipe and fittings.
- Vibration can result in fatigue failures of screwed pipe joints due to the high stress intensification effects caused by the sharp notches at the base of the threads.
- In hazardous piping systems threaded connections should be avoided, if possible. Their vulnerability to fatigue damage is significant, especially where exposed threads are subject to corrosion.
Socket Weld Fittings
A Socket Weld is a pipe attachment detail in which a pipe is inserted into a recessed area of a Valve, fitting or flange. In contrast to buttweld fittings, Socket Weld fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
To join pipe to Valves and fittings or to other sections of pipe, fillet-type seal welds be used. Socket-welded Joints construction is a good choice wherever the benefits of high leakage integrity and great structural strength are important design considerations.
Fatigue resistance is lower than that in butt-welded construction due to the use of fillet welds and abrupt fitting geometry, but it is still better than that of most mechanical joining methods.
Some details of Socket Weld fittings
SW Fittings are family of high pressure fittings are used in various industrial processes.
- They are used for lines conveying flammable, toxic or expensive material where no leakage can be permitted, and for steam 300 to 600 PSI.
- They are used only in conjunction with ASME Pipe and are available in the same size range.
- They are used in areas where pipe-work is permanent and are designed to provide good flow characteristics.
- They are are produced to several ASTM standards and are manufactured in accordance with ASME B16.11. The B16.11 standard covers pressure-temperature ratings, dimensions, tolerances, marking, and material requirements for forged carbon and alloy steel. Acceptable material forms are forgings, bars, seamless pipe, and seamless tubes which conform to the fittings chemical requirements, melting practices, and mechanical property requirements of ASTM A105, A182, or A350.
- They are available in three pressure ratings: Class 3000, 6000 and 9000.
Advantages and disadvantages of Socket Weld fittings
- The pipe need not be beveled for weld preparation.
- Temporary tack welding is no needed for alignment, because in principle the fitting ensures proper alignment.
- The weld metal can not penetrate into the bore of the pipe.
- They can be used in place of threaded fittings, so the risk of leakage is much smaller.
- Radiography is not practical on the fillet weld; therefore correct fitting and welding is crucial. The fillet weld may be inspected by surface examination, magnetic particle (MP), or liquid penetrant (PT) examination methods.
- Construction costs are lower than with butt-welded joints due to the lack of exacting fit-up requirements and elimination of special machining for butt weld end preparation.
- The welder should ensure for a expansion gap of 1/16 inch (1.6 mm) between de pipe and the shoulder of the socket.
ASME B31.1 para. 127.3 Preparation for Welding (E) Socket Weld Assembly says:
In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16" (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.
- The expansion gap and internal crevices left in socket-welded systems promotes corrosion and make them less suitable for corrosive or radioactive applications where solids buildup at the joints may cause operating or maintenance problems. Generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.
- Socket welding are unacceptable for UltraHigh Hydrostatic Pressure (UHP) in Food Industry application since they do not permit full penetration and leave overlaps and crevices that are very difficult to clean, creating virtual leaks.
The purpose for the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal, and to allow for differential expansion of the mating elements.
Branch Connection FittingsBranch Connection fittings (also known as O'lets) are fittings which provide an outlet from a larger pipe to a smaller one (or one of the same size). The main pipe onto which the branch connection is welded is usually called the Run or Header size. The pipe to which the branch connection provides a channel is usually called the Branch or Outlet size. Branch connections are in all sizes, types, bores, and classes, in a wide range of stainless steel, chrome-moly, and other alloys.
Types of Branch Connection Fittings