Flange Face

Three types of flange faces are commonly found. The surface finish of the faces is specified in the flange standards quoted previously:

Raised Face. The raised face is the most commonly used facing employed for steel flanges. The facing on the RF flange has a concentric or a spiral phonographic groove with a controlled surface finish. Sealing is achieved by compressing a flat, soft, or semi-metallic gasket between mating flanges in contact with the raised face portion of the flange.

Ring-Type Joint. This type is typically used for more severe duties than the RF surface, usually ASME classes above 900; however, it is valuable in the lower-pressure classes.

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RTJ for API 6A Type 6B, ASME B16.5 Flanges. The seal is made by plastic deformation of the metallic RTJ gasket into the groove on the flange face, resulting in intimate metal-to-metal contact between the gasket and the flange groove. The faces of the two opposing flange faces do not come into direct contact with each other, because a gap is maintained by the presence of the gasket. Such RTJ flanges normally have raised faces, but flat faces may also be used or specified.

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RTJ for API 6A Type 6BX Flanges. API 6A Type 6BX flanges have raised faces. These flanges incorporate special metallic ring joint gaskets. The pitch diameter of the ring is slightly greater than the pitch diameter of the flange groove. This factor preloads the gasket and creates a pressure-energized seal. A Type 6BX flange joint that does not achieve face-to-face contact will not seal and, therefore, must not be put into service.

Flat Face. Flat-face flanges are a variant of raised-face flanges. Sealing is achieved by compression of a flat nonmetallic gasket between the two serrated surfaces of the mating FF flanges. The gasket covers the entire face of the flange sealing surface. FF flanges are normally used for the least arduous duties, such as low-pressure water piping having class 125 and class 250 flanges and flanged valves and fittings.

Less Commonly Used Flange Faces. Other alternative types of flanges are available; however, due to international standardization in the energy industry, they are very rarely used on projects designed to one of the ASME B31 codes.

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Male-and-Female Facings. The outer diameter of the female face acts to locate and retain the gasket. Custom male-and-female facings are commonly found on the heat exchanger shell to channel and cover flanges.

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Tongue-and-Groove Facings. Tongue-and-groove facings are standardized in both large and small types. They differ from male-and-female facings 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. These are commonly found on pump covers and valve bonnets.

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Check condition of flange faces for scratches, dirt, scale, and protrusions. Wire brush clean as necessary. Deep scratches or dents will require refacing with a flange facing machine.

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Check that flange facing gasket dimension, gasket material and type, and bolting are per specification. Reject nonspecification situations. Improper gasket size is a common error.

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Check gasket condition. Only new gaskets should be used. Damaged gaskets (including loose spiral windings) should be rejected. The ID windings on spiral-wound gaskets should have at least three evenly spaced spot welds or approximately one spot weld every 6 in. of circumference (see API 601).

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Use a straightedge and check facing flatness. Reject warped flanges.

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Check alignment of mating flanges. Avoid use of force to achieve alignment. Verify that

1.

the two flange faces are parallel to each other within $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$32$}\right.$ in. at the extremity of the raised face,

2.

flange centerlines coincide within ⅛ in.

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Check condition of flange faces for scratches, dirt, scale, and protrusions. Wire brush clean as necessary. Deep scratches or dents will require refacing with a flange facing machine.

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Check that flange facing gasket dimension, gasket material and type, and bolting are per specification. Reject nonspecification situations. Improper gasket size is a common error.

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Check gasket condition. Only new gaskets should be used. Damaged gaskets (including loose spiral windings) should be rejected. The ID windings on spiral-wound gaskets should have at least three evenly spaced spot welds or approximately one spot weld every six in. of circumference (see API 601).

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Use a straightedge and check facing flatness. Reject warped flanges.

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Check alignment of mating flanges. Avoid use of force to achieve alignment. Verify that:

1.

The two flange faces are parallel to each other within 1/32 in. at the extremity of the raised face

2.

Flange centerlines coincide within 1/8 in.

The following is a list of typical surface finishes for flange faces;

1.

Stock Finish: 250 to 500 AARH: This finish is produced with a continuous spiral groove generated with a round nosed tool. This finish is suitable for all ordinary service conditions and is the most widely used gasket surface finish

2.

Spiral Serrated: 125 to 250 AARH: This finish is produced with a continuous spiral groove but utilizes a 90° included angle “V” tool. The groove is 1/64” deep and the feed is 1/32” for all sizes.

3.

Concentric Serrated: 125 to 250 AARH: This finish is produced with concentric grooves using the same tools and parameters as the “spiral serrated” finish.

4.

Smooth Finish: 63 to 125 AARH: This finish can be generated with a variety of tooling, but shows no tool marks apparent to the naked eye.

5.

Cold Water Finish: 32 to 63 AARH: This finish is very smooth and has the appearance of a ground finish. It is mirrorlike in appearance.

6.

Flat Face: Stock or Serrated

7.

1/16” Raised Face: Stock or Serrated

8.

¼” Raised Face: Stock or Serrated

9.

Male & Female: Smooth

10.

Tongue & Groove: Smooth

11.

Side Wall of Ring Joint: 63 AARH

For flanged ends (slip-on) for cement-lined pipe and fittings, see Fig. 9.12. For shop-welding, refer Section 9.17.3.4.1.

The body of a dual plate check valve is usually cast before machining of the flange faces and other parts. Machining the hard materials such as duplex requires extra man-hours. The photos in Fig. 9.10 show part of the factory where the body castings of dual plate valves have been received from the foundry for quality control checking. Quality control includes material checking (such as chemical composition and mechanical properties), dimension check, hardness check, PMI, etc.

Each dual plate check valve will be machined (Fig. 9.11) after a visual examination and preliminary quality control.

Valve manufacturers should have established criteria for accepting or rejecting a body casting based on the location, depth, and extension of the defects. Fig. 9.12 shows a sample of criteria for accepting the body casting.

Nondestructive testing (NDT) is done after machining on the body of the valve to make sure that there is no crack or material imperfection on the machined faces. Fig. 9.13 shows a liquid penetration test as a type of NDT done on the body of a dual plate check valve. The first liquid used in NDT is a red liquid called penetrant. The white color is the second liquid used in NDT, and it is called developer. The developer reveals any cracks or material defects.

Sealing pockets of carbon steel body ball valves (Fig. 15.26) including stem and seat sealing grooves, and sometimes flange faces, require an Inconel 625 weld overlay to avoid crevice corrosion. However, Inconel 625 overlay is not applied on the sealing pockets and flange faces of 22Cr duplex body valves due to higher corrosion resistance. Therefore, in case of changing the carbon steel body valves to 22Cr duplex in small sizes, it may be more economical because there is no weld overlay requirement. All carbon steel ball valves in sizes 3″ and below have been changed to 22Cr body duplex in an ongoing Norwegian project to avoid Inconel 625 weld overlay and maybe reduce the cost.