The replacement of asbestos roller pressure plate gaskets is still a technical challenge in the static sealing industry. The current mainstream solution is to treat them differently based on their usage conditions. Metal and semi metal gaskets are used in high-temperature and high-pressure situations, fluororesin gaskets are used in corrosive situations, and non asbestos gaskets are used in general purposes.

This article briefly describes our understanding of the current situation of the use of non asbestos gaskets, in order to be helpful to users.

Non asbestos gaskets are all made by stamping and cutting sheet metal. Non asbestos sheets are made by combining various non asbestos fibers, rubber binders, and other fillers. According to the classification of manufacturing methods, they are generally divided into sheet metal and roller pressure plates.

The manufacturing process of non asbestos sheet is the same as that of paper making. The fibers are beaten into a slurry using a beating machine, mixed evenly with synthetic rubber slurry such as NBR, and then taken by a wet copying machine (paper machine). The sheet is then compressed and formed by heating rollers. The thickness of the copying plate is usually less than 1mm, and its sealing material is uniform, without fiber directionality issues like roller pressure plates. The surface is smooth and soft, so it is suitable for sealing gaskets with complex shapes such as engine hood, automotive oil pan, intake manifold, etc., which have smaller fastening surface pressure.

Problems discovered after replacing asbestos gaskets with non asbestos gaskets


The surface of the gasket made by using a copying plate is smooth and soft, and initial sealing can be achieved with low surface pressure. In addition, the gasket is relatively thin, and the stress relief after long-term use is small. It is mostly used in areas with almost no pressure, so there are fewer cases of failures.


However, the non asbestos roller pressure plate gasket used for pressure pipelines has encountered problems. During continuous operation and additional tightening of steam piping, it is easy to harden and crack due to thermal aging, which was not initially expected. It only replaced the fiber type, resulting in significant differences in usage.


After analysis, it was found that although the manufacturing process is almost the same, the asbestos fiber content in the asbestos roller pressing plate is as high as 65%~80%. Even when used at a heat resistance temperature far exceeding that of the rubber binder, the aging of rubber can still rely on the entanglement of asbestos fibers to obtain very high strength. Therefore, there are many successful cases of using asbestos fibers beyond the heat resistance temperature range of rubber, and sometimes they are even used in high-temperature situations at 500 ℃.

However, the material composition of non asbestos roller pressure plates is different. Due to the poor compatibility between substitute fibers and rubber, it is not possible to add a large amount of fibers like asbestos roller pressure plates, usually with a blending amount of 5% to 15%. Therefore, when rubber undergoes thermal hardening at high temperatures, non asbestos gaskets cannot rely on a large amount of fiber wrapping to maintain strength like asbestos gaskets. The thermal shrinkage of rubber will generate internal stress, Causing small cracks inside the asbestos free gasket, which then expand and cause fracture. In addition, due to the hydrolytic nature of aramid fibers, when used in high-temperature steam environments, as the surface pressure of the gasket decreases, steam penetration will accelerate the aging of non asbestos gaskets, ultimately leading to failure.

Reasonable countermeasures for the use of non asbestos gaskets

Non asbestos gaskets are greatly affected by high temperature aging, and even in general use, they often fail, which initially surprised developers and users. However, their versatility, low slowness, low cost, and simple use are also evident. After several years of follow-up research, Walkar found that non asbestos modification is not simply about modifying the fibers, but requires the establishment of a new set of performance evaluation techniques for non asbestos gaskets in order to fully and correctly understand them.


We believe that we should continue to study and improve the formula and processing methods of non asbestos gaskets. At the same time, in response to the problem of high-temperature hardening, we suggest taking the following measures:


① Controlling the usage temperature of non asbestos gaskets can be relaxed to around 200 ℃ in terms of machine usage for tightening management in factories. For pressure pipelines under construction on site, it is recommended to control the usage temperature below 150 ℃. In situations where additional tightening is required, the usage temperature should be controlled below 100 ℃;


② When the temperature is above 100 ℃, it is recommended to use a thin gasket with less stress relaxation (below 1.5mm);


③ Apply a higher fastening surface pressure of 30-40 MPa during installation, so that there is no need for additional tightening in the later stage;


④ Used in locations that are not easily subjected to stress loads on piping or are easy to replace;


⑤ Apply adhesive to the gasket;


⑥ It is recommended to use a circular gasket with the outer diameter in contact with the inner side of the bolt to provide tightening surface pressure.