Is aditional support required when PWHT a vessel? 3

[ColonelSanders83]

Hello All,

Here is the skinny.

We have a 66" diameter A516 Gr70 vessel with a 3/4" wt.

We have added three nozzles to one end (on the overhung portion of the vessel past the saddle support) and we are about to PWHT the modifications.

The costumers engineer has come out with a question from left field regarding extra support because of the elevated PWHT temperature.

We are now required to prove that we don't need additional support or exactly what support is required before we will be allowed to proceed.

I was under the impression additional support was not required, and I even showed the vessel would have a maximum stress of around 100 psi under a gravity load during an 1100F PWHT, and this is still not enough.

Please let me know your thoughts on the matter or if there are and codes or standard governing this situation.

I have attached a representation of the vessel showing the nozzles.

Any guideance would be greatly appreciated.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[jamesl]

I would not worry about a 66" diameter, 3/4" thick wall vessel under PWHT. You may need additional supports when you have thin wall large diameter vessels or large towers.

I wonder what kind of proof your customer's engineer want.

 

[ColonelSanders83]

The engineer is convinced damage will occur.

I don't think he will accept any proof and has even stated he doesn't believe the AI will sign off on the PWHT if we dont do anything to support it.

I also went back to the original vessel manufacturer and they stated that when performing the original PWHT the vessel was baked on its own saddle supports with no additional supports.

They only way I think I can placate him is if I can point to a code or standard which has a stament reqarding the actual performance of PWHT and when supports are neccassary (or not).

Unfortunatly this is not my field of expertise and it looks like this is governed by industry best practices.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[weldtek]

It's pretty common practice to place supports no farther than 15' apart, but, certain factors may come to bear that warrant placing them closer together than that. It appears that your going to be heating a 360 degree band outside the support so your customer may be concerned that the end should be supported. If so, that would be a pretty simple matter.

 

[jamesl]

I never heard of the 15' rule as a vessel engineer for fabricators and EPC companies. I don't think there is a basis for such a rule without consideration of wall thickness vs vessel diameter.

I have seen calculation done for vessel supports during PWHT. However that was for 30' diameter X 80' long vessel supported by shipping saddles during PWHT.

 

[vpl]

The engineer is convinced damage will occur.

I don't think he will accept any proof and has even stated he doesn't believe the AI will sign off on the PWHT if we dont do anything to support it.

Would doing an "aw shucks" approach and reversing the question work?

In other words, instead of you trying to prove the negative, when it sounds like he's made his mind up, approach him with a puzzled look and ask if he can supply the reference that says the support is needed.

Let him do the wild goose chase, especially if it is going to cost you money to add the support that he doesn't plan to reimburse.

Patricia Lougheed

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[unclesyd]

Here is some information from ASME on WRC 452 that may lead to more information.

If this doesn't workout you might call someone at Pinson Vally. They have always been a great help to me.

Does the latest edition of the ASME Code allow for lower temperatures and extended times for PWHT?
We used this approach numerous times on Pressure Vessels.

http://sections.asme.org/nwhss/PWHT/pwht-wrc452.pdfs

http://www.pvht.com/contacts.asp

 

[jamesl]

unclesyd,

The latest code still allows lower temperatures with extended time for PWHT with certain materials, such as P1 materials.

 

[stanweld]

While the shell will not need support during the local PWHT, there may be a need to support the nozzles due to their location depending on their weight. I have seen a number of instances where 12:00 located nozzles slightly sagged or 6:00 nozzles slightly pulled away during PWHT; both causing distortion of the sheel in so doing. I do not believe that this will actually occur to the detriment of the vessel being out of Codeo roundness tolerance based on the diameter and thickness of the shell.

 

[unclesyd]

jamesl,
thanks for the information.

If OP has any qualms about distortion during the PWHT you can lower the temperature and soak for the appropriate ammount of time. Doing this would alleviate all possibilities of distortion or buckling.

 

[ColonelSanders83]

Hello All,

Sorry for the late update, the letter from the original vessel manufacturer had its intended effect.....eventually.

The PWHT went down with no issues whatsoever. Thanks for the help on the matter.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[trottiey]

Thanks for posting back with the outcome! Glad to hear that reason prevailed.

Stress is a biological mechanism that improves strength at the expense of thought. It is counter-productive to solving technological problems.

 

[racookpe1978]

For the future, I'd recommend approaching the problem (the client guy presenting the problem) the following way:

"PWHT at yyy degrees will heat up the steel for hh hours. At yyy degrees, yield strength is now kkkkk psi. The walls are still 3/4 inch thick, and there is no internal pressure. Therefore, the walls will be under a max of kkk psi (from the dead weight of the head and new nozzles), and therefore will not distort."

And - you may actually find out that a 1500 or 1600 degree PWHT on a thin-wall horizontal vessel cantilevered a long way out past the support might actually allow some distortion. (That same thin-wall vessel sitting vertical might be ok as well.) While a 3/4 thick wall at a more realistic 1150 degrees for 4 hours will not move in either orientation.