In modern industrial production, piping systems play a crucial role, and flange connections are an indispensable component. However, although bolted flange connections designed according to current standards can generally meet sealing requirements under normal conditions, their sealing reliability faces severe challenges under certain extreme operating conditions, especially in high-temperature and high-pressure environments. This is mainly because the combined effect of medium pressure and temperature can easily lead to sealing failure in flange connections. Once leakage occurs, it not only causes waste of raw materials and energy but may also trigger serious economic losses, environmental pollution, and even major accidents. Therefore, the sealing requirements for vessels and pipelines are becoming increasingly stringent, and the problem of sealing failure in high-temperature environments is becoming more prominent, with gasket creep being one of the key factors leading to sealing failure.
Among the many factors affecting gasket performance, high-temperature creep is a critical one. It not only directly affects the sealing effectiveness of gaskets in high-temperature environments but may also trigger a series of chain reactions, leading to failure of the entire sealing system.
Creep is one of the most important indicators for evaluating sealing materials. It reflects the ability of sealing materials to resist stress relaxation and deformation. It is an instantaneous stress-strain relationship closely related to temperature, time, initial stress, and the thickness of the sealing flange. Generally, the slower the creep, the higher the residual compressive stress, and the better the sealing performance. However, in high-temperature environments, creep becomes more pronounced, and the sealing failure of high-temperature connection systems is often caused by gasket creep and stress relaxation. Creep is a special form of inelastic deformation, where deformation caused by stress changes over time under load. Due to creep, the stress state of the material at a certain moment is related not only to the deformation at that moment but also to the deformation history prior to that moment. Even after all external loads are removed, small time-dependent deformations may still occur.
In high-temperature environments, connection system failures are rarely caused by insufficient strength. The main reason is stress relaxation caused by creep of the gasket and bolts. This leads to a decrease in the residual compressive stress of the gasket. When the residual compressive stress of the gasket drops below the minimum pressure required for sealing, the bolted flange connection system will fail and leak. Gasket stress is crucial for maintaining the tightness of the structure under both normal and high-temperature conditions. Especially in high-temperature operations, among the many factors affecting sealing, gasket creep often becomes the primary factor because it may lead to a significant reduction in bolt force and gasket stress.
Sealing gaskets have various failure modes, but in high-temperature environments, creep is often the key factor leading to these failures. Creep not only affects the physical properties of the gasket but also changes its mechanical behavior in actual operation, thereby causing a series of failure issues.
Gaskets are compressed between flange faces by the preload of bolts, generating elastic-plastic deformation to fill the geometric gap between the sealing faces, thus preventing or stopping fluid leakage. However, under high temperatures, gaskets are affected by high-temperature-induced changes such as embrittlement and creep, resulting in reduced sealing capability and eventual leakage. The common failure modes of gaskets are primarily three: creep relaxation failure, high-temperature recovery failure, and high-temperature strength failure.
Creep of a gasket refers to the continuous increase in deformation under constant stress over time. Stress relaxation refers to the gradual decrease of gasket stress over time under the initial compressive load. In fact, stress relaxation is another form of material creep; they interact and depend on each other. Under high-temperature conditions, as time progresses, gasket materials undergo aging, increased thermal degradation, and increased creep. At the same time, internal stresses formed during manufacturing are gradually released, causing stress relaxation. The combined effect of creep and relaxation gradually reduces residual stress on the gasket surface. If the residual stress drops below the working sealing pressure of the gasket, leakage will occur. The rate of creep relaxation reflects the gasket surface’s ability to maintain residual compressive stress under operating conditions. Although both temperature and stress are major factors affecting the gasket, temperature has a much greater influence on creep relaxation than stress.
Material selection of sealing gaskets in high-temperature environments is crucial. Suitable materials can effectively resist high-temperature creep and stress relaxation while maintaining good sealing performance under complex conditions.
Among various gasket materials, corrosion-resistant PTFE gaskets are an ideal high-temperature sealing material. They have excellent corrosion resistance and aging resistance. After special processing, they exhibit good creep resistance and cold flow resistance and can be used safely over a wide temperature range for extended periods. These gaskets can resist various chemicals, adapt to uneven and scratched surfaces, and are suitable for pipelines and equipment in industries such as chemical, power generation, paper making, refining, and machinery. They are particularly suitable for hydrocarbons and highly corrosive environments.
In high-temperature environments, the properties of gasket materials change significantly. To ensure initial sealing during bolt preload, gaskets must have good compressibility. Once the equipment is in operation, with rising temperature and bolt elongation, there is a tendency for relative separation between the flange face and the gasket surface. To ensure that the gasket does not leak under high-temperature working conditions, it must have sufficient recovery to compensate for creep induced by high temperature. Under high temperature and pressure fluctuations, gasket creep intensifies, the elastic modulus and yield strength of the material decrease significantly, compression increases, plastic deformation rises, and recovery decreases. If the sealing medium pressure is also high, the gasket may be pushed out by the medium, eventually causing sudden leakage.
In high-temperature environments with sulfur or chloride corrosion, graphite-metal spiral wound gaskets are commonly used. However, the stainless steel inner reinforcing ring or winding tape in such gaskets is exposed to the medium and may undergo stress corrosion caused by sulfur or chloride ions, eventually leading to fracture of the inner ring or tape and strength failure. Another scenario is that in graphite-metal composite gaskets, excessive sulfur or chloride content in the graphite can cause sudden fracture failure of the metal framework due to stress corrosion. In such corrosive environments, selecting appropriate gasket materials is crucial. Besides graphite-metal spiral wound gaskets, other materials with better corrosion resistance, such as special alloy gaskets or specially treated composite gaskets, can be considered. During design and installation, corrosion resistance and anti-creep properties must be fully considered to ensure long-term stable operation in high-temperature sulfur or chloride environments.
Under high-temperature conditions, gasket performance gradually deteriorates over time, making regular maintenance and management essential. Scientific and reasonable maintenance measures can detect potential problems early and take preventive actions to avoid severe consequences from sealing failure.
Gaskets under prolonged high-temperature conditions experience significant changes in material properties, such as reduced elastic modulus and yield strength, causing separation between flange faces and gasket surfaces. In such cases, bolts need thermal retightening. However, with extended operation, gasket materials may embrittle, harden, or age. During thermal retightening, there is a risk of sudden gasket rupture, resulting in strength failure. Therefore, thermal retightening must be carried out with extreme caution, ensuring proper force and timing to avoid unnecessary damage to the gasket.
In addition to thermal retightening, regular inspection and timely replacement of gaskets are key measures to ensure sealing system reliability. High-temperature conditions significantly affect gasket lifespan, so inspection and replacement schedules must be based on actual operating conditions. Regular inspections can detect gasket deformation, aging, corrosion, and other issues early, allowing for timely repair or replacement to prevent leakage accidents.
The creep problem of sealing gaskets in high-temperature environments is a pressing issue in engineering practice. Creep directly affects the reliability of sealing systems. Under extreme conditions such as high temperature and high pressure, gasket creep can cause sealing failure with serious consequences. Therefore, selecting appropriate gasket materials, such as corrosion-resistant PTFE gaskets, and implementing effective maintenance and management measures, including thermal retightening, regular inspection, and replacement, are crucial for improving sealing system reliability and safety. Meanwhile, with ongoing technological advancement, the development and application of new high-temperature, anti-creep sealing materials will provide more possibilities to address this issue.