Cutting, Bending, Welding, Heat Treatment, and Examination
Cutting, bending, and welding operations in the field parallel those used in the shop. See the section ‘‘Fabrication’’ in this Piping and Fabrication blog in the older post. and also Mechanical and oxyfuel gas cutting are most commonly used in the field. Plasma cutting may occasionally be used in Piping.
Bending, if used at all, is limited to small-diameter piping using relatively simple bending equipment at ambient temperatures. Occasionally in order to correct for misalignment, larger-diameter ferritic piping is bent at temperatures below the lower critical. Please note that this procedure is limited to ferritic materials. Any application of heat to austenitic materials will result in sensitization and loss of corrosion properties. See the section ‘‘Bending.’’ For smaller pipe sizes, torches may be used to supply heat, but for larger, heavier-wall materials and where better temperature control is warranted, heat may be applied by induction or resistance heating units in the same manner as local stress relieving. See the section ‘‘Local Heat Treatment.’’ The heating units are applied to the section of the pipe to be bent. The section of the line upstream of the area to be bent should be anchored to preclude translation or rotation of the installed portion of the line. The anchor should preferably be not more than one or two pipe diameters from the area to be heated. Once the bend area has attained the required temperature, a bending force can be applied on the downstream leg of the pipe until the required bend arc has been obtained. Since most ferritic materials still have reasonably high
yield strengths even at lower critical temperatures, care should be exercised. Large bending forces may damage the building structure or crack the line being bent.
Apply a reasonable force for the conditions and allow the imposed stress in the bend arc to be relieved by the heat. Then repeat. Progress in this fashion until the required bend is accomplished. Some small amount of overbending may be required to offset the deflection which will occur in the unheated section of pipe between the heated arc and the pulling device. When the bend is completed and allowed to cool, all restraints may then be removed. Little if any force should be needed to align the downstream joint; otherwise additional bending may be needed to further correct the situation. No further heat treatment of the bend arc is needed since the temperatures applied in this bending method are below the lower critical temperature. Corrections to lines with large section modulus or where the required bend arc is large should preferably be made in a shop since better controls can be exercised.
Field welding is more often than not in a fixed position. Welders should be qualified in the 6G position since this qualifies for all positions. Welding will be done using SMAW,GMAW,FCAW, and GTAW. Somewelding processes can be automated using orbital welding techniques. Such practice can result in fewer repairs, provided the bevels and alignment are within tolerance and the welding parameters are carefully selected.
Field postweld heat treatment also follows the practices outlined in the section ‘‘Heat Treatment’’ for local stress-relieving of ferritic materials. This usually involves induction or resistance heating units with recording devices. For small pipe welds, torch heating using temperature-sensitive crayons to control temperature is sometimes used. Exothermic heating to stress-relieve welds is still used on occasion for outdoor applications where heating rates are not required to be controlled.
Exothermic materials are preformed to pipe contour and sized to reflect the wall thickness and desired stress-relieving temperature. They are placed around the weld and ignited, attaining temperature in 5 or 10 min. The actual maximum temperature attained may vary. NDE in the field will follow the practices outlined in the section ‘‘Verification Activities.’’ Radiography is usually limited to radioactive isotopes, although occasionally X-ray equipment may find a use. Most surface examination is conducted using liquid-penetrant methods, since magnetic particle equipment is not as convenient in the field. Ultrasonics are used for thickness verification and in certain situations as an alternative to radiography of welds when permitted by the governing code.
Mechanical Joints
Threaded joints probably represent the oldest method of joining piping systems. The dimensional standards for taper pipe threads are given in ASME B1.20.1. This document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters. Thread cutting should be regarded as a precise machining operation. For steel pipe the lip angle should be about 25,
but for brass it should be much smaller. Improper lip angle results in rough or torn threads. Since pipe threads are not perfect, joint compounds are used to provide leak tightness. The compounds selected, of course, should be compatible with the fluid carried and should be evaluated for possible detrimental effects on system components. Manufacturers’ recommendations should be followed.
Threading Die
Where the presence of a joint compound is undesirable, dryseal pipe threads in accordance with ASME B1.20.3 may be employed. These are primarily found in hydraulic and pneumatic control lines and instruments. Flanged joints are most often used where disassembly for maintenance is desired. Agreat deal of information regarding the selection of flange types, flange tolerances, facings and gasketing, and bolting is found in B16.5. The limitations regarding castiron-to-steel flanges, as well as gasket and bolting selection, should be carefully observed. The governing code will usually have further requirements. Gasket surfaces should be carefully cleaned and inspected prior to making up the joint. Damaged or pitted surfaces may leak. Appropriate gaskets and bolting must be used. The flange contact surfaces should be aligned perfectly parallel to each other. Attempting to correct any angular deviation perpendicular to the flange faces while making up the joint may result in overstressing a portion of the bolts and subsequent leakage. The proper gasket should be inserted making sure that it is centered properly on the contact surfaces. Bolts should be tightened hand-tight.
Compression sleeve (Dresser) coupling for plainend cast-iron or steel pipe
If necessary for alignment elsewhere, advantage may be taken of the bolt hole tolerances to translate or rotate in the plane of the flanges. In no case should rotation perpendicular to the flange faces be attempted. When the assembly is in its final location, bolts should be made up wrench-tight in a staggered sequence. The bolt loading should exert a compressive force of about twice that generated by the internal pressure to compensate not only for internal pressure but for any bending loads which may be imposed on the flange pair during operation. For a greater guarantee against leakage, torque wrenches may be employed to load each bolt or stud to some predetermined value. Care should be exercised to preclude loading beyond the yield point of the bolting. In other cases, special studs that have had the ends ground to permit micrometer measurement of stud elongation may be used. Flange pairs which are to be insulated should be carefully selected since the effective length of the stud or bolt will expand to a greater degree than the flange thicknesses, and leakage will occur. Thread lubricants should be used, particularly in high-temperature service to permit easier assembly and disassembly for maintenance. There are a great variety of mechanical joints used primarily for buried castiron pipelines carrying water or low-pressure gas. They are primarily of the bell and spigot type with variations involving the use of bolted glands, screw-type glands, and various types of gasketing. The reader is referred to AWWA Standards C 111, C 150, and C 600, and to catalogs for proprietary types. For reinforced concrete pipe,AWWAStandards C 300, C 301,51 and C 302, should be consulted. Compression-sleeve couplings such as the Dresser coupling and the Victualic coupling are widely used for above- and below-ground services, both with cast-iron and steel pipe. Consult the manufacturers’ catalogs for more information.
