Additional piping fabrication requirements must be considered. Several of these are discussed below.
1.0 Storage and Handling
Improper handling and storage of pipe materials and welding filler metals can cause damage and result in poor
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• Pipe should not be stored directly on the ground to help
prevent rainwater accumulation around the pipe, which could result in corrosion.
• Pipe should not be stacked so high that pipes or their
coatings may be damaged.
• Fittings and valves should be stored in shipping crates or
on racks to provide protection until used.
• End protectors should be firmly attached to prevent
damage to weld bevels, flange faces, threads, or socket- weld ends.
• Lined and coated pipes and fittings should be lifted with
wide fabric or rubber-covered slings and padding to prevent damage.
2.0 Pipe Fitup and Tolerances
Good joint fitup is essential to making a sound weld and minimizing the loads imposed on the piping system and connected equipment. Depending on the welding process used, a slight mismatch may be permissible.
• Pipe fitup for welded joints shall be as required by the
welding procedure.
• The tolerance for axial dimensions, face-to-face, center-
to-face, and location of attachments should be ±1/8 in. maximum.
• Flattening of bends, measured as the difference between
the largest and smallest outside diameter at any cross- section, should not exceed 5% of the nominal diameter of the pipe (3% at the ends).
• Lateral translation of branches and connections from
centerline of run should not exceed ±1/16 in.
• Flange bolt holes shall straddle the centerlines. Rotation
of flanges, measured as the offset between elevation of bolt holes on opposite sides of a flange centerline, should not exceed ±1/16 in.
• The tilt of flanges measured at the periphery across any
diameter should not exceed 1/32 in. from the square position. Use of a 1/64 in. tolerance is often necessary for flanges at load-sensitive equipment.
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3.0 Alignment of Pipe Attached to Load-Sensitive Equipment
Special care must be taken for load-sensitive equipment, especially rotating equipment. Specifically, in attaching pipe to rotating equipment, the installation should avoid putting excessive forces and moments on the machinery nozzles which could result in misalignment.
• Installation of piping that is connected to rotating
equipment should preferably start at the machine nozzle flange. This will reduce the possibility of having a large mismatch between the pipe and machine flanges if pipe installation is begun from the opposite end of the system.
• Bolt on succeeding pipe sections as appropriate up to the
first support. Adjust this support as required to just contact the pipe at its bearing point. Proceed to any other adjacent supports which should be similarly adjusted.
• One or more field welds are typically used to join the
piping nearest to the machine with the rest of the system. The number and location of these field welds are
determined such that they will permit final position adjustments to achieve acceptable flange alignment at the machine nozzle.
• Spring supports should be locked in their cold position
during pipe installation.
• All spring supports will be adjusted in the locked position
just until they contact their respective support points. If spring-support adjustment is insufficient, modifications to associated structural members or shimming will be required.
• Final bolt tensioning of component flanges close to the
machinery should be done after initial alignment of nozzle flanges.
• Piping that requires any sections to be removed for
flushing after completing field welds should have final nozzle alignment and component flange boltup
completed after replacing flushed sections.
• For piping over NPS 3 connected to machinery, flange
147 specified for general piping systems. More stringent limits are required to minimize the loads that are imposed by flange boltup.
• Precautions should be taken to prevent ingress of debris
into machine internals during construction of connecting pipework.
4.0 Flange Joint Assembly
Flange joint assembly procedures directly affect the ability of the flange to be leak-tight in service. In many low-pressure, low-temperature, and/or nonflammable services, many rules of good flanged joint design and makeup can and have been violated with no adverse consequences. However, it is dangerous to break these rules in critical, high-temperature services since the results can be serious leakage problems with consequent fires. The primary factors for successfully making up a flanged joint and controlling leakage are the following:
• Proper selection and design of the flanged joint.
• Proper preparation, inspection, and installation of the
flanged joint.
• Identifying and controlling the causes of leakage.
Flanged joint assembly and leakage control are discussed below.
5.0 Flange Preparation, Inspection, and Installation
The following discusses the primary steps that are required to achieve a properly assembled flanged joint.
• Redo Damaged Surfaces. Warped or badly corroded
flanges should be replaced or refaced. Reface flanges with tool marks or scratches across the gasket seating surface.
• Clean Faces. All gasket and flange surfaces should be
clean. Remove all burrs, rust, and dirt from flange faces with scrapers or wire brushes.
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• Align flanges. Flanges at rest should be within the
alignment tolerances previously discussed, with the flanges practically mating before the bolts are installed. Bringing the flanges into alignment should not leave any residual stresses in the piping system. Residual stresses could lead to flange leakage in service or overload
problems in systems that are connected to load-sensitive equipment. This becomes more important with
increasing pipe diameter, as the residual stress increases with increasing diameter for the same amount of
misalignment.
• Lubricate Threads and Nuts. Lubricate the bolt threads
and the nut faces where they will contact the flange. Lubrication helps increase the amount of bolt load that goes into tightening the flange rather than into
overcoming friction.
• Place Gasket Properly. The gasket must be centered on
the flange faces to achieve a reliable joint, but holding the gasket in place can be a problem. If something must be used to hold the gasket, a high-temperature grease may be used sparingly in systems that operate at less than 200°F. No grease, paste, or adhesive should be used to hold gaskets for systems operating at 200°F or more. The high temperature causes these materials to burn off, which could damage the gasket and cause leakage. Thin cellophane tape may be used on the outside edges of a gasket, but never on the seating surfaces. Tape on the seating surfaces will deform the gasket during joint assembly, burn out at operating temperature, and thus provide a leakage path. Centering rings on spiral-wound gaskets help by allowing the gasket to be supported in the proper position by a few bolts while the other bolts are inserted. Sheet gaskets should be cut so that their outside diameter corresponds to the bolt position, again to help centering.
• Use Proper Flange Boltup Procedure. Flanges may be
made up using a wrench and hammer, an impact wrench, a torque wrench, or a stud tensioner. The most important aspects of a proper boltup procedure, regardless of
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- Use a "criss-cross" pattern bolt-tightening sequence,
as is used when bolting a wheel onto a car. This approach helps to achieve a uniform bolt load around the flange. See Figure 8.4.
- Use at least three rounds of tightening around the
flange, increasing the applied load in each round, with two rounds at the maximum load. This
approach helps achieve uniform bolt load around the flange circumference.
- For the most critical high-temperature or high-
pressure flanges, use a method that permits measuring the applied load (i.e., torque wrench or stud tensioner). In this way, there is greater assurance that uniform bolt load is achieved. For such applications, a maximum stud stress during boltup of 40-50,000 psi is the normal target.
6.0 Causes of Flange Leakage
Most of the primary causes of flange leakage are directly related to poor inspection or installation. These are summarized below:
• Uneven Bolt Stress. An incorrect boltup procedure or
limited working space near one side of a flange can leave some bolts loose while others crush the gasket. This is especially troublesome in high-temperature services, when the heavily loaded bolts relax during operation.
• Improper Flange Alignment. Improper flange alignment,
especially nonparallel faces, causes uneven gasket compression, local crushing, and subsequent leakage.
• Improper Gasket Centering. If a gasket is off-center, it
will be unevenly compressed and more prone to leakage.
• Dirty or Damaged Flange Faces. Dirt, scale, scratches,
protrusions, or weld spatter on gasket seating surfaces provide leakage paths or can cause uneven gasket compression that results in leakage.
• Excessive Loads in the Piping System at Flange
Locations. Excessive piping system forces and moments at flanges can distort them and cause leaks. Common causes of this are inadequate flexibility, using excessive
150 force to align flanges, and improper location of supports or restraints.
• Thermal Shock. Rapid temperature fluctuations can
cause flanges to deform temporarily and leak.
• Improper Gasket Size or Material. Using the wrong
gasket size or material can cause leakage.
• Improper Flange Facing. A rougher flange-surface finish
than specified for spiral-wound gaskets can result in leakage.
Typical "Criss-Cross" Bolt-Tightening Sequence Figure 8.4
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