Intermediate Class Valves, the Forgotten Classification

Valves manufactured in accordance with the Specification for Pipeline and Piping Valves, API 6D, are required to comply with the pressure-temperature ratings for class-rated valves, as listed in the valve standard ASME B16.34. These ratings are grouped by material and list the allowable working pressures and temperatures. For example, group 1.1 materials, which include ASTM A105, ASTM A350 LF2 and ASTM A216 WCB, have a working pressure of 1480 psi for a standard class 600. The listed working pressures start at ambient (-20°F to +100°F) and at higher temperatures the working pressures start to reduce. These pressure and temperature limits are programmed into the piping design software.

In some applications, where the standard pressure-temperature ratings will be exceeded, the use of an intermediate class can be considered if a standard flange is not being used (e.g. compressor station discharge valves or pipelines with significant changes in elevation). In these examples, it can be either the temperature or pressure or both that are higher than normal. This approach can also be beneficial in offshore or other locations where size, weight and space are all limited.

An example of this is shown in the title photograph above, where a 38” ASME 2000 rating was designed. This assembly would have been considerably larger and significantly heavier if the design was required to comply with the traditional higher pressure class of ASME 2500. The weight saving is not just limited to the valve. For every extra ton of valves on an offshore platform, there is typically an extra ton of deck structure to support them and another ton of steel in the jacket to support the deck. Using a typical estimated cost of $160,000 for every ton of valve weight, it’s easy to see the advantages.

In order to apply the API 6D monogram, the values in ASME B16.34 cannot be exceeded, but there are already two ways within the standard to slightly increase the working pressure and temperature without having to design an intermediate class. One simple method is to change to a different material group, e.g. Group 1.2 which allows a working pressure of 1500 psi at temperatures up to 200°F for a standard class 600. This group includes the materials ASTM A350 LF6, ASTM A216 WCC and ASTM A352 LCC. It is also possible to gain a similar advantage without changing materials by utilizing the special class designation, but only if the valve is weld-end or threaded. This special class allows higher pressures, so by applying this group, 1.1 materials can also be rated at a 1500 psi working pressure and at temperatures up to 200°F. It should be noted, however, that when taking advantage of the special class option, additional parent material NDE, in accordance with Section 8 of ASME B16.34, has to be performed. If neither of these easy options are suitable, then the piping system designer would have to use the next higher pressure class or design an intermediate class.

Intermediate class valves are recognized and allowed by API 6D and the appropriate working pressures and temperatures are determined by linear interpolation as described in Appendix B of ASME B16.34. As an example, if the design requirement is for a pressure of 1800 psi at 100°F using ASTM A216 WCB materials then an intermediate class of 728 would be calculated by using the adjacent standard class 600 and 900 values as shown in the table below:

As stated earlier, API 6D does allow the use of intermediate pressure classes but only for weld-end valves or for valves with special end connections. Up-rating of standard ASME B16.5 or ASME B16.47 flanges beyond their listed values is not allowed. In addition, the use of a higher pressure class flange on a lower pressure class body, e.g. a Class 600 valve body with Class 900 flanges, is not allowed due to the risk of the valve being transferred by accident to a different application where the full flange rating is required. In all cases the class, pressure and temperature values must be marked on the valve nameplate.

Exceptions to the Rule

As with every rule there is an exception, and in this case there are three. First of all, the Steel Pipeline Flange standard MSS-SP-44, not listed in API 6D except for the 22” size, does not down-rate working pressures between -20°F and +250°F. So for ASTM A350 LF2 material, we get an automatic increase in working pressure over a wider temperature range with no design or material changes necessary. This is also the case with the Canadian Valve Standard CSA Z245.15 and Australian Pipeline Standard AS 2885.1. If any of these standards are specified, then the valve manufacturer can, of course, comply. However, they would not be able to apply the API 6D monogram which can be a problem, depending upon the country of destination.

Returning to the intermediate class option, there are design considerations that have to be verified before the higher pressures and temperatures can be applied. The first is to make sure that the wall thickness of the pressure- containing parts, e.g. body, closure and bonnet etc., can handle the higher operating and test pressures using the appropriate design code. Secondly, the drive train components need to be checked to verify that the higher torques, thrusts, shear stresses, tensile stresses and bearing loads are less than the allowable limits.

In the case of API 6D valves where the drive train design is based on twice the design, thrust or torque, the effect can be quite significant. Finally, it needs to be confirmed that the internal seal materials are all suitable for the application. Any of these factors could require dimensional changes to the metallic components, changes in the base materials, or both.

In summary, the options are as follows:

For flanged-end valves that need to fully comply with API 6D, the options are to: 1) change the material group, 2) use a higher pressure Class valve, or 3) utilize a proprietary piping connection. For Non-API 6D valves, the use of MSS-SP-44 flanges can also be considered although that only covers flanges up to and including Class 900. If the end user is in Canada or Australia, the valve manufacturer can follow their national standards but only in the case of non-API 6D valves. Finally, for weld end valves we have the added benefit of being able to apply Special Class and, of course, the potential to design an intermediate class.

Lawrence Dec is senior principal engineer - Valves & Measurement at Cameron, a Schlumberger Company.