Why is there a need for a new casting standard? That's the question asked-and answered-by Greg Johnson, president of Houston-based United Valve, during a presentation at VMA's Technical Seminar on March 3. Johnson discussed a "Proposed Specification for Evaluating Standard Class Valve Castings."
"A lot of new foundries are being built, some in new countries, and the paradigm is changing almost daily," Johnson observed. "Right now most foundries are in China and India, but in five years, who knows? There could be a shift to another country."
During the last several years, we've seen a much higher percent of casting failures and rejections, he stated. And the problem is, sometimes you just don't know why, Johnson said, showing a photo of a cracked valve on which his company did forensics. The chemistry was right on; heat treatment wasn't the issue; and even working with a metallurgist, there was no real answer to why the crack occurred, he said. Most likely there were some non-quantifiable metals that in combination caused problems. Essentially, it was a "mystery."
Perception vs. reality
One problem with castings is that the perception (of quality) is the reality. Johnson said that "end users are under much more pressure from management and government entities, so they are looking for reassurance anywhere and everywhere they can."
Many Chinese foundries are not firmly rooted in western quality expectations nor do they comprehend the western manufacturing approach, Johnson said. "Chinese NDE personnel, techniques and equipment are still not universally equal to western standards nor do they fully understand western NDE requirements and specifications." The culture in China requires buyers to have "boots on the ground and eyes on the processes."
Additional guidance for commodity casting quality will help with this situation, Johnson said. "There really are very few requirements for standard class or commodity valve castings," and visual examinations play a major role-but these visual examinations don't reveal everything. Some imperfections just don't show up to the naked eye.
Were castings from the "old" days really better? The short answer to that question, Johnson is: "We think so." However, not much radiography was done on commodity valves 40 to 50 years ago, he explained, and we didn't even have standards until 1959. Furthermore, the requirements were voluntary and not mandatory. "Designs were robust and wall thickness much greater than today. Catastrophic casting failures were almost unheard of," he said.
So what are the requirements?
Unless a specific casting quality level is specified via NDE, Johnson said, "valve castings must only meet MSS SP-55 visual inspection criteria and not leak under hydrotest. But higher casting quality levels are being sought. The most common is "Special Class," in accordance with ASME B16.32. This applies to highly critical applications such as hydrogen and nuclear service, which require much better castings.
Yet the vast majority of steel valve castings - "commodity" or "standard" castings - are not special. How is their quality assessed, and what are the acceptance criteria? Just MSS SP-55 and no leakage! "End users and OEMs want additional guidelines," Johnson said. Castings are evaluated today when API RP591 testing is requested, and the test revision of RP591 has loosened the radiographic acceptance criteria by one level.
More and more, there is an established a quality expectation for standard class valves and some of the requirements of RP591 are now tougher, he explained: 100% of pressure boundary must be radiographed; microstructural examination at 100X; and more extensive chemical analysis.
The proposed MSS Casting standard
The Manufacturers Standardization Society (MSS) is currently working on a proposed new standard, "Quality Standard for Steel Castings Used in Standard Class Steel Valves - Sampling Method for Evaluating Casting Quality." It will be under the auspices of MSS Com. 304, "Quality Standards," of which Johnson is a member.
He summarized the proposed mandatory requirements for pilot castings:
- All pilot castings shall be evaluated without weld repairs.
- All accessible external and internal surfaces shall be dye penetrant (PT) inspected in accordance with MSS SP-93.
- All accessible external and internal surfaces shall be magnetic particle (MT) inspected in accordance with MSS SP-53.
- Critical areas as defined by ASME B16.34 are radiographed (RT) in accordance with MSS SP-54
Among the requirements for production castings, as proposed in this new standard, are the following:
- Each production casting shall have all external and accessible internal areas of the casting visually inspected to the requirements of MSS SP-55.
- Standard production, pressure-containing castings shall be randomly selected based on the sampling plan specified in Annex A.
- The randomly selected casting(s) shall be radiographed and shall meet the radiographic quality levels as listed in Tables 1, 2 & 3, as applicable for critical areas as designated in ASME B16.34
But there's more to the story, as Johnson explained: "Not every plant requirement calls for high- integrity castings, which obviously cost more to produce, so MSS has prepared an additional solution by proposing three classes of valves-Class 1, Class 2 and Class 3-all based on NDE performed and acceptance criteria applied. He then summarized the differences between the classes, which range from the most stringent (Class 1) to the least stringent (Class 3).
"This would create an opportunity for the best manufacturers and the average ones as well," he said. And by offering different standards, the cost of expensive casting upgrades would be reduced.