There are many different types of valves that have different leakage specifications. The most liberal leakage is permitted for control valves. An ANSI/FCI *** standard categorizes control valve leakage specifications. The control valve leakage standard has no application to block valves. Butterfly, ball and other valve specifications often address a leak testing standard. Refer to API/Petro. Standard 594, 598, 599, 600, 602, 603, 608, 609, MSS *** standards.
Industries do not necessarily accept some standards. Check the folloing lenghty text. I began to paraphrase the first paragraph for the MMS (not same as MSS) then left the balance as-is.
Although length and dry, see the excerpt from a 1998 the Department of Interior Minerals Management Service comment on metal-seated API 6D valve changes:
Notice To Lessees and Operators (NTL) of Federal Oil, Gas,
and Sulphur Leases in the Outer Continental Shelf
American Petroleum Institute (API) Specification 6D (SPEC 6D),
Specification for Pipeline Valves (Gate, Plug, Ball, and Check Valves),
21st Edition (March 31, 1994), and Supplements 1 and 2
We are concerned that allowable minimal leakage rates for metal-to-metal valve seats included in Supplements 1 and 2 might diminish the operational safety of OCS pipelines. Therefore, we will not adopt these supplements as documents incorporated by reference under 30 CFR Part 250.
The Minerals Management Service (MMS) regulations published July 1, 1998, at 30 CFR 250.101, incorporate by reference API SPEC 6D, 21st Edition, but not the two supplements. The API Subcommittee on Valves and Wellhead Equipment issued Supplements 1 and 2 to the 21st Edition of API SPEC 6D on December 1, 1996, and December 1, 1997, respectively. The supplements allow minimal leakage rates for pipeline valves having metal-to-metal seats in section 5.3 of SPEC 6D and in section C3 of Appendix C to SPEC 6D. Prior to API’s issuing the supplements, API SPEC 6D allowed no visible leakage from any valves.
The API’s decision to adopt minimal leakage rates for metal-to-metal seats appears to be largely based on the information contained in API Standard 598, "Valve Inspection and Testing"; International Standards Organization (ISO) Standard 5208, "Industrial Valves--Pressure Testing of Valves"; and ISO Draft Industry Standard 14313, "Petroleum and natural gas industries--Ball, check, gate and plug valves--Specification." All of these standards allow minimal leakage rates for metal-to-metal seats.
It may well be that the "no visible leakage" standard contained in the 21st and previous editions of API SPEC 6D is an unreasonably high standard for metal-to-metal seats. Metal-to-metal seats are non-deforming compared to non-metal-to-metal seats; therefore, it may be reasonable to expect that some leakage would occur between facing metal surfaces. Nevertheless, there appears to be no data or agreed-upon formula for predicting an acceptable leakage rate.
Valve leakage within pipelines poses a major safety concern. Multiple fatalities have occurred offshore when workers attempted either to cut into a pipeline or open a pig trap when they believed that combustible hydrocarbons or high pressure had been eliminated from the system. Once a system is bled down and isolated, there is a danger that the system may become re-pressurized if the pipeline valves have "allowable leakage rates." Since all pipeline systems eventually are cut into for repair or modification purposes, "allowable leakage rates" can have deadly consequences for unsuspecting workers.
We are currently reviewing our pipeline safety regulations under 30 CFR 250.1000 through 250.1014. If metal-to-metal seats cannot assure a "leak-proof" seal, we may need to consider allowing only "resilient-seated valves," as they are described in API Standard 598. We realize that there may be many valves in OCS pipelines that contain metal-to-metal seats. We anticipate that any change in our regulations affecting pipeline valves with metal-to-metal seats would likely only affect newly installed valves and old valves being replaced due to normal wear or corrosion. We would not want to require wholesale replacement of metal-to-metal seated valves and thereby increase worker exposure to the potential hazards described in the preceding paragraph.
Our regulations under 30 CFR 250.101 allow you to use a later edition of a document incorporated by reference provided (1) that the lessee demonstrates that compliance with the later edition provides a degree of protection, safety, or performance equal to or better than that which would be achieved by compliance with the listed edition, and (2) that the lessee obtains prior written approval of the authorized MMS official. We do not believe that the allowable leakage rates for metal-to-metal valve seats cited in supplements 1 and 2 to the 21st Edition of API SPEC 6D provide a degree of protection, safety, or performance equal to or better than that provided in the original document. Therefore, we will not incorporate by reference these supplements in our regulations and will not approve requests for their use under 30 CFR 250.101.
Paperwork Reduction Act of 1995 Statement: This Notice to Lessees and Operators (NTL) refers to information collection requirements in 30 CFR Part 250, Subpart A. The Office of Management and Budget has approved the collection of information in these regulations and has assigned OMB Control Number 1010-0030. This NTL does not impose additional information collection requirements subject to the Paperwork Reduction Act of 1995.
If you have any questions about this notice, please contact Mr. Joseph R. Levine on (703) 787-1033.
[signed] Carolita U. Kallaur,
Associate Director for Offshore Minerals Management
This is just a downpayment on the valve leakage topic.
