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Spiral wound gaskets: performing under pressure - Part 1

A spiral wound gasket spiral gasket is a semi-metallic seal that consists of a formed metal strip that is spirally wound or cheap cartier replica watches rolled between metal coils with a soft filler material. The metal strip, a molded V-shape from the inside outwards, provides mechanical strength and resilience to the seal while the soft filler material ensures proper sealing by filling the irregularities of the flange surface. The good resilience makes the spiral gasket suitable for widely varying loads (pressure & temperature).

Fig. 1 : Spiral wound gasket with inner and outer ring.
The soft filler material and the metal strip are wound together under a given pressure. This pressure is important to obtain the proper density of the graphite.
The inner and outer pair of windings have no filling and are intended to strengthen the spiral wound gasket. At the beginning (inside) and end (outside) a spot weld is placed so that the spiral stays in place. The number of turns without filling and placement of the spot welding is described in standard ASME B16.20: at least 3 turns without filling material and the first 2 are attached by spot welding with at least 3 weld points for up to 3 "circumference.
The thickness of the metal strip is usually 0.20 to 0.25 mm.
The most common materials for the metal strip are stainless steel 304, 304L (cheapest), 316 or 316L, and 321 for higher temperatures.
Other materials for the metal strip include Inconel, Hastelloy, Monel, Incoloy, nickel and titanium, of course with matching price tag.
The filler material between the metal strips is compressed and the pressure with which this happens determines the quality of the spiral wound gasket. By varying the pressure, one can obtain different densities of the filler. The filler material (usually graphite) should always be protruding slightly above the metal strip otherwise leakage may occur.
The vast majority of spiral gaskets use graphite as a filler, but PTFE and mica are also used. Graphite is suitable as a filling material because it has a very good chemical resistance, can be used at high temperatures, is not sensitive to aging and has good sealing properties. In addition it meets so-called “fire-safe” requirements.
Sometimes a combination of two materials (eg graphite and PTFE) are needed, and the PTFE is usually used because of its excellent chemical resistance. Mica is mostly used for very high temperatures.
(to be continued in part 2)
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