In industrial production, shell and tube heat exchangers are vital devices for heat exchange. They are widely used in numerous fields such as petroleum, chemical engineering, chemical fiber, coal mining, power generation, and metallurgy, tasked with transferring heat from one fluid to another. Among these components, the gasket, as one of the key parts of the heat exchanger, plays an extremely important role. It not only prevents leakage between different media inside the heat exchanger but also ensures the efficiency and safety of heat exchange. This article will delve into the material selection, design considerations, installation precautions, and maintenance of gaskets for shell and tube heat exchangers, helping readers gain a comprehensive understanding of this seemingly small yet extremely critical component.
In shell and tube heat exchangers, the material and properties of the gasket are the foundation for ensuring sealing effectiveness and the safe operation of the equipment. Choosing the right material can meet the sealing requirements under various working conditions and extend the service life of the gasket.
The materials for shell and tube heat exchanger gaskets are diverse, with common ones including graphite, rubber, and polytetrafluoroethylene (PTFE). Each material has its unique properties and applicable scenarios.
Graphite: Graphite is a material that is resistant to high temperatures and corrosion, maintaining stability in extreme temperature and chemical environments. It has good flexibility and self-lubrication, which allows it to adapt well to minor surface irregularities of flanges, thereby achieving a good sealing effect. Graphite gaskets are widely used in sealing applications involving high temperatures, high pressures, and highly corrosive media.
Rubber: Rubber gaskets have good elasticity, enabling them to achieve sealing under relatively low clamping forces. They have good tolerance for a variety of media, but their temperature resistance is relatively poor, typically suitable for sealing at ambient or moderate temperature ranges. Rubber gaskets are commonly used for sealing water, air, steam, and other media.
Polytetrafluoroethylene (PTFE): PTFE is a high-performance engineering plastic with excellent chemical corrosion resistance, high-temperature resistance, and a low friction coefficient. It can resist the erosion of almost all chemicals and maintain stable performance over a wide temperature range. PTFE gaskets are suitable for sealing highly corrosive media such as strong acids, strong bases, and organic solvents, but their strength is relatively low, requiring the use of composite materials to enhance their mechanical properties.
Metal-clad gaskets are a special type of gasket that encapsulates non-metallic materials (such as graphite, asbestos, etc.) within a metal shell, combining the advantages of both metallic and non-metallic materials. The advantages of this type of gasket include high strength, resistance to breakage, and robustness, allowing it to withstand higher pressures and temperatures. However, its rebound force is limited, requiring a very flat flange surface and a high clamping force to achieve good sealing effects. To overcome this limitation, a graphite sheet can be applied to the outer surface of the metal-clad gasket to enhance its rebound performance and sealing effect.
Metal-clad gaskets can be formed in various configurations based on different material combinations, such as 201, 304, 316 stainless steel clad graphite (temperature-resistant up to 650°C) gaskets, asbestos-clad gaskets, and ceramic fiber (1400°C) clad gaskets. These different forms of clad gaskets can meet a variety of working condition requirements. The materials for clad gaskets include A3 steel, stainless steel (201, 304, 316), copper, aluminum, etc., and the fillers include flexible graphite (80%), asbestos (10%), asbestos-free, ceramic fiber (10%), etc. This type of gasket is mainly used for sealing large-diameter pressure vessels (such as heat exchangers, reactors, cast iron flanges, cylinder heads, boilers, etc.) and is also easy to install and remove, resistant to damage, and can prevent media erosion of the gasket, enhancing pressure resistance.
Metal-graphite wound gaskets are made by alternately winding metal and graphite strips. They have compressive resilience, which allows them to maintain good sealing performance in areas with severe temperature and pressure fluctuations. These gaskets have excellent non-permeability, long service life, and are maintenance-free, making them particularly suitable for sealing applications involving high temperatures, high pressures, and highly corrosive media. The metal strip provides strength and support, while the graphite strip offers good sealing properties. The combination of these two materials ensures that the gasket performs well under complex working conditions.
After gaining an in-depth understanding of the materials and properties of gaskets, the next focus should be on the design considerations for gaskets. Rational design can ensure that gaskets perform optimally in practical applications and meet the sealing requirements under various working conditions.
Shell and tube heat exchanger gaskets are generally divided into several types, including tube box gaskets, floating head gaskets, tube box side gaskets, outer head cover gaskets, and head cover gaskets. The form of the gasket varies according to the requirements of different tube passes. For example, multi-tube pass heat exchangers usually require different forms of ribs on the inner circle of the gasket to enhance the sealing effect. These ribs can be designed and adjusted according to different working conditions to meet the sealing needs of various heat exchanger tube boxes and non-circular pressure vessels.
The performance characteristics of heat exchanger gaskets are closely related to the corrosion resistance and heat resistance of the cladding metal. The non-metallic flexible filling material allows the gasket to achieve good sealing effects under lower clamping forces. The use of gaskets is particularly prominent under working conditions with temperature and pressure fluctuations. Different forms of heat exchanger gaskets have different temperature resistance, medium resistance, and pressure-bearing capabilities. Therefore, when selecting gaskets, it is essential to follow the requirements of the design drawings and not to arbitrarily change the type and material of the gasket.
The installation of gaskets is a crucial step in ensuring the sealing performance of the heat exchanger. The correct installation method can not only improve the sealing effect but also extend the service life of the gasket. Here are several important aspects to pay attention to during the installation of gaskets.
Before installing the gasket, it is necessary to carefully inspect the flange surface to ensure that it is flat, free of scratches, and corrosion-free. Irregularities or defects on the flange surface can affect the sealing performance of the gasket and may even cause leakage. At the same time, inspect the appearance of the gasket to ensure that it is undamaged, undeformed, and that its dimensions meet the requirements.
Position of the Wound Part of the Gasket: The wound part of the gasket must be positioned between the flange sealing surfaces and should not arbitrarily increase the width of the wound part. Otherwise, it will affect the clamping force of the bolts, thereby reducing the sealing performance of the gasket. This is because the inner circle weld points of the typical gasket are prone to cracking under axial clamping force, leading to gasket failure.
Selection of Gasket Material: Never replace a high-grade material gasket with a low-grade one. Gaskets made of different materials have different temperature resistance, pressure resistance, and corrosion resistance properties. Using a low-grade material gasket may cause the gasket to fail during operation, leading to leakage accidents.
Installation Direction of the Gasket: For gaskets with directionality (such as those with ribs), they must be installed in the correct direction. Otherwise, it will affect the sealing effect of the gasket.
After installation, the flange bolts should be evenly tightened to ensure that the gasket is evenly loaded. Before the equipment is put into operation, a leakage check should be performed on the flange connection to ensure good sealing performance. If leakage is detected, the clamping force of the bolts should be adjusted in a timely manner or the gasket should be replaced.
After ensuring the correct installation and use of the gaskets for shell and tube heat exchangers, the next key step is maintenance and care. Good maintenance and care measures can significantly extend the service life of the gasket, reduce the risk of leakage, and ensure the long-term stable operation of the heat exchanger.
The sealing gaskets of heat exchangers can be affected by the surrounding environment. Outdoors, protective covers or insulation layers should be used on the outside of the heat exchanger to prevent erosion of the gasket by rainwater, dust, and other elements. Typically, sealing gaskets should be stored in a cool, dry place, avoiding direct sunlight and minimizing the impact of environmental factors on their aging process.
When working conditions change, the stress generated by the gasket is far higher than that under constant load, and the impact on the internal structure of the material should not be underestimated. Frequent start-ups and shutdowns or long-term storage are not conducive to extending the service life of the gasket. To reduce the impact of temperature load changes, spring devices can be installed. Spring devices can automatically adjust the clamping force of the gasket during temperature and pressure changes, thereby maintaining good sealing performance.
When selecting a sealing gasket, attention should be paid to the usage conditions of the fluid material and consider substances that may damage the physical properties of the sealing gasket, such as mixtures of various additives, inhibitors, and other chemicals. During the design process, it is essential to fully consider and minimize the corrosion of the gasket by the fluid medium. If leakage occurs due to aging of the gasket, temporary measures can be taken, such as tightening the bolts of the combined heat exchanger to adjust the compression force of the gasket between the heat exchangers. However, after temporary treatment, professional maintenance personnel from the heat exchanger manufacturer should be contacted as soon as possible for repairs to ensure the safe operation of the equipment.
Although the gasket for shell and tube heat exchangers is just a small component of the heat exchanger, its role should not be underestimated. It not only prevents media leakage and ensures the efficiency and safety of heat exchange but also maintains stable performance under complex working conditions. When selecting and using gaskets, it is essential to fully consider the working conditions, material properties, design requirements, and details of installation and maintenance. Only in this way can the safe and efficient operation of the heat exchanger be ensured, the service life of the equipment be extended, and a reliable guarantee be provided for industrial production.
