Welding is the most important and most common method of joining all kinds of pipe, from cross-country line pipe to piping used in power plants, refineries and chemical processing plants. Much of this piping operates at high internal pressure, at temperature extremes or in corrosive environments. It is obvious that the welding used to join piping for these various applications must be of the highest quality and integrity.
Pipe welding is a specialized occupation different from plate welding. Welders must pass special tests on pipe welding to qualify for welding pipe in production or on an installation job. Performance tests have been established by the American Petroleum Institute (API) and the American Society of Mechanical Engineers (ASME). These societies have also established specifications for pipe composition and properties, and the
American Welding Society has established filler metal specifications.
Pipe Steels and Welding Electrodes
The API publishes standards covering all aspects of pipe welding procedures, operator qualifications, joint design, testing, inspection and specifications for types
of pipe steel.
In the early days of pipe welding, lap joints, or Bell and Spigot type joints (Figure P-4), were used with fillet welds instead of butt joints, because only bare or lightly coated electrodes which produced marginal weld properties were available at that time. Now, however, with the availability of a variety of coated electrodes and filler wires which provide the necessary weld properties, butt welds are used almost exclusively.
Specifications for pipe steels range from 240 MPa (35 ksi.) min. yield, 414 MPa (60 ksi.) min. tensile for API Grade B X-42; to 448 MPa (65 ksi.) min. yield, 552 Wa (80 ksi.) min. tensile for API Grade B X-65. The higher strength steels allow the use of thinner pipe walls, and improved coatings are available to protect the pipe from corrosion caused by the soil.
Welder Qualification
API pipe welders must make butt welds to specification on API pipe of the same material they are to weld on the job. Four longitudinal specimens are cut from equally spaced locations around the pipe to include the weld at the midpoint of each. The specimens must be ground or machined to provide a uniform width across the weld and into the base metal.
These specimens are tested in tension. The welder is qualified if three of the four, and in some cases, all four, break in the base material. This assures that the weld has at least the strength of the base metal. Welding and testing must be witnessed by an AWS-certified welding inspector. While on the job, the. welding inspector may call for cutting out a pipe weld for subsequent cutting into four specimens and testing. The possibility of testing at any time helps assure that the welder is careful at all times.
Utility Piping
Local building codes must be consulted when welding, soldering, brazing or mechanically joining connections are used in commercial, industrial and residential buildings.
Piping is used in buildings to conduct water for plumbing and sprinkler systems, steam and gas, and for sewer, waste and vent connections. Copper tubing, which is joined by soft soldering (using lead-free solder), or brass pipe, which is joined by threaded connections or brazing, may be used for water piping. Steam heating systems which operate below 103 kPa (15 psi) may use wrought iron or steel piping, which can be welded. See BRAZING, SOLDERING, and COPPER ALLOY WELDING.
Forged Fittings for Welding
Forged steel fittings are made with beveled edges which match the beveled edges of pipes and form V-grooves when butted to the beveled pipe ends. The wall thickness of the fittings matches the wall thickness of the pipe, and presents an unrestricted flow path
for the fluid. For low-pressure steam lines in buildings, pipe may be cut and beveled with an oxyfuel gas cutting torch, but the cut surfaces must be ground to remove scale and roughness. Cutting machines which produce a clean, uniform cut and bevel on pipe should be used when available.
Some city ordinances require pipes to be joined by threading. Where welding is allowed in local codes, cast iron and galvanized pipes may be brazed. Black wrought iron and steel pipe can be arc welded. Galvanized pipe is brazed in basically the same manner as other galvanized steels. See GALVANIZED IRON.

 

Nuclear Power Plants
Piping systems in nuclear power plants operate at approximately 315°C (600°F) under high pressure. These parameters, along with the need to prevent any leakage of radioactive fluids, require high weld integrity. Welders must be nuclear-qualified. To assure complete root penetration and fusion, the root pass is usually made by the manual gas tungsten arc welding (GTAW) process, and the remainder of the groove is filled by the manual shielded metal arc welding (SMAW) process. Submerged arc welding is used for shop welding of pipe where pipe sections can be rotated under the welding head.
Type 304 stainless steel is used extensively in primary piping for nuclear power systems to minimize corrosion and corrosion residue, which may become radioactive in the reactor coolant stream. High ferrite 308 filler rods are used to avoid hot cracking in the weld deposit. Extreme care must be used in the handling of carbon steel and stainless steel electrodes to prevent moisture pickup in the coating. Oven storage facilities should be provided at the job site.
Backing Rings
A backing ring is a device placed against the back side of a pipe joint to support the weld metal or bridge an excessive gap between pipe ends. The material may be partially fused or remain unfused during welding and may be either metal or nonmetal.
Consumable Inserts
A consumable insert is a piece of metal formed into a ring which is fitted into the inside surface of the pipe or tube prior to welding. It should be essentially the same composition as the pipe or tube. The ring serves two purposes: (1) to help align the two pipes to be joined, and (2) to assure complete root penetration and fusion.
Several insert cross sections are available, as shown in Figure P-5.One type of flat ring is designed to fit inside both rings at the joint, another fits in the joint between the pipe ends, and others fit inside the pipes and in the joint. These rings are split and are cut to fit the inside diameter of the pipe. All are tack-welded to both pipe ends to be joined, providing alignment and filler material for the root weld pass, which is made by the GTAW process. Inert gas shielding of the root area of the joint is necessary for best results.
Flat rings may have nubs formed or tack-welded to the outer surface to serve as spacers to establish the joint gap. The other shapes automatically establish the joint gap, align the pipes, and provide filler metal.
Stress Relieving
Stress relieving of pipe weld joints is recommended for lines operating at less than 200°C (400°F).All high-pressure steel pipe welds should be stress relieved at a
temperature of about 620°C (1150°F) for one hour for each inch of wall thickness, then slow-cooled to room temperature. Stress relieving can be done (1) in a furnace, or (2) with a wrap-around electrical resistance heating pad connected to a transformer with a rating of about 200 kVA.
When welding thick-walled pipe, several small passes prove to be more satisfactory than using a few large passes. Depositing thin layers of weld bead helps
prevent porosity and produces a finer grain structure, as a result of the thermal effect of each successive pass on previous passes. Each bead is cleaned with a light chipping hammer and a wire brush to remove all slag particles.
Pipe Welding Processes
All of the standard arc welding processes are suitable for welding pipe, although the shielded metal arc welding (SMAW) process is used for the majority of pipe welds. Oxyacetylene welding was formerly used for welding pipe of less than 100 mm (4 in.) diameter. The advantage of this process is that base metal can be heated without applying filler metal, and heat can be applied independently of the rate of filler metal addition, particularly for making root passes. At present, however, many root passes are made by the GTAW process, followed by one or more shielded metal arc passes to fill the joint. This isme for welding stainless steel and especially nuclear piping, where stainless steel and high-nickel alloys are used. Gas tungsten arc welding is used more frequently for filler passes in these materials, especially for critical highly stressed applications.
Pipes made of the so-called exotic metals, such as titanium, zirconium and tantalum must be arc welded by an inert gas process, GTAW or GMAW, with inert
gas shielding of the weld root. The plasma arc welding (PAW) process uses an inert gas shield, and in many cases can be used in place of the GTAW process. Submerged arc welding (SAW) is used frequently for pipe welding in fabricating shops, where it is con-
sidered to be the most efficient pipe welding process available. It is applied mainly to large-diarneter pipes where it is possible to clamp sections of pipe together and rotate them so that all welding is done in the flat position. Its greatest use is in the double lengthening of line pipe, where two or more sections of pipe are shop-welded into longer sections.
When welding steel piping with the GMAW process, C02or argon-C02 gas mixtures are used. Argon is the primary shielding gas used when welding stainless steel, nickel base alloys and aluminum.

 

Accessories

Many types of clamps, jigs and fixtures are available for holding pipe in position for welding. Rollers allow pipe sections to be rotated together so that the entire joint can be welded from the top in the flat position. It is best that the ends of pipe be cut and joint
preparation be done by machining to provide the cleanest and most accurate fit. Oxyacetylene cutting torches or plasma arc cutting torches can be used to cut bevels on pipe that is rotated under the torch. If the rotating equipment is not available or an irregular cut is required, it may be necessary to do the cutting with a hand-held torch.
Pipe Clamps. Clamps are used to hold pipe ends together in proper alignment for tack welding. A typical bar clamp is shown in Figure P-6. These devices feature quick-release handles for holding a wide range of pipe diameters. Sets of rollers, as shown in Figure
P-7, can be used to rotate the pipe as it is being welded. Usually four sets of rollers are required to support two sections of straight pipe.

 

Miter Joints. The design of miter joints produces a sharp bend in right angle connections unless the joint is made with more than one weld. Miter joints are not recommended and should be avoided, not only because they are difficult and expensive to lay out and
weld, but also because they result in sharp bends that cause turbulence and added resistance to flow. Stresses from thermal expansion and contraction and other loads on the system can be excessive and concentrated at the miter joint welds. Weld fittings are available for every situation where a miter connection would be used and are much simpler to install. See also ORBITAL TUBE WELDING.

 

 

 

 

 

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