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Sealing principle and precautions of flat welding flange

2021-04-24 13:30:58
The two sealing surfaces of the bolt squeeze the flange gasket and form a seal, but it also causes the sealing damage. In order to keep the seal, it is necessary to maintain a huge bolt force, so the bolt will be done more. Larger bolts match larger nuts, which means that larger diameter bolts are needed to create conditions for tightening nuts. However, the larger the diameter of the bolt, the applicable flange will become bent, and the only way is to increase the wall thickness of the flange part. The whole plant will require a great size and weight, which becomes a special problem in offshore environment, because in this case, weight is always the main problem that people must pay attention to. Moreover, essentially, flat welded flange is an invalid seal, which requires 50% of the bolt load to squeeze the gasket, while the load used to maintain the pressure is only 50%. Flat welding flange is usually used in the case of low medium temperature and pressure and strong corrosion of medium. When the medium is corrosive. The rare integral flange has flat welding flange and buttwelding flange. Flange fittings are those with flange (flange or adapter). It can be cast, and the flat welding flange is mainly the part that makes the pipe and pipe connect with each other. According to the structure type. It can also be made up of threaded connections or welding. Flange connection refers to a pair of flange, a gasket and several bolt nuts.
 
Precautions for flat welding flange:
 
1. The steel plate producing flat welding flange shall be inspected by ultrasonic, without delamination defects, and good quality and performance problems shall be ensured. Production and inspection shall be carried out according to certain quality requirements to ensure that the steel plates produced and used are free of quality problems;
 
2. The steel shall be cut into strips along the rolling direction, and be welded into circular rings by bending butt welding, and the surface of steel shall form the cylindrical surface of the ring. When producing flat welding flange, steel plate shall not be directly machined into flange with neck, and certain process shall be used for manufacturing and processing.
 
When the welding of flat welding flange adopts single-sided continuous welding, the circumferential heat-bearing part of flange welding area is greatly contracted when the weld is cooled, which further aggravates the expansion of deformation. The flange forms irregular ellipse in the circumferential direction, and the part is concave and partially convex. At the same time, the inner circumference of flange is reduced due to the welding shrinkage force, while the outer flange is in a lower temperature, the influence of welding heat input is small, the rigidity is good, and there is no shrinkage basically; Thus, the flange section is twisted, forming a phenomenon of flange surface turning over significantly, and the central axis of flange bolt hole is no longer parallel to the cylinder axis. It is proved that the force of the overturning is very large, and it can not be changed after the welding of the groove on the flange and the cylinder.
 
Forging method of flat welding flange:
 
According to the movement of forging die, forging can be divided into swing forging, swing forging, roll forging, cross wedge rolling, ring rolling and skew rolling. Swing, swing and ring can also be precision forged. In order to improve the utilization of materials, roll forging and cross rolling can be used as the previous process of thin materials. The rotary forging is also formed locally as free forging. Its advantage is that it can be formed when the forging force is small compared with the forging size. The finishing of complex forgings can be completed by one or several processes. Without flying edge, the area of the forging will be reduced and the load required will be reduced.
 
In this forging method, including free forging, the material expands from the near die surface to the free surface during the process of flat welding flange. Therefore, it is difficult to ensure the accuracy. Therefore, the product with complex shape and high precision can be obtained by using the computer control of the moving direction and rotary forging process of forging die. For example, the forging of steam turbine blades with many varieties and large size is produced. Forging of flat welding flange can be divided into free forging, upsetting, extrusion, die forging, closed die forging and closed upsetting. Generally speaking, forging in the temperature region with recrystallization is called hot forging, and forging without heating at room temperature is called cold forging. When the flat welding flange is forged at low temperature, the size of the forging changes very little. The scale formed less and the surface was not decarburized.
 
Main functions of flat welding flange:
 
1. The pipeline is in trouble, which is easy to be disassembled and inspected;
 
2. Connect the pipeline and keep the pipeline with good sealing performance;
 
3. If there is a problem with a certain section of pipeline, it is convenient to replace a certain section of pipeline.
 
The flat welding flange has certain technical requirements and technical parameters in production and use, and it is processed in a certain way and method:
 
1. Butt weld of ring shall be heat treated after welding and 100% of the radiographic or ultrasonic testing shall be conducted. The radiographic inspection shall meet the requirements of class II of JB4730, and the ultrasonic inspection shall meet the class I requirements of JB4730. Butt welding flange adopts certain welding standards and technology to produce and process, which meets the corresponding national production standards and requirements;
 
2. The steel plate producing flat welding flange shall be inspected by ultrasonic, without delamination defects, and good quality and performance problems shall be ensured. Production and inspection shall be carried out according to certain quality requirements to ensure that the steel plates produced and used are free of quality problems;
 
3. The steel shall be cut into strips along the rolling direction, and be welded into circular rings by bending butt welding, and the surface of steel shall form the cylindrical surface of the ring. In the production of the flange, it is not allowed to use steel plate to directly machine the flange with neck, and adopt certain process to make and process.
 
Features of flat welded flange:
 
Flat welding flange not only saves space and reduces weight, but also ensures that joint parts will not leak and have good sealing performance. The reduction in the size of the compact flange is due to the reduction of the diameter of the seal, which will reduce the section of the sealing surface. Secondly, flange gasket has been replaced by sealing ring to ensure sealing face matching. This will require only a small pressure to hold the sealing surface down. With the decrease of the required pressure, the size and quantity of bolts can be reduced accordingly. Therefore, a new product with small volume and light weight (70% ~ 80% less than the weight of traditional flange) is designed. Therefore, flat welding flange is a kind of high quality flange product, which reduces the quality and space, and plays an important role in industrial use. However, the main design disadvantage of flat welding flange is that it can not guarantee no leakage. This is the design deficiency: the connection is dynamic, and the periodic load such as thermal expansion and ups and downs will cause the movement between flange faces, affect the function of flange, thus damaging the integrity of flange and eventually leading to leakage. No product can be free of defects, but the product is limited to the minimum. Therefore, the company tries to improve the performance of the products in the production of flat welding flange, so that it can play the greatest role.

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