Sunday, February 3, 2013

Gaskets Materials, Compression and Relation to Bolting

Since it is expensive to grind and lap joint faces to obtain fluid-tight joints, a gasket of some softer material is usually inserted between contact faces. Tightening the bolts causes the gasket material to flow into the minor machining imperfections, resulting in a fluid-tight seal. A considerable variety of gasket types are in common
use. Soft gaskets, such as cork, rubber, vegetable fiber, graphite, or asbestos, are usually plain with a relatively smooth surface. The semimetallic design combines metal and a soft material, the metal to withstand the pressure, temperature, and attack of the confined fluid and the soft material to impart resilience. Various
designs involving corrugations, strip-on-edge, metal jackets, etc., are available. In addition to the plain, solid, and flat-surface metal gaskets, various modified designs and cross-sectional shapes of the profile, corrugated, serrated, and other types are used. The object in general has been to retain the advantage of the metal gasket but to reduce the contact area to secure a seal without excessive bolting load. Effective gasket widths are given in various sections of the ASME Boiler and Pressure Vessel Code.

Gasket Materials

Gasket materials are selected for their chemical and pressure resistance to the fluid in the pipe and their resistance to deterioration by temperature. Gasket materials may be either metallic or nonmetallic. Metallic ring-joint gasket materials are covered by ASME Standard B16.20, Ring-Joint Gaskets and Grooves for Steel Pipe Flanges. Nonmetallic gaskets are covered in ASMEStandard B16.21, Nonmetallic Gaskets for Pipe Flanges. Typical selections of gasket materials for different services are shown in Table below:

* Several gasket manufacturers have introduced nonasbestos, nonmetallic gasket materials for use in
high-temperature service. These materials are proprietary, and therefore the manufacturers should be
consulted for specific applications.

Gasket Compression

In the usual type of high-pressure flange joint, a narrow gasket face or contact surface is provided to obtain higher unit compression on the gasket than is obtainable on full-face gaskets used with low-pressure joints. The compression on this surface and on the gasket if the gasket is used, before internal pressure is applied, depends on the bolt loading used. In the case of standard raised face joints of the steel-flange standards, these gasket compressions range from 28 to 43 times the rated working pressure in the Class 150 to 400 standards, and from 1 to 28 times in the Class 600 to 2500 standards for an assumed bolt stress of 60,000
psi (4200 kg/cm2). For the lower-pressure standards, using composition gaskets, a bolt stress of 30,000 psi (2100 kg/cm2) usually is adequate. The effect of applying the internal pressure is to decrease the compression on the contact surface, since part of the bolt tension is used to support the pressure load.

The initial compression required to force the gasket material into intimate contact with the joint faces depends upon the gasket material and the character of the joint facing. For soft-rubber gaskets, a unit compression stress of 4000 psi (280 kg/cm2) to 6000 psi (420 kg/cm2) usually is adequate. Laminated asbestos gaskets in serrated faced joints perform satisfactorily if compressed initially at 12,000 psi (850 kg/cm2) to 18,000 psi (1260 kg/cm2). Metal gaskets such as copper, Monel, and soft iron should be given initial compressions considerably in excess of their yield strengths. Unit pressures of 30,000 psi (2100 kg/cm2) to 60,000 psi (4200 kg/cm2) have been used successfully with metal gaskets. Various forms of corrugated and serrated metal gaskets are available which enable high unit compression to be obtained without excessive bolt loads. These are designed to provide a contact area that will flow under initial compression of the bolts so as to make an initially pressure-tight joint, but at the same time the compressive stresses in the body of the gasket are sufficiently low as to be comparable to the long-time load-carrying ability of the bolting and flange material at high temperatures.

The residual compression on the gasket necessary to prevent leakage depends on how effective the initial compression has been in forming intimate contact with the flange joint faces. Tests show that a residual compression on the gasket of only 1 to 2 times the internal pressure, with the pressure acting, may be sufficient to prevent leakage where the joint is not subjected to bending or to large and rapid temperature changes. Since joints in piping customarily must withstand both these disturbing influences, minimum residual gasket compressions of 4 to 6 times the working pressure should be provided for in the design of pipe joints.

Relation of Gaskets to Bolting

There is a tendency, as indicated in the ASME Rules for Bolted Flanged Connections, to assign lower residual contact-pressure ratios ranging from about 1 for soft-rubber gaskets to 6 or 7 for solid-metal gaskets. Whereas these are said to have proved satisfactory service for heat-exchanger and pressure-vessel flanges, the more severe service encountered by pipe flanges due to bending moments and large temperature changes is considered by many to warrant designing on the basis of the larger residual gasket compression ratios recommended in the previous paragraph. The lack of understanding of the mechanics of gasket action, the variety of gasket materials, shapes, widths, and thicknesses; the variety of facings used; the variation in flange stiffness; and the uncertainties in bolt pull-up are among the factors that render difficult a precise solution to the problem of gasket design.

Rules for bolting and flange design are contained in Sections III and VIII of the ASME Boiler and Pressure Vessel Code.

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