Local Stresses in Piping 1

[SOROURA]

Of course it is well known that for high temperature service piping (specifically Power Piping), the piping is insulated with a considerable amount of insulation.

If there is a case were we have Main Steam/CRH piping with 165mm-191mm of insulation and, due to a layout problem (interference with a Steel Column) the only solution is to cut through the insulation by as much as 50% at the area of interference (let's say for a length of about a meter or two) to clear the Steel Column (NOTE: this does not mean reducing the insulation thickness to 50%, but just cutting -longitudinally- 50% of the thickness where the pipe and steel are interfering)

My questions are:
1. Since this will definitely result in an unbalanced thermal distribution across the pipe's cross-section, could this induce local stresses in the piping which could be significant?

2. If this could result in excessive local stresses, how can we evaluate these stresses to verify that the piping is not excessively overstressed (i.e. beyond Code Allowables)?

 

[prex]

IMHO it's a no to 1. A circumferentially non uniform temperature in the pipe doesn't give rise to significant stresses and overall expansion stresses are essentially unaffected.
Also, depending on pipe wall thickness, even the circumferential non uniformity of temperature is likely very minor, as conduction in wall will redistribute it.
The only problem I can see could be connected to local condensation conditions in the cold spot, if you can have that.

prex

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[mizzoueng]

I agree with Prex, but you can try other insulators to thin down the insulation and have the same R value.

I think the companies name is Aspen, they make something called aerogel. We used it on a downcomer in a 300MW plant right next to the boiler and saw no heat increases or releases. The material ended up being 1" (25mm) thick.

THey have a full line of stuff you can use and not be worried about your heat releases.

 

[MJCronin]

In many past power projects, I have seen this kind of field "adjustment" of insulation done several places.

I agree w-PREX.... not a local pipe stress issue.

However, there is another issue not mentioned here...

What about the pipe movement ? Was this evaluated ?

Main Steam/CRH piping can easily have thermal movements of up to ~8 inches.

Will this thermal movement force the piping up against the column (a very dangerous situation)or possibly cause the column to "peel away" the insulation ??

-MJC

 

[racookpe1978]

Good point: verify your pipe movement now: The pipe will move, it is just a matter of whether or not you have restrained it strongly enough (and in the correct direction) to prevent it from hitting the steel at that location.

 

[SOROURA]

Of course that's a very good point; however, it is imperative that an interference check is performed during the Hot Position of the piping. We did that (that's why I didn't mention it) because that really is the main concern in these kind of issues, but I wanted to go a bit further and feel satisfied that we are not overlooking a local stress problem.

Thanks for your comment MJCronin