API 594 pdf download Check Valves: Flanged, Lug, Wafer and Butt-welding
4.1.16 When valve design utilizes a stem that extendsbeyond the pressure boundary,it shall be provided with ameans so that,in the event of a structural failure of stem-to-closure attachment items, the stem will not be ejected throughthe pressure boundary while the valve is under pressure.4.2PLATES AND DISC
Valves are classified as follows:
a. A single-plate valve has a plate or disc that closes thevalve when flow reversal or gravity forces the plate or discagainst the valve-body seat. This closure may be aided by theuse of springs or other devices.
b. A dual-plate valve has plates that close the valve with theassistance of one or more springs, when flow reversal forcesthe plates against the valve-body seat.
c. For Type ‘A’ single-plate valves and Type‘B’ valves,when a nut is used to assemble the disc or plate to the hingearm, the nut shall be secured to prevent separation; the use ofa single tack weld, lock washer or lock nut are not acceptablemeans.
4.3.1 The body and plate or disc seating surfaces may be ofdeposited weld metal, integral metal,mechanically retainedmetal, or a resilient material.On Type ‘A’ single-plate valvesand Type‘B’ valves, a resilient seal ring may be fitted eitherto the body or plate seat as specified by the purchaser. Thering shall be designed to give a full metal to metal seal if theresilient seal is inoperative or removed.
4.3.2Welding is not permitted on cast iron or ductile iron.4.3.3 Brazing is permitted on cast iron and ductile irononly for attaching seating surfaces to the body or the plate andonly if agreed to by the purchaser and the manufacturer. Fur-nace brazing is the only type of brazing permitted and may beused only if the parts are heated under closely controlled con-ditions, in a uniform manner, and to a temperature no higherthan the lower critical temperature of the base material.Cool-ing shall be in the furnace or in still air.
4.4EXTERNAL BOLTS AND THREADED HOLES4.4.1 Bolts and threaded holes with a diameter 1 in. orsmaller shall have coarse (UNC) threads. Those larger thanl in. diameter shall be of the eight-thread series (8 UN).Bolt threads shall be Class 2A,and nut threads shall beClass 2B. Threads shall conform to ASME B1.1.
4.4.2For bolts 1 in. diameter or smaller,threaded body-flange bolt holes shall be drilled and tapped according to therequirements for coarse thread series Class 2B in ASME B1.1.For bolts larger than 1 in. diameter,such holes shall bedrilled and tapped according to the requirements for eight-thread series Class 2B in ASME B1.1.
4.4.3 For Type ‘B’ valves,cover flange bolts shall be con-tinuously threaded stud bolts with heavy, semifinished hexa-gon nuts conforming to the requirements of ASME B18.2.2.Hex bolts or cap screws conforming to ASME B18.2.1 mayalso be used for NPS 21/2 and smaller valves.Hex bolts andcap screws shall be suitable for external wrenching only.
4.5 FLOW INDICATION
The valve body shall be furnished with a clearly visiblecast,forged，machined-in, or die-stamped arrow to indicatethe direction of flow through the valve.
Fasteners in the flange seating surface shall be recessed toor below the flange gasket level. Interruptions in the seatingarea of a centered ASME B16.20 spiral wound gasket forvalve sizes NPS 6 and larger shall not exceed the limitationsgiven in Figure 6. The permissible surface interruptions onsmaller size valves shall be as agreed between purchaser andmanufacturer but shall not exceed 50% of the gasket seatingwidth.
Note: The degree of interruption may affect the sealability of a spiral-wound gasket.
5.1BODY AND COVER
The body of Type ‘A’valves and the body and cover ofType‘B’valves shall be made of a material conforming to apurchaser selected material specification listed in the appli-cable ASME standard as referenced in 3.2.
5.2 PLATE AND DISC
Plate and or disc shall be made of a material whose corro-sion resistance is greater than or equal to that of the valve body.
5.3cOVER GASKET (TYPE ‘B’VALVES)
5.3.1 The cover flange gasket shall be a) corrugated or flatsolid metal; b) corrugated or flat filled metal jacketed;