API 594 Check Valve

1. Scope and Applicability

• ‌Types of Valves Covered‌: The standard covers two types of check valves:
  • ‌Type "A"‌ valves: Short face-to-face, available as wafer, lug, or double flanged.
  • ‌Type "B"‌ valves: Bolted cover swing check valves, available as flanged or butt-welding ends.
• ‌Materials‌: Valves are designed for installation between various flange classes and materials including gray iron, ductile iron, steel, nickel alloy, or other alloys.
• ‌Pressure-Temperature Ratings‌: Rated for various classes and sizes as specified in the standard.

2. Design Features

• ‌Body and Cover‌:

  • ‌Minimum Body Wall Thickness‌: Defined for different materials and pressure classes.
  • ‌Face-to-Face Dimensions‌: Specified for both Type "A" and Type "B" valves.
  • ‌Flange Standards‌: Conform to ASME B16.5 or ASME B16.47.
  • ‌Welding Requirements‌: For welded flanges, full penetration butt-welding is required with qualified welding procedures.

• ‌Plates and Disc‌:

  • ‌Single-plate vs. Dual-plate‌: Single-plate valves have one plate or disc, while dual-plate valves have plates that close with the assistance of springs.
  • ‌Disc Assembly‌: Designed to limit disc rotation to less than 360 degrees.
  • ‌Nut Security‌: Positively secured to prevent separation of connecting parts.

• ‌Seating Surfaces‌:

  • ‌Materials‌: Can be deposited weld metal, integral metal, mechanically retained metal, or resilient material.
  • ‌Resilient Seal Rings‌: Optional, designed to provide a full metal-to-metal seal if missing.

• ‌External Bolts and Threaded Holes‌:

  • ‌Bolting Standards‌: Standard inch-series bolting, unless metric-series is specified.
  • ‌Cover Flange Bolts‌: Continuously threaded stud bolts with heavy nuts for Type "B" valves.

• ‌Flow Indication‌:

  • ‌Direction Arrow‌: Clearly visible on the valve body to indicate flow direction.

• ‌Gasket Surface‌:

  • ‌Fastener Recessing‌: Fasteners recessed to or below the flange gasket level.
  • ‌Surface Interruptions‌: Limited to ensure sealability, with specific limits for different valve sizes.

3. Material Specifications

• ‌Body and Cover‌: Made of material conforming to purchaser-selected specifications listed in applicable ASME standards.
• ‌Plate and Disc‌: Corrosion resistance greater than or equal to the valve body.
• ‌Cover Gasket (Type "B" Valves)‌: Various options including solid metal, corrugated, metal ring joint, etc.
• ‌Trim‌: Includes body seating surfaces, plate or disc seating surfaces, and hinge pins. Material specifications provided for different trim options.
• ‌Internal Wetted Parts‌: Corrosion resistance at least equal to the valve-body material.
• ‌Springs‌: Material specified based on valve temperature rating.

4. Inspection, Examination, Testing, and Repair

• ‌Inspection and Examination‌: Visually examined by the manufacturer in accordance with API 598.
• ‌Pressure Tests‌: Pressure tested in accordance with API 598.
• ‌Repair of Defects‌: Defects in cast or forged steel valves may be repaired as permitted by ASTM specifications.

5. Marking and Shipment

• ‌Marking‌: Nameplates attached to the valve body, marked with "API 594" and other required information.
• ‌Shipment‌: Unmachined exterior surfaces painted with rust preventative paint; machined or threaded surfaces coated; protective covers for valve ends; threaded connections fitted with solid plugs.