Flange connection, Pipe and flange connection

What is a flange connection?

Pipe flange connection is to fix two pipes, pipe fittings or equipment on a flange plate first, then add flange pads between the two flange plates, and finally use bolts to tighten the two flange plates. It is tightly coupled with a detachable joint. The connection between stationary pipes and rotating or reciprocating equipment can be realized.

Features of flange connection:

The main features of flange connection are easy disassembly, high strength and good sealing performance. When installing the flange, the two flanges must be kept parallel. The sealing surface of the flange must not be damaged and must be cleaned. The gasket used for the flange must be selected according to the design requirements.

Flange connection method:

Pipe and flange connection methods can generally be divided into five types: flat welding, butt welding, socket welding, loose sleeve, and thread.

1. Flat welding flange
Flat welding flange refers to a flange that is connected to a container or pipe using a fillet weld. It is an arbitrary flange. During design, according to the integrity of the connection between the flange ring and the straight section, the flange is checked as an integral or loose flange. Flange rings are available with or without neck. Compared with the necked butt welding flange, the flat welding flange has a simple structure and saves materials, but its stiffness and sealing performance are not as good as the necked butt welding flange. Flat welding flanges are widely used in the connection of medium and low pressure vessels and pipelines.

2. Butt welding flange
Butt welding flange is a type of pipe fitting, which refers to a flange with a neck and a round pipe transition and is butt welded to the pipe. The inner and outer layers of the flange must be welded, and are generally used in medium and high-pressure pipelines. The nominal pressure of the pipeline is between 0.25 and 2.5MPa.

The butt welding flange is not easy to deform, has good sealing, and is widely used. It has corresponding rigidity and elasticity requirements and a reasonable butt welding thinning transition. The distance between the welding port and the joint surface is large, and the joint surface is protected from welding temperature deformation. It adopts a relatively complex method. The trumpet-shaped structure is suitable for pipelines with large fluctuations in pressure or temperature or pipelines with high temperature, high pressure and low temperature. It is generally used for the connection of pipelines and valves with PN greater than 2.5MPa; it is also used for transporting expensive, flammable and explosive materials. on the media pipeline.

The sealing surface of the butt welding flange connection is concave and convex, and the installation is relatively complicated, so the labor cost, installation cost and auxiliary material cost are relatively high.

3. Socket welding flange
Socket welding flange refers to a flange in which the end of the pipe is inserted into the flange ring step and welded on the pipe end and outside. There are two types: with neck and without neck. The necked pipe flange has good rigidity, small welding deformation and good sealing performance. It is generally used in pipes with a pressure of 1.0~10.0MPa and a nominal diameter of 40mm or less. Type B flange for containers is also a socket welding flange and is used in situations with high sealing requirements. During design, the socket welding flange can be calibrated as an integral flange.

4. Loose sleeve flange
A loose tube flange is a connection between two pipes or between pipes and equipment. Its structure includes a loose tube and two elbow flanges. The loose sleeve refers to the cylindrical part between two pipe elbows. Some loose sleeves have rubber products wrapped around the cylindrical part to ensure sealing.

Loose sleeve flanges work by placing sleeves at the ends of two pipes and bolting the two flanges together to maintain the connection. This structural design allows the pipeline to be freely twisted within a certain angle range to meet the needs of pipeline connection.

Loose flange is a form of loose sleeve flange, which can be divided into three types: flat welding steel ring loose flange, plate flange loose flange, and butt welding ring loose flange. The loose flange uses flanges, steel rings, etc. to put the flange on the pipe end, and the flange can move on the pipe end. The steel ring or flange is the sealing surface, and the function of the flange is to compress them. It can be seen that the loose flange is not in contact with the medium due to being blocked by the steel ring or flange.

The advantage is that no additional moment is generated on the container or pipe when the flange is deformed, and it is easy to manufacture. It can be made of different materials from the container or pipe, which saves precious metals. The disadvantage is that the flange thickness is larger. Suitable for applications where containers or pipes are lined with brittle materials and the pressure is not high. Generally used in pipelines with low pressure but relatively corrosive media, this type of flange has strong corrosion resistance and is mostly made of stainless steel.

5. Threaded flange
Threaded flange refers to a flange that uses threads to connect to pipes. When designing, it can be handled as a loose flange. The advantage is that no welding is required, and the additional moment produced on the cylinder or pipe when the flange is deformed is very small. The disadvantage is that the flange is thick and the cost is high. Suitable for connection of high pressure pipelines.

Threaded flange is a non-welded flange that processes the inner hole of the flange into pipe threads and connects it with threaded pipes. Compared with flat welding flanges or butt welding flanges, threaded flanges are easier to install and maintain, and can be used on pipelines where welding is not allowed on site. Alloy steel flanges have sufficient strength, but are not easy to weld, or have poor welding performance. Threaded flanges can also be selected. However, it is recommended not to use threaded flanges to avoid leakage when the pipe temperature changes rapidly or when the temperature is higher than 260°C and lower than -45°C.

This kind of connection is mainly used for the connection of cast iron pipes, rubber-lined pipes, non-ferrous metal pipes and flange valves, etc. The connection between process equipment and flanges also uses flange connection.

Process steps and precautions for flange connection:

The flange connection process is as follows:

1) The connection between flange and pipe must meet the following requirements:
1. The centers of the pipe and the flange should be on the same horizontal line.
2. The center of the pipe and the sealing surface of the flange form a 90-degree vertical shape.
3. The positions of the flange bolts on the pipeline should be consistent.

2) Use flange gaskets, the requirements are as follows:
1. In the same pipeline, flanges with the same pressure should use the same gasket, so that they can be easily exchanged in the future.
2. For pipes using rubber plates, it is best to use rubber gaskets, such as water pipelines.
3. The principle of gasket selection is: choose as close to the smallest width as possible. This is the principle that should be followed to ensure that the gasket will not be crushed.

3) Connecting flange
1. Check whether the specifications of flanges, bolts and gaskets meet the requirements.
2. The sealing surface should be kept smooth and clean without burrs.
3. The threads of the bolts must be complete without defects, and the fit must be natural.
4. The texture of the gasket should be flexible, not easy to age, and have no surface damage, wrinkles, scratches and other defects.
5. Before assembling the flange, clean the flange to remove oil, dust, rust and other debris, and remove the sealing line.

4) Assembly flange
1. The flange sealing surface is perpendicular to the center of the pipe.
2. Bolts of the same specification have the same installation direction.
3. The installation position of the flange installed on the branch pipe should be more than 100 mm from the outer wall of the riser, and the distance from the wall of the building should be 200 mm or more.

4. Do not bury the flange directly underground, as it is easily corroded. If it must be buried underground, anti-corrosion treatment must be done.

Application areas of flange connection:

1. Pipe and pipeline connection: Flanges are used to connect and fix various parts of the pipeline system, such as pipes and pipes, pipes and valves, pipes and equipment, etc.
2. Shipbuilding and marine engineering: Flanges are used to connect shipbuilding and marine engineering equipment, such as the pipeline system of ships, the pipeline system of offshore oil drilling rigs, etc.
3. Oil and gas industry: Flanges are used to connect oil and gas pipelines, petrochemical equipment and facilities.
4. Chemical industry: Flanges are used to connect chemical industry equipment and pipelines, such as chemical reactors, distillation towers, etc.

5. Water supply and drainage system: Flanges are used to connect pipes and equipment of water supply and drainage systems, such as water pumps, water tanks, sewage treatment facilities, etc.
6. Power industry: Flanges are used to connect equipment and pipelines of power plants and transmission lines.
7. Food and pharmaceutical industry: Flanges are used to connect food and pharmaceutical industry equipment and pipelines, such as food processing equipment, pharmaceutical manufacturing equipment, etc.
8. Construction and municipal engineering: Flanges are used to connect pipe systems in construction and municipal engineering, such as building water supply and drainage systems, HVAC systems, etc.

It should be noted that due to the different designs and characteristics of flanges, flanges used in different fields may also be different.

Advantages and disadvantages of flange connection:

Advantages:
(1) Easy to install and disassemble, the connection method and connection point can be flexibly adjusted to adapt to connections under various complex conditions;
(2) Good sealing, the use of gaskets can effectively avoid oil leakage, air leakage and other problems;
(3) High connection strength and can withstand greater pressure;

Disadvantages:
(1) High maintenance requirements, and gaskets need to be replaced regularly;
(2) Relatively high cost, and a large number of flanges, bolts, gaskets and other accessories need to be purchased.

Flange connection has the characteristics of high reliability and wide application range, and is often used in industrial pipelines, ships, petrochemicals and other fields. When using flange connection, it is necessary to select the appropriate flange model, material and gasket according to the specific situation to ensure the reliability and sealing of the connection.

Read more: Pipe elbow and flange welding connection or Pipe flange material selection