Mixing American Petroleum Institute (API) pressure safety values with those from the American Society of Mechanical Engineers (ASME) can create potential hazards. Especially during an overpressure event, it is imperative to understand the nuances and differences inherent in the two standards.
While both standards address elements in pressure safety systems, the differences are sufficient to cause problems when used interchangeably in designing and constructing projects involving pressure safety. However, this should not be taken as to preclude the use of one standard or the other as they are different but complementary standards and when both are applied properly, contribute to the overall safety and success of a project.
In simple terms, API pressure safety standards provide a practical way of meeting the standards as specified under the ASME standards. For example, API 520 Part One covers the preliminary sizing of pressure safety relief valves which are designed to comply with the standard set forth in ASME BPVC Section Eight.
However, in API 520 Part Two it is dictated that the final valve sizing is done under the ASME orifice size to continue compliance with ASME Section Eight. In other words, API standards supply the practical methodology to comply with the standards set forth under ASME standards to achieve said standards.
Certified And Binding
Another difference—and one that can be critical in the use of API standards concerning pressure safety systems—is that the API standards are not certifiable but the ASME BPVC standards are certifiable. Another way to perceive this difference is that the ASME standards are the ones to achieve or design towards while the API standards are the recommended practices but that these practices are not legally binding whereas once the pressure system has been ASME certified the ASME certification is legally binding.
Process Safety Implications
Because of the subtle nuances and differences that exist between the recommended practices of API standards and meeting the ASME specifications for certification it is necessary to ensure that both API and ASME units are used properly throughout the design and construction process. This includes ensuring that the same values under each standard are correctly applied when making estimates and design characteristics and that at no time should the values be mixed or used interchangeably, otherwise over or under estimates can occur which will produce eventual failure points.
While both standards have their differences in practicality, they are complimentary and in order to achieve quality process safety and controls, both are required in their appropriate uses. Using API standards to achieve and ensure ASME certification is an excellent way to make sure your pressure vessel systems are not only compliant with current methodologies but also allow your project management teams that assurances they need that they can deliver on time and on budget.