How to Choose Gasket Seals (1)

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This post was last edited by Xin Ruifangxiao on 2025-6-13 08:55. Last time’s discussion showed us that gaskets are needed for sealing; so how should we choose the right gaskets? Sealing gaskets need to be selected considering three aspects: the gasket itself, the flange, and the fasteners. We will cover this in three phases, and in this first phase we will start with the gasket. Temperature: It primarily affects the physical, chemical, and mechanical properties of gaskets. Physical properties: The overall physical characteristics of a gasket or individual component include the material state, oxidation point, and general elasticity ； Mechanical properties: include creep relaxation (sometimes referred to as stress relaxation) and the resulting torque retention capability ； Chemical properties: The chemical properties of the components are highly influenced by temperature, and this needs to be taken into consideration specifically ； Pressure: working pressure, design pressure, operating pressure, and test pressure. Working pressure refers to the pressure rating of a flange at a given material and operating temperature. Work pressure is governed by recognized codes and standards, such as those specified by ASME, CEN, JIS, DIN, etc. For practical purposes related to gasket selection, these specifications define the temperature and pressure ratings (limits) as well as the relevant flange dimensions. The higher the pressure rating, the stronger the flange becomes, providing a greater load capacity ； Design pressure: The system design pressure is the pressure limit specific to a process or equipment, and it is usually the same as the setting of the pressure relief device. The system design pressure takes into account system failures, deviations, and pressure peaks ； Operating pressure: Operating pressure refers to the pressure under which normal operation takes place under normal conditions. This pressure is below the system’s design pressure, taking into account system changes caused by process control, as well as safety and pressure fluctuations. It is important to distinguish between these two limits, as a specific gasket material suitable for a given operating pressure may not be suitable for the system’s design pressure ； Test pressure: The test pressure is a safety check pressure used to verify the integrity of the equipment before the system is started up. Hydrostatic testing is usually carried out at a given factor higher than the operating pressure (usually 1.5 times) ； Media: The media (fluid) can be a gas or a liquid; it can range from hot to cold, from high pH to low pH, from corrosive to neutral, from flammable to non-flammable, as well as all substances that fall between these extremes. The washer must be chemically compatible with the process fluid it contains, as well as with any fluids used for pressure testing or flushing. Each gasket material has a different range of chemical compatibility, which can vary depending on the process temperature, fluid composition, and concentration. Compatibility depends on whether the physical and chemical properties of the washer are adversely affected by exposure to the medium. These adverse effects include changes in the material’s color, shrinkage or weight loss, as well as expansion or softening. These effects are the result of direct chemical erosion, particularly oxidation and corrosion. Direct chemical oxidation is a very complex topic that can be influenced by many different variables, such as temperature, concentration, and exposure time. Generally, as the temperature rises, the choice of gasket becomes more critical, as the rate of chemical corrosion usually increases with rising temperatures. Gasket thickness: The thickness of metal and semimetal gaskets generally does not change the pressure and temperature (PT) ratings of the gasket. Therefore, gasket thickness is not a key factor in the selection of metal and semimetal gaskets. Non-metallic (soft) gaskets; the thickness of the gasket can be selected based on the flange material or flange conditions. Advantages of thick gaskets: More \"compression\" to fill irregularities on the flange surfaces, thereby sealing any leakage paths between the gasket and the flange and reducing leakage rates. They can also tolerate greater misalignment of the flanges. Advantages of thin gaskets: Smaller surface area, resulting in better leak prevention performance; smaller surface area leads to reduced permeation leakage rates. They also offer better resistance to creep relaxation and maintain torque better. PS: Thick gaskets are better at filling surfaces with rough textures, providing improved sealing effects ； Misalignment: Not aligned horizontally or vertically ； Smaller surface area: It refers to a smaller area in contact with the medium, which allows it to withstand greater pressure, thus providing better blowout prevention performance. Gasket stress: Gasket stress is a commonly used term to describe the unit load on the surface of a gasket. It is one of the most important parameters for bolted connections, as it directly affects the sealing capability of the washers. Flange surface: The gasket needs to be compressed only to the minimum extent in order to be fixed to the flange surface; it must adapt to the irregularities of the flange in order to function effectively. If the flange is completely flat and smooth, a gasket may not be necessary. For larger defects, more compression is required of the gasket to seal them, which demands a greater stress on the gasket. Gasket penetration: The gasket fills the surface of the flange, and additional compression may be required to prevent any penetration inside the gasket body. The sealing performance of gaskets made from different types of materials varies greatly, but in almost all cases, the leakage rate decreases as the compressive load increases. The required stress, especially in a gaseous environment, will increase depending on the degree of tightness required for sealing. Internal pressure: When non-metallic gaskets are used, the ability of the bolted connection to maintain internal pressure depends on friction, which in turn is related to the compressive load on the gasket. The minimum compressive stress must be high enough to maintain the friction required to prevent the washer from bursting due to internal pressure. Operating temperature: Temperature factors must be taken into account when determining the installation stress. An increase in temperature will cause the washer to loosen, which in turn leads to a reduction in the bolt load. Some load losses can be as high as 50% of the initial stress of the gasket. The initial installation stress must be high enough to compensate for this effect. This is why some washer manufacturers recommend retightening them according to the type of washer after the first heating cycle. That’s all for today’s sharing. See you next time~