MDMT 1

[meca]

We have a cold separator vessel which has an MDMT of -50 Deg. F, which was achieved by impact testing the material. We are now wanting to further decrease the MDMT of the vessel to -55 Deg. F. My question is that as I understand ASME Sec. VIII Div. 1 and API-579, you either impact test the material or you use the impact exempt curves to set the MDMT. I know you can set the MDMT based upon Fig UCS-66 and then reduce the MDMT if you design to a lower stress by using Fig UCS-66.1; however, can the reduction in Fig UCS-66.1 be used to further reduce the MDMT determined from impact testing? I don't think this is allowed by ASME or API-579, but that's what I'm trying to verify? I can't seem to find any other feasible alternative other than impact testing all components and welds in the existing vessel, or building an entirely new vessel. Any ideas would be appreciated.

 

[weldtek]

This doesn't answer your question directly, however, here's a couple of ideas. One, if your not already PWHT'ing per Code you could consider applying the provision in UCS 68(c)
Second, dependent on the material in question, you may be able to apply Table UG 84.4. If you were using SA516-70 this may get you where you need to be.

 

[metengr]

thread292-116022
may be of value to you - look for the post with the link that contains an attached file. This paper does a nice job of summarizing impact testing requirements and exemptions under ASME Section VIII.

I believe the reduction in MDMT would be considered an alteration of this vessel under the rules of the NBIC. An alteration would require approval of the AI and possibly the Jurisdiction. I believe you are also right in that you would need to perform additional impact testing of the vessel material to support a lowering of the MDMT. One option is to use equivalent materials and welding processes and test coupons to determine if the material could achieve the desired impact results at -55 deg F.

 

[weldtek]

I believe Metengr is correct in saying that lowering the MDMT of an existing vessel constitutes an alteration by NBIC rules, certainly so if additional testing is required. My comments pertained to new fabrication, not an existing vessel, sorry for the confusion.

 

[meca]

Thanks for the feedback. I actually just found the answer to my problem. UCS 66(i) allows you to use the impact tested MDMT, and further reduce by using Fig UCS 66.1 if you design to a lower stress. This is exactly what I was trying to determine. I realize it will be considered an aleration, but we wanted to make sure that we had a valid approach before we went down this path.

Thanks Again!

 

[Shmulik]

Hi Chris,
I'm not sure if the following is relevant for your case, but maybe the necessity of PWHT should be taken into consideration.
As far as I recall, PWHT becomes mandatory acc. to UCS-68(b) when MDMT is colder than -55 ºF and coincident ratio in UCS-66.1 is 0.35 or greater. Thus, in case PWHT wasn't carried out in your vessel, maybe now it is required.

Regards,
Shmulik, B&P