Exemptions in MDMT and Economic Cost Impact

[Sawsan311]

Dear All,

I have been going thoroughly through ASME SEC VIII and ASME B31.3 UCS-66.1 and Section 323.2.2 respectively.. I would like to understand why some designers tend to favor reporting the minimum exposure cryogenic temperature at pressure levels less than the design pressure stating that there will be a cost optimization benefit.. when I look at ASME SEC VIII, I see that impact testing is in fact not required if the MDMT >-48 C at all stress ratio levels and even also not required if the stress ratio < 0.35 and the MDMT is between -48 C and more than -104 C. In other words, if stress ratio <0.35, one can directly nominate the Exemptions in MDMT are allowed for stress ratio >0.35 subject to the below membrane stress condition to be met and shall be justified with supported calculations. Additionally, the maximum exemption level is by 50 C as per figure UCS-66.
if the designer tends so selected a minimum design temperature warmer than MDMT limits of the material then he has to justify that by stress wall thickness calculations and demonstrate that the membrane stress doesn't exceed 50 N/mm2.

I understand that selecting lower design temperature warmer than the MDMT limiting of the material dictates safety procedures of not starting the equipment whilst cold such that it doesn't get pressurized beyond the ductile brittle transition point. However, I need to understand the details behind the cost optimization in the above cases where already some MDMT limits of CS grade materials have latent exemptions already allowed.

Thanks

Regards,

 

[georgeverghese]

From a greenfield design perspective, adopting such an approach is not a good idea. It increases operating risk, as you have pointed out, and that risk is borne by Plant Operations for the entire design life of the facility. There must be a large CAPEX benefit for this approach to be taken, and Operations acceptance is also required.

From a brownfield design perspective, Operations acceptance is key. TSLL loop may have to be set as a 1oo2 voting configuration. Operators may be impatiently waiting for this system to warm up before restart/ repressurisation is possible - so we are looking at production loss also. They may ask you to quantify this warmup wait time.

In piping systems, it is difficult to estimate what the actual stress levels are on the pipe when the pipe is cold - I have not seen anyone taking this approach on piping systems for this one unknown.

 

[Sawsan311]

Thank you very much georgeverghese for your reply..
I also wanted to understand where the CAPEX savings are really embedded when applying the exemptions approach.. is it in impact testing.. thicnkness.. anyways the vessel/ piping needs to be designed for MAWP.. can you please elaborate how CAPEX saving is actually achieved.. even if the material design temperature > -48 C and hence as per ASME SEC VIII UCS-66.1, impact testing is not required.

Regards,

 

[georgeverghese]

Was under the impression the comparison between CS and duplex SS, however, if it is only between normal killed CS (not impact tested) and impact tested LTCS, then yes, there is most likely almost no CAPEX savings.