Pipe support design for fiberglass piping follows the same same fundamentals as other metallic piping. Supports are designed to limit the deflection and allowable stresses within the design limits. Manufacturers recommendation are to be followed for supporting fiberglass piping.
Fiberglass reinforced pipe is an anisotropic composite material which results in different modulus values in tensile, bending and compression. The modulus varies again depending upon type of resin, amount of glass, and reinforcement orientation used. Care must be taken to ensure that the appropriate modulus is used, which should come from the manufacturer.
Fiberglass piping does not have the same outside diameter as steel piping. This requires the support components such as clamped shoes, U-bolts etc. to be customized for fiberglass piping. The thermal expansion of fiberglass piping is two to five times that of steel. Additionally the axial expansion due to internal pressure can be significant for fiberglass piping. However, note that the loads due to pressure expansion apply only at direction changes.
General Guidelines for Supporting
For supporting fiberglass piping systems, the following general guidelines should be given due consideration:
- Fiberglass piping should not be supported directly on steel structure.
- Supports should be designed to avoid point loads on the fiberglass piping.
- Supports must be stiff enough to prevent pipe deformation.
- Support components must be sized to properly fit the pipe.
- In-line equipment should be independently supported.
- Supports should not be located on pipe joints.
- Pipe shall be protected against abrasion at support locations.
Effect of Modulus values on Stress and Supporting
The modulus values of fiberglass piping are significantly less than those of steel piping. The axial modulus of elasticity is less than 1/20th that of steel. This results in fiberglass piping being significantly more flexible than steel pipe. This property makes offset legs in fiberglass piping systems very effective at absorbing thermal expansion of the piping. However, this property also limits support spans to lower values than for steel pipe to limit the deflection between the supports. Since fiberglass piping has low modulus values, the end loads created due to thermal and pressure expansion are lower than that of steel.
Expansion loops can also be employed in fiberglass systems to introduce flexibility in the piping system.
Resting Supports
Fiberglass piping should be supported at spans recommended by the manufacturer. Rule of thumb is to limit the deflection at the center of span to 0.5 inch or 12.5 mm. This deflection will usually limit the bending stress on pipe and bearing stress and supports to within allowable values. The supports should be located on straight sections of pipe and not on joints and fittings which can obstruct free movement of pipe during thermal expansion.
Supports should provide sufficient bearing area to distribute the load over a reasonable area of the pipe. It is common to specify a minimum length of a support to be 30% of the diameter of the pipe to limit the bearing stress to less than 50 psi.
Pipes supported on sleepers or structure are provided with saddles which are bonded to the pipe to protect the pipe against abrasion damage due to piping thermal displacements. The saddle angle can vary between 120° to 180° depending on the size of the pipe. Large diameter pipes require larger saddle angle. Saddles also provide sufficient stiffness to the pipe at support locations and prevent the pipe from getting ovalized or deformed at the support location. Shoes if required are clamped type shoes with neoprene liner on the inside. Spacers may be provided at the clamp bolts to ensure that the clamping forces do not crush the pipe. Valves and inline special components must be supported independently. Most fiberglass manufacturers recommend supporting heavy components at the flange bolts to avoid over-stress on the adjacent fiberglass piping.
Anchors and Line Stops
An anchor must positively restrain the movement of the pipe against all applies forces. Anchors can be installed in both horizontal and vertical directions. Anchors or Line stops can be designed on fiberglass piping by adding encirclement saddles or pads on either sides of the clamp. On small bore piping, the saddle can be a 180° saddle instead of full encirclement. When applying unrestrained joints or mechanical couplers, pipe lines must be anchored at each change of direction.
A combination of anchors and guides are an effective way of dealing with expansion in fiberglass piping. The relatively low modulus of fiberglass pipe allows it to absorb thermal stresses as compressive stresses in the pipe wall.