Insulate Flanges or Not?

[butelja]

I have a situation where a 12" 304H pipe operates at ~15 PSIG and 1250 °F. Gaskets are spiral wound, 347 SS windings with composite mica/graphite/mica filler. Bolting is A193 B16. Piping is insulated with calcium silicate insulation. In areas where the flanges are not accessible (no personal protection issues), would it be better from a gasket sealing standpoint to insulate over the flanges or not? I've heard arguments for both cases, and would like to have a discussion on this issue. Also, heat loss is not a concern, as the process deliberately "dumps" the waste heat to the ambient surroundings anyway.

 

[fredb]

Hello butelja,

I would not insulate the flanges for several reasons, one is the cost, another is that any leak may remain hidden beneath insulation long enough to damage a flange or the gasket or both.

 

[StressGuy]

You also can take credit for the reduced temperature in considering the allowable stresses in the bolting. Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[KernOily]

I'm guessing you're concerned about bolt load (pretension), yes? If the flanges are insulated, won't the bolt temperature be higher than if they were uninsulated? You could build an FEM model to model the heat transfer, estimate the relaxation of the pretension, and then adjust your pretension/post-tension procedure accordingly. That would save you the cost of insulation. You didn't say how many flanges you had, so maybe cost is no issue.

Is this a critical application? At one plant I worked in, they had H2 at 600° F and 5000 psig. They left the flanges uninsulated so they could see when they had flange leaks since the H2 always caught fire when it leaked to atmosphere. Thanks!
Pete

 

[01101998]

I had read some artice in Hydrocarbon processing some years ago that discussed stress failures due to change in temp due to insulation discontinuties.
Why don't you intelligent guys give it a thought.I would like to know your views about that.

 

[apiguy]

butelja;

I would be a little more concerned about Chloride-Stress Corrosion from the cal. sil. insulation. If the lines are subject to temperature variations or are steam traced any moisture may promote the iniation of cscc. We have seen several cases of this in our stainless steel systems that lead to many feet of piping to be replaced. I would recommend stripping the insulation approx. 2' on either side of the flanges and perform liquid penetrant inspection at the next available shut-down opportunity.

Just my two cents worth, Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[quark]

Hi All!

Stainless steels are very much prone to creep. The larger the temperature difference greater is the problem. Further I feel there may be likely chances of leakage from the flange joint. Generally spiral wound gaskets are used with tongue and groove flanges and replacement of these gaskets is a hard task.

I agree with 01101998.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.