Last updateThu, 18 Oct 2018 4pm


The Proposed MSS Standard for ‘Standard-Class’ Castings

The issue of “commercial” casting quality has been argued at length at meetings, seminars and after-hours watering holes. But no one could really state what quality requirements needed to be met in order to call a casting a quality, commercial-grade valve casting.

The first thing that needs to be addressed is the word “commercial.” Commercial is not a technical casting term, so out it goes. In its place we use the term “standard class,” to differentiate them from their high-end “special class” brothers.

In the American Society of Mechanical Engineers (ASME) B16.34 standard, “Valves, Flanged, Threaded, and Welding End,” there are very specific NDE (non-destructive examination) testing requirements that must be met in order to label a casting “special class.” This means that all the other non-special class valve castings are by default, “standard class.” But the question is how to prove that these castings are of good quality and suitable for their intended service. As it stands now, these valve castings must only meet a visual examination and not leak under hydrotest.

The problem with the “looks-good, no-leaks” examination is that it not very thorough, and the results are often very inconclusive. This has resulted in end users not wanting to pay for higher quality special class valve castings but still demanding good quality that they can count on and not lose sleep over. So the dilemma is how to prove a company’s common, everyday valve castings have good repeatable quality. To this end, the Manufacturer’s Standardization Society (MSS) is developing a standard to identify these castings that make up the vast majority of ferrous and stainless-steel castings in use today. The purpose of the standard is to provide valve manufacturers with evaluation methods, non-destructive testing standards and acceptance criteria, as well as sampling plans, to help them achieve respectable, repeatable quality in the standard class valves they manufacture.

These goals are met in two ways. The first is by the testing of pilot, or first-run castings. These castings must meet the NDT requirements and radiography acceptance criteria of the standard without casting repairs. The second involves production testing that is dictated by a strong sampling plan prescribed in the standard.

The NDE requirements for both testing phases include dye penetrant (PT), magnetic particle (MT) and, as applicable, visual examination and radiography (RT). Probably the stiffest test is the radiography, even though the acceptance criteria have been loosened one level from the RT requirements for special class valves. This loosened RT acceptance criteria is already specified in the API RP591, “Process Valve Qualification” recommended practice and is also equal to the RT acceptance criteria in one level of the new API 20A, “Carbon Steel, Alloy Steel, Stainless Steel, and Nickel Base Alloy Castings for Use in the Petroleum and Natural Gas Industry.”

Hopefully, the recently amended draft of the MSS document will achieve approval when it is balloted this summer. If so, it will be a useful tool to help valve manufacturers walk through the treacherous minefield of questionable casting quality and end-user quality and repeatability concerns.

Greg Johnson is president of Houston-based United Valve and serves as first vice president for the Manufacturers Standardization Society.

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