Pipe Support friction plays a significant role in pipe stress analysis. All piping stress engineers must be aware that while modeling supports or restraints we have to enter the frictional coefficients at the pipe support surfaces. The value of this coefficient depends on the supporting surface material and surface roughness. During the project bidding stage (ITB Document) the client generally provides the information regarding which friction factor to be used for which surface.
Also, every EPC organization prepares its own guideline for using standard friction factors in case not available in the ITB document. The following write-up will try to provide an idea regarding which coefficient of friction to be used in what situation. This can be used as a guide only. However, project-specific data or information will override any word mentioned here.
What is Friction Coefficient?
Coefficient of friction provides a measure of the amount of friction existing between two sliding surfaces. Coefficient of fiction is defined as the ratio of Normal force to the resisting force. As the friction coefficient is a ratio of two forces, it is unit less or non-dimensional.
Coefficient of Friction for Various Surfaces
The coefficient of friction factor depending upon the supporting interface (i.e, the junction between Top of Steel and Bottom of Pipe or Bottom of Shoe/Cradle) shall be applied at all vertical restraints (+Y or Y supports) locations as mentioned below. But if ITB for any project provides separate data then those data shall be considered.
- Carbon Steel to Carbon Steel: 0.3
- Polished Stainless Steel to Polished Stainless Steel/Graphite: 0.15
- Teflon to Teflon/ Polished Stainless Steel: 0.10
- Concrete to Carbon Steel: 0.5
- Pipe to Roll Support: 0.01 to 0.05
- Teflon to Carbon Steel: 0.2
- Pipe on Sand Soil (pipe laying on the sand): µ=0.4
There is various philosophy among EPC companies regarding the use of co-efficient of friction for guide and directional anchor supports. Some organizations prefer not to use any frictional co-efficient for horizontal supports. However if used the same can be taken from the above table (normally 0.3 is used if no special arrangement is made).
Using Support Friction in Caesar II
Refer to Fig. 1 to understand the Support Friction Co-efficient application philosophy in Caesar II input screen.
Friction coefficient must be applied for all vertical sliding restraints or Rest type of supports. However, there is variation in the philosophy of friction coefficient application for guide and line stop supports. Some organization uses friction co-efficient for guide and limit stops while some organizations does not use it.
Again there are differences in support friction considerations for occasional stress calculations. Some organizations make friction multiplier in caesar ii load cases as zero. They believe that occassional events like seismic, wind, slug, etc being dynamic events the application is quick and friction does not get time to act in those instance. Where some organization keeps the friction multiplier value as 1. So, follow what your organization is following during analysis.
Significance of Support Friction
As you can see in Fig. 1, In CAESAR II, the friction is activated at any restraint by entering a non-zero Mu value. Generally, no friction factor is used while supporting using rigid hangers.
In general, friction will always be present whenever a pipe will move. The frictional force will act against the pipe movement. The maximum frictional force against the pipe sliding will be Mu (Coefficient of friction) times the vertical restraint force. If a pipe movement is in +X direction the frictional force will act in -X direction. Friction creates a non-linear effect on piping system.
As friction opposes pipe displacement, it will create high stress on the pipe and higher loads on the connecting equipment.
Many a times in Caesar you may find that due to non-linear effect the iteration in Caesar II analysis does not converge. In that situation by deleting the friction from that specific support node can converge the solution and result is obtained.
Reducing Pipe Support Friction Forces
To optimize structural design, qualify expansion stresses, and qualifying nozzle loads, it may be required to reduce frictional loads. As we know that,
Frictional force=Coefficient of Friction (Mu) * Vertical Normal Force at the support location.
As vertical force is usually constant, the friction force can easily be reduced by changing the friction coefficient.
This is the reason that pipe stress engineers sometimes used SS/PTFE/Graphite plates at the contact surfaces to reduce friction coefficient which in turn reduces the horizontal frictional force. Special support design arrangements are made to use PTFE/Graphite/SS plates.
However, some organizations argue that over the times the reduced friction coefficient does not remain valid and friction coefficent increases. Also, there may be inclusion of sand particles in between the contact surfaces where frequent sand storm is expected. So, many organizations does not prefer to use PTFE/Graphite slides plates to reduce frictional loads.
Sometimes, pipe roller supports or Spring Hanger Supports are used in place of normal resting supports to reduce support friction.
Some Important Consideration related to Pipe Support Friction Coefficient
In case when Sliding Plate is required, add the comment as “(PTFE/Graphite) Sliding Plate Required” and mention friction factor μ=0.1 /0.15 respectively depending on temperature” on stress sketch. Use Teflon (PTFE) Slide plate up to a Temperature of 204 degrees Centigrade, above which use graphite plate (up to 540-degree Centigrade).
Normally the friction factor shall not be applied when modeling bottom type spring. But sometimes ITB document/Client could insist on friction modeling of bottom type springs, in that situation friction factor could be applied as per requirement.
When the pipe/shoe is supported on the welded rod on the structure, a friction factor of 0.25 shall be considered.
For buried or underground pipes frictional forces are very high.
For Compressor and Turbine piping systems, some organization wants to qualify the equipment nozzle loads in both cases, using friction and without friction.
For Zigzag pipelines its preferrable to check pipeline displacements without support friction and decide the sleepper or support dimensions. There are many instances of pipe movement at the support location to be more than the pipeline design displacements considering support friction. So, by designing support dimensions without considering friction effect solves the problem.
Further Studies on Support Friction in Pipe Stress Analysis
To learn more on pipe support friction and its implication the following paper by L C Peng is highly beneficial: Treatment of Support Friction in Pipe Stress Analysis by L C Peng