Materials for Metallic Gaskets
When specifying the material for a metalic gasket it is important to analyze the metal or alloy properties and its reactions under stress and temperature. Special attention must be given toThe following are the most common alloys used in manufacturing metallic gaskets, their characteristics, temperature limits and approximate Brinell harness
Commonly used in jacketed gaskets and ring joints. Carbon Steel has a low corrosion resistance and should not be used in water, diluted acids or saline solutions. Carbon Steel can however be used in alkalis and concentrated acids. Temperature limits: 900EF (500EC). Hardness: 90 to 120 HB.
An alloy that contains 18% Cr (Chromium) and 8% Ni (Nickle) and is one of the most common in the maufacturing of metallic gaskets because of its excellent resistance to corrosion, its low cost and is readily available. Its maximum operating temperature is 1400EF (760EC). Because of stress and intergranular corrosion, the continuous service temperature is limited to 790EF (420EC). Hardness: 160 HB.
The metallic strip that holds the filler has a standardized thickness of 0.008 in. (0.20 mm) and the width according to the gasket thickness. The most common materials used for metallic strips are:
Stainless steel AISI 304 and 304L: the most widely used material. Good corrosion resistance and is affordable.
Stainless AISI 316 and 316L for chemical service.
Stainless steel AISI 321 for high temperature service.
Other usable materials can include: Monel, Nickle 200, Inconel, and Titanium.
The filler material is what provides gasket sealability. The edge of the filler should be flush or above the edge of the metal strip, never below. Here are some materials used as filler
Flexible Graphite: This material is great for spiral wound gaskets due to its lowpermeability, thermal stability, low creep and chemical resistance. In neutral or reducing services it can be used from 330EF (-200EC) up to 5430EF (3000EC). When used in oxidizing services at a temperature above 660EF (450EC) it is necessary to encapsulate the gasket to protect it from direct contact with the oxidizing medium. Flexible Graphite can be used in oxygen, in any concentration, up to 930EF (500EC) as long as the sealing element is completely enclosed between the flanges. Its operational temperature limit for steam and hydrocarbons rich in hydrogen is 1200EF (650EC) . Flue gas service should be avoided at this temperature. Flexible graphite is also rated as fire-safe by The Pressure Vessel Research Comittee (PVRC).
PTFE: This material is used when higher chemical resistance is needed. PTFE filled gasketsshould be confined in a grooved flange or with an inner reinforcing ring to increase its mechanical resistance.
Mica-Graphite: A chlorite, graphite and cellulose based Beater Addition paper with anNBR latex binder. This material is used as a subsitute for Asbestos paper because it has similar sealability and overall performance up to 450EF (232EC). In higher temperatures it degrades. NOT RECOMMENDED for use in refineries, chemical plants or any service with flammable media where a fire-safe gasket is required.
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