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Types of Flange faces

Various types of flange faces serve as the contact surfaces for accommodating the sealing gasket material. ASME B16.5 and B16.47 define a range of flange facings, including the raised face and the large male and female facings, which have identical dimensions to offer a relatively large contact area.

These standards also encompass other flange facings, such as the large and small tongue-and-groove facings, as well as the ring joint facing specifically designed for ring joint type metal gaskets.

Raised Face (RF)

The Raised Face flange is commonly used in process plant applications and is easily recognizable. It earns its name because the gasket surfaces are raised above the bolting circle face. This type of face accommodates a wide range of gasket designs, including flat ring sheet types and metallic composites like spiral wound and double jacketed types.

The purpose of an RF flange is to concentrate more pressure on a smaller gasket area, thereby enhancing the pressure containment capability of the joint. The diameter and height are determined by ASME B16.5 based on pressure class and diameter, with the height of the raised face being determined by the flange’s pressure rating.

The typical flange face finish for ASME B16.5 RF flanges ranges from 125 to 250 µin Ra (3 to 6 µm Ra).

Raised Face height

For all described dimensions of flanges on this website, except for the Lap Joint flange, it is crucial to understand and remember the following regarding the height measures H and B.

In pressure classes 150 and 300, the height of the raised face is approximately 1.6 mm (1/16 inch). In these two pressure classes, nearly all flange suppliers include the H and B dimensions, including the raised face height, in their catalogs or brochures (see Fig. 1).

In pressure classes 400, 600, 900, 1500 and 2500, the height of raised face is approximately 6.4 mm (1/4 inch). In these pressure classes, most suppliers show the H and B dimensions excluding the raised face height. (Fig. 2)

Flat Face (FF)

The gasket surface of the Flat Face flange is positioned in the same plane as the bolting circle face. Flat face flanges are commonly used in applications involving mating flanges or flanged fittings made from castings.

It’s important to note that flat face flanges should never be bolted to a raised face flange. According to ASME B31.1, when connecting flat face cast iron flanges to carbon steel flanges, the raised face on the carbon steel flange must be removed, and a full face gasket is necessary. This precaution is taken to prevent the thin and brittle cast iron flange from being deformed by the raised face of the carbon steel flange.

6061 flange

Ring-Type Joint (RTJ)

The Ring Type Joint (RTJ) flanges are commonly utilized in high-pressure (Class 600 and higher rating) and/or high-temperature services exceeding 800°F (427°C). These flanges feature grooves on their faces to accommodate steel ring gaskets. Tightening bolts compress the gasket between the flanges into the grooves, deforming the gasket to establish intimate contact inside the grooves, thus creating a metal-to-metal seal.

An RTJ flange may have a raised face with a ring groove machined into it, but this raised face does not contribute to the sealing mechanism. When RTJ flanges seal with ring gaskets, the raised faces of the connected and tightened flanges may come into contact with each other. In such cases, the compressed gasket will not bear additional load beyond the bolt tension, and any vibration or movement will not further crush the gasket or reduce the connecting tension.

Ring Type Joint gaskets

Ring Type Joint (RTJ) gaskets are metallic sealing rings designed for high-pressure and high-temperature applications. They are specifically used with special accompanying flanges to ensure effective and reliable sealing, achieved through the appropriate selection of profiles and materials.

These gaskets are engineered to create a seal through “initial line contact” or wedging action between the mating flange and the gasket. When pressure is applied to the seal interface through bolt force, the “softer” metal of the gasket flows into the microfine structure of the harder flange material, resulting in a highly secure and efficient seal.

Most applied type is style R ring that is manufactured in accordance with ASME B16.20 used with ASME B16.5 flanges, class 150 to 2500. Style ‘R’ ring type joints are manufactured in both oval and octagonal configurations.

The Octagonal ring 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 RX type is suitable for pressures up to 700 bar. This RTJ is capable of sealing itself. The outer sealing surfaces make the first contact with the flanges. A higher system pressure causes a higher surface pressure. Type RX is interchangeable with the standard R-models.

The BX type is suitable for very high pressures up to 1500 bar. This ring joint is not interchangeable with other types, and is only suited for API type BX flanges and grooves.

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.

Choice of material

The table below indicates the most commonly used materials for ring type joints.

  • Soft iron
  • Carbon steel
  • SS (Stainless steel)
  • Nickel alloys
  • Duplex steel
  • Aluminum
  • Titanium
  • Copper
  • Monel
  • Hastelloy
  • Inconel
  • Incoloy

Tongue-and-Groove (T&G)

The matching of the Tongue and Groove faces on these flanges is crucial. One flange face features a raised ring (Tongue) machined onto the flange face, while the mating flange has a corresponding depression (Groove) machined into its face.

Tongue-and-groove facings come in both large and small standardized types. Unlike male-and-female facings, the inside diameters of the tongue-and-groove do not extend into the flange base, which helps retain the gasket on its inner and outer diameter. These types of facings are commonly found on pump covers and valve bonnets.

Tongue-and-groove joints also offer the advantage of being self-aligning and acting as a reservoir for the adhesive. The scarf joint keeps the axis of loading in line with the joint and does not require major machining operations.

It’s important to note that general flange faces such as RTJ, Tongue and Groove, and Male and Female should never be bolted together. This is because the contact surfaces do not match, and there is no gasket designed to accommodate one type on one side and another type on the other side.

Male-and-Female (M&F)

With this type the 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.

The female face is 3/16-inch deep, the male face is1/4-inch high, and both are smooth finished. The outer diameter of the female face acts to locate and retain the gasket. In principle 2 versions are available; the Small M&F Flanges and the Large M&F Flanges. Custom male and female facings are commonly found on the Heat Exchanger shell to channel and cover flanges.

Advantages and Disadvantages of T&G and M&F flange faces

Advantages

Better sealing properties, more precise location and exact compression af sealing material, utilization of other, more suitable sealing and spezialized sealing material (O-rings).

Disadvantages

Commercial availability, cost, and complexities related to piping design, including considerations for valve and flange connections. It’s important to weigh the availability and cost of components such as valves, flanges, and sealing materials when making decisions. Additionally, the design of piping systems requires adherence to specific rules, such as determining whether valves should have female ends on both sides or only on one side, and whether male ends should be oriented in the flow direction. Similar considerations apply to flanged joints and vessel connections. These factors can add complexity to the design and procurement process.

why a connection between an RF and RTJ flange is not acceptable?

When connecting RTJ flanges with RF flanges, it’s important to note that RTJ flanges have grooves in their sealing faces, while RF flanges do not. If you attempt to make a connection between the two flanges, only a flat gasket can be used. While this flat gasket will make full contact with the sealing surface of the RF flange, it will not fully contact the sealing surface of the RTJ flange due to the groove. As a result, a portion of the gasket will go unused, making it impossible to achieve a properly sealed flange connection. Additionally, the RTJ flange may damage the gasket, especially when using a spiral wound or metal jacketed gasket. Considering the potential for injuries or fatalities, it’s not worth risking an improper flange connection.

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