Pipe welding involves the welding of pipe for the handling of oil, gases, water, and other substances, the operating conditions of which range from high vacuum to pressure of several thousand pounds per square inch. Mechanical joints are not satisfactory for many of these services. Electric arc or oxyacetylene welding provide effective joints in these services and also reduce weight, increase the strength, and lower the cost of pipe installations.
Preparation for Pipe Welding
a. Pipe Beveled by Manufacturer. Pipe to be welded is usually supplied with a single V bevel of 32-1/2 degrees with a 1/16-in. (1.6-mm) root face for pipe thicknesses up to 3/4 in. (19.1 mm). A single U groove is used for heavier pipe. If the pipe has not been properly beveled or has been cut in the field, it must be beveled prior to welding.
b. Cutting of Pipe. This operation is necessary when pipe must be cut to suit a specific length requirement. To ensure a leak proof welded joint, the pipe must be cut in a true circle in a plane perpendicular to the center line of the pipe. This may be accomplished by using a strip of heavy paper, cardboard, leather belting, or sheet gasket material with a straight edge longer than the circumference of the pipe to be welded. The material is wrapped around the pipe and overlapped and the pipe marked along the edge of the material with a soapstone pencil. Pipe with a wall thickness exceeding 1/8 in. (3.2 mm) should be cut first with a straight cut, then beveled with a hand torch to a 30 to 35 degree angle, leaving a shoulder of approximately 1/8 in. (3.2 mm).
c. Cleaning of Pipe. After beveling, remove all rust, dirt, scale, or other foreign matter from the outside of the pipe in the vicinity of the weld with a file, wire brush, grinding disk, or other type of abrasive. If the bevels are made by oxyacetylene cutting, the oxide formed must be entirely removed. The inside of the pipe in the vicinity of the weld may be cleaned by a boiler tube and flue cleaner, by sandblasting, by tapping with a hammer with an air blast followup, or by any other suitable method, depending on the inside diameter of the pipe. Care must be taken to clean the scarf faces thoroughly.
d. Aligning the Joint.
(1) A pipe lineup clamp should be used to align and securely hold the pipe ends before tack welding. A spacing tool to separate the pipe ends can be made from an old automobile spring leaf. The spacing for oxyacetylene welding should be approximately 1/8 in. (3.2 mm); for arc welding, the spacing depends on the size of the electrode used for the root pass.
(2) If a pipe lineup clamp is not available, the pipe section must be set in a jig so that their center lines coincide and the spacing of the pipe ends is uniform prior to tack welding. An angle iron (fig. 12-34) will serve as a jig for small diameter pipe, while a section of channel or I-beam is satisfactory for larger pipe.
(3) When a backing ring is used and it is desired to weld to the backing ring, the spacing should not be less than the diameter of the electrode used for the root pass. When welding to the backing ring is not desired, the spacing should not exceed one half the electrode diameter, and varies from this diameter to zero, depending on whether a small or large angle of bevel is used.
e. Backing Rings and Tack Welding.
(1) The purpose of a backing ring is to make possible the complete penetration of the weld metal to the inside of the pipe without excessive burning through, to prevent spattered metal and slag from entering the pipe at the joint, and to prevent the formation of projections and other irregular shaped formations of metal on the inside of the joint. Backing rings also aid materially in securing proper alignment of the pipe ends and, when used, are inserted during assembly of the joint. Backing rings are not used when the pipe service requires a completely smooth inner pipe surface of uniform internal diameter.
(2) There are several types of backing rings: the plain flat strip rolled to fit the inside of the joint; the forged or pressed type (with or without projections); the circumferential rib which spaces the pipe ends the proper distance apart; and the machined ring. All shapes may be of the continuous or split ring types. Several backing rings are shown in figure 12-35.
(3) Backing rings should be made from metal that is readily weldable. Those used when welding steel pipe are usually of low carbon steel.
(4) When the pipe ends have been properly aligned, four tack welds should be made. They should be one-half the thickness of the pipe and equally spaced around the pipe.