Per ASME B16.34, pressure-temperature ratings are designated by class numbers. Each class number is further identified as Standard, Special, or Limited Class. Pressure-temperature ratings are tabulated for Standard and Special Class Pressure Rating Designation numbers 150, 300, 600, 900, 1500, 2500, and 4500.
Before we dive into the differences between each class, it’s important to note a few points:
- Flanged-end valves shall be rated only as Standard Class, and furthermore flanged-end valves larger than NPS 60 are beyond the scope of this Standard
- Class 4500 only applies to welding-end valves
- A class designation greater than 2500 or a rating temperature greater than 1000 degrees Fahrenheit applied to threaded end valves is beyond the scope of this Standard.
Standard Class Valves
- Valves that conform to the requirements of B16.34, except for those meeting the additional requirements of Special Class or Limited Class valves shall be designated as Standard Class. Depending on material, these valves range from -20F to 1500F.
Special Class Valves
This standard only applies to threaded or welded end valves that conform to all the requirements of Standard Class valves, but have successfully passed the NDE requirements of ASME B16.34 Section 8.
- Nondestructive examinations (NDE) are to be performed on cast and forged material
- These valves will be marked ‘SPL’ and have different pressure/temperature rating tables than the Standard class for each B16.34 material — these include higher ratings for the same temperature.
Limited Class Valves
- This standard only applies to threaded or welded end valves NPS 2.5 and down.
- Valves will be marked “B16.34LTD”
- Limited class ratings shall not be used for flanged-end valves.
- Threaded end valves rated above Class 2500 and Socket-weld-end valves rated above Class 4500 are not within the scope of this standard.