MSS - Manufacturers Standardization Society of the Valve and Fittings Industry.
Scope
SCOPE AND FIELD OF APPLICATION This Standard Practice covers cast or fabricated@ bonneted or bonnetless knife gate valves designed and manufactured to serve as a ??Double Block and Bleed?? single valve system in pipelines. These valve types shall have flanged body designs compatible with ASME B16.5 flanges for NPS 2 (DN 50) through NPS 24 (DN 600)@ ASME B16.47 Series A flanges for NPS 26 (DN 650) through NPS 36 (DN 900)@ and ASME B16.42 for sizes greater than NPS 36 (DN 900). The valves identified in this Standard Practice are intended for use in applications where a ??Double Block and Bleed?? type of isolation is required@ and where two (2) individual isolation valves with a removable or drain-containing spool piece between them is typically used. NOTE: For the purposes of this Standard Practice@ ??Double Block and Bleed?? refers to a valve (or valve application) with two seating surfaces that@ in the closed position@ provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces.