The Basic Knowledge of the spiral wound gasket

What is a spiral wound gasket?

A spiral wound gasket is one of the most widely used gaskets currently. A spiral wound gasket comprises a mix of metallic and filler material. A gasket has a metal wound outwards in a circular spiral with the filler material (generally a flexible graphite) wound in the same manner but starting from the opposing side. A gasket is applied to high temperature, high pressure, ultra-low temperature and vacuum conditions. The problem of the gasket being corroded by all kinds of medium can be solved by changing materials of gaskets. The structural density can be made according to the requirements of different clamping forces. A spiral wound gasket has very good sealing effects, long service life and good practicality, which makes the spiral wound gasket widely used in various industries.

There are four types of metal wound gaskets, that is, the basic type, the inner and outer ring type, the inner ring type and the outer ring type.
Spiral wound gaskets are usually made of stainless steel strips such as SUS304 and SUS316, and graphite, asbestos, polytetrafluoroethylene, non-asbestos and other soft materials are wound around stainless steel strips. The beginning and the end of the metal strip are fixed by spot welding. The basic spiral wound gasket doesn’t have inner and outer rings, and it is usually used for flanges with tongue and groove faces, plain faces or male and female faces in medium and low pressure.
Materials
1. Basic spiral wound metal strips
A. SUS304, SUS316L and SUS321 cold rolled steel strips with thickness from 0.15mm to 0.25mm can be adopted for basic spiral wound metal strips.
B. The metal strip is the preformed V-type or W-type narrow strip, and its surface should be smooth and clean. There should not be defects such as roughness, cracks, scratches, steel pits and rusty spots.
C. Hardness of stainless steel should be equal to or more than 200.
2. Basic spiral wound non-metallic strips
A. Non-metallic are filled with special asbestos, flexible graphite, PTFE, non-asbestos fiber, mica and other strips. The thicknesses of the strips are from 0.3 to 1.0mm.
B. Loss on ignition of special asbestos must be equal to or more than 25%.
C. The chloride ion content of non-metallic strips should be equal to or more than 100PPM.
Performance
Spiral wound gaskets have good compression and resilience performance and functions of multi sealing and certain self tightening. They are not sensitive to surface defects of flanges’ sealing surfaces and don’t stick to sealing surfaces of flanges, which can eliminate partial pressure. Spiral wound gaskets can keep their good sealing performance in the harsh conditions of high temperature, low temperature, high vacuum and shock vibration.
Applications
Static sealing of flanges’ connection parts for pipes, valves, pressure vessels, condensers, heat exchangers, towers, manholes, hand holes of petroleum, chemical engineering, metallurgy, electricity, shipping, fuel gas, nuclear energy, aerospace, textile, pharmaceuticals, machinery and other industries.
Types, code names and applicative flanges of spiral wound gaskets
The basic type is used for tongue and groove faces, male and female faces, flat and groove faces.
The inner ring type is used for tongue and groove faces, male and female faces, flat and groove faces.
The locating ring type is used for flat faces or female faces.
The inner ring and locating ring type is used for flat faces or female faces.
Leakages of spiral wound gaskets
The gap on the contact surface is the root cause of the leak, and the pressure difference and concentration difference on both sides of the contact surface are the driving forces for leakages. There is a gap on the sealing surface due to the influence of the form of the sealing surface and the processing accuracy. This will cause the sealing surface not to be fully fit, resulting in leakages.
It is necessary to maximize the cross-sectional area of the leakage passage and to increase the leakage resistance in order to reduce leakages. Applying a compressive load on the sealing surface to create a compressive stress can have better sealing performance. When the stress is increased enough to cause significant plastic deformation of the surface, the clearance of the sealing surface can be filled to block the leakage path.

The recommended temperature and pressure for various media and non-metallic strips
Material Abbreviation Temperature ℃ Maximum working pressure Applicable media Compression rate% Resilience rate% Stress relaxation%
Expanded Graphite F.G. -220–550 250 BAR Corrosive Media 18–30 ≥17 ≤18
Special asbestos ASB -150–450 100 BAR Media with airtightness 18–30 ≥19 ≤28
Polytetrafluoroethylene PTFE -200– 250 100 BAR Corrosive Media 18–30 ≥15 ≤13
Non-asbestos NAF -100–350 100 BAR Media with airtightness 18–30 ≥19 ≤28
Mica MICA -150–800 100 BAR Super high temperature 18–30 ≥19  
Ceramic CER -200–1090 100 BAR Ultra High Temperature 18–40 ≥19  

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