There are many different pipe support designs, types, and styles. They include hanger type, support shoes, saddles, bearing surfaces (e.g., structural members, concrete plinth blocks, etc.), spring type, dummy legs (trunnions), slide plates, sway braces/snubbers/struts, stands, sleeves, rollers, straps, clamps, and restrictive guides or anchors.
An understanding of the function and design of pipe supports is required to manage their integrity and piping systems’ integrity. Pipe supports can be subject to various damage mechanisms as well as significant stresses from static loading and thermal movements that can affect the pipe support itself and the supported piping and piping components.
Piping supports are usually designed to carry the weight of piping, including valves, insulation, and the fluid in the pipe, INCLUDING HYDROSTATIC TEST CONDITIONS.
Properly designed piping supports must ensure that:
- pipes and piping components are not subjected to unacceptable stresses from sustained loads, external loads, or vibration;
- the piping does not impose an unacceptable load on the connections to the equipment it services (e.g., pressure vessels, pumps, turbines, tanks);
- thermal movement is controlled within allowable displacements so as not to interfere with adjacent piping or equipment and be maintained within allowable stress levels;
- the potential for corrosion, cracking, and other in-service damage is minimized.
Pipe support design considerations can differ depending on the support type or style. While some pipe support manufacturers offer innovative and proprietary designs to eliminate or minimize some of the potential damage mechanisms, the following is a list of some special piping support design parameters to take into consideration.
Pipe Shoes—It is important that the shoe is long enough and/or guides or stops are provided on the structural steel to prevent the shoe from coming off the support, which could cause tearing or other damage to the pipe. Also, some pipe shoes may trap water between the pipe and shoe (e.g., clamp-on, bolt-on, saddles that have been stich welded, etc.) and make inspection difficult to determine the condition of the pipe.
Pipe Sleeves—Pipe sleeves are often used where pipe passes through a wall, under a roadway, or through an earthen berm. When used, design precautions should be taken to prevent corrosion on both the pipe, as well as the pipe sleeve. Centering devices should also be considered to keep the inner pipe centered and prevent coating damage and corrosion. Fully welded and/or sealed sleeves may be considered if loss of containment detection and control are necessary. It should be noted that sleeves can make future pipe inspections and examinations much more difficult.
Doubler Plates, Half Soles, and Wear Pads—Additional plates may be attached to a pipe system at points where the pipe rests on bearing surfaces. Plates should be fully welded to avoid crevice corrosion except in hydrogen charging environments, where a weep hole should be included that will not lead to moisture ingress. The use of adhesive bonded stainless steel or composite half soles may be considered, but it is very important to make sure that the adhesive is fully bonded and maintained so as to effectively eliminate water entrapment. Galvanic corrosion should also be considered when using dissimilar materials for this
purpose.
Dummy Legs (Trunnions)—Historically, dummy leg (trunnion) supports were simple open-ended lengths of pipe welded to a piping system from which the piping system was supported. An open-ended design can allow moisture and debris to become trapped inside the support and cause corrosion of the support itself and of the pipe. The dummy leg design should include, as a minimum, drain holes no smaller than 1/4 in. (6 mm) located at a low point, with the unattached end of the support being fitted with a fully welded cap or end plate to prevent debris or animals from entering. Trunnion design can be improved by using solid sections such as “C” channels or “І/H” beams to reduce the risk of this problem. However, even solid member sections can trap water and debris depending upon their design and orientation. Incorporating a fully welded doubler pad to the pipe at the trunnion attachment location can provide additional corrosion protection and may help to more evenly distribute loads. The end of a dummy leg support that is not attached to the pipe may or may not be anchored or restrained.
Supports on Insulated Lines—Special attention is necessary for the design of supports on insulated lines so as to minimize the possibility of water ingress and wicking of water into the insulation.
Accessibility—The accessibility, and therefore inspectability/maintainability, of pipe supports should be considered during design.
Welding—Paths for water ingress into hollow supports can be minimized with the use of fully welded seams. Avoid welding undercut or excessive penetration. Welding defect associated with supports can contribute to loss of containment events and, in some cases, be of sufficiently small size so as to make leak
detection and source identification difficult. In hydrogen charging environments, a weep hole should be provided to avoid buildup of pressure between the plate and pipe.
Anchors and Restraints—The attachment of an anchor or restraint to a pipe should preferably encircle the pipe in order to distribute the stresses evenly about the circumference of the piping component(s).
Piping Support Damage Mechanisms
External Corrosion at Supports
Corrosion of supports, and their associated pipe work, may occur in areas of protective coating breakdown where water and airborne debris become trapped (often referred to as “touch point corrosion”). Support design (i.e. support beams) can significantly contribute to this issue. Corrosion rates can be increased by local factors.
Elevated temperatures from hot piping (e.g. steam piping) can increase corrosion, including fireproofed supports. Other factors such as heat tracing or steam trap drain outlets, or where moisture is increased such as from proximity to cooling towers, and vegetation (creating a wet environment on the underside of the pipe and on any supports in the proximity) all can contribute to locally high corrosion rates.
Crevice corrosion can occur under any partially or nonwelded shoe, doubler plate, wrapper, or half-sole plate. Considerations should also be given to intermittent environmental conditions such as testing of fire suppression deluge systems, etc.
Dummy leg supports may trap water and airborne debris, leading to corrosion of both the support and the pipe.
When constructed using pipe, consideration should be given to capping all open-ended supports with fully welded caps or plates and providing a drain hole no smaller than 1/4 in. (6 mm) at the lowest position. For horizontal dummy legs, drain holes should be provided at both ends and the dummy leg should slope slightly
away from the pipe it is supporting.
Corrosion at Supports
The chilling effect of support on an elevated temperature pipe may be sufficient to cause product or water condensation inside the pipe. In some process services, this condensation may contribute to accelerated internal corrosion.
Fretting, Overstress, or Coating Damage at Supports due to Thermal Expansion
Thermal expansion and contraction due to temperature changes can damage protective coating systems and/or overstress both pipe and pipe supports.
Galvanic Corrosion at Supports
Galvanic corrosion is associated with the use of two or more materials of differing values in the galvanic series in close proximity to each other. For example, carbon steel supports welded to stainless steel piping may be subject to corrosion at a higher rate than stainless steel piping.
CUI at Supports
Supports that penetrate insulation systems may provide a potential for water ingress and subsequent CUI due to poor sealing at the penetration.
Environmental Cracking at Support
In predominantly alkaline process environments (e.g., amines and caustic), welding of supports to carbon steel piping either with or without post weld heat treatment (PWHT) can cause internal environmental cracking as a result of residual stresses.
External Cracking at Supports
Stainless steel piping may be susceptible to external chloride cracking where there is a source of chlorides above a threshold temperature. Pipe supports that trap water against the pipe can contribute to the susceptibility of cracking.
Foundation/Concrete Plinth Deterioration (Including Subsidence)
Deterioration of foundations and plinths is often a direct result of overloading the support and/or extended service life.
Vibration/Movement/Misalignment
Pipe vibration, movement, and misalignment can create a potential for fatigue, fretting, and/or overstressing of pipe and support members. Proper anchors, restraints, and movement allowances/guides should be considered during support design. This includes available travel of spring hangers.