Stainless Steel Pipe Fittings

The sealing principle of flat welding flange and the matters needing attention

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Flat welding flange and pipe connection: is to insert the pipe flange hole to the appropriate position, then take welding. Flat welding flange is suitable for the pressure level is lower, the pressure fluctuation, the vibration and shock were not serious in the pipeline system. Flat welding flange advantages in flat welding flange welding assembly for more easily, and the price is cheaper, thus has been widely used.

The Differences between Flat Weld Flange and Butt Weld Flange

The flat weld flange applies to the pipes with nominal pressure no more than 2.5Mpa, such like the cycling water pipe and low pressure air pipe. It has the sealing face of FF, MFM, and TG types, and the FF is most popular. Its biggest trait is the low price.

The butt weld flange has the compact structure, high rigidity and high strength, so that it can withstand the drastic temperature fluctuation and repeated bending. The butt weld flange with the nominal pressure from 0.25Mpa to 2.5Mpa mostly adopts the MFM type sealing face.

The welding methods are different. The flat weld flange has a socket RF with the diameter bigger than the pipe, so that the pipe can be inserted and welded inside. The butt weld flange’s wall thickness and diameter are required to be the same with the pipe, so that both the inside and outside of the weld seam can be welded up.

The sealing principle of flat welding flange

Two seal face mutual extruding flange gasket bolt and form a seal, but it also led to the destruction of the seal. In order to keep the seal, will have to maintain a huge bolt force, therefore, the bolt will be bigger. And a bigger bolt will match the bigger nut, which means the larger diameter bolts to create conditions to tighten the nut. However, the diameter of the bolt, the greater the applicable flange will become bent, the only way is to increase the flange part of the wall thickness. The whole device will requires a great deal of the size and weight, this in the offshore environment has become a special problem, because in this case the weight of people must concern has always been the main problem. And, ultimately, flat welding flange is an invalid sealing, it needs to be 50% of the bolt load for dummy blocks, and used to keep the pressure load is only 50%.

Attention to the production of flat welding flange

1, the production of flat welding flange plate should be via ultrasonic testing, with no defects, guarantee the good quality and performance problems, according to certain quality requirements of the production and inspection, ensure the quality of the production and use of the steel plate no problem;

2, should be along the direction of steel rolling cut into strips, the bending welded into a circular ring, and to form the surface of the steel ring cylinder. In the production of flat welding flange shall not adopt steel plate machining directly into bring neck flange, adoption of a certain craft production and processing.

Flat welding flange during welding using single continuous welding, flange welding area circumferential weld cooling to heating part under toroidal welding shrinkage is very big, the further expansion of the deformation of the flange in the ring to form irregular ellipse phenomenon, local concave and convex part. At the same time due to the welding contraction force smaller flange inside the perimeter, and flange lateral due to low temperature, weld heat input effect is small, good rigidity, basic no shrinkage; So that flange section, forming flange face significant reverse phenomenon, at the same time cause the flange bolt hole is no longer the central axis parallel to the cylinder axis. Flip the force, it turns out, it is very big, at the completion of flange welding groove and the cylinder of after, still can’t make the deformation of any change.

Flat welding flange of main design drawback is that it does not guarantee no leakage. This is insufficient on its design: the connection is dynamic, and such as thermal expansion and the ups and downs of cyclic load can create the flange surface between the moving, affect the function of the flange, to destroy the integrity of the flange is damaged, eventually lead to leakage. Any product may not have the flaw, not just the lack of control products to a minimum as far as possible, so the company in the production of flat welding flange to improve the performance of the product as far as possible, make it play a biggest role.

Source: Yaang Pipe Industry Co., Limited (

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