Abstract: Flange sealing relies on bolt preload and gasket deformation to prevent leaks. Key factors include gasket type (metal/non-metal), surface design (flat, groove, ring), and operational conditions. Proper installation ensures safe, efficient pipeline connections.
Introduction:
In petrochemical plants, flanges are one of the most important and common forms for connecting pipelines, fittings, valves, instruments, and equipment. Although this type of connection has the advantages of easy disassembly and no need for hot work, due to the cooperation of flanges, bolts, and gaskets to achieve sealing, any component failure can cause leakage, not only causing waste, but also polluting the environment, and even causing casualties and property damage. Therefore, today we will explain in detail the relevant knowledge of flange sealing.
1. Sealing of flanges
More precisely, flange connection should be a bolt flange gasket connection, and its sealing is achieved through the synergistic effect of the three, with the gasket being the core component for achieving sealing.
2. Sealing principle of flanges
By pre-tightening the bolts, sufficient pressure is generated between the gasket and the flange sealing surface, so that the deformation on the gasket surface is sufficient to fill the micro unevenness of the flange sealing surface and achieve the purpose of sealing.
The minimum unit compressive force applied to the gasket to achieve the above objectives is called the specific pressure. When the pipeline connected to the flange reaches the operating pressure, the two flanges tend to separate due to the axial force of internal pressure, while the bolts tend to elongate, reducing the compressive force acting on the gasket.
When the compressive force acting on the effective cross-section of the gasket drops to a critical value, the seal can still be maintained, and the remaining compressive force on the gasket is the effective fastening force of the gasket. When the clamping force on the gasket is less than the effective fastening force, the flange will leak and even blow the gasket away.
Therefore, the effective fastening force of the gasket must be greater than the operating pressure of the pipeline. The distance between the two flange sealing surfaces is greater in the operating state than in the initial state, and the tightness between the gasket and the flange sealing surface is ensured by the rebound force of the cushion plate.
It can be said that in the initial sealing stage, the surface plastic deformation of the gasket plays a decisive role in filling the micro-unevenness of the flange sealing surface. In the operating state, the sealing of the flange and the elastic recovery inside the gasket play a dominant role.
3. Common forms of flange sealing
Face flanges
Advantages: Simple structure and easy processing
Disadvantages: It has a large contact area, requires a high preload ratio, and has a large bolt load capacity. Therefore, flanges and other parts have high requirements and are bulky. Gaskets are easy to extrude, and the sealing performance is poor. It cannot be used in toxic, flammable, or explosive media with a pressure of P ≤ 2.5Mpa.
Male-and -female flanges
Advantages: Easy to align, able to prevent gasket extrusion, is susceptible to medium erosion and corrosion. It is suitable for high-pressure situations.
Disadvantages: It is not convenient for replacing gaskets.
Tongue and groove face flanges
Advantages: Composed of mortise and tension surfaces and groove surfaces, the gasket is placed in the groove and will not be squeezed out of the compression surface, thus being tension susceptible to erosion and corrosion by the medium. Due to the narrow sealing surface, the required bolt force is small, making it suitable for situations with higher pressure and strict sealing requirements, such as flammable, explosive, and highly or extremely hazardous toxic media.
Disadvantages: The structure is complex, and the gasket is easily squeezed into the groove and difficult to remove, making it difficult to replace the gasket. Additionally, it should be noted that the tenon flange should be protected during transportation to avoid collision and deformation.
Ring connection flanges
Advantages: The ring connection surface is mainly used on neck welding flanges and integral flanges, with good sealing performance and general installation requirements, and is used in high temperature and high pressure conditions.
Disadvantages: The processing accuracy of the sealing surface is extremely high.
4. Factors affecting the sealing of flanges
The leakage of flange seals mainly occurs between the sealing surfaces. The possibility of leakage caused by the capillary action of the gasket is very small. After the gasket is compressed by external force, the elastic and plastic deformation generated by the gasket itself fills the small uneven gaps on the sealing surface, making the resistance of the medium passing through the sealing surface greater than the pressure difference on both sides of the sealing surface, thus achieving sealing. However, when affected by external conditions, the seal is damaged, resulting in leakage. The main factors affecting sealing are as follows.
(1) The impact of operating conditions. The operating conditions are pressure, temperature, and the physicochemical properties of the medium. In petrochemical plants, there are many low-pressure flanges, and the influence of pressure or medium factors on flange leakage is often not the main factor. The problem only becomes serious when combined with temperature. When the temperature changes repeatedly, the possibility of seal failure is greater.
(2) The influence of gasket coefficient and compression ratio in design parameters. At present, China selects two parameters by checking the table "Steel Pressure Vessels". However, even for the same material, the gasket coefficient and compression ratio are still related to factors such as gasket width, pre-tightening pressure, medium performance, flange sealing surface width and roughness. Therefore, there is currently no authoritative standard for how 5) The coefficients to choose from.
(3) The influence of bolt pre-tightening force. Increasing the pre-tightening force of bolts can enhance the sealing performance of the gasket, but excessive bolts and bolts can cause the gasket to lose its elasticity, or even crush or damage the foundation, which cannot guarantee sufficient elasticity in its working state.
(4) The impact of gasket performance. The gasket deformation includes elastic deformation and plastic deformation. The gasket material is the main factor affecting the performance of the gasket, so the quality of the gasket will affect the sealing of the flange.
(5) The influence of flange stiffness. Insufficient flange stiffness can lead to excessive warping deformation, which is often one of the reasons for seal failure.
(6) The impact of the sealing surface. The shape and roughness of the flange sealing surface should match the gasket. The flatness of the sealing surface and the perpendicularity between the sealing surface and the flange axis are prerequisites for ensuring uniform compression of the gasket. Therefore, the influence of inference or eccentric force caused by pipeline thermal expansion must be considered in the design and installation. Otherwise, it is easy to cause poor flange sealing and leakage.
5. Leakage of flanges
There are generally two ways for flange leakage:
The first type is the permeation effect of the gasket itself.
The second type is leakage between the gasket and the flange sealing surface.
Due to the continuous emergence of new sealing materials, the first type of leakage can be basically solved. The vast majority of daily occurrences are of the second type of leakage. Under a specific operating condition, the leakage rate that flange connections can achieve is lower than a specific indicator leakage rate. Or under specified leakage conditions, flange connections can withstand specific operating conditions. Flange connections that meet this indicator of leakage rate or specific operating conditions are considered non-leaking or tight, while those that do not meet this indicator are considered leaking or not tight.
The influence of flanges and bolts on flanges is very complex in actual working conditions. Bolts, gaskets, and internal pressure all directly exert forces on flanges, which may lead to irregular deformation of flanges. These forces are not a problem in terms of strength, but deformation may cause flange leakage. Therefore, in flange design, it is not only necessary to meet the strength requirements, but also to meet the minimum deformation required for the flange to not leak, that is, to design according to the stiffness theory.
As we all know, tightening bolts can improve sealing effectiveness and reduce leakage rates, but due to the strength and stiffness limitations of flanges and bolts themselves, this method becomes ineffective to a certain extent. Improper bolt installation procedures can also cause leaks, and the correct bolt tightening sequence (cross-equidistant symmetry) is extremely important to ensure the sealing of flange connection points and shorten the tightening operation time.
6. Gaskets
The most effective existing method to improve sealing effect and reduce leakage is to enhance the performance of the component - gaskets.
Flange gaskets are generally divided into metal gaskets and non-metal gaskets.
Non-metallic gaskets are made of non-metallic materials such as asbestos, rubber, synthetic resin, etc.
A non-metallic gasket wrapped with a layer of synthetic resin or other materials.
Half metal gaskets are gaskets made of metal and non-metal materials, such as spiral wound gaskets and metal coated gaskets.
a. Spiral wound gasket is a gasket made by sandwiching a non-metallic strip with a V-shaped or W-shaped cross-section metal strip and winding it in a spiral manner.
1) The inner ring is a metal ring set inside the wrapped gasket.
2) The outer ring is a metal ring set on the outer ring of the wrapped gasket.
b. Metal coating
Metal gaskets are gaskets made of metals such as steel, aluminum, copper, nickel, or Monel alloy.
Metal coated gaskets are particularly suitable for high-temperature and high-pressure sealing parts such as heat ex-changers and pressure vessels. It effectively prevents the gasket from falling apart and the erosion of the medium, while also improving the pressure resistance.
