Section Three: Joints

Transcription

1 Section Three: Joints SPIRALWELD PIPE

2 AMERICAN offers a variety of steel pipe joints to meet the designer s needs, allowing the designer to select the most effective joint type(s) for the project s installation and performance requirements. The joint type is generally identified as one of the following: field welded, bell and spigot O-ring gasket, flange, or coupling. Field Welded Joints Field welded joints are available for all diameters and wall thicknesses of AMERICAN pipe. These joints are generally used where restraint of longitudinal thrust is required for buried pipe. In addition, some specifiers prefer to use field welded joints throughout an entire piping system in lieu of a system including both gasketed and welded joints. Field welded joints do not allow for in-service flexibility, but they can accommodate, during installation, deflections that are generally larger than those for O-ring gasket joints. Field welded joints can be supplied as bell and spigot lap type, butt strap type, and butt weld type. Requirements for field welded joints are defined in the AWWA standard for field welding, C206. Although each joint type employs welding in some manner, the manufacturing process, assembly, and performance limitations differ. Lap Weld ( ) The bell and spigot lap weld joint has a proven performance history in steel pipe. It is an economical method for restraining longitudinal thrust in buried pipes. Lap weld joints can be configured for welding inside only, outside only, or both when design conditions dictate. A single full fillet weld is adequate for restraint of longitudinal thrust forces due to internal pressure. Conditions involving other longitudinal forces such as from thermal stress, Poisson s ratio of hoop stress, or bending stress require additional consideration. Consult an AMERICAN representative when presented with such conditions. The joint can be configured for welding after backfill of the installed pipe. This configuration is generally used for joints employing a single inside fillet weld. The installation of pipe, exterior joint wrapping, and trench backfilling can progress uninterrupted by the welding process, which takes place after consolidated fill has been placed to at least one foot over the top of the pipe. This process improves installation efficiency and reduces project costs. It also significantly reduces the level of longitudinal thermal stress in the steel cylinder, as the steel can achieve thermal equilibrium 1

3 with the surrounding soil prior to welding. The weld after backfill method can be employed on joints requiring a fillet weld both inside and outside, but the cost benefit is less significant and the reduction in thermal stress would not be achieved. Lap weld joints offer flexibility for varying installation conditions. Alignment correction can be achieved by providing field trim at strategic locations such as major horizontal or vertical alignment changes, connections to existing pipes, specific appurtenance placements, connections to pipes at structures, and closures. In areas where post-installation access is difficult or undesirable, such as inside casings, tunnels, or encasements, the joint can be configured for the application of one full structural fillet weld, one seal weld, and an air test hole. The joint can then be tested to assure that the structural weld is watertight prior to installation. When required, the joint also can be configured to function as a temperature control joint. Refer to AMERICAN s Field Service guide for assembly recommendations for a lap weld joint. Manufacture A lap weld joint consists of a bell and spigot integrally formed into the parent cylinder, without the use of separate, welded-on joint rings. The parent cylinder steel is formed beyond its elastic limit to exacting tolerances to form the bell, and, when necessary, to size the spigot. The tolerances are small to maintain a tight fit between the outside of the spigot and the inside of the bell to minimize the weld and eccentricity of the joint. The joint is configured to provide a minimum engagement equal to the smaller of 1 plus the design joint pull, or 3 times the thickness of the bell plus the design joint pull. Additional engagement can be accommodated in instances where the joint will be used as a temperature control joint. The bell and spigot are generally formed prior to hydrostatic testing. When expansion takes place after the parent cylinder has been hydrostatically tested, the weld seams in the formed section of a bell or spigot are nondestructively examined visually (VT) and by the magnetic particle method (MT). Angular Joint Deflection A lap weld joint can be disengaged on one side, commonly referred to as deflecting or pulling, to accommodate small alignment changes and curves required for installation. AMERICAN recommends a maximum design pull of 1 for all pipe sizes. Where additional deflection is required, weld bell pipe ends can be miter-cut prior to expansion. Such miter-cut ends are generally limited to approximately 5. Consult an AMERICAN representative regarding specific miter-cut bell limitations. The angular deflection for a 1 joint pull and the resulting minimum radius for a pulled joint curve installation are computed as follows: Angular Deflection, Q = tan -1 (joint pull/pipe O.D.) Minimum curve radius = [(pipe length)/2]/sin(q/2) 2

4 Based on a 1 pull, the resultant angular deflections and associated minimum curve radii based on these angular deflections are as shown below. Nominal Diameter Angular Deflection Q (degree) Joint Pull Offset per 50 Length Lap Weld Joint Deflection Table Radius of Curve* (ft) Max. Mitered Deflection Angle** (degree) Max. Offset per 50 Length Radius of Curve* (ft) , * Curve radius shown is directly proportional to pipe length. Use of shorter pipe lengths will result in a proportionally smaller radius. For example, if the radius using 50 long pipes is 1690, then the radius using 25 long pipes will be approximately 845. ** Angle shown is the maximum possible as a combination of miter cut and joint deflection. Mitered lap joints can be furnished at resultant angles between approximately 0.5 and

5 Butt Strap (4-144 ) The butt strap joint is an economical method for connecting two butting pipe ends at closures, post-installation access points, and repair areas. The straps are generally furnished in two halves that are assembled around the butting pipe ends and fillet welded in place. The strap can be fillet welded inside only, outside only, or both inside and outside. In instances where butt straps are to be welded both inside and outside, each half of the strap can be furnished with air test holes to facilitate pneumatic testing of the finished fillet welds. This is especially beneficial at closures completed after the installed pipe has been hydrostatically tested. In addition to the circumferential fillet welds, this configuration requires two longitudinal complete-joint penetration welds where the two strap halves meet. Angular Joint Deflection Butt strap joints are typically located in areas where the two butting pipes are on the same slope. Designed deflections in this type joint are rare, but are acceptable, as are installed joint deflections, as long as gap tolerances between the strap and the pipes are maintained in accordance with the requirements for bell and spigot lap weld joints as defined in AWWA C200 and AWWA C206. Butt Joint (4 144 ) The welded butt joint can be furnished in any diameter or wall thickness and is suggested for exposed joints that will be subject to excessive bending, in-service thermal cycling, or vibration. Most steel pipe installed using horizontal directional drilling (HDD) methods are furnished with butt joints. The joint can be welded from either the inside or outside, or from both sides. Single welded butt joints can be furnished with or without backing. Backing assists in placement of a sound root pass. Backing also aids in aligning pipe ends during assembly and welding, although it limits joint deflection. Design joint deflections are accomplished by miter cutting one or both ends of the mating pipes. Miter cutting only one pipe end is acceptable as long as the assembled joint between the resulting ellipse of the cut end and the mating pipe square end does not yield an edge offset that violates the guidelines of the governing welding standard, code, or specification. 4

6 Bell and Spigot Rubber Gasket Joints A bell and spigot rubber gasket joint is an efficient, economical method for joining steel pipe. The joints remain flexible in service, providing for limited longitudinal and angular movement due to ground settlement or thermal fluctuations, while remaining watertight. They are generally used in areas where restraint of longitudinal thrust is not necessary. For areas where restraint of longitudinal thrust is necessary and welding is neither possible nor desired, the joints can be restrained by harness lugs and rods, or other suitable methods to develop a longitudinal resistance force, including AMERICAN s restrained rubber gasket joint presented below. All weld seams present in the faying surfaces of each joint are ground flush with the cylinder surface to facilitate forming and to effect an acceptable sealing surface for the gasket. The gasket size and configuration varies based on pipe diameter, wall thickness, and spigot configuration. Given these variables, as-manufactured pipe dimensions are evaluated for each pipe diameter, thickness, and spigot configuration combination on each project to properly size the gasket to the resultant groove dimensions. Requirements for O-ring type gaskets are defined in AWWA standard C200 for steel pipe. Rolled Groove O-Ring (24 84 ) The rolled groove O-ring gasket joint has decades of proven performance history with steel water pipes. The rolled groove joint consists of a bell and spigot that are formed integrally into the parent pipe cylinder, yielding a joint that is capable of sustaining pressures equal to that of the parent cylinder pipe. The O-ring gasket is contained in the formed spigot groove, and upon insertion of the spigot into the bell, the gasket is compressed between the two ends to form a watertight seal. Given the form of the ends, a properly assembled rolled groove joint is self centering. The rolled groove O-ring joint has a proven history of successful service across a wide range of pressures. Further, historical testing has shown that the parent cylinder will yield and fail prior to failure of the joint itself. Refer to AMERICAN s Field Service guide for assembly recommendations for a rolled groove O-ring gasket joint. 5

7 Manufacture A rolled groove joint consists of a bell and spigot integrally formed into the parent cylinder, without the use of separate, welded-on joint rings. The parent cylinder steel is formed beyond its elastic limit to exacting tolerances for each end configuration. The tolerances are small to maintain a tight fit between the outside of the spigot and the inside of the bell, and assure in-service joint integrity. The bell is shaped by using expansion dies to form the steel cylinder, including an end flare designed to aid in assembly of the joint. Expansion of the bell configuration offers uniformity and consistency of joint dimensions over the full range of cylinder thicknesses available for the rolled groove joint. The spigot is shaped by using rolling dies to form the groove in the steel cylinder. After the bell and spigot are formed, the parent cylinder is hydrostatically tested. Size and Pressure Performance Limitations Because of the formation process of rolled groove spigot ends, the current nominal thickness limitation of such ends is 3/8. Based on this limitation, the maximum pipe diameter for this type of joint is generally 84. Further, the material grade for rolled groove spigot ends is limited to steels with minimum specified yield strengths less than or equal to 46 ksi. The assembled joint can sustain the same operating and transient pressures as the pipe cylinder. Angular Joint Deflection A rolled groove gasket joint can be disengaged on one side, commonly referred to as deflecting or pulling, to accommodate alignment changes during installation. AMERICAN recommends design pulls not exceeding 1 for nominal pipe sizes 30 and less, and 3/4 for nominal pipe sizes greater than 30. Contact an AMERICAN representative regarding pulls exceeding these general values. The angular deflection for a given joint pull and the resulting minimum radius for a pulled joint curve installation are computed as follows: Angular Deflection, Q = tan -1 (joint pull/pipe O.D.) Minimum curve radius = [(pipe length)/2]/sin(q/2) 6

8 Based on the recommended design pulls noted above, the resultant angular deflections, and associated minimum curve radii for these angular deflections, are as shown below. Nominal Diameter Angular Deflection Q (degrees) O-Ring Joint Deflection Table Joint Pull Offset per 50 Length Radius of Curve* (ft) ** ** * Curve radius shown is directly proportional to pipe length. Use of shorter pipe lengths will result in a proportionally smaller radius. For example, if the radius using 50 long pipes is 1690, then the radius using 25 long pipes will be approximately 845. ** 78 and 84 o-ring joints have special pressure and service limitations. Contact AMERICAN for specific applications. Steel Lok-Ring Restrained Joint AMERICAN s restrained rubber gasket joint is a proprietary design that provides for installation of rubber gasketed restrained joint pipe in areas where welding is undesirable or logistically not feasible. This design combines the advantages of field proven snap-ring restraint with O-ring seals that can be seal tested prior to filling the line. The joint consists of a bell integrally formed into the parent pipe cylinder after the inside bearing ring and outside stiffening ring are attached; and a spigot consisting of a double gasket spigot ring and bearing ring welded to the parent cylinder of the mating end. Restraint is accomplished by placing and expanding a locking ring in the assembled joint between the two bearing rings. The O-ring gaskets are contained in the spigot ring grooves, and upon insertion of the spigot into the bell the gaskets are compressed between the two ends to form a watertight seal. 7

9 Manufacture A restrained rubber gasket joint consists of a bell integrally formed into the parent cylinder and a gasketed spigot ring welded to the parent cylinder. After the restraint and reinforcement rings are attached, the parent cylinder steel is formed beyond its elastic limit to exacting tolerances for the bell end configuration. The tolerances are small to maintain a tight fit between the outside of the spigot and the inside of the bell, and assure in-service joint integrity. The bell is shaped by using expansion dies to form the steel cylinder. Expansion of the bell configuration offers uniformity and consistency of joint dimensions over the full range of cylinder thicknesses. The spigot consists of an extruded gasket retaining ring and restraint ring that are welded to the steel cylinder. After the bell is formed and the spigot rings are welded on, the parent cylinder is hydrostatically tested. Size and Pressure Performance Limitations The restrained rubber gasket joint is available in nominal pipe sizes 36 through 84, in thicknesses up to 1/2. The rated working pressure for this joint is 250 psi. Angular Joint Deflection This joint is flexible and can be extended slightly on one side, commonly referred to as deflecting or pulling, to accommodate alignment changes during installation. AMERICAN recommends design angular deflections after snap ring engagement not exceeding the values shown in the table below. The angular deflection for a given joint pull and the resulting minimum radius for a pulled joint curve installation are computed as follows: Angular Deflection, Q = tan -1 (joint pull/pipe O.D.) Minimum curve radius = [(pipe length)/2]/sin(q/2) 8

10 Based on the suggested angular deflections noted below, the resultant minimum curve radii are as shown below. Nominal Diameter Angular Deflection Q (degrees) Steel Lok-Ring Joint Deflection Table Joint Pull Offset per 50 Length Radius of Curve* (ft) * Curve radius shown is directly proportional to pipe length. Use of shorter pipe lengths will result in a proportionally smaller radius. For example, if the radius using 50 long pipes is 1690, then the radius using 25 long pipes will be approximately 845. Flange Joints (4 144 ) A flange is used at a joint that may require future disassembly such as connections to valves and meters, access port covers, and connections to existing pipe. Flanges are generally shop attached but can be furnished loose for field attachment when such flexibility is required. Connections are made using bolts that connect two flanges separated by a gasket. Given this configuration, flanges afford the designer a convenient location to provide electrical isolation between sections of installed pipe by virtue of insulating kits. Flanges can be furnished as either of two types, ring or hub, with ring flanges being the industry standard. Blind flanges are configured without a center hole and are generally used for covers at access ports, or for closure at the end of a pipeline. Requirements for flanges, bolts, nuts and gaskets for standard water pipe service are defined in the AWWA standard for flanges, C207. For conditions outside of the guidelines of AWWA C207, use of alternate standards may be appropriate, including ASME B16.5 Pipe Flanges and Flanges Fittings, ASME B16.47 Large Diameter Steel Flanges, or MSS SP-44 Steel Pipeline Flanges. It is important to note that use of ASME flanges will likely require the use of special, nominal O.D. pipe that does not maintain the larger inside diameter of typical AWWA steel pipe. 9

11 Coupling Joints Coupling joints provide flexible, removable means of connecting two adjacent pipe ends. Generally couplings are located at connections between buried piping and piping in structures, and adjacent to in-line valves and meters. Some coupling types provide resistance to longitudinal thrust forces, while other types rely on mechanisms external to the coupling when restraint is needed. Similarly, some couplings are capable of providing electrical isolation between the two mating pipe ends, while others cannot provide such isolation. Most couplings have a recommended axial gap between the pipe ends (contact coupling manufacturer for gap requirements). Couplings are available in general classifications as bolted sleeve-type or split-sleeve. Bolted Sleeve-Type Coupling (4 144 ) Bolted sleeve-type couplings offer no internal resistance to longitudinal thrust forces. Such forces must be mitigated by other means, or the coupling must be restrained externally via harness bolts and attached restraint lugs. Whether restrained or unrestrained, these couplings can be configured to provide electrical isolation between the two adjacent coupled pipe ends. When electrical isolation is required, the couplings are furnished with one or two rubber boots that are placed on the pipe end(s) to halt the flow of electrical current between the adjacent pipes. Harnessed couplings requiring electrical isolation also require washer/sleeve insulation inserts at the connection of the harness bolts to the restraint lugs. Requirements for bolted sleeve-type couplings are defined in AWWA C219. Requirements for external harness restraint are defined in Steel Water Pipe: A Guide for Design and Installation, AWWA M11. Split-Sleeve Coupling (4 144 ) Split-sleeve couplings can be designed to function in a non-restrained manner, or to provide restraint of longitudinal thrust forces within the confines of the coupling itself. Restraint is achieved by welding a ring to each mating pipe end at a specified location so that the coupling will encapsulate and lock against the rings. Given the internal locking configuration of this type of coupling, electrical isolation is only possible with the non-restrained type of split-sleeve coupling. For non-restrained split-sleeve couplings, electrical isolation is achieved by use of one or two rubber boots, similar to that for bolted sleeve-type couplings. Requirements for split-sleeve couplings are defined in AWWA C227. Joint Assembly Detailed assembly recommendations for joints are available in AMERICAN s Field Service Guide. 10

12 A Subsidiary of AMERICAN P.O. Box 2727 Birmingham, AL Phone: ASWP (2797) Note: The contents of this Field Service Guide are provided for informational purposes and convenience. It remains the responsibility of the installing contractor to comply with the requirements of the project s plans and specifications, and in the event of a conflict, the terms of the plans and specifications shall govern and control. However, to the extent any sales representative or other agent or representative of AMERICAN makes any statement that conflicts with the contents of this Field Service Guide, the contents of the Field Service Guide shall govern and control, and it shall be the responsibility of the installing contractor to comply with the terms of the Field Service Guide, subject, however, to the preceding sentence.