Five connection methods of flange

Flange is a part that connects pipes to each other and is connected to the pipe end. There are holes on the flange, and bolts can be inserted to tightly connect the two flanges, and the flanges are sealed with gaskets.

There are two types of connections between container parts: non-detachable (welding) and detachable. Due to production process requirements and for the convenience of manufacturing, transportation, installation and maintenance, detachable flange connections are commonly used between cylinders and cylinders, cylinders and heads, pipes and pipes, and pipes and valves.

Although the detachable structure includes flange connection, threaded connection and socket connection, etc. However, flange connections are most widely used in chemical and pharmaceutical equipment and pipelines due to their unique advantages of reliable sealing, high strength, and wide adaptability.

Working mechanism:

By pre-tightening the bolts, the gasket between the flange sealing surfaces is compressed and solidified, and the uneven gaps on the flange sealing surfaces are filled, preventing the permeability leakage and leakage of media through the capillary pores inside the gasket. Leakage at the interface between gasket and sealing surface. Thus achieving pre-sealing.

The conditions to ensure that the flange connection is leak-proof: when pre-tightening, the pre-tightening specific pressure acting on the gasket is not less than the pre-tightening sealing specific pressure; during operation, the remaining specific pressure acting on the gasket is not less than the pre-tightening specific pressure. Lower than working seal specific pressure.

Five connection methods of flange:

1. Flat welding:

Only the outer layer is welded, without welding the inner layer; it is generally used in medium and low pressure pipelines, and the nominal pressure of the pipeline is less than 0.25MPa. There are three types of sealing surfaces for flat welding flanges, namely smooth, concave and convex, and tongue and groove. Among them, the smooth type is the most widely used, and it is affordable and cost-effective.

2. Butt welding: 

The inner and outer layers of the flange must be welded. It is generally used in medium and high-pressure pipelines. The nominal pressure of the pipeline is between 0.25 and 2.5MPa. The sealing surface of the butt welding flange connection is concave and convex, and the installation is relatively complicated, so the labor cost, installation method and auxiliary material cost are relatively high.

3. Socket welding: 

Generally used in pipes with nominal pressure less than or equal to 10.0MPa and nominal diameter less than or equal to 40mm.

4. Loose sleeve: 

Generally used in pipelines where the pressure is not high, but the medium is relatively corrosive, so this type of flange has strong corrosion resistance and is mostly made of stainless steel. 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 process equipment adopts flange connection.

5. Thread:

Threaded connection method is a common connection method, suitable for connecting metal flanges and seamless  pipes. This connection method tightly connects the flange and the pipe through threads, and has high reliability. Common threaded connections include internal threads and external threads. The internal threaded flange connects to the external threads on the pipe, while the external threaded flange connects to the internal threads on the pipe. The threaded connection method has the advantages of simple installation and convenient disassembly, and is widely used in the connection of various steel pipes.

Flange connection process

Although the flange connection methods are different, their connection processes are not much different. The specific flange connection process is as follows:

1. Connection between flange and pipe

The following requirements need to be met: the midpoint of the pipe and the flange should be on the same horizontal line; the midpoint of the pipe should be in a 90-degree vertical shape, and the sealing surface of the flange should be the same as the sealing surface of the flange; the flange bolts should be in the pipeline should be in the same position.

2. Gasket flange gasket

Requirements: In the same pipeline, the gaskets selected for flanges under the same pressure should be the same so that they can be exchanged with each other in the future; the gasket selection principle should be as close as possible to the small width selection, that is, without crushing the gasket. The gasket principle should be followed.

3.Connecting flange

Requirements: Check whether the specifications of flanges, bolts and gaskets meet the requirements; the sealing surface should be smooth and clean, without burrs; the bolt threads should be complete without gaps, and the chimera should be natural; the texture of the gasket should be soft, not easy to age, and the surface should be smooth. Damage, wrinkles, scratches and other problems; before assembling the flange, the flange should be cleaned of oil, dust, rust and other debris, and the sealing line should be removed.

4. Assembly flange

Requirements: The flange cover is perpendicular to the middle point of the pipe; bolts of the same specifications are installed in the same direction; 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 building wall should be more than 200 mm ; The flange must not be buried directly underground as it is prone to corrosion. Therefore, if it is buried underground, anti-corrosion treatment needs to be done.

5.Flange disassembly

Requirements: It is confirmed that the pipeline should not fall after the flange is disassembled, otherwise it should be temporarily installed to prevent the pipeline from deforming due to stress and gravity. According to the torque tightening sequence of flange bolts, loosen the bolts every 2 to 3 times (as slowly as possible to prevent the residual media from blowing out); after all bolts are loosened, slowly remove the gasket to avoid damaging the flange cover.

The main advantages of flange connections

1. Reliable connection: Because the flange has good sealing performance, it ensures that the pipeline will not leak and ensures the safe operation of the equipment and pipelines. At the same time, it adopts a reliable connection method and is easy to connect.

2. Low cost: Because of its low cost, it is widely used. It is also the most common connection method at present.

3. Strong adaptability: The flange can be designed according to different engineering needs, has good sealing performance, and is suitable for a variety of media.

4. Wide range of applications: Flanges can connect all types of pipes, such as pipe joints, etc.; they can also be used to weld butt-welded pipes and connect flanges. The flange is suitable for nominal pressure PN≥4.0 MPa and ≤150 MPa.

5. Good designability: it can be designed according to different working conditions; it is also the most commonly used and effective method to solve complex stress problems.