Selection of low friction supports

[garfio]

I would like some guidance regarding the selection of low friction supports: I need supports for a steam lines (800 F, from 6" to 20") to be mounted on a pipe rack, and I am looking at rollers and shoes with low-friction pads. I found that for both types, the manufacturers recommend friction factors of about 0.15. However, I also found some comments saying that after some time exposed to ambient (my pipes are outdoors), the friction factor for rollers will increase due to corrosion and dust.

Anybody has experience to confirm or deny this claim?

Besides, I would like some input on the following:

In the case of rollers: When a guide is required in the piping system, is it possible to take credit for the lateral load that the roller could take due to its curved shape? If not, what type of guides are typically used?

For shoes: The manufacturer limits the temperature at the anti-friction pad. Is there some practical rule/method to estimate the temperature at the pad of a shoe to verify the compliance with this limitation?

In general: In determining the need or not of low friction supports, is there a rule of thumb that suggests that low friction is not needed for small diameters? My thought here is that for small diameters, the additional forces imposed to the pipe rack due to friction will probably be low enough to not being of concern.

Thanks

 

[unclesyd]

Piping Technology just had a Webinar last week on piping slide plates so you might contact them to get a copy of the program.

http://www.pipingtech.com/products/ptpcat/slideplate/ptfe.htm

 

[garfio]

Any comment on rollers?

 

[DSB123]

garfio,
Unless you have a comprehensive maintenance regime to routinely grease the rollers and replace damaged rollers then they will not work as intended. I have seen many types of roller design in the field after years of use and where maintenance has not been sufficient then they became useless. I have seen the rollers with "Flats" worn on them - so they don't roll.

On the low friction pads (PTFE) to get the optimum friction factor (i.e. as low as you can) then normally you have to achieve an effective bearing stress on the PTFE so the old adage of putting large PTFE pads in is not the case. Also outside if you do not ensure the shoe is large enough to cover the PTFE in all extreme pipe movements debris can get onto the PTFE and then it's a "goner". Also you will require some sort of heat break to ensure the heat from the steam does not get to the PTFE or do some calcs to check the temperature at the contact surface. Normal practice is to have SS polished plate welded to the bottom of the shoe which contacts the suitably sized PTFE pad.