Flange Facings

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 Flange Facings

1) Flat Face (FF):


  • Flanges with flat face are those that have a flat level connecting surface
  • The Flat Face flange has a gasket surface in the same plane as the bolting circle face.
  • Flat face flanges are never to be bolted to a raised face flange.
  • This is used for 150# flanges, especially when connected to cast iron equipments, valves etc.

2) Raised Face (RF):

  • The most common face type, This flange has a prominent raised surface.
  • With shallow grooves etched into the raised surface, this flange face assures a positive grip with the gasket.
  • Flanges rated 150# and 300# have a 1/16" raised face, while Flanges rated 400# and above have a 1/4" raised face.

3) Ring Type Joint (RTJ):

  • Ring joint does not use a gasket to form a seal between connecting flanges. Instead uses a round metallic ring that rests in a deep groove cut into the flange face.
  • The donut shaped ring can be oval or octagonal in design.
  • As the bolts are tightened, the metal ring is compressed, creating a tight seal.
  • Although most expensive, it is considered to be the most efficient flange used in process piping systems.
  • These are used for high pressure services, normally 900# and above. 

4) Tongue & Groove:

  • These are used mainly for vaccum services, high temperature/pressure services. 
  • However these are not recommended for use now-a-days due to corrossion in the groove and also the problem of matching male & female flanges at each installation.

Type of Finish on Flange Face:

The face finish depends on the type of gasket used. There are two types of finishes done to flange facings:
  1. Smooth Finish
  2. Serrated Finish
Smooth Finish flanges are specified when metallic gaskets are used.
Serrated Finish are specified when non metallic gaskets are used.
The finish is specified by a term called Arithmetic Average Rough Lines Height (AARH).
AARH indicates the depth of the serration in micro inches.
Smooth finish will have a face finish of 125 - 250 AARH.
Serrated finish for a raised face flanges will have 250 - 500 AARH.
The serration provided on the facing could be concentric or spiral concentric serrarations are insisted, when fluid being carried has very low density and can leak through these points.

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