PTFE lined pipe calculations

[Grzegorz.T]

Greetings,

I have to prepare documentation for notifed unit and I encountered some problems. I have to calculate minimum wall thickness and flange connection acc. to ASME for PTFE lined pipe (A105). It's already prefabricated with flanges and manufacturer guarantee it like CL150 pressure resistance.

Should I calculate it like normal steel pipe made of A105 carbon steel or is it another procedure for that kind of pipes?

Thanks in advance
Grzegorz

 

[LittleInch]

Govern the large difference in Youngs modulus between steel and ptfe, the steel is doing 95%+ of the work so normal practice is to ignore the liner for stress but include it fit weight and ID flow modelling

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[Grzegorz.T]

Thanks for that information. It's really helpful. I'm wondering only about one more thing: vent holes. Do We completely ignoring them because of their small dimensions?

 

[mk3223]

The design requirement of the vent connection in the piping system is the same regardless the pipe lined or not. A flanged spool with a small bore branch and PTFE lining may be used for the vent and drain connections.

 

[BigInch]

PTFE is what DuPont calls Teflon. Ignore the PTFE for pressure design calculations.

According to my somewhat limited experience in their use for pressure piping, coatings, liners and claddings are always ignored for pressure design, unless of course should the application of the cladding somehow weaken the base pipe.

"He's declaring war on the planet itself."- Vicente Fox

 

[ehbadger]

As others mention, the lining is effectively ignored when doing pressure calculations. The steel is doing the overwhelming majority of the work.

Regarding the vent holes, I am assuming you mean the small holes in the steel pipe wall for permeation evacuation. These are generally ~1/8" and are ignored, in my experience (I don't generally do ASME piping calcs first-hand but I've never heard it come up as a factor). However, depending on the process you may end up with some corrosion over time at these exit points. There will be some limit to how large this vent hole in the steel can become before your PTFE is at risk of blowout. Obviously, this is going to be depending on pressure, temperature, and your particular liner attributes.

You could also reach out to resistoflex or other lined pipe manufacturer, they will generally have some good guidance on how they calculate things to fit ASME code. One example is on PTFE lined tees; the way they line them basically means a full-pen weld of the steel is not possible, so they did some modeling to prove it is still strong enough and safe and have some sort of variance through ASME to still be able to say it qualifies as B31.3 piping.

 

[BigInch]

An approved thread-o-let will handle metal loss from drilling holes for the piping at vent holes.

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3. Write down the answer.

 

[ehbadger]

Yes, these added couplings are becoming more and more standard for lined pipe manufacturers. CRP has them standard, as does Baum (I believe). Resistoflex's ATL line does as well. The nipple extensions serve to vent the permeants further away so the vapors don't interact with moisture in the air right at the pipe wall and cause corrosion. They are also necessary for insulated piping (which is rather helpful for hot PTFE service to prevent liner blistering).

Also note, some smaller diameter piping as well as the "loose" lined sections i.e. "Thermalok" are fabricated with threaded flanges and the liner flared over; on shorter runs they won't require vent couplings since the vapors can vent through the threaded flange connection.