An ASME B16.5 Class 1500 blind flange is a solid steel disc used to seal the end of a high-pressure piping system or pressure vessel nozzle. Unlike standard weld neck or slip-on flanges, a blind flange contains no center bore for fluid flow. Instead, it serves as a structural end-cap, absorbing the significant mechanical stresses exerted by high internal pressures and bolt tension. HT PIPE supply all different types and scales of flanges. Contact us market@htpipe.com for more information and a free quote!
Raised Face vs Ring Type Joint: Which One for Class 1500?
This is probably the question I get asked most often by engineers switching from lower pressure classes into Class 1500 territory. At Class 150 and 300, raised face is the default and nobody thinks twice. At Class 1500, the decision has consequences.
Raised Face (RF)
Standard finish: 125–250 μin Ra (3.2–6.3 μm) for spiral-wound gaskets
Works with spiral-wound, Kammprofile, double-jacketed, and sheet gaskets
Easier to install and replace gaskets in the field
Lower flange cost (no groove machining)
Limitation at Class 1500: Gasket blowout risk increases at large diameters and high temperatures. ASME B16.20 requires an inner ring on spiral-wound gaskets for Class 900 and above.
Suitable for: steam, water, hydrocarbons below auto-ignition temperature, general process
Ring Type Joint (RTJ)
Uses a solid metal ring (oval or octagonal cross-section) seated in machined grooves
Metal-to-metal seal; the ring deforms plastically under bolt load
Ring is single-use. Remove the flange and you replace the ring. No exceptions.
Higher flange cost (precision groove machining required)
RTJ grooves must be inspected for damage before every assembly. A scratched groove face will leak.
Standard at Class 1500 for: Hydrogen service, toxic fluids (ASME B31.3 Category M), gas service above 600°F, wellhead connections per API 6A
Flange Installation Process
Inspect Seating Surfaces
Verify the raised face phonographic serrations or the RTJ groove paths are entirely free of gouges, tool marks, or radial scratches. Any path across the face will become a high-pressure leak pathway.
Align and Insert Gasket
Center the high-pressure gasket (typically a spiral-wound metal gasket with an inner ring or a solid metallic RTJ octagonal ring). Never reuse a compressed gasket.
Lubricate Studs
Apply a nickel-based anti-seize compound uniformly to the stud threads and nut bearing faces. This ensures uniform friction factors during torque application.
Torque in Progressive Stages
Cross-tighten the nuts using a star pattern or hydraulic tensioners. Achieve full torque incrementally:
Stage 1: Tighten to 30% of target value.
Stage 2: Tighten to 60% of target value.
Stage 3: Tighten to 100% of target value.
Final Pass: Execute a rotational clockwise sweep at 100% torque to ensure equilibrium across all bolting positions.
