What are Butt Weld (BW) Fittings
Pipe fittings are forged or fabricated pieces of piping components that are used for various purposes such as making changes of direction (elbow), branching from a main pipe (tee), or making a reduction in line size (reducer). There are several ways in which these fittings can be joined to each other and to the pipe. When the fittings are butt welded to each other or to the pipe they are termed as butt weld (BW) fittings. There are other types of fittings such as socket welded or threaded fittings which are explained in other articles.
Butt weld fittings are most commonly used to connect piping components. They offer the most practicable way to join pipes to fittings as the butt weld joints are reliable and leakproof when welded as per the specifications. Butt weld fittings allow a high-integrity, full-penetration, circumferential butt-weld to be completed.
How are Butt weld Joints made
The end of the pipe is beveled as shown in figure below. Fittings are similarly supplied with beveled ends by the manufacturer. Butt weld fittings have beveled ends (BE) configured as per ASME B16.25. In piping material specifications the fittings are specified with equivalent material and same wall thickness as the matching pipe. The two parts are aligned with a proper gap, usually 1.6mm (1/16") root gap. The two parts are tack welded and then a continuous weld is made to complete the joint. The first pass of weld is called the root weld. Butt weld fittings must not be supplied with lower wall thickness than the run pipe as this can affect the integrity of the weld joint. When pipes and fittings are supplied with different wall thickness, one of the components must be machined or ground internally to align the butt joints. The internal machining will reduce the wall thickness and the pipe wall thickness at the weld joint may not be suitable for the design conditions. Hence, the designer must pay close attention to this requirement.
Sketch below shows typical piping assembly with pipes and fittings.
Industry Standards and Specifications
Standard butt weld fittings include elbows, equal tees, reducing tees, concentric reducers, eccentric reducers which are covered in standard ASME B16.9 - Factory-made wrought buttwelding fittings. ASME B16.9 standard covers overall dimensions, tolerances, ratings, testing and markings for wrought factory-made buttwelding fittings in sizes NPS 1/2 through 48 (DN 15 through 1200). There are other fitting types covered in ASME B16.9 - such as reducing elbows, 180 degree returns, crosses which do not find very wide application. Fittings covered under this standard are supplied with butt weld ends to ASME B16.25.
In the oil and gas and petrochemical industry, most companies follow the project piping material specifications that establishes the connection requirements. These specifications typically specify pipes and fittings 2 inches and larger to be fabricated with butt-welded connections and pipes and fittings smaller than 2 inches to have screwed or socket-weld connections. Figure below shows the various types of butt weld fittings covered in ASME B16.9.
Elbow Fittings
Elbows are used when a pipe changes direction and can turn up, down, left, right, or any angle in between. Elbows are available as long radius and short radius elbows.
Long-Radius 90 Degree and 45 Degree Elbows
Long Radius 90 Degree elbows are the most frequently used fittings. 90 Degree elbows can be classified as long-radius elbows, short-radius elbows, reducing elbows, or mitered elbows, with the long-radius elbows being the most commonly used type.
The length of an elbow is defined by the centerline to the end of fitting dimension. For butt-weld 90 degree elbows, there is a direct relationship between the nominal size and the length of the elbow. The elbow length is calculated as the nominal pipe size multiplied by 1.5 as per formula below:
Nominal pipe size × 1.5 = Elbow length
For example, an 8-inch elbow would have a length of 8" × 1½ = 12".
The 45-degree elbow is another important fitting used to make directional changes within the piping system. Since the 45-degree elbow is one-half of a 90-degree elbow, it is obviously shorter. The 45-degree elbow is normally used to make an offset in the piping. It is not the preferred fitting to use for change in direction. However its use becomes inevitable to achieve certain layout configurations.
Short-Radius 90 Degree and 45 Degree Elbows
The 90-degree short-radius elbow may be used under certain circumstances and with owners permission. The short-radius elbow makes a sharper turn than the long-radius elbow. It also creates a larger pressure drop inside the line and does not have the smooth flow characteristics of the long-radius elbow. For these reasons, the short-radius elbow is not used often in the industry. It may be used in specific instances where layout constraints do not permit a long radius elbow to be installed.
The center-to-end dimension for a 90-degree short-radius elbow can be calculated using the simple formula below:
Nominal pipe size × 1 = Elbow length.
For example, an 8-inch elbow would have a length of 8" × 1 = 8". When a short-radius elbow is used, the abbreviated note "S.R." must always be placed adjacent to the drawing symbol.
Mitered Elbows
The mitered elbow is not an actual fitting but rather an elbow fabricated usually in the fabricators shop. This elbow is made by making angular cuts in a straight run of pipe and subsequently welding the cut pieces together. Mitered elbows may be classified as two, three, or four weld miters, with the number of welds depending on the smoothness of flow required through the turn. Mitered elbows are
Tee Fittings
Weld Tee
The weld tee gets its name from its resemblance to the letter T. It is a three-way fitting used to make branch connections to a pipe at 90 degree to the run pipe. Lines that connect to the main run of pipe are known as branches, while the main run of pipe is called the header or run pipe.
The weld tee requires three welds to be made for installation. Two types of tees are used in the piping industry:
- Straight or Equal tee - all three outlets are the same pipe size
- Reducing tee - branch outlet is a smaller pipe size
A reducing tee is identified by specifying the header and branch sizes, with the header size (the larger size) shown first.
Stub-In
An alternative method of making a branch connection is called a stub-in. The stub-in is most commonly used as an alternative to the reducing tee and is not an actual fitting but rather a description of how the branch connection is made. A hole is bored into the header pipe, either the size of the OD or ID of the branch, and the branch is then stubbed into it. The two pipes are fitted together and then welded.
Although the branch connection can be the same pipe size or smaller than the header, it cannot be larger. The chief advantage of the stub-in over the tee is cost - not only can the cost of purchasing a fitting be avoided, but the stub-in requires only one weld whereas the tee requires three. The stub-in type connection is not normally used in the oil and gas industry.
A general rule for stub-in placement is to allow a minimum of 3 inches between welds. This means a minimum of 3 inches should be allowed between the outsides of branches made from a common header, and a header should be attached no closer than 3 inches to a fitting.
Reducer Fittings
When the piping designer has to reduce the diameter of a straight run of pipe, a reducing fitting must be used. The reducer is available in two styles namely concentric and eccentric reducer.
Concentric Reducer
A concentric reducer has a common centerline and maintains the same centerline at both the large and small ends of the fitting. When a concentric reducer is installed in horizontal piping, the centerline of the pipe remains unchanged. Reducers are frequently used at pump suction and discharge piping where the pump nozzles are smaller in size than the suction and discharge piping. When there is a large reduction in pipe diameter one reducer may not be sufficient and two reducers have to be used together. For example, if a reduction in line size from 12 inches to 4" is required, you will have to use a combination of two reducers as one reducer for this size reduction is not available in ASME B16.9. Accordingly you may use a combination of 12"x8" and 8"x 4" reducer to make this size reduction.
Eccentric Reducer
An eccentric reducer has offset centerlines that will maintain a flat side on the top or the bottom of the fitting. The eccentric reducer is used in pipe racks to maintain a constant bottom of pipe (BOP) and on pump suction nozzles with flat side on the top to keep entrained air from entering the pump. When the designer routes piping on the rack, it is preferable to use a eccentric reducer to maintain the same bottom of pipe.
The dimensional difference between the two centerlines of an eccentric reducer must be included when calculating the elevations of pipe in a pipe rack. The formula for calculating this difference is:
Offset = (Large OD - Small OD) / 2
Weld Cap
The weld cap is used to seal an open end of pipe. When dimensioning the positional location of a weld cap on a drawing, only the length of the run of pipe should be indicated. The cap will be welded to the end and need not be included in the length dimension of the run of pipe.
Dimensioning Guidelines
The general rules for placing dimensions on piping drawings are:
- Pipe should be dimensioned from center of fitting to center of fitting, or
- Pipe should be dimensioned from center of fitting to the end of pipe
All welds must be shown on drawings, using weld dots on single-line pipe symbols and weld lines on double-line pipe symbols. While welds may seem insignificant to the designer, a piping facility could not be built without them.
Conclusion
Butt weld fittings are essential for laying out piping systems, providing the necessary branch connections, directional changes, and size transitions required for efficient fluid transport. Understanding the proper selection, dimensioning, and installation of these fittings is crucial for successful piping system design and construction. The standardization of fitting dimensions and installation practices ensures consistency and reliability across the industry while maintaining safety and operational efficiency.