Flange Types and Contact Surface Finish on Pipe System (continued)

Beside still with Flanges Types, Piping & Fabrication also will explain about contact surface finish and I hope all of this articles will be useful to all of us. And with Flange Types and Contact Surface Finish on Pipe System we will continue.

Optional-Type Flanges. 

This type covers designs where the attachment of the flange to the nozzle neck or the vessel or pipe wall is such that the assembly is considered to act as a unit, which shall be calculated as an integral flange, except that for simplicity the designer may calculate the construction as a loose-type flange, provided that stipulated load values are not exceeded.

It is important in flange design to select materials and to proportion dimensions of bolts, flanges, and gaskets to ensure that the necessary compression will be maintained on the joint faces over the expected life of the equipment.

Several distinct phases of the problem are involved: (1) type of flange facing, (2) finish of contact surfaces, (3) gasket type and proportions, (4) bolt load required to secure and maintain a tight joint, and (5) proportions of flange needed to support the bolt load.

Types of Flange Facing. 

There are numerous types of contact facings for flanges, the simplest of which is the plain face provided with a ‘‘smooth tool finish.’’ Class 125 cast-iron flanged fittings are provided with this type of facing. For steel flanges and fittings, the typical facings (Fig. A2.19) are taken from the American Standard for Steel Pipe Flanges and Flanged Fittings, ASME B16.5 and ASME B16.47. The raised face, the lapped, and the large male-and-female facings have the same dimensions, which provide a relatively large contact area. Where metal gaskets are used with these facings, the gasket area should be reduced to increase the gasket compression.

FIGURE A2.18 Commonly used flanged joints. (a) Screwed flange to fitting
joint, plain face; (b) screwed flange pipe joint, male-and-female face; (c) lapped
pipe to fitting joint, square corner; (d) lapped pipe to pipe joint, round corner;
(e) ring and groove joint, welding neck flange to fitting; ( f ) lapped pipe to fitting
joint, Sarlun seal welded; and (g) lapped pipe to fitting joint, Sargol seal welded

FIGURE A2.19 Typical flange facings (for dimensions, see ASME B16.5)

The flange-facing types illustrated in Fig. A2.19 range in size and contact area in the following order: large tongue-and-groove, small tongue-and-groove, small male-and-female, and ring joint. Because of the small gasket contact area, a tight joint may be secured with the ring-type facing using low bolting loads, thereby resulting in lowered flange stresses (ASME B16.5).
The Sargol and Sarlun facings, which have lips for seal welding, are used frequently for severe service conditions. Seal welding is not always performed since, if it is properly made, a tight joint often can be maintained without the welded seal, thus facilitating disassembly. Typical facing dimensions for Sarlun and Sargol joints are shown in Fig. A2.20. Special types of facing of individual design intended for a specific service are numerous. Economic considerations generally make it desirable to use a standard facing wherever possible.

FIGURE A2.20 Typical facing dimensions for Sargol and Sarlun joints, Class 150 to 2500 flanges

Selection of the type of facing depends to a considerable extent on the nature of the service. However, it is not possible to determine exactly which facing should be used. Prior experience is usually relied on as a guide. Plain-face joints with red rubber gaskets have been found satisfactory for temperatures up to 220_F (105_C), whereas serrated raised-face joints with graphite-steel-composition gaskets are commonly used for temperatures up to 750_F (400_C). For high temperatures and pressures, faces giving a high contact pressure for a given bolt load are customary, such as the tongue-and-groove and ring joints. However, with high contact pressures, the gasket load must be checked to ensure that the gasket is not overcompressed. An equally successful joint for most types of service can be made by using a profileserrated metal gasket contacting the flange facing, which may be the plain male to-male raised-face type.

Contact Surface Finish. 

The surface finish is an important factor in determining the extent to which a gasket must flow to secure an impervious seal. Bolting that results in adequate gasket flow to form a satisfactory seal with a smooth contact surface may be inadequate to secure a tight joint with a rough surface. The finish may vary from that produced by rough casting surfaces to that produced by grinding and lapping. Less gasket flow will be necessary for the latter than for the former. The finish most frequently provided on cast-iron and steel pipe flanges is the smooth tool finish. A serrated finish frequently is provided for steel flanges, particularly when using a graphite-composition gasket with a wide contact area such as is furnished on raised, lapped, or large tongue-and-groove facings. The serrated finish consists of spiral or concentric grooves, usually about 1/64 in (0.4 mm) deep with 12.5 serrations per cm (32 serrations per inch). Where metal gaskets are used, a smooth surface produced by grinding or lapping is usually provided. The Sargol and Sarlun facings mate metal to metal without a gasket, in which case a mirrorlike finish is necessary. This is usually produced by grinding and lapping. It is evident that the surface finish varies with the type of contact face and gasket used and, therefore, should be specified accordingly.