Main factors affecting the sealing performance of flanges

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Bolt preload is an important factor affecting sealing. The preload must compress the gasket to achieve initial sealing. Appropriately increasing the bolt preload can enhance the sealing capability of the gasket, as a greater preload helps maintain a higher contact surface pressure on the gasket under normal operating conditions. However, the preload should not be excessive; otherwise, the gasket as a whole may yield, losing its resilience, or even get extruded or crushed. Additionally, the preload should be applied to the gasket as uniformly as possible. Typically, measures such as reducing the bolt diameter, increasing the number of bolts, and employing appropriate pre-tightening methods are taken to improve the sealing performance. Gasket performance: The function of a gasket is to fill the gap between the sealing surfaces of two flanges, thereby preventing fluid leakage. The types of gaskets include non-metallic gaskets, composite gaskets of non-metallic and metallic materials, and metallic gaskets. The appropriate gasket material must be able to undergo the necessary elastic deformation under an appropriate preload, without being crushed or extruded ； During operation, the distance between the flange sealing surfaces increases; therefore, the gasket material must possess sufficient resilience to ensure that its surface remains in close contact with the flange surfaces, thereby maintaining good sealing performance ； When selecting gasket material, the working medium and operating temperature should also be taken into consideration. The width of the gasket is also an important factor affecting sealing. The wider the gasket, the greater the required preload force; consequently, larger sizes of bolts and flanges are needed. ①Non-metallic gaskets such as rubber, asbestos rubber, and polytetrafluoroethylene are commonly used on flanges of medium- and low-pressure equipment and pipelines. They exhibit good corrosion resistance and flexibility, but have poor strength and temperature resistance. They are usually cut from a single sheet of gasket material; the overall shape of the gasket is circular, with a rectangular cross-section. ②To improve the strength and heat resistance of gaskets, wound gaskets are created by winding thin steel strips together with asbestos strips (or polytetrafluoroethylene strips or flexible graphite strips), or metal-clad gaskets are made by covering asbestos or other non-metallic materials with thin metal sheets. These gaskets provide multiple layers of sealing, exhibit good resilience, can be used in high temperature and pressure conditions, and maintain effective sealing even under fluctuations in pressure and temperature, which is why they are widely used. Wound gaskets are formed by alternately winding steel strips with filler strips such as asbestos, polytetrafluoroethylene, or flexible graphite. To prevent loosening, weld the starting and ending ends of the metal strip. To increase the elasticity and resilience of the gasket, both metal and non-metal strips are rolled into a wave shape. The shapes of the waves are V-shaped and W-shaped. Type A -- also known as the basic type, without reinforcing rings; used for mortise and tenon sealing surfaces. Type B -- with internal reinforcing ring, used for male/female sealing surfaces. Type C -- with an external reinforcing ring, for flat sealing surfaces. Type D -- has reinforcing rings on both the inner and outer sides, used for flat sealing surfaces. ③A metal-lined gasket consists of an asbestos rubber sheet as the core, surrounded by a thin metal sheet with a thickness of 0.2 to 0.5 mm (as shown in the figure below). The material for the metal sheet can be aluminum, steel, or their alloys; stainless steel or high-quality carbon steel can also be used. Metal jacketed gaskets are also used only on Type B butt-weld and long-neck butt-weld flanges. ④Metal gaskets are commonly used on the flanges of high-pressure equipment and pipelines, with materials including soft aluminum, copper, soft steel, and stainless steel. In addition to metal gaskets with rectangular cross-sections, there are also metal ring gaskets with elliptical or octagonal cross-sections, as well as other special shapes. Metal gaskets can be used when the operating pressure is very high or stringent leakage requirements apply, as well as in situations where the temperature is high or the environment is highly corrosive. The specific pressure value for metal gaskets is quite high; to reduce the bolt load, the sealing surface must be very narrow. To maintain a good seal with such a narrow sealing surface, it is necessary to have a low surface roughness, with Ra ≤ 2.5 μm to Ra ≤ 0.63 μm. Flange seal face type: The contact surface that achieves sealing by inserting a gasket between the flanges and compressing it is known as the flange seal face or compression surface. The choice of seal face type depends on operating conditions, the consequences of leakage, and the properties of the gasket; the most common structural types are as follows. ①Planar sealing surface: Its structure is as shown in Figure (a) below. The sealing surface is not a smooth plane; it usually has 2 to 4 grooves with a triangular cross-section that are arranged concentrically on this plane (i.e., the flange waterline). The planar sealing surface has a simple structure, is easy to manufacture, and facilitates the application of anti-corrosion linings. Secondly, the width of the sealing surface in this type of structure is large; therefore, non-metallic or soft metallic gaskets are commonly used in operation. However, after the bolt is tightened, the washer material tends to spread to both sides. It is used in applications where the required clamping force is not high and the medium is non-toxic. ②Convex-concave sealing surfaces: The structure of these sealing surfaces is shown in Figure (b). Essentially, it involves a pair of flanges with flat sealing surfaces; one of them has a raised platform that serves as a compression surface. This flange is referred to as the convex flange. The other flange has a concave surface and is called the concave flange. A gasket with dimensions identical to those of the concave surface is placed within it; this facilitates proper alignment. The height of the convex surface is slightly greater than the depth of the concave surface, and bolts are used to apply pressure and achieve sealing. This structure can limit the radial deformation of the gasket, prevent it from being pushed out, and improve the sealing performance to a certain extent. Suitable for high-pressure applications. ③Tenon-and-socket sealing surface: In the middle of a pair of planar sealing surfaces in the width direction, one surface is designed with a cross-section similar to a tenon, while the other has a cross-section similar to a socket; these two surfaces are used together. As shown in figure (c), the former is called a tenon-faced flange, and the latter is called a socket-faced flange. The grooved compression surface can limit the radial deformation of the gasket, ensuring good sealing performance; at the same time, the gasket is less subject to erosion and corrosion by the medium. However, the tenon surface is prone to damage. It is commonly used in environments with flammable, explosive, or toxic media, as well as in situations with high pressure. In addition, there are trapezoidal groove sealing surfaces and conical sealing surfaces. The former uses a ring-shaped metal gasket with an oval cross-section, while the latter is equipped with a lens-shaped ring-shaped metal gasket. The above two types of structures feature forced sealing and are commonly used in high-pressure pipelines. The shape of the flange sealing surface and its surface properties play a crucial role in determining the sealing efficiency. The flatness of the flange sealing surface, as well as its perpendicularity to the flange’s centerline, directly affect the uniform distribution of forces on the gasket and the quality of contact between the gasket and the flange. The roughness of the flange sealing surface should match the requirements of the gasket. The surface must not have any radial cuts or scratches, let alone surface cracks. Flange stiffness: Excessive warping deformation caused by insufficient flange stiffness (as shown in the figure below) is often one of the main reasons for seal failure in bolted flange connections in actual production. Flanges with high stiffness experience less deformation, allowing the bolt preload to be evenly distributed across the gaskets, thereby improving the sealing performance of the flanges. Flange stiffness is influenced by many factors; among them, appropriately increasing the thickness of the flange ring, reducing the diameter of the bolt circle, and increasing the outer diameter of the flange ring can all enhance flange stiffness. Using a flange with a neck or enlarging the size of the conical neck portion can significantly improve the flange’s bending resistance. However, increasing the flange stiffness without principle will make the flange bulky and raise its cost. Operating conditions: The effects of pressure, temperature, and the physicochemical properties of the medium on sealing performance are complex. Pressure and the medium alone have a modest impact on sealing, but when combined with temperature, especially fluctuating high temperatures, they can severely affect sealing performance, even leading to complete failure of the seal due to fatigue. Because at high temperatures, the viscosity of the medium is low and its permeability is high, making leakage more likely ； The corrosive effect of the medium on gaskets and flanges is intensified, increasing the likelihood of leaks ； Flanges, bolts, and gaskets all experience significant high-temperature creep and stress relaxation, which leads to seal failure ； Certain non-metallic gaskets can also accelerate aging and deterioration, or even burn out.