Steel pipe fittings are often sold along with steel pipe. Fitting manufacturers create seamless construction fittings (WPB) and welded construction fittings (WPW). In seamless construction, the steel pipe is heated and molded into its final shape. Special equipment is used to create the necessary bevels and markings. In welded construction, on the other hand, a steel plate is cut and shaped. The two sides are then welded together before they are beveled. Common steel pipe fittings include: standard butt weld fittings, socket weld fittings and threaded fittings.
Standard Butt Weld Fittings
Standard butt weld fittings are required to change direction and connect to all of the other equipment and devices required to make them function. Standard butt weld fittings are designed with varying wall thicknesses and are beveled for welding. Standard butt weld fittings are constructed in both seamless and welded. The most common wall thicknesses are standard and extra heavy. Heavy wall, or special end fittings, are used for high temperature services, such as mainstream nuclear power.
Butt weld fittings allow for direction changes to enable for bigger pipe sizes, branching and ending. They can be assembled onsite and help the pipe system safely transport its content.
Socket Weld Fittings
A socket weld is a pipe attachment detail in which a pipe is inserted into a break area of a valve, fitting or flange. Construction costs are typically lower for socket weld joints than with butt weld joints. Socket weld fittings can also be substituted for threaded fittings and have a smaller risk of leakage. Unlike standard butt weld fittings, socket weld fittings are typically used for small pipe diameters, NPS 2 or smaller. With socket weld fittings, a specially sized pipe is inserted into the recessed area of a socket weld joint. Socket weld fittings are used simultaneously with ASME Pipe and they are available in the same size range. The weld bead joins the inner socket surface with the outer pipe surface. Socket weld fittings are available in class 3000, 6000 and 9000. The fittings available include: elbows, reducers, tees, crosses, laterals, couplings, caps and socket welding reducer inserts.
The main disadvantage of these fittings is they tend to leave the pipe interior with hard to clean surfaces. There are mainly used for lines conveying flammable, toxic, or expensive material where no leaking can happen. As a result, their use in transporting food materials should be avoided.
Threaded Steel Pipe Fittings
Threaded steel pipe is used to transport a variety of gases and liquids. The threaded steel pipe, alone, cannot direct the materials it transports without the help of additional devices, such as threaded fittings. It is used in low-cost applications such as domestic water, fire protection, and industrial cooling water systems. Threaded fittings assist in regulating flow direction, as well as restricting the movement of the materials, both temporarily or permanently.
Threaded fittings are typically designed for use with smaller threaded pipes. Threaded fittings can be made from different materials, so they can be used with a variety of threaded pipe, threaded steel pipe, and threaded PVC pipe. They are usually made of cast gray or malleable iron, cast brass or bronze, forged alloy, and carbon steel.
The term “threaded” refers to the thread pattern, or screw pattern, of the threaded pipe or threaded fitting. These thread patterns are designed based on predetermined specifications. The most common thread pattern is the NPT or the National Pipe Thread. Some other threaded fitting categories available are: couplings, plugs, screw plugs, nipples, hose nipples, crosses, caps, elbows, reducers, nuts, tees, unions, and arches.
The plug or cap seals the end of the pipe, while the coupling joins two pipes. The union is used during installation or maintenance, and the elbow makes varying degrees of changes in direction. The tee and the cross make branches from the main run. And, the bushing can be used to reduce the size of a threaded fitting. Some fittings are offered with galvanized coating.
Advantages and Disadvantages of Threaded Fittings
Threaded joints are among the oldest methods for connecting pipe systems. They are often applied in low-cost, noncritical systems, where vibration and extreme temperatures are unlikely. Specialized installation skills are not required.
Threaded fittings are vulnerable to fatigue damages and, therefore, should not be used in the transportation of hazardous materials. Additionally, leaking may occur as a result of rapid temperature changes.
Both threaded fittings and socket weld fittings are forged fittings. Forged steel pipe fittings are manufactured using a process called forging. In this process, A105 carbon steel is heated and molded into the desired shape. After the completion of the forging process, the molded steel is finished to meet required specifications.
A105, or SA 105 (A/SA 105), is a standard specification for carbon steel forgings in pipe applications. Forgings made to this specification have weight restrictions (maximum weight of 10,000 pounds).