Pipe Supports and Restraints – Types and Functions

In the design of piping systems, the piping engineer or designer is responsible for locating the pipe supports and restraints to take care of the loads and displacements imposed on the piping system for the various load cases. A piping system may have several types of supports and restraints to take care of the various loads imposed on it and to allow the piping system to function safely under operating, design and occassional load conditions.

Difference between Pipe Support and Restraint

ASME B31.3 does not explicitly define the difference between a pipe support and a restraint. However, paragraph 319.2.1 implies the definition of a restraint by stating: “Any change in temperature will cause a piping system to undergo dimensional changes. If connected equipment and restraints such as guides and anchors constrain the system from freely expanding or contracting, it will displace from its unrestrained position.” Therefore, by definition, a restraint is a structural element that constrains or limits the thermal movement of a piping system.

On the other hand, one can define a support as a structural element or assembly that absorbs the weight loads of the piping system and contains the sustained longitudinal stress within allowable limits. Its function is to sustain the weight of the pipes, fittings, flange assemblies, valves, and the pipe contents, including any additional loads from insulation or external or internal coating/lining on the pipe.

Besides the function of a restraint to control of thermal movement, it may be required to achieve one or more of the following functions:

• Limit or redirect the piping movement due to thermal expansion.
• Keep the pipe thermal expansion stresses within allowable limits.
• Limit the transfer of thermal loads to the connected equipment.
• Absorb other loads imposed on the piping system such as wind loads, earthquake loads, water hammer loads and other dynamic loads and thereby limit the piping deflections and resultant stresses within allowable limits.

Depending on the piping configuration, a combination of support and restraint may be installed at one location. For example a shoe support and guide is a combination of a support and restraint.

Pipe supports may be broadly classified as rigid supports and resilient or flexible supports. A brief understanding of various types of pipe supports is provided below.

Rigid supports

Pipe Shoe Supports

The majority of pipe supports in a plant facility fall under the category of rigid supports. Engineers primarily use rigid supports to carry the weight of the pipe and its components. Therefore, their purpose is to prevent the downward movement of the pipe. Designers select support intervals or spans so that the pipe does not sag excessively under its own weight, along with the weight of its contents and insulation. A bare pipe may rest directly on a structural member, or workers may provide a shoe for an insulated pipe. In the latter case, the bottom of the shoe rests on the structural member. A shoe support offers a larger contact surface with the structural steel member, whereas a bare pipe resting on a structural member creates only line contact. The figure below shows the most commonly used examples of shoe support configurations.

The size of the pipe and the weight of the pipe and its contents determine the choice of shoe type. For smaller pipe sizes and lighter loads, fabricators weld a single T-section cut from a structural steel member, such as an IPE or HEA section, to the bottom of the pipe to form a shoe support. As the pipe size increases and the pipe becomes heavier, designers configure the shoe support with two vertical members. This design spreads the pipe load over a larger surface area of the pipe wall, which results in lower local stresses in the pipe wall. The saddle-type shoe support uses a reinforcement saddle plate that workers weld to the bottom of the pipe. This saddle plate usually subtends an arc of 90° or 120°. Fabricators can make the saddle plate from the parent pipe material. However, if the parent pipe has a high wall thickness, they form the saddle plate from plate material, such as ASTM A36 for carbon steel pipes. A pipe saddle spreads the pipe load over a much larger portion of the pipe surface area and substantially reduces the local pipe wall stresses.

Pipe Trunnion or Dummy Supports

Another example of rigid pipe support is a dummy support also called trunnion support. A dummy support consists of a tubular member which is welded to the pipe instead of structural member.

Two dummy supports conveniently support a vertical pipe by resting on structural members. Depending on the load and the available structural steel member, one may use a single dummy support instead of two.
At control valve stations or pump piping, one typically uses a base-type dummy support. Depending on the piping configuration and design, the dummy support base may rest on a structural steel member, a concrete foundation with a base plate, or a spring support.
As shown above, one may also weld a trunnion support to an elbow in the horizontal direction to support the piping. To minimize the bending moment and stresses at the weld joint on the pipe, the trunnion length should be kept as short as possible.
For heavy pipe loads, welders may weld trunnion supports onto reinforcing pipe saddles to distribute the pipe load over a larger pipe wall area.

Flexible or Resilient Pipe Supports

When thermal movements are substantial, upward movement may lift the pipe off the support, while downward movement may subject the pipe to large expansion stresses if the support restricts vertical downward movement. Flexible or resilient supports allow piping movement while still supporting the pipe. Thus, we can define flexible supports as pipe supports that maintain a supporting force throughout the expansion and contraction cycle of the piping system. Spring supports serve as examples of flexible or resilient supports. We require flexible supports when the pipe needs both support and the ability to move vertically. In general, when vertical movement is low to medium and the variation between hot load and cold load is less than 25%, engineers recommend variable spring supports. For large vertical movements, they recommend constant spring supports. Other factors, beyond the scope of this article, also determine the type of spring support to use.

Stainless Steel Guide

In a variable spring support, the load that the spring imposes on the pipe varies with the pipe deflection. The magnitude of this load variation depends on the spring rate (or spring constant) of the support. We calculate the load variation as the product of the spring constant and the pipe thermal movement. When a system requires low load variation, such as on pump piping supports, designers should choose a spring support with a low spring constant.

In a constant spring support, the load that the spring imposes on the pipe remains constant throughout the pipe’s range of thermal movement. Engineers use constant spring supports when pipe deflection is large (above 50–75 mm), especially near strain-sensitive equipment such as pumps and turbines. The results of piping stress analysis will determine whether to use a variable or constant spring support.

Pipe Restraints

1）Pipe Guide Restraint

Pipe support guides permit axial pipe movement while restraining lateral movement. Designers may configure guides to limit lateral movement in either one direction or two directions. When a guide restrains pipe movement in two directions, it also limits rotational movement to some extent, depending on the gap between the guide and the pipe shoe. These guides can take the form of structural members on shoe supports, as shown in the figure below, or appear as U-bolts, clamps, or structural members if the pipe does not have a shoe. For vertically routed pipes, guide supports typically consist of boxed structural members around the pipe or pipe shoe.

Installers provide guides whenever the pipe must maintain its position on supports, such as on piperacks. Without guides on rack-supported piping, the pipe may snake after repeated expansion and contraction cycles. Guides may also be necessary on vertical pipe runs to handle wind loads. Engineers frequently install guides on pump suction nozzles to minimize the transfer of thermal loads to the pump nozzles.

2）Pipe Axial Limit Stop Restraint

An axial limit stop restrains the pipe movement in the axial direction but allows movement in a direction at right angles to the axial direction. Engineers also commonly refer to it as a line stop. Depending on the requirements of the stress analysis, designers may set different gaps between the limit stops or line stops on either side of the structural steel member. This gap ensures that the pipe can expand due to thermal growth for a specified distance until the gap closes and the structural steel member limits further thermal movement. Engineers may use a limit stop in combination with a guide support to create an anchor support.

3）Pipe Anchor Restraint

An anchor is a restraint that prevents pipe translational movement and rotational movement in all the three degrees of freedom, relative to the structure of the anchor. An anchor (structural member) may have flexibility or the anchor itself may have imposed displacement and rotation. For example a nozzle on an equipment is treated as an anchor while carrying out stress analysis. However the nozzle itself may have displacement and rotation due to thermal expansion of the equipment. Further the nozzle also may have certain amount of flexiblity depending on the diameter and wall thickness of the equipment. The pipe connected to the nozzle in such cases is treated as an anchor with imposed displacements.