Stainless Steel Pipe Fittings

Flat welding flange and butt welding flange

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What is a flat welding flange?

A flat welded flange is a type of flange that is joined to a vessel or pipe by a fillet weld. It is an arbitrary type of flange. According to the degree of integrity of the connection between the flange ring and the straight section, the design is checked according to the integral or loose flange. The flange ring is available in both neck and neck. Compared with the neck-welded flange, the flat-welded flange has a simple structure and is material-saving, but the joint is not as good as the neck-welded flange. Flat welded flanges are widely used in the connection of medium and low pressure vessels and pipes. Flat welding flange is suitable for steel pipe connection with nominal pressure not exceeding 2.5MPa. The sealing surface of flat welding flange can be made into smooth, concave and convex and grooved. The application of smooth flat welding flange is the largest. It is mostly used in cases where the medium condition is moderate, such as low pressure non-purified compressed air and low pressure circulating water.

Type of flat welding flange:

Plate flat welding flange
Advantages: easy to take, simple to manufacture, low cost, widely used
Disadvantages: Poor rigidity, so it should not be used in chemical process piping systems with high and extreme hazard requirements for supply, demand, flammability, explosive and high vacuum.
The sealing surface type has a flat surface and a convex surface.
Neck flat welding flange
Advantages: It is convenient to install on site, and the process of welding seams can be omitted.
Disadvantages: The neck of the flat welded flange has a low neck height, which improves the rigidity and load carrying capacity of the flange. Compared with the butt welding flange, the welding work is large, the welding rod consumption is high, and the high temperature and high pressure and repeated bending and temperature fluctuation cannot be withstood.
Flat welding ring loose flange
The flat welding ring loose flange is a movable flange. It is usually matched on the water supply and drainage fittings (the most common on the expansion joint). When the factory leaves the factory, there is a flange at each end of the expansion joint, directly in the project. The pipes and equipment are bolted. The purpose of using a flat welding ring to loosen the flange is generally to save material. The structure is divided into two parts. One end of the pipe is connected to the pipe, one end is made into a flange, and the flange is partially placed on the flange. The flanges are made of low-grade materials, and the tubes are made of the same material as the pipes to save material.
Flat welding flange features:
The flat welded flange not only saves space, reduces weight, but more importantly ensures that the joint is not leaked and has good sealing performance. The reduced size of the compact flange is due to the reduced diameter of the seal which will reduce the cross-section of the seal face. Second, the flange gasket has been replaced by a seal ring to ensure that the seal faces the sealing surface. In this way, only a small amount of pressure is required in order to press the cover tightly. As the required pressure is reduced, the size and required number of bolts can be reduced accordingly, so a new design that is small in size and light in weight (70% to 80% lighter than conventional flanges) is designed. product. Therefore, the flat welding flange type is a relatively high quality flange product, which reduces the quality and space and plays an important role in industrial use.
Sealing principle of flat welding flange:
The two sealing faces of the bolt press each other against the flange gasket and form a seal, but this also causes damage to the seal. In order to maintain the seal, it is necessary to maintain a huge bolt force, for which the bolt is made larger. Larger bolts are matched to larger nuts, which means that larger diameter bolts are needed to create the conditions for tightening the nuts. However, the larger the diameter of the bolt, the more suitable the flange becomes, the only way to increase the wall thickness of the flange. The entire device will require a great size and weight, which becomes a special problem in the offshore environment, because weight is always a major concern for people in this situation. Moreover, fundamentally, the flat weld flange is an ineffective seal that requires 50% of the bolt load to be used to squeeze the gasket, while the load used to maintain the pressure is only 50%.
The main design disadvantage of flat welded flanges is that they are not guaranteed to be leak free. This is the design flaw: the connection is dynamic, and cyclic loads such as thermal expansion and undulations cause movement between the flange faces, affecting the function of the flange, thereby impairing the integrity of the flange and ultimately leakage. It is impossible for any product to have no defects, but to minimize the shortage of the product as much as possible, so the company should try to improve the performance of the product when producing flat welding flanges, so that it can play its biggest role.

What is a butt weld flange?

butt weld flange is a type of pipe and refers to a flange with a neck that has a transition of a round pipe and is butt welded to the pipe. Butt welding flange is not easy to be deformed, sealed, widely used, has corresponding rigidity and elasticity requirements and reasonable butt welding thinning transition, the welding joint is away from the joint surface, the joint surface is protected from welding temperature deformation, it takes more complicated It is suitable for pipelines with high pressure or temperature fluctuations or high temperature, high pressure and low temperature pipelines. It is generally used for the connection of pipelines and valves with PN greater than 2.5MPa. It is also used for expensive, flammable and explosive transportation. On the tubing of the media.

Classification of welded flanges

Welding Neck Flanges
The sealing faces of the neck butt weld flange are: face (RF), concave (FM), convex (M), face (T), groove (G), full plane (FF).
Advantages: the connection is not easy to be deformed, the sealing effect is good, and the application is wide. It is suitable for pipes with high temperature or pressure fluctuation or pipes with high temperature, high pressure and low temperature, and also for conveying expensive medium, flammable and explosive medium and toxic gas. on
Disadvantages: The neck-welded flange is bulky, heavy, expensive, and difficult to install. Therefore, it is easier to bump during transportation.
Butt ring loose flange
The loose flange of the welding ring is a movable flange. It is usually matched with the water supply and drainage fittings. When the factory leaves the factory, there is a flange at each end of the expansion joint, which is directly connected with the pipeline and equipment in the project.
Function: The purpose of loosening the flange of the welding ring is generally to save material. The structure is divided into two parts. One end of the pipe is connected to the pipe and the other is made into a butt weld ring. The flanges are made of low-grade materials, and the tubes are made of the same material as the pipes to save material.
Advantages: cost saving and easy construction.
Disadvantages: low pressure. Low strength at the weld ring (especially when the thickness is less than 3mm)

The difference between the welding flange and the flat welding flange

  1. Different weld joint forms: welded welds with neck welded flanges and flanges are in the form of fillet welds, while welded welds with neck butt welds and pipes are in the form of girth welds;
  2. The material is different: the neck flat welding flange material is made of ordinary steel plate machine with the thickness meeting the requirements, and the material with neck butt welding flange is machined by forging steel parts;
  3. The nominal pressure is different: the nominal pressure of the neck flat welding flange is: 0.6—4.0MPa, and the nominal pressure of the neck butt welding flange is: 1–25MPa grade.
  4. Another point is that the weld with the neck butt weld flange and the joint is a type B joint, and the weld with the neck flat weld flange and the joint is a type C joint, and the non-destructive testing after welding is different.
  5. The neck flat welding flange and the neck butt welding flange can not be replaced casually, in terms of manufacturing angle, the neck flat welding flange (SO is SLIP ON abbreviation), the internal diameter is large, which means the weight is small, the cost is low, and The neck-welded flange with a nominal diameter of more than 250 mm (WN is the abbreviation of WELDING NECK) is to be tested. The flange of the SO does not need to be tested, so the cost is low.

Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (

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