Bolt preload is an important factor affecting sealing. The preload must compress the gasket to achieve the initial seal. Properly 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 specific pressure on the contact surface under normal working conditions.
However, the preload force should not be too large, otherwise, the gasket will yield as a whole and lose its resilience, or even squeeze out or crush the gasket. In addition, the preload should be applied to the gasket as uniformly as possible. Usually, measures such as reducing the diameter of the bolts, increasing the number of bolts, and taking appropriate pre-tightening methods are used to improve the sealing performance.
The gasket is an important element that constitutes a 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 to prevent fluid leakage. The types of gaskets include non-metallic gaskets, non-metallic and metal combined gaskets and metal gaskets.
Appropriate gasket material requires that the gasket can not only produce the necessary elastic deformation under the action of appropriate pre-tightening force, but will not be crushed or extruded; the distance between the flange sealing surface is enlarged during operation, and the gasket material should have enough resilience to make the gasket surface in close contact with the flange surface 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 preload, and the greater the size of the bolts and flanges.
①Non-metal gaskets such as rubber, asbestos rubber, and PTFE are commonly used on medium and low pressure equipment and pipeline flanges. They have good corrosion resistance and flexibility, but poor strength and temperature resistance. They are usually cut from the entire gasket sheet, the shape of the entire gasket is a ring, and the cross-section is rectangular.
②In order to improve the strength and heat resistance of the gasket, use thin steel strips and asbestos strips (or Teflon strips or flexible graphite strips) to form wound gaskets or wrap asbestos or other non-metallic materials with metal The sheet is made of the metal-clad gasket, which has multiple sealing functions and good resilience. It is widely used in high temperature and pressure ranges and can maintain good sealing under pressure and temperature fluctuations. Spiral wound gaskets are made by intertwining steel strips with filler strips such as asbestos, polytetrafluoroethylene, or flexible graphite. 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 a wave shape. There are two types of waves, V-shaped and W-shaped. As shown in the figure below, it is V-shaped, and there are 4 types of structures.
Type A – also known as the basic type, without reinforcing ring, used for tongue and groove sealing surface.
Type B – with inner reinforcing ring, used for concave and convex sealing surface.
Type C – with outer reinforcing ring for flat sealing surface.
Type D–Internal and external reinforcement rings are used for flat sealing surfaces.
③The metal clad gasket is made of asbestos rubber sheet as the inner core, and the outer casing is composed of a thin metal sheet with a thickness of 0.2 to 0. 5mm (as shown in the figure below).
The material of the metal plate can be aluminum, steel and its alloys, or stainless steel or high-quality carbon steel. Metal clad gaskets are also only used on B-type flat weld and long neck butt weld flanges.
④On the flanges of high-pressure equipment and pipelines, metal gaskets are commonly used, and the materials are soft aluminum, copper, mild steel and stainless steel. In addition to metal gaskets with a rectangular section, there are also metal ring gaskets with an oval or octagonal cross-section and other special shapes.
When the operating pressure is very high or the leakage rate is very strict, and the temperature is very high or the corrosiveness is very 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.
Type of sealing surface
The contact surface where the gasket is placed between the flanges and pressed to play a sealing role is called the flange sealing surface or the pressing surface. The selection of the sealing surface type is related to the operating conditions, the consequences of leakage and the nature of the gasket. Common structures are as follows.
①Flat sealing surface: Its structure is shown in figure (a) below. The sealing surface is not a smooth plane. There are often 2~4 concentric grooves with triangular cross-sections on the plane (that is, the flange waterline).
The flat sealing surface has a simple structure, is convenient to manufacture, and is convenient for anti-corrosion lining. Secondly, the width of the sealing surface of this structure is large, so non-metallic or metal soft gaskets are often used in use. However, when the bolts are tightened, the washer material tends to stretch to the sides. It is used for occasions where the required pressing force is not high and the medium is non-toxic.
②Concave-convex sealing surface: The structure of the sealing surface is shown in the figure (b) below. It is equivalent to a pair of flat sealing surface flanges, one of which is made into a pressing surface with a raised platform, and the flange is placed on the flange. It is called a convex flange, and the other correspondingly made concave is called a concave flange. The gasket with the same size as the concave is embedded in it, and the gasket is easy to center.
The height of the convex plane is slightly larger than the depth of the concave surface, and it is pressed with bolts to seal.
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.
③Tonite and groove sealing surface: in the middle of the width direction of a pair of flat sealing surfaces, one of which is made into a tenon-like section, and the other section is paired with the pressing surface of the groove, as shown in the figure (c) below, the former is called tenon surface Flange, the latter is called groove face 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 be damaged. Commonly used in flammable, explosive, toxic media and high pressure occasions.
In addition, there are trapezoidal groove sealing surfaces and conical sealing surfaces. The former uses a ring-shaped metal gasket with an elliptical cross-section, and the latter uses a lens-type 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 impact of the sealing effect. The straightness 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 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 tool marks or scratches are not allowed on the surface, let alone surface cracks.
Excessive warpage 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. The flange with high rigidity has small deformation, which can evenly transmit the bolt pre-tightening force to the gasket, thereby improving the sealing performance of the flange.
Flange stiffness is related to many factors. Among them, appropriately increasing the thickness of the flange ring, reducing the diameter of the center circle of the bolt and increasing the outer diameter of the flange ring can improve the flange stiffness, adopting a neck flange or increasing the size of the tapered neck part, which can significantly improve the bending resistance of the flange. However, increasing the rigidity of the flange unprincipled 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 are very complicated. The influence of simple pressure and medium on the sealing is not significant, but under the combined action of temperature, especially the fluctuating high temperature, 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 large, and it is easy to leak; the corrosion effect of the medium on gaskets and flanges is intensified, increasing the possibility of leakage; flanges, bolts and gaskets will generate large high temperatures Creep and stress relaxation make the seal fail; some non-metallic gaskets will also accelerate aging, deterioration, and even burn.
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