As a piping system undergoes thermal expansion or contraction during its operating cycle, friction forces are generated at the pipe support locations. The friction force acts opposite to the direction of pipe displacement and could contribute significantly to reduction in piping flexibility and increased loads on anchors or connected equipments.
Friction forces must be included in the design of structural members of pipe racks and other structural supports. For small bore piping, the magnitude of friction forces may be insignificant and may be safely ignored. However, friction forces should not be ignored for large diameter, heavy wall thickness pipes where friction forces can become significant, restricting thermal movement of pipes and increasing the loads on restraints or connected equipments. Use of computer programs makes it easy to include the effect of friction in stress analysis calculations. Unless the pipe is restrained at the support point by a guide or line stop, the friction forces are considered to act in the longitudinal as well as lateral direction. When a piping designer submits the piping loads to structural department, the designer in the worst case scenario may assume all the friction loads to act simultaneously.
Some of the commonly encountered surfaces and corresponding friction factors are tabulated below:
| Materials | Friction Factors |
|---|---|
| Steel to Steel | 0.3 - 0.4 |
| Teflon to Steel | 0.1 - 0.2 |
| Teflon to Teflon | 0.1 |
| Sand to Pipewrap | 0.25 |
| Sand to Plastic coating | 0.20 |
| Sand to Concrete | 0.40 |
The significance of Friction Loads
Consider a piping system as show in figure below connected to two equipment nozzles. Let us assume that the friction forces are ignored and the reaction force on nozzle A due to themal expansion effects is RA. Now if friction forces are to be considered, the equipment nozzle A has to additionally overcome friction force at each support location. Hence the total reaction force at nozzle A considering friction is equal to:
Total reaction force FA = RA + μR1 + μR2 + μR3 + μR4 = RA + μΣR
For large bore and heavy wall piping, the friction forces can be significant and add to the reaction loads on the nozzle. Where the piping system is connected to strain sensitive equipment such as pump or compressor, it may be then prudent to reduce the reaction forces by reducing the friction loads. Similarly on pipe rack, the anchor bays have to overcome the friction forces in addition to the thermal loads. Large horizontal loadings on pipe rack structures are not desirable due to requirement of heavy structure, bracings and large foundations to overcome them. Hence measures for reducing friction loads can provide an effective means for reducing loads on structures and equipment nozzles.
Reducing Friction Loads
At times, specially when supporting heavy pipes and valves, it may be necessary for the piping designer to reduce the friction loads acting on the support to optimize the structural and foundation design. The magnitude of friction force is dependent on the weight of piping and the coefficient of friction.
Friction Force = Coefficient of friction (μ) * Normal Force (F)
where μ is the coefficient of friction and
N the normal force at the contact surface
Thus the friction loads on the structure can be reduced by reducing the coefficient of friction or the friction loads can be eliminated by avoiding contact between the sliding surfaces. The reduction in friction loads or its elimination can be achieved through several means:
- Using materials at the interface of pipe support and structure that have a low coefficient of friction.
- Using roller supports instead of sliding supports (since coefficient of rolling resistance is generally much smaller than the coefficient of sliding friction).
- Using hanger supports thereby eliminating the sliding contact.
Pipe Rolls
Pipe rolls are roller devices used to support horizontal piping when it predominantly undergoes axial movement and small vertical or lateral movements. The movement of the roller converts the sliding action into rolling action. Schematic of Adjustable Pipe Roller support is shown below. This type of arrangement may be used to support piping where:
- Significant axial movement of the piping occurs due to thermal expansion or contraction of the piping.
- Structural support is available below the piping to install the roller support.
- Vertical adjustment of the roll is possible to ensure the pipe rests on the roll.
Slide Plate Assembly
Low friction sliding plates can be used to significantly reduce the friction loads in the piping system. A typical slide plate assembly comprises of two sliding plates, the upper plate having a polished stainless steel surface and the lower plate embedded with low friction material such as teflon sheet, bronzephite, graphite or other low friction materials. The low friction material can be either bonded on the surface or contained within a recess of the bottom plate. The upper plate containing the polished stainless steel surface is welded to the pipe support element such as shoe and is mobile whereas the lower plate is welded to the fixed structure and is stationary. The dimension of the upper stainless steel surface is larger than the lower surface such that it is fully in contact with the upper stainless steel surface at the extreme displaced position of the pipe support. If the stainless steel surface is not large enough to handle the anticipated lateral movements, the plate edges can touch the teflon sheet and cause a dent and damage its surface. Additionally in such a situation, the full surface area of teflon sheet is not available and results in an increase in bearing pressure.
For pipe supports in the vicinity of strain sensitive equipment such as pumps and compressors, a sliding plate assembly may be provided at the base support to achieve a low friction coefficient of 0.1 and consequently reduce the loads on the nozzles.
Hanger Supports
Supporting the piping from higher elevation using a rigid pipe hanger is another way to keep the pipes from rubbing against other metal surfaces. However, this type of support may not be feasible where no overhead structure is available to attach the pipe hanger. Further, rigid hangers restrict vertical movement along the hanger rod that connects the pipe to the structural steel.