ASME BPVC SEC.VIII DIVI TABLE UCS-66 MDMT DETERMINATION
GRAPHIC READING TOOL FOR TABLE UCS-66. The impact test exemption rules of UCS-66 are some of the most important and yet most misunderstood provisions of the ASME Code. Because of its importance, minimum design metal temperature (MDMT) is one of just a few critical parameters stamped onto vessel nameplates. The rules of UCS-66 guard against vessel failure by brittle fracture, a low probability high consequence event. Brittle fracture failures exhibit “break before leak” behavior and can be catastrophic in that they entail high risks and occur without warning. Carbon steel pressure vessels are most susceptible to brittle fracture during start up, shut down, hydro test (test water that is too cold) and rapid depressurization (auto-refrigeration). Given the consequences of this type of failure, it is important that all the relevant factors listed in UCS-66 be properly addressed. These include consideration of the coldest temperature expected, metallurgy, material thickness, post weld heat treatment, impact testing, applied stress and weld pass size.
UCS-66 is a section of the ASME Boiler and Pressure Vessel Code that provides guidelines for the design and construction of pressure vessels subjected to external pressure. The section includes specific exemption rules for certain types of pressure vessels.
The exemption rules of UCS-66 are intended to provide relief from certain design requirements and testing procedures for pressure vessels that meet specific criteria. The following are some of the exemption rules of UCS-66:
Pressure vessels with a diameter less than or equal to 6 inches and a length less than or equal to 15 times the diameter are exempt from the requirements of UCS-66.
Pressure vessels that are not intended for human occupancy and are designed to operate at a maximum allowable working pressure (MAWP) less than or equal to 15 psi are exempt from the requirements of UCS-66.
Pressure vessels that are used for the storage of compressed air or other gases that are not hazardous to human health or property are exempt from the requirements of UCS-66.
Pressure vessels that are designed and constructed according to a recognized standard or code, such as API 510, ASME B31.3, or ASME Section VIII, Division 1, are exempt from the requirements of UCS-66, provided they meet specific criteria.
Pressure vessels that are designed and constructed according to an approved safety code or standard, such as ANSI/ASME B31.1 or ANSI/ASME B31.5, are exempt from the requirements of UCS-66, provided they meet specific criteria.
It is important to note that the exemption rules of UCS-66 are not applicable to all pressure vessels, and the specific exemptions and requirements may vary depending on the design, size, and operating conditions of the vessel. It is always recommended to consult the latest version of the applicable codes and seek the advice of a licensed engineer for any specific design or construction requirements.
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