Design considerations for pipe supports and the influence on thermal performance

A pipe support or pipe hanger is a component that transfers the load from a pipe to the supporting structures. The load includes the weight of the pipe itself, the content that the pipe carries, the pipe fittings fixed to the pipe, the insulation system, and finally, for external applications, possible snow and ice loads. The main functions of a pipe support are to anchor, guide, absorb shock, and support a specified load.

Pipe supports are typically made from a rigid material, such as wood or high-density insulation materials with protective shields. They are used in high and low temperature applications with load and operating conditions as their main design considerations. The compressive strength of the support resting on the shield must uniformly support the pipe insulation (i.e, no point loading), and it must not damage the insulation and keep coatings or vapor-retardant jacketing intact. The ideal pipe insulation support prevents direct transfer of moisture or heat from the shield to the actual pipe.

Pipe hangers that are installed in direct contact with the surface of the pipe, such as a pipe clamp, should have insulation applied over the hanger as well as the pipe. Cold piping supported by direct contact hangers should be insulated, to the maximum extent possible, to prevent condensation on the pipe and pipe hangers. Hot piping supported by direct contact hangers should have the hanger insulated for personnel protection as appropriate for the operating temperature and exposure conditions.

Pipe saddles are commonly used on large diameter piping where the load is exceptionally heavy or when contraction-expansion movement is expected. Pipe saddles are welded to the pipe and are used in conjunction with roller supports or on beam supports. Where possible, pipe saddles should be sized to be flush with the insulation material. If rollers are used and suspended from above, hanger rods should be spaced to allow clearance for the insulation. Insulation may be inserted into the void between the pipe and saddle if desired.

Several factors govern the pipe span and size of the cradle (length, rolled width and thickness).

Following are the formulae to make the necessary calculations:

Maximum span limited by pipe stress

Where:

l = maximum allowable span between supports in m or ft

S = maximum allowable pipe stress, in MPa or lb/in²

Z = section modulus of pipe, in cm³ or in³

W = uniform weight of pipe, contents, insulation, in kg/m or lb/ft

Cradle design is based on a maximum bearing capacity that is calculated by applying a safety factor (typically 5) to the compressive strength. For standard grade FOAMGLAS® ONE™ material (compressive strength of 620 kPa / 90 psi) this would give a maximum bearing value of 124 kPa (18 psi) which would translate to a maximum bearing value of 1.3 kg/cm², allowing up to 3.8 mm (0.15 in. ) deformation in the cradle area without adversely affecting the insulation

A 120° arc has been established as the effective bearing contact angle for insulation in clevis hangers. Use a 30° contact angle for rollers or point load applications – which include rollers, flat beams, legs of a channel welded together to form a truss, and structural tees. The insulation/pipe interface is the surface of greatest stress and the surface on which the 1.3 kg/cm² (20 lb/in²) limit applies. Cradles should cover the bottom 180° of insulation to facilitate installation and retention.

Contact area of interface

Where:

CA = Contact area of insulation/pipe interface, measured in cm² or in²

SL = Shield length, in cm or in.

AL = Arc length, in cm or in.

Maximum allowable insulation stress

Where:

S = Maximum allowable insulation stress

Ws = Weight of span including pipe, insulation, and contents, in kg or lbs

CA = Contact area of insulation/pipe interface, measured in cm² or in²

A mastic finish may be used under a pipe cradle. If metal jacket is used over an insulation system without a finish, the outer surface of the insulation in the area under the cradle may be coated with a mastic to provide more uniform bearing, prior to application of the metal jacket.

When excessive movement is expected, such as in steam systems, above 93 °C (200 °F), a direct contact pipe saddle may be used. However, pipe saddles that are installed in direct contact with the service piping, are generally not recommended for use on cold piping that requires a vapor barrier, or in shallow trench, vault, and tunnel systems where flooding may occur.

Typically, clevis hangers and rollers are only available up to 609 mm (24 in.) in diameter. On lines where the outer diameter of the pipe and insulation exceeds 609 mm (24 in.), a support designed by qualified engineers with provisions for movement is recommended. Typically these are clamshell type construction with high density insulation material inserts and slide plates welded to the support structure.

Penetrations, pipe hangers, supports and gaps/joints all represent significant design and application problems for hot work, as well as for cold work. In both cases, they can be potential paths for moisture entry as well as sources of air leakage via induced natural convection. This can eventually lead to issues with energy loss and the formation of corrosion. Corrosion at pipe supports is one of the most common causes of external piping corrosion failures in processing environments and can seriously endanger the structural integrity of the pipeline.

Finally, a common mistake is not using the correct insulation material for a particular support. Ensure that the insulation material is suitable for the demands of the system and code requirements. Only closed-cell insulation should be allowed in below-ambient conditions. Also evaluate the flame and smoke ratings, which will differ for some materials depending on the insulation thickness.

From an economic point of view, FOAMGLAS® insulation will keep its thermal efficiency over time as it is not susceptible to cell aging or decreasing thermal performance due to moisture penetration. This will help to eliminate hot or cold spots and help ensure that the overall system can run economically and operating costs are kept at a minimum.

FOAMGLAS® insulation is most suited to use in “dead weight load” support applications where the pipe is resting in a band style hanger, or on a rolled plate cradle. It is not suited for use in guides, (spider or cryogenic service) where thermal forces (live loads) can be in a magnitude much greater than simple dead weight load.