How to choose a flange?

2025-04-11

A flange, also known as a flange disc or a boss, is a part used to connect pipes to each other and is attached to the pipe ends. There are holes on the flange, and bolts are used to tightly connect the two flanges. A gasket is used for sealing between the flanges. Flanges are classified into threaded (screwed) flanges and welded flanges.

What is a Flange
A flange, also known as a flange disc or a boss, is a part used to connect pipes to each other and is attached to the pipe ends. There are holes on the flange, and bolts are used to tightly connect the two flanges. A gasket is used for sealing between the flanges. Flanges are classified into threaded (screwed) flanges and welded flanges.
A flange is a disc-shaped part that is most commonly seen in pipeline engineering, and flanges are always used in pairs. In pipeline engineering, flanges are mainly used for the connection of pipelines. For the pipelines that need to be connected, a flange disc is installed on each end. Threaded flanges can be used for low-pressure pipelines, while welded flanges are used for pipelines with a pressure of more than 4 kilograms. A sealing point is added between the two flange discs, and then they are tightened with bolts. Flanges with different pressures have different thicknesses and use different bolts.
When pumps and valves are connected to pipelines, the local parts of these equipment are also made into corresponding flange shapes, which is also called flange connection. Any connecting parts that are bolted around the periphery and closed at the same time between two planes are generally called “flanges”, such as the connection of ventilation pipelines. This type of parts can be called “flange-like parts”. However, this kind of connection is only a part of a piece of equipment. For example, in the connection between a flange and a pump, it is not appropriate to call the pump a “flange-like part”. Smaller equipment such as valves can be called “flange-like parts”.

How to Select a Flange

Currently, there are 4 flange standards existing simultaneously in China, which are:
(1) National flange standard GB/T9112~9124——2000;
(2) Flange standard of the chemical industry HG20592~20635——1997;
(3) Flange standard of the machinery industry JB/T74~86.2——1994; (Refer to: Flange standard of the Ministry of Machinery)
(4) Flange standard of the petrochemical industry SH3406——1996.
Now, taking the national standard as an example, let’s illustrate the selection of flanges. National standard flanges are divided into two major systems: the European system and the American system. The nominal pressures of flanges in the European system are: PN0.25, PN0.6, PN1.0, PN1.6, PN2.5, PN4.0, PN6.3, PN10.0, and PN16.0MPa; the nominal pressures of flanges in the American system are: PN2.0, PN5.0, PN11.0, PN15.0, PN26.0, and PN42.0MPa.
The basis for selecting a flange
(1) The nature of the conveyed medium, including general media, special media, toxic and harmful media, and flammable and explosive media;
(2) According to the parameters of the medium, such as the working pressure and working temperature. When the medium is determined, the nominal pressure PN of the flange is determined according to the working temperature and working pressure of the medium.
(3) Determine the connection method between the flange and the pipe and the form of the sealing surface according to the place of use and the connection conditions.
(4) Determine the flange specification according to the connected object
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