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 pipe support and restraint. However the definition of restraint is implied in para. 319.2.1 which states "A piping system will undergo dimensional changes with any change in temperature. If it is constrained from free expansion or contraction by connected equipment and restraints such as guides and anchors, it will be displaced from its unrestrained position". Thus restraint by definition is a structural element used to constrain or limit the thermal movement of a piping system. On the other hand, support can be defined as a structural element or assembly that is required to absorb the piping system weight loads and contain the sustained longitdunal stress within the allowable limits. The function of a support is to sustain the weight of the pipes, fittings, flange assemblies, valves and the pipe contents including additional loads arising due to 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
Majority of pipe supports in a plant facility will fall under the category of rigid supports. A rigid support is primarily used to support the weight of pipe and its components. Hence its purpose is to prevent the downward movement of pipe. Support intervals or spans are chosen such that the pipe does not sag excessively due its self weight and the weight of its contents and insulation. A bare pipe may be allowed to rest on a structural member or if the pipe is insulated it may be provided with a shoe. In the later case the bottom of the shoe will rest on the structural member. A shoe support provides a larger contact surface with structural steel member as against bare pipe resting on structural member which results in line contact. Figure below shows examples of shoe support configurations which are most commonly used.
The choice of shoe type depends on the size of pipe and the weight of the pipe and its contents. For smaller pipe sizes and lesser loads, a single T-section cut from structural steel section such as IPE or HEA member is welded to the bottom of pipe to form a shoe support. As the pipe size increases and the pipe becomes heavier the shoe support is designed with two vertical members which allows the pipe load to be spread over a larger surface area of the pipe wall resulting in lower local stresses in the pipe wall. The shoe support with saddle uses a reinforcement saddle plate that is welded to the bottom of the pipe. The saddle plate usually subtends an arc of 90° or 120°. The saddle plate can be fabricated from the parent pipe material. However if the parent pipe has high wall thickness, the saddle plate is formed 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 shoes also serve as convenient means to provide slope on lines routed on pipe racks by using shoes of different heights to achieve the required slope. This allows the pipe rack to be kept at a constant elevation for the other lines which are not required to be sloped.
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.
A vertical pipe is conveniently supported by using two dummy supports which rest on structural members. Depending on the load and the available structural steel member one dummy support may be used instead of two dummy supports. A base type dummy support is used at control valve stations or pump piping. The base of the dummy support may rest on structural steel member, concrete foundation with base plate or on a spring support depending on the piping configuration and design. A trunnion support may also be welded to a elbow in horizontal direction to support the piping as shown above. The trunnion length should be kept as short as possible to minimize the bending moment and stresses it can cause at the weld joint on the pipe. Trunnion supports may be welded on reinforcing pipe saddles for heavy pipe loads to distribute the pipe load over a larger pipe wall area.
Flexible or Resilient Pipe Supports
When thermal movements are substantial, the pipe may lift off the support if the thermal movement is upwards or the pipe may be subject to large expansion stresses if the thermal movement is downwards and the support does not allow vertical downward movement of pipe. Flexible or resilient supports allow movement of piping while still providing support to the pipe. Thus flexible supports can be defined as pipe supports that provide supporting force throughout the expansion and contraction cycle of the piping system. Spring supports are examples of flexible or resilient supports. Flexible supports are necessary when the pipe needs to be supported and at the same time permitted to move in the vertical direction. In general, when the permitted vertical movement is low to medium and the variation between hot load and cold load is less than 25%, variable spring supports are recommended to be used. When the permitted vertical movement is large, constant spring supports are recommended to be used. There are other factors which will determine the type of spring supports to be used which are beyond the scope of this article.
In a variable spring support, the load imposed by the spring on the pipe varies with the pipe deflection and the magnitude of the load variation depends on the spring rate (or spring constant) of the support. The load variation is the product of spring constant and pipe thermal movement. If a low variation in load is required such as on pump piping supports, it is desirable to choose a spring support with a low spring constant.
In a constant type spring support, the load imposed by the spring on the pipe remains constant throughout the range of thermal movement of the pipe. Constant spring supports are used when the pipe deflection is large (above 50-75mm) and may be used near strain sensitive equipments such as pumps and turbines. The choice of using a variable or constant spring support will be based on the results of piping stress analysis.
Pipe Restraints
Pipe Guide Restraint
Pipe support guide permit axial movement of pipe but restrain the movement of pipe in the lateral direction. Guide may be designed to limit lateral movement in only one direction or lateral movement in two directions. When a guide is used to restrain pipe movement in two directions, the guide also limits the rotational movement of pipe to a certain extent depending on the gap between the guide and the pipe shoe. Guides may be provided as structural members on shoe supports as shown in figure below or they may be provided in the form of U-bolts, Clamps or structural members if the pipe is not provided with a shoe. For pipes routed in vertical direction, guide supports usually take the form of boxed structural members around the pipe or pipe shoe.
Guides are provided whenever the position of pipe has to be maintained on the supports such as on piperacks. If guides are not provided on piping routed on racks, the pipe may snake after some cycles of expansion and contraction. Guides may be required on vertical runs of pipe to take care of wind loads. Guides are frequently provided on pump suction nozzles to minimize transfer of thermal loads on pump nozzles.
Pipe Axial Limit Stop Restraint
An axial limit stop restrains the pipe movement in axial direction but allows pipe movement in a direction at right angles to the axial direction. It is also commonly referred to as a line stop. Depending on the requirements of stress analysis, the gap between the limit stops or line stops could be different on either sides of the structural steel member. This gap will ensure that the pipe is permitted to expand due to thermal growth for the specfied distance until the gap is closed and the structural steel limits further thermal movement. A limit stop may be used in combination with guide support to provide an anchor support.
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.
It may be noted that the ASME B31.3 now explicity permits the flexibility of restraints to be taken into consideration. As per para.319.2.1 of ASME B31.3 "If restraints are not considered rigid, their flexibility may be considered in determining displacement stress range and reactions".