Guide to Gasket Leakage: Causes and Solutions

In the industrial field, whether it is large pressure vessels, complex process equipment, various power machines, or connecting pipelines, all rely on a crucial form of sealing structure, gasket sealing. Widely used at various detachable joints, it serves as an important line of defense to ensure safe equipment operation and prevent medium leakage. This article provides an in-depth discussion on the working principles of gasket sealing, causes of leakage, material selection, and optimization strategies, helping readers gain a comprehensive understanding of this important industrial sealing technology.

Gasket sealing mainly consists of flanges, gaskets, and connecting bolts and nuts, collectively known as a flange sealing joint. This structure applies a compressive load to deform the gasket material, thereby filling the tiny irregularities on the flange sealing surface to achieve sealing. Under the action of the compressive load, the gasket material can easily undergo plastic deformation, filling the gaps on the sealing surface and blocking leakage channels.

In the safety valve flange sealing joint, the gasket is the main sealing element. For non-metallic gaskets, sealing is achieved by tightening the bolts. Bolt tightening generates large compressive stress on the contact surfaces between the flange and gasket, as well as inside the gasket. On the one hand, this stress allows the gasket surface to closely fit the flange surface, filling the micro-gaps on the flange; on the other hand, it reduces the porosity of the gasket material, thereby reducing the leakage channels of the sealed medium.

Although gasket sealing is widely used in the industry, leakage remains a common issue. Leakage refers to the undesired phenomenon in which a medium flows from inside a confined space to the outside, or from outside into a confined space. The fundamental cause of leakage is the presence of gaps at the contact surfaces, while the pressure difference or concentration difference between the two sides of the contact surface serves as the driving force for leakage.

Leakage can be divided into two main forms: interface leakage and permeation leakage.

Interface leakage occurs between the gasket and the flange sealing surface. Its causes include:

Insufficient gasket compressive stress: If the bolt tightening force is inadequate, the gasket cannot closely adhere to the flange surface, resulting in leakage.

Rough flange sealing surface: If the surface machining accuracy is poor and tiny irregularities exist, the gasket cannot completely fill these gaps, causing leakage.

Thermal deformation, mechanical deformation, and vibration of the pipeline: These factors lead to uneven distribution of contact pressure between the flange and gasket, or even separation between the gasket and flange, resulting in leakage.

Bolt deformation and elongation: Due to temperature and pressure during operation, bolts may deform or elongate, reducing gasket compressive stress and causing leakage.

Gasket creep relaxation and reduced resilience: Under high temperature or long-term operation, the gasket material may undergo creep relaxation, losing resilience and failing to compensate for flange separation, causing leakage.

Gasket material aging and deterioration: Over time, gasket materials may age, crack, or deteriorate, losing original sealing performance and causing leakage.

Permeation leakage mainly occurs within the gasket material. Non-metallic gaskets are typically made of plant fibers, animal fibers, mineral fibers, or chemical fibers bonded with rubber, or made from porous materials such as flexible graphite. These materials have loose structures, low density, and countless micro-gaps between fibers, making them easily penetrated by media. Under pressure, the medium will permeate through these micro-gaps, resulting in permeation leakage.

Sealing gaskets are widely used in static sealing of joints in pipelines, pressure vessels, and various housings. According to material type, sealing gaskets can be classified into three main categories: non-metallic sealing gaskets, composite non-metallic–metal sealing gaskets, and metallic sealing gaskets. Common materials include rubber, leather, asbestos, cork, PTFE, iron, steel, aluminum, copper, and stainless steel.

Non-metallic sealing gaskets are usually made of cellulose-based materials, such as cotton, hemp, asbestos, leather, or paper. These materials have loose structures and low density, with micro-gaps between fibers that can be easily penetrated by media, causing permeation leakage. To reduce permeation leakage, gaskets can be impregnated with materials such as grease, rubber, or synthetic resins.

These gaskets combine the plasticity of non-metallic materials with the strength of metals, offering improved sealing performance. For example, PTFE has excellent chemical resistance and a wide operating temperature range (-190–250°C), but poor resilience and a tendency to cold flow. By adopting composite structures, such as PTFE encapsulation with internal fillers like compressed asbestos board, rubber sheets, or metal inserts, these drawbacks can be mitigated, forming a “sandwich” structure. This not only protects the gasket from corrosion but also improves resilience and reduces cold flow issues.

Metallic sealing gaskets are typically used in high-temperature, high-pressure, or extremely demanding sealing applications. Metal gaskets have high strength and good anti-creep properties, but poor plasticity, requiring higher compressive stress to achieve sealing. Therefore, selecting metallic gaskets requires optimization based on specific operating conditions.

Leakage in gasket sealing is influenced by multiple factors, including the physical properties of the sealed medium, operating conditions, flange surface roughness, compressive stress, and gasket characteristics, dimensions, and loading/unloading history.

Different media have different physical properties that significantly affect leakage rates. For example, gas leakage rates are typically higher than liquids, and hydrogen leaks more easily than nitrogen due to its lower viscosity. Viscosity has the greatest influence among physical properties: the higher the viscosity, the greater the leakage resistance, and the lower the leakage rate.

Operating conditions include fluid pressure and temperature. Pressure difference is the main driving force of leakage, the larger the pressure difference, the easier the leakage. Temperature also significantly affects sealing performance. Studies show that gasket elastic and plastic deformation increase with temperature, resilience decreases, and creep increases. Additionally, temperature affects medium viscosity: liquid viscosity decreases with temperature, while gas viscosity increases. Thus, higher temperature generally leads to more leakage.

Flange surface roughness significantly affects leakage rate. Generally, the smaller the surface roughness, the lower the leakage. Ground flange surfaces seal better than unground ones because small irregularities are easier to fill.

Higher compressive stress on the gasket increases deformation. This fills flange irregularities, reducing interface leakage, and compresses internal capillaries, increasing leakage resistance. However, excessive stress may crush the gasket, causing loss of resilience and sharp leakage increases. Thus, compressive stress must be maintained within an appropriate range.

Gasket performance includes mechanical properties, compression, resilience, creep, and stress relaxation, and inherent sealing performance. Compression reflects the ability of the gasket to conform to the flange surface and form initial sealing. Under operating conditions, due to bolt elongation and flange deformation, separation occurs, and sealing depends heavily on gasket resilience. Creep and relaxation reflect the time-dependent deformation and stress decrease under load and temperature. They are major causes of increased leakage in high-temperature sealing.

Gasket thickness and width significantly affect sealing. With the same compressive load and medium pressure, leakage decreases as gasket thickness increases because thicker gaskets have greater compression and resilience. However, greater thickness also increases permeation leakage due to a larger cross-sectional area.

Within a certain range, leakage decreases linearly with increasing gasket width because leakage resistance increases with the length of the leakage path. But wider gaskets require greater bolt load to achieve the same compressive stress.

To improve gasket sealing reliability and reduce leakage, the following optimization strategies can be adopted:

Selecting Appropriate Gasket Materials: Choose materials based on operating conditions and medium characteristics. For high-temperature, high-pressure, or corrosive media, PTFE or metallic gaskets can be used. For applications requiring good resilience, rubber or elastomeric materials are suitable.

Optimizing Flange Surface Machining Accuracy: Improving the machining accuracy and reducing flange surface roughness effectively reduces interface leakage. Ground flange surfaces perform significantly better in sealing than unground ones.

Controlling Compressive Stress: During installation, compressive stress must be controlled precisely. Excessive stress may crush the gasket, while insufficient stress fails to achieve proper sealing.

Using Composite Gasket Structures: Composite gasket structures overcome the disadvantages of single materials, enhancing sealing reliability. For example, PTFE-encapsulated “sandwich” gaskets with internal fillers like compressed asbestos, rubber sheets, or metal inserts can offer both plasticity and elasticity, improving sealing performance.

Regular Maintenance and Inspection: Regular maintenance and inspection of sealing systems help detect and replace aged or damaged gaskets, effectively reducing leakage risks. In high-temperature, high-pressure, or long-term operating equipment, bolt tightness must also be checked regularly to prevent leakage from bolt deformation or elongation.

Gasket sealing plays a vital role in industrial equipment. Its sealing performance directly affects equipment safety and production efficiency. By understanding the working principles, causes of leakage, material selection, and optimization strategies of gasket sealing, reliability can be significantly improved and leakage risks reduced. In practical applications, appropriate gasket materials should be selected based on specific operating conditions and medium characteristics, while optimizing flange surface machining accuracy, controlling compressive stress, adopting composite gasket structures, and performing regular maintenance to ensure long-term stable operation of sealing systems.