<p>The current ASME Safety Standard for Pressure Vessels for Human Occupancy (PVHO-1) contains procedures for designing polymethyl methacrylate (PMMA), or “acrylic”, components in critical pressure vessel service for conditions from North Sea winter environments to controlled, protected medical environments. These procedures are based on an empirical method and do not use material properties. &nbsp;&nbsp;This makes it challenging to change the process, so much so there have been no major changes to the design method since its first publication in 1977.&nbsp; &nbsp;PVHO-1 has a proven safety record that sets the standard for several industries, including deep sea diving, submersibles, and medical hyperbaric health. Given there are people within the pressure boundary, any pressure boundary failure will cause severe injury or death.&nbsp; This makes the risk management critical. &nbsp;PVHO-1 is also used for conventional pressure vessels such as liquid chromatography laboratory equipment for pharmaceutical manufacturing.&nbsp; The PVHO-1 design system is locked into specific shapes and cannot be adjusted to account for differences in yield strength, ultimate strength, elongation prior to failure, and other conventional material properties. This has proven to be problematic over the years, particularly given those not operating in jurisdictions requiring PVHO-1 are able to use modern engineering methods and improved materials.&nbsp;&nbsp; An ASME task group is developing a Design By Analysis (DBA) methodology for glassy polymers in order to allow for optimization in current applications, expand the range of materials beyond a single type of acrylic, and allow for innovation in existing and novel applications.&nbsp; Acrylic has been used in this service because it will yield and crack before failure, unlike glass and transparent ceramics.&nbsp; Being a transparent pressure boundary material, acrylics give the operator the opportunity to detect cracks and take measures to reduce the load (pressure) to prevent catastrophic failure.&nbsp; This characteristic is an example of an implicit risk management consideration that is imbedded in the PVHO-1 design method by limiting the material to PMMA, replacing the glass and synthetic sapphire used in earlier technology. &nbsp;There are now many types of glassy polymers that can respond in the same manner as the the PVHO-1 PMMA specification. This paper presents proposed design margins as part of an overall risk assessment process that considers material properties, service conditions, and other factors not currently incorporated in the existing design method.&nbsp; ASME Boiler and Pressure Vessel Code (BPVC) and the historical works used to develop PVHO-1 are used as benchmarks for the proposed design margins and risk assessment method.&nbsp; The intent is to ensure using a DBA process will not reduce the design’s safety and reliability below the level of the existing PVHO-1 methodology.&nbsp; In order to do this, the initial risk assessment must fully quantify the service conditions, emergency conditions, maintenance considerations, and other factors that can increase or decrease the design margin for the specific application.&nbsp; The outcome will be quantified design requirements listed in the User Design Specification to guide the stochastic analyses and other subsequent design work.</p>

Attempting To Establish Design Margins for Glassy Polymers In Critical Structural Service

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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