1、 Core Definition
A flange, also known as a flange, is a disc-shaped mechanical component with evenly distributed bolt holes. It is a key component for achieving detachable connections in industrial pipelines, valves, pumps, and equipment. Its core function is to firmly connect two independent components (such as two pipelines) into a continuous system, and form a seal between the connecting surfaces to prevent leakage of internal media (liquids, gases, powders).
A flange connection usually consists of three parts: a pair of mating flanges, a sealing gasket sandwiched in the middle, and bolts and nuts used for fastening.
2、 Main Categories and Characteristics
Flanges are mainly classified based on their connection method with pipelines or equipment, and each type has its own structure and applicable scenarios.
1. Welding flange
This flange has a conical long neck and is connected to the pipeline through butt welds. Due to its structure resembling a reinforced beam, it can effectively disperse stress and therefore has high structural strength. It is a pressure resistant, temperature resistant, and fatigue resistant type, especially suitable for pipeline systems with high pressure, high temperature, or severe fluctuations in working conditions, such as main steam pipelines in the petrochemical and power industries. Its disadvantages are high cost and welding requirements.
2. With neck flat welding flange
This is a compromise solution between butt welded flanges and flat welded flanges. It has a short neck and is connected to the pipeline through a fillet weld. It has better rigidity and higher strength than flat welding flanges, and is also easier to install and align than butt welding flanges. It is a type of application in medium and high pressure pipeline systems.
3. Flat welding flange
This flange is flat without a neck and is directly connected to the outer wall of the pipeline through a fillet weld. Its structure is simple, the cost is low, and it is easy to process. However, due to its poor rigidity, the flange itself is prone to deformation when subjected to force, making it only suitable for low-pressure, mild medium applications, such as ordinary water supply and drainage or low-pressure air systems.
4. Threaded flange
The inner hole of the flange is machined with threads, which can be directly connected to externally threaded pipes without welding. Its major advantage is easy installation and disassembly, especially suitable for use in situations where welding is not possible or convenient, such as connecting galvanized pipes, certain alloy steel pipes, or for maintenance and replacement. But its sealing performance is not as good as welding under high temperature and pressure.
5. Loose flange
The special feature of this flange is that its disc body is not directly fixed to the pipeline. It is composed of an active flange and a welding ring or flange that is welded to the pipeline. During installation, the flange can rotate freely, making it very easy to align the bolt holes. Meanwhile, expensive flange materials such as stainless steel do not directly contact the medium inside the pipeline, which can save costs. Commonly used for non-ferrous metal pipelines or pipe sections that require frequent disassembly.
6. Blind flange
This is a solid disc flange without a hole in the center. It is not used for connection, but specifically for sealing the end of pipelines, equipment interfaces, or ports behind valves, acting like a sturdy "plug". It is crucial during system isolation, pressure testing, and equipment maintenance.
3、 Key sealing surface form
The sealing surface is the area where the flange and gasket come into direct contact to achieve sealing, and its form determines the sealing level.
Protruding surface: It is a common type where a raised ring is formed on the flange surface. It has a simple structure, easy processing, and wide applicability.
Full plane: The entire flange end face is a sealing surface. Commonly used for low pressure cast iron flanges or connections to equipment bases.
Concave convex surface: used in combination with a pair of concave and convex flanges. This restricts the gasket within the groove, making it less likely to be blown out by internal pressure and providing good neutrality.
Mortise and groove surface: Paired with delicate tenons (tongues) and grooves (grooves), the gasket is completely enclosed in a narrow space. This form of sealing has excellent sealing performance and is an honest merchant of flammable, explosive, highly toxic or high vacuum medium pipelines.
Ring connection surface: finely machined circular grooves on the flange surface, used in conjunction with metal octagonal or elliptical ring pads. This is one of the high pressure resistance levels among all types, suitable for extreme high pressure and high temperature conditions, such as key parts in petrochemicals and nuclear power.
4、 Selection requires parameters
When ordering or selecting flanges, it is necessary to clarify the following core parameters to confirm matching and safety:
Execution standards: Clearly define which standard system to follow, such as Chinese national standards, American standards, European standards, etc.
Connection type: Select specific forms such as butt welding, flat welding, threading, etc. according to the working conditions.
Nominal diameter: refers to the nominal flow diameter of a pipeline, such as DN100.
Pressure rating: indicates the pressure capacity that the flange can withstand, such as PN16 (European standard), Class150 (American standard).
Sealing surface type: Select according to the characteristics of the medium and sealing requirements, such as RF (convex surface), MFM (concave convex surface).
Material: Choose carbon steel, stainless steel, alloy steel, etc. based on the corrosiveness and temperature of the medium.
5、 Application industry
Flanges are the cornerstone components of almost all industrial industries. From the city's water and heating supply network around us to the ship's engine system; From clean pipelines in food and pharmaceutical factories to intricate reaction equipment in chemical plants; From boiler pipelines in power plants to oil pipelines in oil fields, flange connections and seals are indispensable. It can be said that wherever there are pipelines and equipment, there are flanges.