- Published on Monday, 03 December 2012 09:27
- Written by Greg Johnson
To be of value, valves must be attached to a piping system. How that attachment occurs has changed over time. For decades, screwed or flanged connections were the only way to go. However, the perfection of fusion welding techniques in the 1930s led the way for new methods of joining piping components, including valves. These new ways are the butt-weld (BWE) (Figure 1) and socket-weld (SW) (Figure 2) end connections.
Both of these joining techniques create a homogenous entity composed of the two components to be joined and filler metal. Since there is no gasketed connection, the chance of joint leakage is virtually nil as long as the welding is performed correctly and the proper welding procedure is used. These welded joints are much preferred for high-temperature applications such as steam or critical refinery processes.
These two types of welded connections have different joining philosophies. The BWE connection is completed by performing what is called a “full penetration weld” to connect the two components. The SW end connection uses what is called a “fillet weld” to connect the two components. Although both joints create a potentially leak-free attachment, the BWE is a higher integrity joint than the SW.
Why is the BWE better? The butt-weld joint has greater strength because the full area of the pipe (inside diameter to outside diameter or ID to OD) is completely filled with weld deposit, and the filler material becomes one with both joined components. Meanwhile, the fillet-weld is a much smaller, triangle-shaped weldment that connects the outside of the pipe with the slightly larger outside and end of the socket through which it is slipped. Another disadvantage for the SW joint is that it retains a void between the inserted pipe and the socket, which can retain fluid and act as an inception point for corrosion.
While the BWE is a higher integrity joint, it is also more difficult and costly to field weld on pipe connections less than NPS 2, which are commonly joined via socket-type welds. However, some piping designers choose the BWE on small-diameter pipe connections in highly critical or lethal service applications. Where fugitive emissions are an overriding concern, the combination of a welded-bonnet bellows-sealed valve with butt-weld ends provides the ultimate protection.
The fillet-welding process is straightforward and relatively easy for a qualified welder to perform. A pipe is slipped into the socket of a SW end valve or fitting and a scratch mark is made around the pipe after it reaches the bottom of the socket. The pipe is then retracted about 1/16th of an inch, to allow for expansion of the pipe as it is heated up. The fillet weld is then applied with either the gas tungsten arc (GTAW), also called the “TIG” process or the shielded metal arc (SMAW) also called the “stick” process.
The full-penetration butt weld is more difficult to learn and to perform. The pipes or pipe and valve are aligned ID to ID and OD to OD, with a tolerance of 1/16 to 1/8 of an inch, depending on pipe size. Next, a tack weld procedure is used to deposit several small dots of weld around the periphery of the joint. After that, a small layer of weld metal called a root pass is deposited at the bottom of the groove all the way around, in a manner that ensures the weld melts into and through both components being joined. After the root pass is complete and perfect, final cover passes of weld material are added to fill up the groove area.
THE RIGHT DIMENSIONS
The dimensions of SW sockets and butt-weld ends have to be precise and within tolerance. In the case of the SW, they should be machined in accordance with American Society of Mechanical Engineers (ASME) B16.11, Forged Fittings, Socket-Welding and Threaded. However, recently occasional batches of imported pipe have been noted as exhibiting an outside dimension that is out of tolerance and will not easily slip into a correctly machined socket.
Butt-weld end dimensions are detailed in two primary documents. For compact valves, American Petroleum Institute (API) 602, Steel Gate, Globe & Check Valves for Sizes NPS 4 and Smaller for Use in the Petroleum and Natural Gas Industries, has a table and drawing of the correct BWE design. For larger valves and other non-API 602 valves, ASME B16.25, Buttwelding Ends is the governing document for BWE dimensions and tolerances.
One area of concern with cast valves is the integrity of the welding area that will mate with other piping components. This beveled area must be sound and free of severe gas pockets or shrinkage that could create quality issues, which would affect the welding to be performed above and adjacent to the weld area. Oftentimes these weldments are radiographed, and many cases have been found where porosity in the valve was interpreted as bad welding by the welder after the radiograph showed defects in these areas. Bad indications require the time-consuming task of grinding out the defect, repairing the weld and performing additional non-destructive evaluation. This is the reason many construction codes require that BWE areas be radiographed prior to welding.
Another issue related to BWE integrity on cast valves is the procedure where the flanges of cast raised-face valves are cut off and then machined to a beveled BWE for joining by welding. Because of the location of the cutting and machining, an area highly prone to casting defects is now exposed or hidden slightly below the surface. While these defects may be satisfactory on a cast valve with flanges intact, they are unacceptable on butt-weld ends that will be exposed to the thermal welding process. If flanged cast valves are to be modified to BWE, it is important that the ends be radiographed prior to any welding.
Generally, SW ends are not radiographed, but if non-destructive examination is required, either the magnetic particle (MT) or dye penetrant (PT) examination process usually is employed.
One area where welding ends are exclusively used is underground and underwater in the pipeline industry, where the highest integrity joints are mandated for these hard-to-observe pipe connections. This requirement applies to pipeline valves as well. However, one concern in field-welding large-diameter, elastomer-seated valves is that the delicate seating surfaces can be damaged. As a result, most pipeline companies prefer their valves have several feet of pipe, called “pups,” welded onto each end of the valve before it reaches the job site. This makes the ultimate field-welding process less harmful for the valve.
The welding of pipe pups onto the ends of smaller elastomer-seated ball valves is also commonly performed. This allows the potentially damaging high-temperature welding operation to be performed in a controlled shop environment, where the soft seats can be kept cool while the pups are welded into the valve ends or welded into disassembled valves prior to final testing.
The obvious disadvantage of welded-in valves is that they are not easy to remove from the line for replacement or repair. This means that costly field service repairs may be necessary if the valves require renovation.
For this reason, while welded-in valves are very useful tools in the piping designer’s tool-chest, they still will be joined for a long time to come with the old standbys, the screwed and flanged end valves.