Spiral Wound gaskets are made from any type of metal coupled with a filler (graphite, PTFE, ceramic fibre, glass or Mica) wound uniformly with a constant winding tension. The special metal profile has an elastic action that ensures a perfect seal in all conditions of fluctuating temperatures and pressures. The seals can be assembled with an inner or outer centering ring or both. The use of rings has very specific purposes: the inner ring has the function of anti-turbulence, it prevents material from settling between the interstices of the flanges, it is normally made of the same material as the spiral, protecting against corrosion and eliminating erosion of the flange; the outer ring has the function of centering between the bolts, it prevents lateral expansion of the spiral and serves as a reference shim for correct assembly of the spiral. Spiral Wound gaskets can be coated with a layer of softer material, usually graphite, to further improve sealing.