An important factor affecting sealing. The preload must compress the gasket to achieve an initial seal. Appropriately increasing the bolt pre-tightening force can increase the sealing ability of the gasket, because increasing the pre-tightening force can make the gasket retain a larger contact surface-specific pressure under normal working conditions.
However, the pre-tightening force should not be too large, otherwise, the gasket will yield as a whole and lose its resilience, and even the gasket will be squeezed out or crushed. In addition, the preload should act on the gasket as evenly as possible. Measures such as reducing the diameter of bolts, increasing the number of bolts, and adopting appropriate pre-tightening methods are usually taken to improve the sealing performance.
The gasket is an important element that constitutes the seal, and the main types are shown in the figure below.
The function of the gasket is to seal the gap between the sealing surfaces of the two flanges and prevent fluid leakage. The types of gaskets are non-metallic gaskets, non-metallic and metal composite gaskets and metal gaskets.
A suitable gasket material requires that the gasket can produce the necessary elastic deformation under the action of an appropriate pre-tightening force without being crushed or extruded; the distance between the flange sealing surface is enlarged during work, and the gasket material should be It has sufficient resilience to make the surface of the gasket closely contact with the flange surface, so as to continue to maintain good sealing performance;
The working medium and working temperature should also be considered when selecting the gasket material. The width of the gasket is also an important factor affecting the seal. The wider the gasket, the greater the required pre-tightening force, and thus the larger the size of the bolts and flanges.
① Non-metallic gaskets such as rubber, asbestos rubber, and polytetrafluoroethylene are commonly used on medium and low-voltage equipment and pipeline flanges. They have good corrosion resistance and softness, but poor strength and temperature resistance. They are usually cut from a full sheet of the spacer, which is circular in shape and rectangular in cross-section.
②In order to improve the strength and heat resistance of the gasket, thin steel strips and asbestos strips (or polytetrafluoroethylene strips or flexible graphite strips) are wound together to form wound gaskets or asbestos or other non-metallic materials are covered with metal The thin sheet is made of the metal clad gasket, which has a multi-channel sealing effect and good resilience. It is used in a relatively high temperature and pressure range, and can maintain a good seal under pressure and temperature fluctuations, so it is widely used.
Wound gaskets are made of steel strips and filled strips such as asbestos or polytetrafluoroethylene or flexible graphite. In order to prevent loosening, the beginning and end of the metal strip are welded dead. In order to increase the elasticity and resilience of the gasket, both the metal strip and the non-metal strip are rolled into waves. There are two types of wave shapes, V-shape and W-shape. As shown in the figure below, it is V-shaped, and there are 4 structural types.
Type A – also known as the basic type, without a reinforcing ring, is used for tongue and groove sealing surfaces.
Type B–With internal reinforcement ring, used for concave and convex sealing surfaces.
Type C – with external reinforcement ring, used for flat sealing surface.
Type D – There are reinforcing rings inside and outside, which are used for flat sealing surfaces.
③The metal-clad gasket is made of an asbestos rubber plate as the inner core, and the outer shell is composed of a thin metal plate with a thickness of 0.2-0.5mm (as shown in the figure below).
The material of the metal plate can be aluminum, steel and their alloys, stainless steel or high-quality carbon steel can also be used. Metal-clad gaskets are only used on Type B flat welding and long neck butt welding flanges.
④ On the flanges of high-pressure equipment and pipelines, metal gaskets are commonly used, and the materials include soft aluminum, copper, mild steel and stainless steel. In addition to metal gaskets with rectangular cross-sections, there are metal ring gaskets with oval or octagonal cross-sections and other special shapes.
When the operating pressure is high or the leakage rate is very strict, and the temperature is high or the corrosion is extremely strong, metal gaskets can be used. The specific pressure value of the metal gasket is very large. In order to reduce the bolt force, the pressing surface must be very narrow, relying on the extremely narrow pressing surface to maintain a good seal, it must have a small surface roughness, Ra≤2.5μm to Ra ≤0.63μm.
Sealing surface type
The contact surface between the flanges where the gasket is placed and pressed to perform the sealing function is called the flange sealing surface or the pressing surface. The choice of the sealing surface type is related to the operating conditions, the consequences of leakage and the nature of the gasket. Common structural types are as follows.
①Plane sealing surface: its structure is shown in figure (a) below. The sealing surface is not a smooth plane, and there are often 2 to 4 concentric grooves with triangular cross-sections distributed on the plane (that is, the flange waterline).
The flat sealing surface is simple in structure, easy to manufacture, and convenient for anti-corrosion lining. Secondly, the width of the sealing surface of this structure is relatively large, so non-metal or metal soft gaskets are often used in use. But when the bolt is tightened, the gasket material tends to stretch to the sides. It is used in occasions where the pressing force required is not high and the medium is non-toxic.
②Concave-convex sealing surface: The structure of the sealing surface is shown in figure (b) below, which is equivalent to a pair of flanges with flat sealing surfaces, one of which is made into a pressing surface with a raised platform, and the flange It is called a convex flange, and the other correspondingly made concave is called a concave flange, and a gasket of exactly the same size as the concave surface is embedded in it, and the gasket is convenient for centering.
The height of the convex plane is slightly greater than the depth of the concave surface, and it is compressed with bolts to play a sealing role.
This structure can limit the radial deformation of the gasket, prevent the gasket from being extruded, and improve the sealing performance to a certain extent. Suitable for high-pressure occasions.
③Tongue and groove sealing surface: In the middle of a pair of flat sealing surfaces in the width direction, one of them is made into a cross-section like a tenon, and the other cross-section is like a groove. Flange, the latter is called groove flange.
The groove-shaped pressing surface can limit the radial deformation of the embedded gasket, and the sealing performance is good, and the gasket can be less eroded and corroded by the medium. But the tenon surface part is easy to damage. Commonly used in flammable, explosive, toxic media and high-pressure occasions.
In addition, there are trapezoidal groove sealing surfaces and tapered sealing surfaces. The former uses a ring-shaped metal gasket with an oval cross-section, and the latter uses a lens-shaped ring-shaped metal gasket. The above two structures are forced seals and are commonly used in high-pressure pipelines.
The form and surface properties of the flange sealing surface play a crucial role in the sealing effect. The flatness of the flange sealing surface and the perpendicularity between the sealing surface and the center line of the flange directly affect the uniformity of the force on the gasket and the good contact between the gasket and the flange.
The roughness of the flange sealing surface should match the requirements of the gasket. Radial knife marks or scratches are not allowed on the surface, let alone surface cracks.
Excessive warping deformation due to insufficient flange stiffness (as shown in the figure below) is often one of the main reasons for the failure of the bolted flange connection seal in actual production. Flanges with high rigidity have small deformation and can evenly transmit the pre-tightening force of the bolts to the gasket, thereby improving the sealing performance of the flange.
Flange rigidity is related to many factors, among which appropriately increasing the thickness of the flange ring, reducing the diameter of the bolt center circle and increasing the outer diameter of the flange ring can all improve the flange rigidity. , can significantly improve the bending capacity of the flange. However, increasing the rigidity of the flange without principle will make the flange bulky and increase the cost.
The influence of pressure, temperature and the physical and chemical properties of the medium on the sealing performance is very complicated. The influence of simple pressure and medium on the sealing is not significant, but under the combined effect of temperature, especially at fluctuating high temperatures, it will seriously affect the sealing performance. , or even cause the seal to fail completely due to fatigue.
Because at high temperature, the viscosity of the medium is small, the permeability is high, and it is easy to leak; the corrosion effect of the medium on the gasket and flange is intensified, which increases the possibility of leakage; flanges, bolts and gaskets will all produce greater high temperature. Creep and stress relaxation will make the seal invalid; some non-metallic gaskets will also accelerate aging, deterioration, and even burnout.