What is the use of spiral wound gaskets for blind plate valves

Spiral wound gaskets have been used abroad since the 1930s, and have been widely produced and promoted in China since the 1960s. The early spiral wound gaskets used asbestos as the filling material. Because the sealing performance of the spiral wound gaskets filled with asbestos is not as good as that of expanded graphite, and asbestos is a carcinogen, the current spiral wound gaskets mostly use expanded graphite as the filling material.

The expandable graphite spiral wound gasket has good sealing performance, can adapt to the requirements of high temperature and high pressure parameters, and has good resilience. It can still maintain good sealing performance in situations of temperature and pressure changes (its sealing performance index usually can reach 1 x 10-4Cm3N/S). It is a common gasket used in petroleum, chemical, power and other systems in the past.

However, due to the structure and manufacturing of the blind valve, the defects of the spiral wound gasket are also very obvious. As some experts have pointed out, "This type of gasket is currently used the most, but it also causes the most problems", which is commonly referred to as the "loose frame" and "collapse (or" instability ") problem.

The so-called 'scattered shelves' include two situations:

One is that the gasket is damaged due to scattering during manufacturing, transportation, and installation. This is because the spiral wound gasket structure is only formed by intertwining and interlocking, making it unable to form a solid overall structure. Therefore, it is easy to scatter and damage, causing difficulties in the transportation and installation of the gasket, especially for large-sized gaskets (such as those on cold exchange equipment).

The second reason is that the spiral wound gasket is subjected to significant compression force during use, and the welding points used for fixing the outer layer are prone to detachment and damage under circumferential force, causing the entire gasket to "fall apart" and ultimately leading to sealing failure.

Due to the proportional relationship between the magnitude of circumferential stress and diameter, the larger the size of the gasket, the easier it is to pull off and damage.

The so-called "crushing" (or "instability") refers to the gasket being subjected to a large compression ratio during use, and the internal stress generated by the compression of the filling material (expanded graphite) acts on the inner steel strip towards the inner diameter direction, causing the steel strip to be subjected to external pressure.

When the external pressure exceeds a certain value, the steel strip ring enclosed by the steel strip is like a thin-walled container that has lost stability due to external pressure, with local areas sinking inward, causing damage to the gasket and leading to sealing failure. The higher the compression force, the larger the gasket size, and the more likely it is to "collapse".

From on-site investigations, it has been found that many spiral wound gaskets used in cold exchange equipment have been "scattered" or "collapsed" when disassembling the flange after use, resulting in leaks often occurring from these parts. Therefore, in order to overcome the defects of "loose frame" and "crushing", inner and outer strengthening rings can be installed at the inner and outer diameters of the winding pad, making it a winding pad with inner, outer, or inner and outer rings.