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Flange Face Surface Finish

Flanges provide the necessary connections to link pipelines. Faces are the mating surface of a flange.

Different types of flange faces are used as the contact surfaces to seat the sealing gasket material. ASME B16.5 and B16.47 define various types of flange facings, including the raised face, the large male and female facings which have identical dimensions to provide a relatively large contact area.

Flange faces have to be smooth enough to ensure a tight, leak-free seal for bolted flanges. Flange face

The most used types are:

Other flange facings covered by these standards include the large and small tongue-and-groove facings, and the ring joint facing specifically for ring joint type metal gaskets.

Raised Face (RF)

The Raised Face type is the most applied flange type, and is easily to identify.

Raised Face height

It is called raised face because the gasket is raised 1/16" to 1/4" above the bolt circle face. This face type allows the use of a wide combination of gasket designs, including flat ring sheet types and metallic composites such as spiral wound and double jacketed types.

The purpose of a RF flange is to concentrate more pressure on a smaller gasket area and thereby increase the pressure containment capability of the joint.

Flat Face (FF)

The flat face (full face) flange has a gasket surface in the same plane as the bolting circle face.

Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting.

Flat face flanges are never to be bolted to a raised face flange. When connecting flat face cast iron flanges to carbon steel flanges, the raised face on the carbon steel flange must be removed, and that a full face gasket is required. Flat face flanges are used on pump facings or on fiberglass flanges where the torque of compressing the gasket will damage the flange body and on cast iron flanges sometimes found on mechanical equipment that can cause complications due to the brittle nature of cast iron. Forged steel flat face flanges are often found 150# and 300# ratings.

The Flat Face flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting.

Ring-Type Joint (RTJ)

The Ring Type Joint flanges are typically used in high pressure (Class 600 and higher rating) and high temperature services above 800°F (427°C).

RTJ flanges have grooves cut into their faces. An RTJ flange may have a raised face with a ring groove machined into it. This raised face does not serve as any part of the sealing means. For RTJ flanges that seal with ring gaskets, the raised faces of the connected and tightened flanges may contact each other. In this case the compressed gasket will not bear additional load beyond the bolt tension, vibration and movement cannot further crush the gasket and lessen the connecting tension.

Ring-type joints (RTJ) are considered to be the most efficient flanges for use in pipeline design. Rather than using a gasket between connecting flanges, RTJ have a deep groove in a ring shared around the face.

Ring type gaskets must be used on this type of flange. Ring Type Joint gaskets are metallic sealing rings, suitable for high-pressure and high-temperature applications.

The most applied rings are:
1. Type R-Oval according to ASME B16.20
The original style of metallic ring joint. Used on round bottomed groove flanges though can also be used on later, flat-bottomed groove flanges. Suitable for ASME B16.5 flanges from Class 150 to 2500.
2. Type R-Octagonal according to ASME B16.20
An improved design over the original Oval design. However these can be used only in flat-bottomed groove flanges. Suitable for ASME B16.5 flanges from Class 150 to 2500.

The octagonal cross section has a higher sealing efficiency than the oval and would be the preferred gasket. However, only the oval cross section can be used in the old type round bottom groove. The newer flat bottom groove design will accept either the oval or the octagonal cross section.
The sealing surfaces on the ring joint grooves must be smoothly finished to 63 Microinches and be free of objectionable ridges, tool or chatter marks. They seal by an initial line contact or a wedging action as the compressive forces are applied. The hardness of the ring should always be less than the hardness of the flanges.
Style R ring type joints are designed to seal pressure up to 6,250 psi in accordance with ASME B16.5 pressure ratings and up to 5,000 psi.

Tongue-and-Groove (T&G)

With this type the flanges must be matched. One flange face has a raised ring (Tongue) machined onto the flange face while the mating flange has a matching depression (Groove) machined into it′s face.



These facings are commonly found on pump covers and valve bonnets.

Tongue-and-groove facings are standardized in both large and small types. They differ from male-and-female in that the inside diameters of the tongue-and-groove do not extend into the flange base, thus retaining the gasket on its inner and outer diameter.

Tongue-and-groove joints also have an advantage in that they are self-aligning and act as a reservoir for the adhesive. The scarf joint keeps the axis of loading in line with the joint and does not require a major machining operation.

Male-and-Female (M&F)

This type of flanges also must be matched. One flange face has an area that extends beyond the normal flange face (Male).

The other flange or mating flange has a matching depression (Female) machined into it′s face. Custom male and female facings are commonly found on the heat exchanger shell to channel and cover flanges. The female face and the male face are smooth finished. The outer diameter of the female face acts to locate and retain the gasket.

Advantages:
Better sealing properties, more precise location and exact compression of sealing material, utilization of other, more suitable sealing and specialized sealing material.

Disadvantages:
Normal raised faced is far more common and ready available both regarding Valves, flanges and sealing material. Another complexity is that some rigid rules must be applied to the piping design.

What is the difference between a RTJ, FF, and RF flange on seals and thermowells?

The raised face, RF, is the standard process connection on our third party seals and thermowells. Both have options for a RTJ and FF type flange connections. The Raised Face (RF) is the most common type used in process plant applications.

The gasket surface of the flange is raised above the bolting circle face. A Ring-type Joint (RTJ) can also have a raised gasket face with the difference being the ring groove machined in this face.

This groove will accommodate a steel ring gasket for flange mating. The Flat Face, FF, flange has a gasket surface that is in the same plane as the bolting circle face.

None of these three flange types are interchangeable between types, i.e. RTJ flange cannot be mated to a RF flange. ​

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