Recommendations for Establishing the Minimum Pressurization Temperature (MPT) for Equipment
D.A. Osage, P.E., ASME Fellow D. W. Spring, Ph.D. T.L. Anderson, Ph.D., P.E., ASME Fellow S.R. Kummari, Ph.D., P.E. P.E. Prueter, P.E. K.R.W. Wallin, Dr. Tech., ASTM Fellow
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Pressure vessels, piping and storage tank construction codes provide design rules to ensure that brittle fracture will not occur at design conditions. The risk of brittle fracture is typically highest during start-up and shutdown conditions because the equipment may be exposed to the highest pressure (i.e., stress) and lowest metal temperature (i.e., lowest fracture toughness) events possible. In order to minimize or eliminate these effects, a Minimum Pressurization Temperature (MPT) envelope is developed to ensure adequate toughness exists during pressurization or depressurization. MPT envelopes may be developed based on the ASME B&PV Code, Section VIII, Division 1 (VIII-1), paragraph UCS-66 and Section VIII, Division 2 (VIII-2), Part 3, paragraph 3.11.
This Bulletin provides the basis to determine an MPT curve for pressurized equipment based on a fracture mechanics approach consistent with API 579-1/ASME FFS-1, Part 9, 2016 Edition. The approach is an extension of that used in WRC 528 to create the toughnes exemption curves for the ASME B&PV Code, Section VIII, Division 2. The fracture toughness estimation in this Bulletin is based on the Wallin Fracture Toughness Master Curve. The fracture toughness estimation includes temper embrittlement, hydrogen effects on fracture toughness, and considers both fast and slow fracture in hydrogen environments so that MPT curves can be generated for high-pressure hydrogen processing vessels constructed from carbon steel, 1.25Cr-0.5Mo, 2.25Cr-1Mo. 2.25Cr-1Mo-V, 3Cr-1Mo and 3Cr-1Mo-V with and without austenitic stainless steel cladding. Loading rate dependent effects may also be included. The MPT curves are based on membrane stress from pressure and residual stresses in accordance with API 579-1/ASME FFS-1, Annex 9D, and the effects of cladding. Solutions are provided for two reference flaws in cylindrical and spherical shells: t/4 and t/8 depth with an aspect ratio of six. Provisions are also provided to incorporate the effects of flaws found during in-service inspection into the MPT curve. In general, the MPT curve developed using this Bulletin permits a lower or colder MPT curve than that achievable using the VIII-1 or VIII-2 Codes while maintaining a definable safety margin, which results in increased operational flexibility for startup operations. The Warm Prestress (WPS) effect that describes the effect of a prior loading on the subsequent effective fracture toughness is also included. Finally, a procedure is provided for using the MPT model to generate exemption and temperature reduction curves that may be incorporated into construction codes. Example problems are provided to demonstrate the technology.