Evaluating Standard Class Valve Castings

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."

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.

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

"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.

For more information on the proposed MSS Standard, contact Greg Johnson at greg1950@unitedvalve.com.