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element. Failure of a mechanical seal is defined as an inoperative seal before wear-out of the sacrificial surface. The reliability of a seal design is determined by the ability of the seal to restrict the flow of fluid from one region to another for its intended life in a prescribed operating environment. The evaluation of a seal design for reliability must include a definition of the design characteristics and the operating environment in order to estimate its design life. Section 3.2 discusses the reliability of gaskets and other static seals. Procedures for evaluating the reliability of dynamic seals are contained in Section 3.3. Procedures for evaluating mechanical seals for reliability are included in Section 3.4. 3.2 GASKETS AND STATIC SEALS A gasket is used to develop and maintain a barrier between mating surfaces of mechanical assemblies when the surfaces do not move relative to each other. The barrier is designed to retain internal pressures, prevent liquids and gases from escaping the assembly, and prevent contaminants from entering the assembly. Gaskets can be metallic or nonmetallic. Flange pressure compresses the gasket material and causes the material to conform to surface irregularities in the flange and is developed by tightening bolts that hold the assembly together. Gasket reliability is affected by the type of liquid or gas to be sealed, internal pressure, temperature, external contaminants, types of surfaces to be joined, surface roughness, and flange pressure developed at the joint. To achieve the barrier to a potential leakage path the seal must be sufficiently resilient to conform to cavity irregularities and imperfections, while remaining rigid enough to provide the required contact force needed to ensure a tight seal. This contact force is a function of the seal cross section, as well as the compression of the seal between the mating cavity faces. The load on the gasket must be distributed evenly over the whole area of the gasket rather than have a few points of high load with reduced stress at midpoints between the fasteners. Therefore, a larger number of small bolts is better than a few larger bolts. Use of a torque wrench during installation is always a necessity. An O-ring is a mechanical gasket in the shape of a torus. It is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. The combination of the O-ring and the gland that supports the O-ring constitute the classic O-ring seal assembly. A typical O-ring configuration is shown in Figure 2. While static seals in most cases are designed for "zero-leakage" semi-static seals for applications where there is limited movement are not designed or intended to be "zero-leakage" seals. Their contact or compression force is typically an order of magnitude lower than a static seal. Seals and Gaskets 3-3 Revision G

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