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Flange Design

Flange Facing Types: RF, FF, RTJ, M&F, T&G Selection Guide

The facing type determines how your flange seals, which gasket it accepts, and how much pressure it can hold. Choosing the wrong facing is a leading cause of gasket blowout and flange leaks.

1. What is a Flange Facing?

The facing is the machined surface of a flange that contacts the gasket and mating flange. It is not cosmetic—its geometry directly controls:

  • Gasket compression: Raised surfaces concentrate bolt load for higher sealing stress
  • Gasket retention: Grooves and recesses prevent gasket blowout under pressure
  • Alignment: Tongue-and-groove or male-female facings self-align and prevent lateral movement

ASME B16.5 defines standard facing types. API 6A adds BX ring grooves for wellhead equipment. Choosing the correct facing is as important as choosing the correct flange class.

2. The Five Standard Facing Types

RF

Raised Face

Most common facing. A 1/16" (1.6mm) or 1/4" (6.4mm) raised ring on the flange face concentrates gasket compression for a tight seal.

Best for: Class 150–2500, general process piping

Gasket: Spiral wound, ring joint, non-metallic

FF

Flat Face

Full-face contact with no raised portion. Used where flange bending must be minimized, typically with cast iron or plastic equipment.

Best for: Class 125 cast iron, plastic-lined piping

Gasket: Full-face non-metallic (rubber, PTFE)

RTJ

Ring Type Joint

Precision-machined groove holds a metal ring gasket. Deforms plastically under load for a metal-to-metal seal.

Best for: Class 900–2500, high-pressure gas, subsea

Gasket: R, RX, BX metal rings (soft iron, SS, Inconel)

M/F

Male & Female

One flange has a raised male face, the mating flange has a recessed female face. Gasket sits in the female recess, protected from flow erosion.

Best for: Class 150–600, corrosive or erosive service

Gasket: Non-metallic flat gaskets

T/G

Tongue & Groove

One flange has a raised tongue, the other a matching groove. Provides precise gasket alignment and blowout resistance.

Best for: Class 150–600, toxic or hazardous media

Gasket: Non-metallic flat gaskets (confined)

3. Facing Selection by Service & Class

ServiceClassFacingGasket Type
Water, air, low-pressure150–300RF or FFNon-asbestos sheet
Steam, oil, general process150–600RFSpiral wound
High-pressure gas900–2500RTJMetal ring (R/RX)
Corrosive chemicals150–600M/F or T/GPTFE, graphite
Cast iron equipment125FFFull-face rubber
Subsea pipeline900–2500RTJ (BX)BX ring

Critical rule: Never mate RF with FF flanges. The raised face will crush the flat face flange, causing catastrophic failure. Always match facing types.

4. RF Face Heights by ASME B16.5

Raised face height depends on pressure class. Taller faces provide more gasket compression area and better sealing at high pressure.

Class 150 & 300

  • Height1/16" (1.6 mm)
  • Surface finish125–250 µin AARH
  • Gasket ODMatches RF OD

Class 400–2500

  • Height1/4" (6.4 mm)
  • Surface finish125–250 µin AARH
  • Gasket ODMatches RF OD

5. RTJ Ring Groove Dimensions

Ring Type Joint grooves are precision-machined to accept metal ring gaskets. The three standard ring types:

Type R (Oval & Octagonal)

Most common. Oval for older flanges, octagonal for modern. Sizes R-11 to R-105 covering NPS 1" to 24".

Type RX (Pressure-Energized)

Similar to R but with a wider cross-section that pressurizes the groove. Used in high-pressure API 6A flanges.

Type BX (Subsea & Ultra-High Pressure)

Square cross-section with pressure passage hole. For API 6BX flanges in subsea and 15,000+ psi service.

6. Surface Finish Requirements

Facing finish is measured in Arithmetic Average Roughness Height (AARH) or Root Mean Square (RMS):

  • 125–250 AARH (3.2–6.3 µm): Standard for RF flanges with spiral wound or non-metallic gaskets
  • 63–125 AARH (1.6–3.2 µm): For metal-to-metal seals and some RTJ applications
  • 32–63 AARH (0.8–1.6 µm): Mirror finish for lens ring joints or special applications

Phonographic (concentric serrated) finish is standard for RF. Spiral serrated is acceptable but may require harder gaskets.

7. FAQ

Can I convert an RF flange to RTJ?
Yes, by machining a ring groove into the raised face. However, this reduces flange thickness and may require engineering approval. Some shops offer "RTJ conversion" services, but verify that the remaining flange thickness meets ASME B16.5 minimums.
What is the difference between AARH and RMS?
AARH (Arithmetic Average Roughness Height) and RMS (Root Mean Square) both measure surface roughness. RMS values are approximately 11% higher than AARH for the same surface. 125 AARH ≈ 140 RMS. Always confirm which unit your specification uses.
Can I use a spiral wound gasket on an FF flange?
No. Spiral wound gaskets require a raised face or ring groove to compress properly. On FF flanges, use full-face non-metallic gaskets. The spiral wound's metal windings will damage the flat face.
Which facing is best for high-vacuum service?
Male & Female or Tongue & Groove with a soft metal gasket (copper, aluminum) provides the best vacuum seal. The confined gasket cannot extrude into the vacuum stream, and the precise alignment prevents leaks.

Need Flanges with Specific Facing?

RF, FF, RTJ, M/F, T&G—get a quote for any facing type in Class 150–2500, carbon or stainless steel.

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