Selection principle of common steel pipe flanges

The article introduces the basic types and applications of various steel pipe flanges often used in the salt chemical industry, focusing on the nominal diameter, nominal pressure and flange sealing surface of steel pipe flanges and other characteristics as well as the principles of selection and selection methods in the design of pipelines.

1. Preface

Steel pipe flanges are mainly used for the connection of pipelines and chemical equipment, and are widely used in the salt chemical industry. At present, there are more flange standards, including national standard GB/T9112-9124-2000; chemical industry standard HG/T20592-20635-2009; mechanical industry standard JB/T74-90-1994; American standard ASME B16.5 and so on. The article takes the chemical industry standard HG/T20592-20635-2009, which is widely used in the salt chemical industry, as an example, to explain the basic types, selection principles and selection methods of various steel pipe flanges. The flanges of other standards are roughly the same as the chemical industry standards, and some of them can be used interchangeably, so careful checking is required when matching flanges.

2. The basic characteristics of the flange

Steel pipe flange, also known as flange, is a metal disk, the use of welded or threaded connection with metal pipes as one. The basic features of the flange are shown in Figure 1, consisting of bolt holes, sealing surfaces, connecting pipe holes, bolt hole center distance, flange outer diameter, nominal size, nominal pressure and other major parameters.
The above parameters are different for different flanges, so attention should be paid to the use of pairs when matching. For example, the number of flange bolt holes and the diameter of the bolt holes should be the same, otherwise, the pair can not be assembled, can not complete the flange connection and sealing function.
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Figure.1 Flange
In flange, nominal size DN and nominal pressure PN are two basic parameters that are not visible. Nominal size DN neither refers to the diameter of the flange connection hole, nor the outside diameter of the steel pipe connected with the flange, it is the nominal size of the flange, indicating the size of the flange, and the nominal size of the steel pipe connected with it in numerical terms. The nominal pressure of the flange PN limits the pressure range of flange use. Standard flange will be in the flange cylindrical surface in the form of a steel seal to the flange nominal size DN and nominal pressure PN and other parameters of the specific value printed out, to facilitate the user to choose.
The unit of pressure in the standard is “bar”. (1bar = 0.98kgf/cm2, 1bar = 0.1MPa). The unit of size is “mm”. 3.

3. Types of flanges

There are six types of flanges in common use, as shown in Figure 2.
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Figure 2 flange types
Plate type flat welding flange: indicated by the letter “PL”. The flange is connected to the pipeline by a single-sided fillet weld. Allowed to use steel plate, so this flange is convenient to take materials, simple manufacturing, low cost, but the rigidity is poor, only for the occasion of conveying the general media pressure is not high.
Slip on flange: with the letters “SO” said. Also belongs to the flat welding flange, because with a short neck, thus increasing the strength of the flange, improve the bearing strength of the flange. So can be used for higher pressure pipeline. Flange is usually made of forgings, more expensive than the plate flat welding flange.
Welding neck flange: the letters “WN” said. Pipeline and flange using butt welding connection. Butt welding connection is not easy to deformation, high strength, good sealing. Suitable for pressure or temperature fluctuations in the pipeline or high temperature, high pressure and low temperature pipeline, suitable for conveying expensive and non-general media occasions. Flange is usually forgings.
Integral flange: The letter “IF” is used. Similar to the neck butt weld steel pipe flange. Often flange and equipment, pipe, pipe fittings, valves, etc. into one, this type is commonly used in equipment and valves. The overall flange is mostly castings.
Socket weld flange: with the letters “SW” said. Socket welding flange flange hole with a limited table, to facilitate the insertion and positioning of the connection tube, welding for single-sided fillet weld, used for pipe diameter is not greater than DN50 and no gap corrosion occasions. Flange is usually made of forgings.
Threaded flange: the letter “Th” is used. Flange and steel pipe connection is a threaded connection, is a non-welded flange. It is a non-welding flange. Compared with the flat welding flange or butt welding flange, threaded flange has the characteristics of easy installation and maintenance, and can be used in some sites where welding is not allowed on the pipeline. Threaded flange is only used with the imperial series pipe. Suitable for the transport of general media, smooth working conditions of the occasion.
In addition to the above six types of flanges, there are butt-welding ring loose flange and flat-welding ring loose flange. Loose flange sealing surface and flange is composed of two separate parts, the seal and pipeline welding, flange wear on the pipe, can be free to move on the pipe, the flange only play a fastening role, and not in contact with the medium. The material of the seal is usually corrosion-resistant, mainly for corrosive media. In some special occasions the use of loose flange, such as loosening the bolt after the need to adjust the pipeline, can facilitate the disassembly of the pipeline.

4. Sealing surface of flange

In order to meet the needs of different production conditions, the flange has five different types of sealing surfaces, respectively, full plane FF, the protruding surface RF, concave and convex surface MFM, tongue and groove surface TG, ring connection surface RJ. specific structure as Figure 3.
Full plane FF: The compression surface is flat. This type of sealing surface is simple in structure and easy to process. However, it has a large contact area and requires a large preload force, and the gasket is easily extruded, resulting in poor sealing performance. It can not be used in toxic, flammable and explosive media, but only in general media and under moderate operating conditions; it is usually connected with cast iron flange or non-metallic flange.
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Figure.3 Flange sealing surface
RF: is the most widely used type, it is often used with butt welding and socket welding type, commonly used in the PN16, PN25 pressure level.
Concave Convex Face MFM: It consists of a concave face and a convex face. The gasket is placed in the concave side. Easy to align and prevent gasket extrusion. It is often used in combination with butt-weld and socket type, mainly for pressures higher than PN16. It is not convenient for gasket replacement.
Tongue and groove surface TG: the use of the basic situation in the concave and convex surface flange.
Ring connection surface RJ: often used with butt welding connection type (not with socket welding), mainly used in high temperature, high pressure or both higher working conditions. Often used in the occasion of PN ≥ 40.

5. The choice of flange

In production practice, steel pipe flanges can be selected according to the chemical industry standard HG/T20592-2009.
First, in accordance with the specific circumstances of the project and the relevant data provided by the production process, determine the nominal size of the pipe DN and the highest working pressure of the pipe. When selecting the flange, the nominal size of the pipe is used as the nominal size of the flange, and the highest working pressure of the pipe is rounded up to the standard pressure level (6.0 bar, 10 bar, 16 bar, 25 bar, 40 bar, 63 bar), and then raised a grade as the nominal pressure PN of the flange. Flange type and sealing surface. Attention should also be paid to the matching of the medium and the conditions of qualification. Non-general media refers to: flammable, explosive; high vacuum requirements; toxic hazards and leakage will occur after the gasification of the medium. General media refers to: non-general media other than the media with safety properties. Flanges for non-general media can be used in general media occasions, and vice versa.

6. Flange materials

The common materials of plate flat welding flange are: Q245R, Q345R, S30408, S31603, etc., which are allowed to be made of steel plate.
Other flange materials are forgings, (castings are only applicable to the overall flange.) Commonly used are: 20#, 16Mn, S30408, S31603, etc.

7. Flange marking

The selected flange should be marked to facilitate procurement, management and use of installation.
Flange markings to be written in accordance with the following provisions, the indispensable items: standard number, flange, type code, nominal diameter DN, nominal pressure PN, sealing surface code (neck butt-welding flange should be marked steel wall thickness), material grade.
Marking example 1: nominal diameter DN300, nominal pressure PN6, with the metric pipe (of B series pipe) of the sudden panel type flat welded steel pipe flange, material Q245R.
Marked as: HG/T20592 flange PL300 (B)-6RFQ245R
A series of steel pipe (A) can be omitted, B series of steel pipe (B) must be marked.
Marking example 2: nominal diameter DN300, nominal pressure PN25, with imperial pipe (of A series pipe) convex surface with neck flat welded steel pipe flange, the material is 20 steel.
Marked as: HG/T20592 flange SO300-25M20.

Table.1 gives the A, B two series of steel pipe nominal size and outside diameter dimensions.

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Table.1 Nominal size and outer diameter of steel pipe

8. Conclusion

Flange selection should comply with the provisions of more, including pressure vessels, pressure piping in the selection of flanges should be designed by qualified professionals, in use pressure vessels, pressure piping flanges should be used in accordance with the original design drawings of the program for maintenance and repair, and shall not be changed without permission, not to allow the flange with a low pressure level instead of a high pressure level flange, to ensure the safe and reliable operation of production.
Author: Dong Fenglin, Wang Dan

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

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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