- Published on Monday, 28 April 2014 13:13
- Written by Carlos E. Davila, PE
The EPA has been reducing the allowable fugitive emissions on valves in the last few years. Due to the upcoming emission legislation, most refineries, chemical and process industry plants will have to monitor their emissions more closely. Additionally, new industry standards, both API and ISO, are being published or revised for type testing of valves to improve emissions in new valves.
Many aspects of the processes will be addressed to ensure tight seals and minimal emissions to comply with new legislation. Better valves and sealing elements will replace older designs during shutdowns as pressure seal designs can have a big impact on emissions and gasket leaks.
Pressure Seal (MSS-SP-144)
Valve pressure seal design was last included as Appendix A in API 600 Standard, 10th Edition, published February 1997. Appendix A covered gate valves, both Style A and Style B, but it was not exclusive; the industry used it for other valve types as well. This standard addressed the design using both API 600 as well as ASME B16.34 wall thicknesses. That was the last published pressure seal standard from API. Later API cooperated with ISO to publish combined API/ISO co-branded documents
After ISO review of API 600, the appendix was not included in the API 600/ISO 10434 co-branded document published as the 11th Edition of API 600. API decided not to include the design in the 12th edition published in 2009 as only an API 600 and not co-branded with ISO. With continued requests for a published document, eventually API started discussions with the Manufacturers Standardization Society (MSS).
MSS agreed to look into preparing such a standard and was placed in the TC 114, steel valves agenda, for development. It was prepared and published as SP-144 with an edition date of 2013. It covers the original requirements of appendix A in the 10th edition of API 600, which includes construction requirements for steel and steel alloy valves. It applies to gate, globe and check valves and may be used in conjunction with other valve specific product standards.
Pressure Seal Valve Highlights
These valves use the process pressures to compress the bonnet gasket. A typical design is shown in Figure 1. This eliminates leak paths to the atmosphere and prevents fugitive emissions.
MSS SP-144 includes parent standards in the standard practice. They are:
- API 594, check valves; flanged, lug, wafer, and butt-welding
- API 600, steel gate valves- flanged and butt-welding ends, bolted bonnets
- API 603, corrosion-resistant, bolted bonnet gate valves – flanged and butt-welding ends
- API 623, steel globe valves – flanged and butt-welding ends, bolted bonnets (This standard came out because steel globe valves were not keeping up with the changes in the industries.)
- ASME B16.34, valves-flanged, threaded, and welded end
MSS-SP-144 is not intended to replace requirements of the parent valve standard except for modifications for pressure seal. MSS-SP-144 includes sizes NPS 2 (DN 50) through NPS 50 (DN 1250) and pressure classes 600, 900, 1500, 2500 and 4500.
Parent valve standard API 600 covers two styles of pressure seal bonnet valves while parent valve standard API 594 covers wafer single and dual plate valves as well as swing check valves. In the case of parent valve standard API 603, the valve must comply with all of the requirements, there is no differentiation between style A or B. Parent valve standard API 623 was recently published by API. It specifies stem diameters larger/different than API 600, and in this case the valve also must comply with all of the requirements. Style A or B is not covered.
Under MSS SP-144, the body and bonnet are to be an ASME B16.34 Group 1, Group 2 or Group 3 material. Gasket material is to be the manufacturer’s standard, but limited to ductile iron, soft steel, austenitic stainless steel, vermiculite or flexible graphite. Pressure seal bonnet retention/sealing combination is to be the manufacturer’s standard based on a tested and proven design. Wetted metal parts internal in the valve must have a corrosion resistance at least equivalent to the valve body material and adjustment bolting material must be selected, or be coated, to provide continuous serviceability during the valve’s service life.
For API 600 Style A gate valves, the body and bonnet wall thickness must conform to the requirements of ASME B16.34. For API 600 Style B gate valves, double disc gate is not permitted, unless otherwise specified. API 600 gate valves require body and gate guide surfaces to minimize wear and ensure alignment during full travel in all orientations.
Only ASME B16.34 standard pressure class valves are covered, and it specifies that body-to-bonnet joint design requires that seal force increase as internal pressure increases. It also requires a bolting adjustment mechanism to pre-compress gasket to allow standard production tests to proceed.
These were also requirements of Appendix A in API 600 when it first published.
2013 Edition of ASME B16.34
This edition was approved by the ASME B16 Committee and published on Feb. 19, 2013. The three-year cycle of this standard is set to align with B16.5 and B16.47 so that the material tables with pressure ratings could be aligned with each other. In previous editions they were published on different dates so there was always some conflict.
There were no major changes in this edition. The 2013 edition increased the size range from NPS 24 to NPS 50 and added reference to ASME B16.47 for flanges due to size range increase. Some materials changed groups or were deleted:
- Casting ASTM A351 Gr. CF3 moved from group 2.3 to 2.1
- Casting ASTM A351 Gr. CF3M moved from group 2.3 to 2.2
- Casting ASTM A351 Gr. CF8C deleted from group 2.5
- Changes are proposed for the 2016 edition of ASME B16.34, including:
- An alternative ceiling pressure for B16.34 Group #3, Nickel Alloy materials. The discussion has been that many of the materials offered today have much higher properties. Increasing the pressure temperature ratings for these materials will take advantage of their properties without violating the "deflection rule." This will not apply to flanged valves.
- Review material group alignment for 316 and 316L casting material
- Review and update ASTM material description changes
- Expanding valve sizes to NPS 60“ to match ASME B16.47