Stress Relieving

[jcfoley]

A colleague of mine is upset with a request of our ASME QC manager. It involves the min hold time and temperature for stress relieving a large number of pressure vessels.

My colleague did the calcs and drawings with each showing the min required hold time and temperature for each vessel. Each of these vessels is different in both overall size and wall thickness. In order to keep costs down, he wanted to fill the oven completely. The QC manager wants all the calcs and drawings redone to include a hold time and temperature based on the thickest vessel in each group so that the times and temperatures all match. This involves engineering doing all the planning for what vessels go into the oven at the same time which is a bit silly to me.

The calcs and drawings only show the minimums required. Does anyone know of a specific code reference that requires the drawings and calcs to match the temperature charts exactly in this manner? Has anyone else run into problems like this?

Thanks

 

[metengr]

Follow the advice of your QC Manager to avoid any nonconformance. The minimum hold times for post weld heat treatment are based on nominal thickness. If you have vessels of varying thickness, the thickest wall thickness should be used for heat treatment in the furnace. If you mix pressure retaining items of varying thickness during PWHT, you may not meet the minimum hold times.

 

[stanweld]

Simultaneous heat treating of multiple vessels in a furnace is often done for economy. Care must be taken to assure that the minimum hold time at temperature for the thickest vessel is obtained and that the thinnest vessel is not overheated or exposed to an excessive hold time at temperature, especially if the vessel is subject to impact testing.

While there is no Code mandate that the thinner wall vessels show the higher minimum hold times for the thickest wall vessel, it is considered a good idea to avoid confusion.

 

[jcfoley]

Gentlemen,

I appreciate the response. I have listed an example below with a few more questions at the end.
Example:

Project Manager A has a job with 2 vessels of the same thickness.

Project Manager B has a job with 6 vessels with the maximum one having a thickness of 1.5"

If, for economy, the oven is loaded with both jobs, does Project Manager A have to redo all the calculations and prints based on Project Manager B's job? There are no maximum temperatures listed in UG-56 that I know of. The prints refer specifically to min times and temperatures, not the maximums. If the hold time and temperatures exceed the mins, then why would there be a problem?

All this being said, stanweld has mentioned that no ASME requirements require this. If this is required as a matter of practice, I would expect this to be detailed in the QC manual.

Overall, this seems to be a matter of preference, not code. I do not want engineering doing oven planning so some compromise is required. I may have to set both and upper and lower limit like "400 deg F (+75 deg - 0 deg) for 4 hours (+1 hour - 0 hours)"

 

[metengr]

The designer and/or fabricator have sufficient leeway to optimize a particular fabrication process, as long as minimum Code requirements are met.

Regarding PWHT, the requirements in Table UCS-56 can be applied to one or more components simultaneously. One has to be aware of the effects of PWHT temperature and time on impact and/or mechanical properties of materials, if applicable. Longer PWHT times can certainly be used with caution noting a reduction in impact or certain mechanical properties (which may or may not be applicable in your application). Also, there are maximum temperatures for PWHT, these being the lower critical transformation temperatures for the material.

The Code does not care about the designer or fabricator saving money or production time, it is about assuring safe operation of the component in service. If you can optimize your furnace loading and still meet the necessary PWHT time at temperature minimum requirements with some leeway in time, you have met your objective.

 

[jcfoley]

Metengr, I wonder if you read minds. This is exactly what I was thinking when I wrote the above including the critical transformation temperatures as the max.

However I need a correction to my previous as I had UG-56 which is actually UCS-56.

Thanks!

 

[jamesl]

This could be a code issue or QA/QC issue.
ASME code does not have limit on PWHT temperature or holding time. However, for certain mateials, it does require test specimens to go through simulated PWHT. The production PWHT cannot deviate so much away from the the simulated one.
A vessel shop always have a internal QA manual. If your drawings call for a a PWHT temperature/time incosistent with PWHT chart, QA manger has the right to ask the engineering to revise the drawings to make them consistent.

 

[GenB]

You do not have to change any drawings,
The Code addresses only minimum requirements, your thicker vessel will manage the time in the furnace by staying longer,
the thiner vessels will stay there until the cycle has finished and there is not penalty for that, I do not know how to put it more clearly.
ER

 

[GenB]

Also your QCMgr is probably right in his/her approach to you but he/she certainly does not know the Code procedures,
that's why there are Engineers out there,(and consultants),
even if they are not always right in what they do because the Code is so large no one knows everything in this ASME Business.
... Then is your job to fix it and not by getting upset.
I think the QCMgr is always right even if he/she 's wrong.
Myself I am still learning the basics.
regards, er