Sawsan311
Chemical
- Jun 21, 2019
- 303
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,
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,