Sealing Challenges of Gasket in High-Temperature Application

In the industrial sector, whether in chemical processing, power generation, papermaking, or oil refining, the sealing performance of pipes and equipment is a crucial factor in ensuring production safety and efficiency. Gaskets, as the core components of sealing systems, play a particularly vital role in high-temperature applications. This article will delve into the failure modes, influencing factors, and strategies for dealing with gaskets in high-temperature environments, helping readers to better understand and apply relevant knowledge.

The sealing principle of gaskets is relatively straightforward but of great significance. It relies on the pre-tightening load from bolts to compress the gasket against the flange surface, causing it to undergo elastic or plastic deformation. This fills any gaps between the flange sealing surfaces, cutting off or preventing fluid leakage. Under normal operating conditions, gaskets need to have good compressibility and resilience to ensure effective sealing. However, in high-temperature environments, the performance of gaskets can be affected by various factors, leading to a decline in sealing capability and even leakage.

Under high-temperature conditions, gaskets may experience different failure modes due to various factors. These failure modes can not only reduce the sealing performance of gaskets but also lead to serious leakage accidents. Therefore, understanding these failure modes is essential for preventing and solving sealing problems.

When subjected to a constant stress over a long period, gaskets will deform, a phenomenon known as creep. At the same time, under the initial clamping force, the stress on the gasket will gradually decrease over time, which is referred to as stress relaxation. Creep and stress relaxation are actually two manifestations of gasket material under long-term stress, and they interact and depend on each other. Under high-temperature conditions, as time increases, the gasket material will age, the thermal loss will increase, and the creep will gradually grow. Meanwhile, the internal stresses formed during the manufacturing process will also be gradually released, leading to stress relaxation. The combined effect of these two phenomena will cause the residual stress on the gasket surface to gradually decrease. If the residual stress falls below the working sealing pressure of the gasket, leakage will occur. The magnitude of the creep relaxation rate reflects the gasket's ability to maintain residual clamping stress under operating conditions. Although both temperature and stress affect the performance of gaskets, the impact of temperature on creep relaxation is much greater than that of stress.

Once the equipment is put into operation, as the temperature rises, the gasket will experience creep, and the bolts will elongate, causing a tendency for the flange and gasket surfaces to separate. To ensure sealing effectiveness, the gasket must have sufficient resilience to compensate for the creep caused by high temperatures. However, under high-temperature and high-pressure conditions, fluctuations in temperature and pressure will exacerbate the creep of the gasket. Its material's elastic modulus and yield limit will significantly decrease, the compression will increase, and the plastic deformation will grow, leading to a decline in resilience. If the pressure of the sealing medium is also high at this time, the gasket may be blown out by the medium pressure, ultimately resulting in a leakage accident.

Under prolonged high-temperature exposure, the properties of gasket materials will change significantly. Both stress fluctuations and corrosive effects of the medium can lead to strength failure of the gasket. For example, in high-temperature sulfide or chloride corrosion environments, graphite metal wound gaskets are commonly used. However, if the stainless steel inner reinforcement ring or the inner winding steel strip of the gasket is exposed to the medium, it will be susceptible to stress corrosion by sulfides or chlorides, eventually leading to the breakage of the inner ring or steel strip and causing strength failure. Another situation is with graphite metal composite gaskets, where if the sulfur or chloride content in the graphite material exceeds the standard, the metal skeleton of the gasket will also suddenly break due to stress corrosion and fail.

The performance of gaskets is significantly affected under high-temperature conditions. Therefore, selecting appropriate gasket materials and conducting proper maintenance are key to ensuring sealing performance. Correct selection and maintenance can not only extend the service life of gaskets but also effectively prevent leakage accidents.

When facing high-temperature applications, choosing the right gasket material is crucial. It is recommended to use corrosion-resistant PTFE gaskets, which have excellent corrosion resistance and non-aging properties. After special processing, these gaskets exhibit good creep resistance and cold flow resistance, allowing them to be safely used over a wide temperature range for an extended period. They can withstand various chemical media and adapt to uneven and damaged surfaces, making them widely applicable in industrial settings such as chemical processing, power generation, papermaking, oil refining, and machinery, especially in hydrocarbon and corrosive environments.

Under high-temperature conditions, the elastic modulus and yield limit of gaskets will significantly decrease, leading to separation between the flange and gasket surfaces. To avoid this, hot-tightening of bolts is necessary. However, as operating time extends, the material properties of the gasket will change considerably, such as becoming brittle, hardening, or aging. During hot-tightening, these changes can cause the gasket to suddenly burst, resulting in strength failure. Therefore, when performing hot-tightening operations, caution must be exercised to ensure that the operators have sufficient experience and professional knowledge to avoid unnecessary damage to the gasket.

In high-temperature environments, the working temperature of flanges is usually higher than that of bolts, causing the axial elongation of flanges to be greater than that of bolts, thus inducing thermal stress in the bolts. This thermal stress, combined with the tensile stress caused by pressure, can lead to yielding or creep of the bolts. Additionally, under the action of high-temperature fluids, the wall temperature of the cylinder increases, causing expansion deformation and flange deflection, which increases the bolt load. The elongation of bolts and deformation of flanges will reduce the clamping force on the gasket, ultimately leading to leakage. Therefore, when designing and installing flange connections, the impact of thermal stress must be fully considered, and appropriate measures must be taken to mitigate its negative effects on sealing performance.

Self-tightening seals, such as lens-type metal gaskets, can also experience leakage problems under high-temperature conditions. This is because when the temperature rises, the flange, having a larger volume, heats up slowly, while the lens gasket heats up quickly. The thermal expansion of the lens gasket is constrained by the flange, generating significant thermal stress. This thermal stress, combined with the pre-tightening stress on the gasket, can easily exceed the yield limit, leading to stress relaxation and residual deformation of the gasket. During cooling, since the gasket cools faster than the flange, it can result in insufficient clamping force and leakage. If this cycle occurs multiple times, the residual deformation will accumulate continuously, making the clamping force increasingly insufficient and ultimately leading to leakage. Therefore, when using self-tightening seals, the impact of temperature changes on sealing performance must be fully considered, and appropriate measures must be taken to ensure the reliability of the seal.

The load on the gasket can change due to temperature differences between the cylinder, flange, and bolts. When the internal pressure on the flange connection increases from low to high, the gasket load will decrease. However, if the flange temperature is higher than the bolt temperature, the gasket load will increase; whereas if the cylinder temperature is higher than the flange temperature, the gasket load will decrease. If the gasket load is less than the required load for sealing, leakage will occur. Therefore, the greater the temperature difference between the shell and the flange, the more necessary it is to increase the pre-tightening load or re-tighten the bolts during operation. During operation, the deformation and load of the gasket are in a balanced state, but when the operation stops, the balance is disrupted, and the gasket and its clamping surface will under excessive pressure, easily causing gasket fatigue and bolt relaxation. Therefore, during operation and maintenance, it is essential to closely monitor changes in gasket load and take appropriate measures to ensure sealing performance.

Asymmetric thermal deformation can also cause flange leakage. When there is a temperature difference around the flange, its deformation will vary, and the gasket load will change circumferentially. When pre-tightening the bolts, the load cannot be uniform. Combined with uneven thermal deformation, if the local gasket load is less than the required load for sealing, leakage will occur. Therefore, when designing and installing flange connections, it is necessary to minimize the circumferential temperature difference of the flange as much as possible to ensure the uniform distribution of gasket load and improve the reliability of sealing.

The sealing performance of gaskets under high-temperature conditions is crucial for the safety and efficiency of industrial production. By gaining a deep understanding of the failure modes, influencing factors, and strategies for dealing with gaskets in high-temperature environments, we can better select and maintain gaskets to ensure their sealing performance under high-temperature conditions. In practical applications, it is essential to fully consider the impact of temperature, pressure, and medium corrosion on gasket performance and take appropriate measures to mitigate the negative effects of these factors. Only in this way can we ensure the safe, efficient, and stable operation of industrial production.