Supports
Figure 2. It&#;s good practice to have supports near valves and other heavy items attached to piping.
Movement of the piping must be controlled. A fixed-point anchor restricts all axial and rotational movements whereas a cross guide constrains displacements of piping along the axis perpendicular to its centerline. Support design should consider many details, such as the type of machinery connected to the piping, details of the machinery package, nearby equipment and other items attached to the piping. For example, machinery packages that include shell-and-tube heat exchangers (e.g., oil and water coolers, inter/after coolers, etc.) should have an anchor support on the side from which the tube bundle will be pulled out for maintenance work and also should consider the thermal expansion of piping connected to the exchanger(s).
Typical vertical supports to carry deadweight are:
&#; support hangers;
&#; rod hangers;
&#; resting steel supports; and
&#; variable and constant spring hangers (which should be used where other options aren&#;t effective).
Rod hangers and resting steel supports fully restrain downward pipe movement but permit pipe to lift up. Variable spring hangers usually use coiled springs to support a load and allow piping movement; the resistance of the coiled springs to a load changes during compression. In contrast, a constant spring hanger provides consistent support force by having two moment arms pivoted about a common point. The load is suspended from one of these arms and a spring is attached to the other. An appropriate choice of moment arms and spring properties can provide a resisting force nearly independent of position. Constant support hangers principally are used to support pipes and equipment subject to vertical movement due to thermal expansion (or contraction) at locations where transfer of load/stress to other supports or equipment can be critical. As an indication, the maximum recommended variation from the operating load is around 25&#;30% for variable spring hangers. If the variation exceeds 30%, a constant support hanger might be used.
Undesirable movements can occur due to many phenomena, such as sympathetic vibration, rapid valve closure, relief valve opening and two-phase flow. It may be necessary to limit this type of deflection to prevent generation of unacceptable stresses and high loads on equipment nozzles. A sway brace, which essentially is a double-acting spring housed in a canister, is a cost-effective means of restricting pipework deflection. It isn&#;t intended to carry the weight of piping systems but only to limit undesirable movements. It acts like a rigid strut until a small preload is reached, then the restraining force increases in proportion to the applied deflection. A sway brace does provide some resistance to the thermal movement of a piping system; so specifying it requires care. Installation of a sway brace raises the fundamental frequency of vibration of a pipework system, which likely will reduce undesirable deflections. The devices often are used to solve unforeseen problems of resonant vibration.
Piping systems also may face loads imposed by occasional events such as severe wind, earthquake or a fluid hammer. To protect piping from wind or earthquake (which usually occur in a horizontal plane), normal practice is to attach lateral supports (instead of axial restraints) to piping systems. Protecting piping from fluid hammer loads may call for both lateral supports and axial restraints.
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To carry sustained loads, vertical supports normally are required. For thermal loads, having no supports gives zero stresses &#; so, the fewer the number of supports, the lower the thermal stresses. Only use axial restraints and intermediate anchors to direct thermal growth away from equipment nozzles.
Pipe Stress Analysis
Such an evaluation is an important step for machinery piping designs as well as many piping systems. In simple terms, it is used to:
&#; Ensure the stresses in all piping components (including piping supports) in machinery package(s) and connected systems are within allowable limits;
&#; Solve dynamic problems developed due to mechanical vibration, pulsation, etc.; and
&#; Address issues, such as displacement stress range, nozzle loading, etc., due to higher or lower operating temperatures.
Internal pressure, whether design or operating, usually causes uniform circumferential stresses in the pipe wall; pressure/temperature ratings enable determining the appropriate pipe wall thickness. Internal pressure also gives rise to axial stresses in the pipe wall. These axial pressure stresses depend upon pressure, pipe diameter and wall thickness. Because all three are set at initial stages of design, the axial stresses can be determined at that point; changing the piping layout or the support scheme usually can&#;t alter these stresses. A pipe&#;s deadweight causes it to bend (generally downward) between supports and nozzles. This produces in the pipe wall so-called bending stresses, which vary more-or-less linearly across the pipe cross-section &#; being tensile at either the top or bottom surface and compressive at the other surface. If the piping system isn&#;t supported in the vertical direction (i.e., in the direction of gravity) except for equipment nozzles, pipe bending due to deadweight may create excessive stresses in the pipe and impose large loads on equipment nozzles, thereby increasing susceptibility to &#;failure by collapse.&#; Various international piping standards and codes impose stress limits on these axial stresses generated by deadweight and pressure to avoid problems; to keep calculated actual stresses below such allowable stresses for sustained loads may require provision of more supports for the piping system.
Thermal loads (expansion and contraction loads) are important forces in piping design; Piping will expand or contract as it goes from one thermal state to another, e.g., from ambient conditions (while idle) to normal operating temperature and then back to ambient. If the piping system isn&#;t restrained in the thermal growth (or contraction) directions &#; for example, in the axial direction of pipe &#; then, for such cyclic thermal loads, the piping system expands or contracts freely. In this case, no significant internal forces, moments and resulting stresses and strains result. If, on the other hand, the piping system is restrained in the directions it wants to thermally deform, such as at equipment nozzles and pipe supports, cyclic thermal stresses and strains develop throughout the system as it goes from one thermal state to another. When such calculated thermal stress ranges exceed the allowable thermal stress range specified by various international piping standards or codes, then the system is susceptible to &#;failure by fatigue&#; or other modes of failure.
To avoid such failures due to cyclic thermal loads, the piping system should be made flexible. This often involves introducing bends or elbows into the layout to add flexibility. Having connected equipment nozzles offset from each other provides one avenue for this. If the two nozzles are in line, then the straight pipe connecting these nozzles will be very stiff. In contrast, offset nozzles will require piping with a bend or elbow; such an &#;L-shaped&#; piping layout is much more flexible. Another option is to use expansion loops (with each loop usually consisting of four bends or elbows) to absorb thermal growth or contraction. If these options aren&#;t feasible, alternatives such as expansion joints (bellows, slip joints, etc.) might make sense. However, expansion joints are expensive and require some attention by maintenance and operations.
Cyclic thermal loads also impose loads on nozzles of rotating equipment and machines. Some of these units are sensitive to nozzle loads, with excessive loads impairing operation and even causing damage. So, reduction in nozzle loads is an important topic for the piping-in of machinery or rotating equipment packages. A number of methods can help keep nozzle loads within limits:
&#; Adding elbows or using other techniques to increase the flexibility of the piping connected to machinery and, consequently, reduce the nozzle loads;
&#; Putting in axial restraints, which constrain piping in its axial direction, at appropriate locations to direct thermal growth (or contraction) away from nozzles; and,
&#; Installing intermediate anchors, which restrain piping movement in the three or four translational and three or four rotational directions, at appropriate locations so regions (such as expansion loops) away from equipment nozzles absorb thermal deformation.
AMIN ALMASI is a rotating equipment consultant based in Sydney, Australia. him at [ protected].
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