Welding is the simplest and easiest way to join sections of pipe. The need for complicated joint designs and special threading equipment is eliminated. Welded pipe has reduced flow restrictions compared to me-chanical connections and the overall installation costs are less. The most popular method for welding pipe is the shielded metal-arc process; however, gas shielded arc methods have made big inroads as a result of new advances in welding technology.
Pipe welding has become recognized as a profession in itself. Even though many of the skills are comparable to other types of welding, pipe welders develop skills that are unique only to pipe welding. Because of the hazardous materials that most pipelines carry, pipe welders are required to pass specific tests before they can be certified.
In the following paragraphs, pipe welding positions, pipe welding procedures, definitions, and related information are discussed.
PIPE WELDING POSITIONS
You may recall from and earlier lesson that there are four positions used in pipe welding. They are known as the
PIPE WELDING PROCEDURES
Welds that you cannot make in a single pass should be made in interlocked multiple layers, not less than one layer for each 1/8 inch of pipe thickness. Deposit each layer with a weaving or oscillating motion. To prevent entrapping slag in the weld metal, you should clean each layer thoroughly before depositing the next layer.
Butt joints are commonly used between pipes and between pipes and welded fittings. They are also used for butt welding of flanges and welding stubs. In making a butt joint, place two pieces of pipe end to end, align them, and then weld them. (See fig. 7-42.) When the wall thickness of the pipe is 3/4 inch or less, you can use either the single V or single U type of butt joint; however, when the wall thickness is more than 3/4 inch, only the single U type should be used.
Fillet welds are used for welding slip-on and threaded flanges to pipe. Depending on the flange and type of service, fillet welds may be required on both sides of the flange or in combination with a bevel weld (fig. 7-43). Fillet welds are also used in welding screw or socket couplings to pipe, using a single fillet weld (fig. 7-42). Sometimes flanges require alignment. Figure 7-44 shows one type of flange square and its use in vertical and horizontal alignment.
Another form of fillet weld used in pipe fitting is a seal weld A seal weld is used primarily to obtain tight-ness and prevent leakage. Seal welds should not be considered as adding strength to the joint.
JOINT PREPARATION AND FIT-UP
You must carefully prepare pipe joints for welding if you want good results. Clean the weld edges or surfaces of all loose scale, slag, rust, paint, oil, and other foreign matter. Ensure that the joint surfaces are smooth and uniform. Remove the slag from flame-cut edges; however, it is not necessary to remove the temper color.
When you prepare joints for welding, remember that bevels must be cut accurately. Bevels can be made by machining, grinding, or using a gas cutting torch. In fieldwork, the welding operator usually must make the bevel cuts with a gas torch. When you are beveling, cut away as little metal as possible to allow for complete fusion and penetration. Proper beveling reduces the amount of filler metal required which, in turn, reduces time and expense. In addition, it also means less strain in the weld and a better job of design and welding.
Align the piping before welding and maintain it in alignment during the welding operation. The maximum alignment tolerance is 20 percent of the pipe thickness. To ensure proper initial alignment, you should use clamps or jigs as holding devices. Apiece of angle iron makes a good jig for a small-diameter pipe (fig. 7-45), while a section of channel or I-beam is more suitable for larger diameter pipe.
Make certain the welding current is within the range recommended by the manufacturers of the welding machines and electrodes.
At temperatures between 0°F and 32°F, heat the weld area within 3 inches of the joint with a torch to a temperature warm to the hand before beginning to weld.
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy
Copyright © 2001-2006 SweetHaven Publishing Services