Introduction to Pipe Supports

Introduction

This post gives some of the basic information about Introduction to pipe supports, design of pipe supports, types of pipe supports, location of supports, pipe spacing requirements, piping expansion etc.

All pipes have to be supported adequately. Pipe supports is a general term that actually is split into two families.

Primary pipe support systems

The primary pipe supports systems are those supports that are a part of the infrastructure and fall under the prime responsibility of the civil or structural department.

These primary pipe supports systems may also be referred to as piperacks, pipeways, pipe alleys. These support systems may be major or minor and they may be overhead or sleeper pipe racks.

Secondary pipe support systems

The secondary pipe support systems are more a part of the piping systems and as such fall under the prime responsibility of the piping department. Consists of a broad family of devices and actually include more than just supports.

(a) “engineered” devices

The term “engineered” pipe supports relates to devices that are non-static, one-of-a-kind, location and condition specific.

Constant support spring hangers ,snubbers and sway braces are some of the devices in this category. The piping stress engineer is the party/person who is responsible for the engineering of these.

(b) “miscellaneous” pipe support devices

Refers to a broad array of devices that includes items such as Anchors, Base Supports, Cradles, Dummy Support Legs, Guides, Hanger Rods, Pick-ups, Shoes, Trunnions, etc.

While finalizing the pipe supports, the following shall be considered,

1. Provision must be made for thermal expansion and contraction. Other wise this will result in thermal loads for the system.
2. All process and utility lines within the process area and all yard lines outside the process area shall generally be routed above ground, on civil supports.
3. All piping shall be adequately supported and restrained so as to prevent undue vibration, deflection, stresses or loads on equipment.
4. Piping shall be supported from below rather than being hung from another structure.
5. Smaller pipes may be supported from larger pipe lines.
6. Insulated pipes 50NB and larger shall be supported on 100 mm shoes. Bare lines shall be directly laid on the structure.

Design of Pipe Supports

The design of supports and restraints is based on concurrently acting loads like self-weight, loads due to thermal expansion, etc. It shall be designed for maximum loading conditions, including test loads, when temporary supports are not envisaged. Supports for relief valves must be adequate to withstand the jet reaction produced by their discharge.

The piping code ASME B31.3 states further that supports shall fulfill the following criteria

• Adequate to avoid piping stresses in excess of those permitted in the code.
• Avoid excessive interference with the flexibility for thermal loads.
• Shall not cause any leakage at joints or excessive pipe sag.
• Designed to prevent over stress, resonance or disengagement due to variation of load.
• Complete release of piping load shall not occur in case of a spring or support failure, and in case of any unintentional disengagement from the supports.
• Excessive thrusts and moments on connected equipment and in the supporting elements shall not exist.
• Should not undergo resonance with the imposed or fluid induced vibrations.
• Shall be constructed of steel.
• Shall be constructed of Alloy Steel when temperature limits are exceeded.
• Have threads for screw adjustment which shall confirm to ANSI B1.1

Between two fixed points most piping systems have a point where thermal displacements are zero and are ideal for locating anchors. Anchors shall be located to provide fixed stability to the system. Anchors shall be designed to withstand the algebraic sum of the forces at the maximum pressure and temperature at which the joint is to be used. The forces are,

• Pressure thrust (effective thrust x maximum area).
• Force required to compress or extend the joint in an amount equal to the calculated expansion movement
• The force required to overcome the static friction from the installed to the operating positions.

Supports fabricated from structural, hangers, springs, pipe straps, U – bolts, hydraulic supports, etc., are some of the more frequently used supports in the process industry.

Spring hangers are important in piping systems since they support the weight of the piping as it thermally expands.

Pipe spans are taken into consideration for locating the supports. A deflection of 1” (25.4 mm) is the maximum, which can be permitted.

A supporting element used as an anchor shall be designed to maintain an essentially fixed position.

Location of pipe supports

Supports for piping are located depending on the requirements of the piping configuration and availability of structures for supporting. The piping system should be considered as a whole for this purpose.

Location of supports is an iterative process in which several support configurations are investigated to arrive at a suitable one.

Pipe supports spacing

Spans are specified for spacing of supports based on a combined bending and shear stress considering that the pipe is filled with water. Tables are available for the same. When changes in a piping direction takes place the support span considered is 0.75 of the straight span.

Supports should be placed as close as possible to concentrated loads such as valves, strainers, etc., in order to keep piping stresses to a minimum.

Types of Pipe Supports

The major criteria for support selection is the magnitude of the expected load, displacements and space limitations. The design temperature for selection of pipe clamps, u bolts and other steel is the same as that for the fluid in the pipe. The support hardware must be designed to accommodate for insulation when the same has been provided.

The piping attachments and supporting structure in contact must be of compatible materials in order to reduce galvanic action. Sometimes, inert packing material may be required to be place in between.

The anticipated movement at each support point dictates the basic type of support required. The simplest support must be selected after evaluating the vertical and horizontal movement of the pipe.

Piping which is subject to significant vertical movement may require spring supports.

Pipe supports should be as simple as conditions allow. Stock items are used wherever possible, especially for piping held from above. To support piping from below, supports are usually made to suit from plates, pipes and pieces of structural steel. Following hardware is used to create supports.

• Rest Support : The weight of the piping is usually carried on supports made from structural steel, or steel and concrete.
• Hanger Support : It is a device which suspends piping (usually a single line) from structural steel, concrete or wood. These are generally adjustable for height.
• Anchor Support : A rigid support which prevents movement of a pipe or to control the direction of movement (thermal, vibratory etc.) along piping. Construction may be from steel plate, brackets, flanges, rods, etc. Attachment of anchors to pipe should preferably encircle the pipe and be welded all around as this gives better distribution of stress in the pipe wall.
• There are two types, fixed and directional. In fixed anchor support, both movement and rotation of the pipe or the pipe shoe is restricted in all directions. Directional anchors are used to force movement to occur in one direction while preventing it in other directions
• Tie Support : It is an arrangement of one or more rods, bars etc. to restrain movement of piping.
• Dummy Leg Support : In this an extension piece (of pipe or rolled steel section) is welded to an elbow in order to support the line. This part rests or anchors on some steel member.
• Guide : When total restriction of the pipe is not required, guides are used. They confine movement along the pipes linear axis only. This is a means of allowing a pipe to move along its length, but not sideways. Mostly they are used to maintain proper spacing of the pipe especially in pipe racks. Unlike shoes, they are not welded on to the pipe. These are welded on the structural support. When the pipe is on shoes, the shoes are guided.
• Shoe : It is a piece of metal attached to the underside of a pipe, which rests on supporting steel. It is primarily used to reduce wear from sliding for lines subject to movement. It permits insulation to be applied to pipe.
• Pipes are often insulated to prevent heat loss or for protection from thermal gradients. While supporting these pipes they cannot be allowed to settle on to the support as it will destroy the insulation. In order to protect the insulation, pipe shoes (“T” shaped structurals) are welded on to the pipe at the location where it rests on a support. This is welded prior to insulation. The pipes are elevated of the support due to this reason.
• Saddle : It is a welded attachment for pipe requiring insulation, and subject to longitudinal or rolling movement (resulting from temperature changes other than climatic). Saddles may be used with guides.
• Slide Plate : In this two slide plates of graphite, teflon or some special materials, fixed to steel plates, are fixed to the flat surface of the pipe support. These plates are faced for low friction able to withstand mechanical stress and temperature changes.
• Constant Load Hanger : This device consists of a coil spring and lever mechanism in a housing. Movement of the piping, within limits, will not change the spring force, holding up the piping; thus no additional force will be introduced to the piping system. Constant effort spring supports are used when the load variation is higher the 25%.
• Variable Spring Hanger : This device consists of a coil spring in housing. The weight of the piping rests on the spring in compression. The spring permits limited amount of thermal movement. A variable spring hanger holding up a vertical line will reduce its lifting force as the line expands toward it. A variable spring support would increase its lifting force as the line expands towards it. Both place load on piping system, and where this is undesirable, a constant-load hanger can be used instead.Variable spring supports are used when the load variation is less than 25%.
• Hydraulic Dampner : These are also called as shock snubber or sway suppressor. One end of the unit is attached to the piping and the other to structural steel or concrete. The unit expands or contracts to absorb slow movement of piping, but is rigid to rapid movement.
• Sway Brace : This is also called as sway arrestor. It is essentially a helical spring in a housing which is fitted between piping and a rigid structure. Its function is to buffer vibration and sway.

Expansion of piping

Expansion due to temperature gradients can be significant in piping especially for long runs and when they are connected to equipment nozzles. To accommodate this expansion loops, offsets or expansion bellows are often incorporated.

Lines resting on structural must be carefully positioned so that it does not slip away from the support due to expansion.