Flange Facings

Raised Face

Steel flanges with a raised face are extensively used because of the simplicity of the design.

Moderate unit gasket pressure is maintained with conventional machining and assembly methods and facing is suitable for average service conditions. However for severe service involving high pressure, high temperature, thermal shock or cyclic operation, this type of flange facing may not be satisfactory.

Flanges with rating of ASME 150 and 300lbs have 0.06 inch high raised face which is included inthe minimum flange thickness. However, flanges having higher ratings have 0.25 inch highraised face which is additional to the minimum flange thickness.

The raised face is machined with spiral or concentric grooves. Depth and spacing of these grooves vary depending upon finish required to suit the gasket used for making joint. The edges of these grooves serve to deform and hold the gasket.

As per ASME B16.5, for raised face, either a serrated concentric or serrated spiralfinish having from 24 grooves/inch to 40 grooves/inch shall be used. The cutting toolemployed shall have an approx.0.06 inch or larger radius. The resultant surface finish shall have a 125 micron to 500 micron roughness. The finish of contact face shall be judged by visual comparison with Roughness Average ‘Ra’ standards and not by instruments havingstylus tracers and electronic amplification (Refer ASME B46.1)

The facing should not have any radial tool mark since the same would provide shortest path for fluid to escape.

The facing should be properly protected during transportation and handling at site.

Flat ring composition gaskets are normally used having a width equal to the width of the raised face whereas flat metal gaskets may be used having a width equal to that used with large tongue-and-groove type face.

Flat face:

Flanges  with  flat  faces  using  full-face  gaskets  are  usually  used  for   making connections to ASME 25lb and 125 lbs. Cast iron flanges and flanged components. This decreases the leverage in bolting, thereby reducing the tendency for the flange to crack.

Male & Female Facings:

They have the disadvantage that the two mating flanges are not identical. For thisreason, these flanges are not as widely used as raised face flanges.

Male & female facings have the advantage of confining the gaskets thereby minimizing the possibility of blow-out of the gaskets. The outer diameter of the female face serves to locate and retain the gasket.

Male & female facings offer no protection against forcing the gasket into the line orvessel.

Tongue-and-Groove Facings:

Tongue-and-Groove facings have the advantage of confining the gasket. The presence ofretaining metal on either side of the gasket gives protection against possibility of blow out of the gasket as well as protection against deforming soft gaskets into the interior of the line or vessel. This is an advantage over the male-and- female type of face.

The Gasket is less subject to erosive or corrosive contact with the fluid.

In service, the tongue is more likely to get damaged than the groove, therefore thetongue should be placed on the part that can be easily replaced.

Usually for services, where tongue-and-groove facings are used, all  equipment nozzleflanges, valve flanges, and instrument flanges etc. are provided with groove face andmating piping flanges with tongue face.

Pressure-Temperature Ratings

Pressure-Temperature ratings are determined as per Annexure D of ASME B 16.5 Foreasy reference, ASME B16.5 Table 1A and Table 2 - 1.1 (rating for Group 1.1 material) are attached in section 7.0.

For any temperature below -20°F, the rating shall be no greater than the rating shown for –20 O F.

Socket welding and screwed flanges are not recommended for service above 500 O F or below –50 O F if severe thermal gradients or thermal cycling are involved.

At temperature in the creep range, gradual relaxation of flanges, bolts & gaskets may progressively reduce bolt loads. It may be necessary to arrange for periodic tightening of bolts to prevent leakage.

When used above 400 O F, class 150 flanged joints may develop leakage unless care is taken to avoid imposing severe external loads and/or severe thermal gradients. For other classes, similar consideration should be given above 750° F.