In modern industrial piping systems, flanges are one of the most important connecting components. They not only provide connection and sealing between pipes, valves, pumps, and pressure vessels, but also must remain reliable under complex conditions such as high temperature, high pressure, corrosion, and vibration. Although a flange looks like a “simple ring,” it actually integrates knowledge from materials science, mechanics, welding processes, and sealing technology.
This guide systematically introduces the definition of flanges, types, standards, pressure ratings, facing types, common materials, and selection guidelines, combined with real-world applications, providing engineers, purchasers, and operators with a “basic handbook on flanges.”
What is a Flange?
Definition: A metallic component used to connect pipes, valves, and equipment reliably with bolts + gaskets to achieve tight sealing.
Mechanism: Relies on bolt preload to compress the gasket to the target surface stress, resisting internal pressure and relaxation caused by temperature, vibration, or thermal cycles.
Key Terms:
- NPS/DN: Nominal Pipe Size (e.g., NPS 6″) / Nominal Diameter (e.g., DN150).
- Schedule (Pipe Wall Thickness): Affects inner bore matching and weld bevel prep (critical for WN/SW).
- Class / PN: Pressure rating systems (ASME “Class,” European “PN”) – numbers must not be interpreted directly as pressure values.
Three Elements of a Flanged Connection
- Flange body: Includes flange ring, neck (for WN), facing (RF/FF/RTJ, etc.), bolt holes.
- Gasket: Soft, semi-metallic, or metallic (e.g., spiral wound, RTJ metal rings); determines required sealing stress and media compatibility.
- Fasteners: Bolts, nuts, and washers – material grade must match service conditions (e.g., A193 B7 bolts + A194 2H nuts).
Common Flange Types (Usage + Pros & Cons)
Weld Neck (WN):
- Pros: Butt-welded, excellent stress distribution, long fatigue life; ideal for high-temp/high-pressure critical lines.
- Notes: Control bevel alignment, root gap, and ensure NDE after welding.
Slip-On (SO):
- Pros: Easy alignment, low cost, quick installation; widely used in low-to-medium pressure.
- Notes: Requires double fillet weld (inside + outside); check bore-pipe OD fit to avoid step misalignment.
Blind (BL):
- Pros: Closes pipe ends or vessel nozzles; useful for hydrostatic testing.
- Notes: Bolt load high on large sizes – check stresses.
Threaded (TH):
- Pros: No welding needed; suitable for small-diameter, low-pressure, no-hot-work areas.
- Notes: Not recommended for high-temp, high-pressure, or vibration service.
Socket Weld (SW):
- Pros: Good concentricity, often used for ≤NPS 2 high-pressure piping.
- Notes: Leave expansion gap (~1.6 mm) before welding.
Lap Joint (LJ):
- Pros: Used with stub ends; easy to dismantle and align; common in lined or corrosive service.
- Notes: Lower load capacity vs WN.
Orifice / Reducing / Spectacle Blind: For metering, reducing, or isolation – selected per design codes.
Pressure Ratings (Class/PN) and Temperature Derating
- ASME B16.5: Class 150/300/600/900/1500/2500 (NPS 1/2″–24″).
- ASME B16.47: Series A/B (26″–60″).
- EN 1092-1: PN6/10/16/25/40/63/100…
Key Points:
- Class ≠ direct psi value – allowable pressure depends on material group & temperature, must consult pressure-temperature charts.
- Allowable pressure drops significantly at higher temperatures; curves differ by material grade.
- Selection sequence: Service P/T → consult P-T ratings for chosen material → determine Class/PN.
Major Standards and Scope
- ASME B16.5: Steel pipe flanges & fittings, NPS 1/2–24.
- ASME B16.47: Large diameter flanges, Series A (ex-MSS SP-44) / Series B (ex-API 605).
- EN 1092-1: European standard, PN-based system, flange types 01/02/05/11…
- ISO 7005-1: Equivalent/close to EN system (e.g., Type 11 = WN flange).
- API 6A/6B/6BX: High-pressure flanges for wellhead & Christmas tree equipment (non-B16.5).
- JIS B2220 / DIN: Japanese/German standards; cross-standard mixing requires checking bolt circle, hole number, facing compatibility.
Flange Facing Types & Surface Roughness (Critical to Sealing)
RF (Raised Face): Most common.
- Height: 1.6 mm (Class 150/300), 6.4 mm (Class ≥400 typical).
- Roughness: 125–250 AARH (≈ Ra 3.2–6.3 μm), concentric/spiral serrated.
- Gaskets: Spiral wound, graphite, CNAF, PTFE.
FF (Flat Face): For cast iron/non-metallic flanges to prevent localized crushing.
RTJ (Ring Type Joint): Metal ring (R, RX, BX types); high-temp/high-pressure; requires machined grooves.
TG / FG (Tongue & Groove / Male-Female): Prevents gasket blow-out; used in special services.
Tip: The harsher the service (P/T/cycles), the more likely RTJ or high-performance gaskets are used; common chemical services favor RF + spiral wound gaskets.
Common Materials & Service Guidance
- Carbon Steel: ASTM A105/A105N (ambient-medium temp); low-temp service A350 LF2.
- Stainless Steel: A182 F304/F316 (corrosion-resistant); chloride service prefers 316/316L.
- Alloy Steel: A182 F11/F22/F91 (high-temp steam/boiler).
- Duplex Steel: A182 F51 (2205)/F53/F55 (seawater, chloride stress corrosion + high strength).
- Nickel Alloys: Inconel, Hastelloy, Monel (severe corrosion, high temp).
Selection factors: Media (corrosivity, chlorides), temperature, pressure, weldability, availability, cost.
Bolts, Nuts & Gasket Compatibility
Fasteners:
- Common: A193 B7 bolts + A194 2H nuts (high-temp carbon/alloy steel).
- Low-temp: A320 L7/L7M.
- Corrosive: Stainless B8/B8M (304/316).
- Note: Match grade to service P/T; apply coatings/lubricants as needed.
Gaskets:
- Soft: CNAF, PTFE, graphite (general, low-medium pressure).
- Semi-metallic: Spiral wound (most common, versatile).
- Metallic: RTJ rings (high-temp, high-pressure, oil & gas).
Assembly: Use lubricant; tighten in star pattern, multiple stages (30%→60%→100%), then final circular pass; hot torquing/retightening may be required.
Selection Workflow (Practical Checklist)
- Service conditions: media, P/T, corrosion, vibration/cycles, design life
- Standard: ASME B16.5/B16.47 or EN 1092-1/ISO 7005
- Size: NPS/DN & matching pipe schedule (affects WN/SW welding)
- Pressure rating: select Class/PN from P-T charts
- Facing: RF/FF/RTJ/TG, match gasket
- Type: WN, SO, BL, TH, SW, LJ…
- Material: A105/A105N, F304/316, F51, etc.
- Bolts & gaskets: proper grade & dimensions
- Documentation: MTC (EN10204 3.1/3.2), NDE, dimension reports, pressure test (if required)
- Packaging/marking: rust-proof, moisture-proof, stamping, wooden crates, traceability
Common Mistakes to Avoid
- Treating Class = psi: must check P-T ratings, varies by material.
- Mixing RF and FF: risk of crushing/leaks.
- SO bore mismatch: creates step → turbulence, corrosion, stress concentration.
- SW no expansion gap: thermal stress → cracking.
- Mixing standards: ASME vs EN/JIS have different bolt circle & hole patterns.
- Reusing old gaskets: high leakage risk.
- Incorrect bolt tightening: uneven gasket stress → early leakage.
Inspection & Documentation (Mandatory Deliverables)
- MTC (EN 10204 3.1/3.2): Heat no., chemistry, mechanicals, heat treatment, NDE, dimensions.
- NDE reports: UT/MT/PT (RT if required).
- Marking: Material, size, pressure class, HT, heat no., manufacturer ID.
- Third-party inspection: SGS, BV, TÜV, DNV, LR (per project specs).
Installation Essentials (Practical Onsite Guidance)
Alignment & gasket positioning: avoid misalignment; RTJ ring must match groove.
Bolt tightening: clean–lubricate–torque in star sequence, multi-stage (30%/60%/100%), final pass.
Welding:
- WN butt weld: follow WPS/PQR, post-weld NDE, control heat input/distortion.
- SW: keep expansion gap, root pass + cover pass, clean root to avoid slag.
Hydrostatic test: Pressurize, hold, check leakage & pressure drop; hot torquing/retightening if needed.
Maintenance & Operation
- Retorque after thermal cycles, esp. for large-diameter/high-temp flanges.
- External corrosion protection: repainting, wraps, shields.
- Planned replacement: manage gasket & fastener lifespans; maintain traceability records.
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