Flat Gasket and Spiral Wound Gasket

1. Flat Gaskets

Flat gaskets are the most simple and inexpensive ones of the different types of gaskets. As its name described, flat gaskets are machined with a simple outside diameter, inside diameter, and a certain height. For the most part, these gaskets adapt easily to any irregularities in metal surfaces of the joint(raised face, flat face, RTJ, etc) due to its elasticity or plastic deformation. Flat gaskets are best used for general service applications without severe temperature or pressure considerations. Flat gaskets can be made from industrial plastics such as polytetrafluoroethylene(PTFE) or chlorotrifluoroethylene(CTFE), or soft metals, such as aluminum, copper, silver, soft iron, lead or brass. Some metal flat gaskets are applied to high-temperature service, such as nickel(760℃ ), Monel(815 ℃) or Inconel(915 ℃).

2. Spiral-Wound Gaskets

Spiral wound gaskets are all-purpose, medium-priced gaskets that consist of alternate layers of metallic and non-metallic materials wound together. The metal strip winding is normally V-shaped and is set on edge with the filler material sandwiched between windings. Spiral-wound gaskets combine the elastic properties of flat gaskets with the inclusion of soft metal windings, which adds strength to prevent possible gasket blow-out high-pressure-high-temperature applications. The strength of spiral wound gaskets can be varied by material specified. The strength is also determined by the number of windings: the higher the number of windings, the greater the pressure load handled by the gasket. Wehn spiral-wound gaskets are compressed, the metal layers are crushed, providing an effective seal even with uneven gasket surfaces. However, because the metal strips are deformed during compression, spriral-wound gaskets can never be reused.

As a general rule, spiral-wound gaskets should never be used with soft-seat or soft-seal designs, where the closing device, such as a plug or disc, seats against a nonmetallic surface in valves. The  force needed to compress the spiral-wound gasket is partially transmitted through the soft insert, which is more compressible than the gasket. Therefore, the soft insert is likely to extrude before the spiral-wound gasket is fully compressed. Unfortunately, the outcome is usually a damaged soft insert or the joint connection leaks.

In the past, a common filler material for high-temperature spiral wound gaskets has been asbestos. However, due to the controversial health and legal aspects of this material, many clients of Hebei Metals Industrial Limited now require a non-asbestos-filling design. Newer substitute filler materials have been developed or used, such as ceramic fiber. Gaskets with this new filler have been known by the generic term asbestos-free gaskets(AFG), which can be substituted for gaskets with asbestos filler in most high-temperature applications. Their ability to seal at high temperatures is very similar to a spiral wound gasket that contains graphite. Safety controversies and legal issues aside, asbestos gaskets are occasionally specified by users, especially by those in power generation industry. As noted earlier, because asbestos spiral wound gaskets are used primarily for high-temperature applications, they are typically installed in stainless steel, carbon steel, and chrome-molybdenum flange or valve connections. Besides asbestos, common filler materials include polytetrafluoroethylene, graphite, mica or ceramic.

Graphite spiral-wound gaskets are used for high-temperature-high-pressure applications associated with valves, flanges and pipe fittings in severe service. Either 316 stainless steel or Inconel can be used for the metal windings, denpending on the process fluid. Spiral-wound gaskets can be also custom-made depending on the process fluid and its interaction with metal windings or filler. In addition to those noted earlier, windings can be made from the following materials: SS304, SS316, SS347, SS321, Monel, Pure Nickel, titanium, Alloy 20, Alloy 31, Inconel, carbon steel, Hastelloy B, Hastelloy C-276, phosphor bronze, copper, or platinum.

3. Temperature-Pressure Specifications of Typical Gaskets