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 co-efficient 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.
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: 0.4
- Pipe to Roll Support: 0.01
- Teflon to Carbon Steel: 0.2
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).
- No friction factor to be used while supporting using rigid hangers.
- 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 Temp 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 then friction factor of 0.25 shall be considered.