Hi there,
In my opinion, PTFE Pad provided for friction reduction purpose between relatively sliding parts of pipe supports should be considered for plane contact surfaces only, meaning for shoe-type (or equivalent, such as saddle or trunnion-type) attachments to the pipe components.
If you have a bare pipe sliding on the plane top-surface of the supporting structural member (with/without reinforcing pad attached to pipe and/or isolation pad fixed to top of steel), then there is a quasi-linear contact between the sliding components. You cannot count on the friction factor decrease here.
Certainly, this is my personal opinion, and I've applied it on all projects I've worked so far. Although I've met initial opposite opinions trying to make the second option acceptable, finally I've never met a documented and fully argued professional opinion to demonstrate the validity of that second case.
In all standard supports collections used by internationally recognized companies I've worked with so far, the standard supports intended to employ reduced friction factors were developed based upon plane contact surfaces (usually polished SS Plate welded on the bottom surface of pipe support attachment - shoe, saddle, trunnion sliding on he Teflon Pad bonded on the top surface of supporting structural member).
Maybe other Forum's members might have other opinions based on other criteria or circumstances.
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Dorin Daniel Popescu
Lead Piping Stress Engineer