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Flange Facing & Gasket Sealing

Flange Face Types Explained: Raised Face (RF), Flat Face (FF), RTJ & Tongue-and-Groove

Remax Forge Sealing Technologies 8 min read • ASME B16.5 Facing Standards

In a bolted flange joint, the flange facing provides the contact surface where the gasket is compressed to form a leak-tight seal. Under ASME B16.5 and ASME B16.47, specifying the proper facing geometry—whether Raised Face (RF), Flat Face (FF), Ring Type Joint (RTJ), or Tongue & Groove (T&G)—is critical to preventing gasket blowout, catastrophic fluid leaks, and flange cracking.

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1. The Four Primary Flange Facing Types

1. Raised Face (RF) — The Industry Standard

The most widely used facing in process plants. The gasket surface is elevated above the bolting circle face:

  • Class 150# and 300#: 1/16 inch (1.6 mm) raised face height.
  • Class 400# through 2500#: 1/4 inch (6.4 mm) raised face height.
  • Sealing Mechanism: Concentrates bolt load onto a smaller gasket area, creating high sealing stress without excessive bolt torque.

2. Flat Face (FF) — Protecting Fragile Equipment

The entire mating surface lies in the same plane as the bolting circle. Uses full-face gaskets:

  • Mandatory when mating with cast iron (ASTM A126), bronze valves, plastic (PVC/FRP), or fiberglass equipment nozzles.
  • Prevents excessive bending moment caused when tightening raised face flanges against brittle cast iron.

3. Ring Type Joint (RTJ) — Extreme High Pressure & Temperature

Features a precision-machined metallic groove in the face designed to house a solid metal ring gasket (octagonal or oval profile):

  • Standard in high-pressure oil & gas exploration (Class 900#, 1500#, and 2500#).
  • Metallic ring deforms into the groove under intense bolt load, providing a reliable metal-to-metal seal up to 1000°C.

4. Tongue and Groove (T&G) & Male-to-Female (M&F)

Matings feature interlocking male and female profiles that confine the gasket inside an enclosed groove:

  • Completely eliminates gasket extrusion under extreme pressure.
  • Common in ammonia refrigeration, explosive toxic gas lines, and high-vacuum services.

2. Surface Finish Requirements (AARH / Ra)

ASME B16.5 specifies exact surface finish serrations to ensure gasket bite:

  • Stock Finish / Serrated Concentric or Spiral: 125 to 250 microinches AARH (3.2 to 6.3 micrometers Ra). Used with non-metallic sheets and spiral wound gaskets.
  • Smooth Finish: 63 to 125 microinches AARH (1.6 to 3.2 micrometers Ra). Used with solid metal gaskets or critical vacuum sealing.
  • RTJ Groove Finish: 63 microinches Ra maximum roughness along the 23° groove sidewalls.

Frequently Asked Questions

Can you bolt a Flat Face flange to a Raised Face flange?

Engineering codes strongly advise against it when the flat face component is cast iron or bronze, as the cantilever bending stress induced by the 1/16" gap can crack the cast iron flange. If unavoidable, the raised face must be machined off or a full-face spacer ring used.

Why do RTJ ring gaskets need to be softer than the flange?

The metallic ring gasket must plastically yield and deform into the groove to form a seal without galling or damaging the expensive flange groove faces. For example, a soft iron ring (max 90 HBW) is used with carbon steel flanges, or a 316L ring (max 160 HBW) with stainless steel flanges.

Custom Machined Flange Facings in Mumbai

Remax Forge machines RF, FF, RTJ, and Tongue & Groove facings on CNC lathes to exact ASME B16.5 surface roughness tolerances with complete EN 10204 3.1 certification.

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