MDMT
MDMT
(OP)
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.





RE: MDMT
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.
RE: MDMT
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.
RE: MDMT
RE: MDMT
Thanks Again!
RE: MDMT
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