46 CFR §56.20-15

1. (a)A valve in which the closure is accomplished by resilient nonmetallic material instead of a metal-to-metal seat must comply with the design, material, construction, and testing for valves specified in this section.
2. (b)Valves employing resilient material are divided into three categories: Positive shutoff, Category A, and Category B, and must be tested and used as follows:
   1. (1)Positive shutoff valves. The closed valve must pass less than 10 ml/hr (0.34 fluid oz/hr) of liquid, or less than 3 l/hr (0.11 cubic ft/hr) of gas per inch nominal pipe size through the line at full rated pressure after being subjected to the fire test requirements of API 607 (incorporated by reference; see § 56.01-2). Packing material must be fire resistant. Piping subject to internal head pressure from a tank containing oil must be fitted with positive shutoff valves located at the tank in accordance with § 56.50-60(d). Positive shutoff valves may be used in any location in lieu of a Category A or Category B valve.
   2. (2)Category A valves. Category A valves may be used in any location except where positive shutoff valves are required by § 56.50-60(d). To be qualified as a Category A valve, the valve must meet the fire test and leakage requirements of API 607.
      1. (i)Category A valves are required at vital piping system manifolds;
      2. (ii)Category A valves must be used in isolation valves in cross-connects between two piping systems, at least one of which is a vital system, where failure of the valve in a fire would prevent the vital system(s) from functioning as designed; and
      3. (iii)Category A valves must be used for valves providing closure for any opening in the shell of the vessel.
   3. (3)Category B valves. The closed valve will not provide effective closure of the line or will permit appreciable leakage from the valve after the resilient material is damaged or destroyed. Category B valves are not required to be tested and may be used in any location except where a Category A or positive shutoff valve is required.
3. (c)Resiliently seated valves previously accepted by the Commandant or the Marine Safety Center may continue to be used within the service restrictions of their acceptance.