A flange, also called a flange plate or flange, is a component used to connect pipes to each other. It is attached to the pipe end. Flanges have holes for bolts to pass through, tightly connecting two flanges. A gasket is used for sealing between flanges.
Connections between container components can be non-removable (welded) or detachable. Due to production process requirements and for ease of manufacturing, transportation, installation, and maintenance, detachable flange connections are commonly used between cylinders, cylinders and heads, pipes and pipes, and pipes and valves.
Although detachable structures include flange connections, threaded connections, and socket connections, flange connections are the most widely used in chemical and pharmaceutical equipment and pipelines due to their unique advantages such as reliable sealing, high strength, and wide applicability.
Flange Connection Structure and Sealing Mechanism
Working Mechanism: By pre-tightening the bolts, the gasket between the flange sealing surfaces is compressed and compacted, filling the uneven gaps on the flange sealing surfaces, preventing permeation leakage of the medium through the capillary pores inside the gasket and interface leakage between the gasket and the sealing surface. This achieves pre-sealing.
The conditions for ensuring a leak-proof flange connection are: during pre-tightening, the pre-tightening pressure acting on the gasket is not lower than the pre-tightening sealing pressure; during operation, the residual pressure acting on the gasket is not lower than the working sealing pressure.
Flange Types
Classification based on the degree of integration between the flange and the equipment or pipeline:
1. Integral Flange
Slip-on flange: Used in applications with relatively low temperature and pressure. (Specific temperature and pressure values are provided in the standard.)
Butt-welded flange: Suitable for critical applications with higher pressure and temperature, and for toxic, flammable, and explosive materials. Higher cost.
Differences between slip-on and butt-welded flanges:
1. Different weld types: Slip-on welds cannot be radiographically inspected, while butt-welded welds can; slip-on welds consist of two corner ring welds, while butt-welded welds consist of a single butt ring weld.
2. Different Nominal Pressures: The nominal pressure of a slip-on flange is 0.6-4.0 MPa, while that of a weld neck flange is 1-25 MPa. Clearly, the slip-on flange is suitable for lower pressure ratings.
3. Different Connection Methods: Slip-on flanges can generally only be connected to pipes, not directly to weld neck fittings; weld neck flanges can generally be directly connected to all weld neck fittings (including elbows, tees, reducers, etc.), including pipes.
2. Loose Flanges
Characteristics: The flange is not connected to the pipe or container. It is suitable for low-pressure applications and does not generate additional bending stress on non-ferrous metals and stainless steel equipment and pipe shells. When made of carbon steel, it can save on the consumption of precious metals. However, loose flanges have poor rigidity.
Flange Connection Methods to Containers or Pipelines:
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Sleeve over a flange
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Sleeve over a weld ring
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Sleeve over a ring
3. Threaded Flanges
Widely used on small-diameter, high-pressure pipelines, the flange exerts relatively low additional stress on the pipe wall.
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Flange Shapes
Generally, flanges are available in disc or necked disc shapes, and some are square or oval (waist-shaped).
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Square Flange
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Round Flange
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Oval Flange
Factors Affecting Flange Sealing
The main factors affecting flange sealing include: bolt preload, sealing surface type, gasket performance, flange rigidity, and operating conditions.
Sealing Surface Types:
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Flat Face Type: Simple structure, easy to process, convenient for corrosion protection or lining. However, the gasket is easily extruded and difficult to tighten, suitable for low-pressure, small-size, non-toxic applications with low sealing requirements.
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Raised Face Type: Good centering, good sealing performance, suitable for medium-pressure and high-temperature applications.
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Tongue and groove type: Good centering, reliable sealing, small gasket width, and low clamping force required, suitable for flammable, explosive, toxic, and high-pressure applications. Machining and maintenance are relatively difficult.
The selection principle for the clamping surface is: ensure reliable sealing, strive for ease of machining, convenient assembly, and low cost.
Gasket Performance
In flange connections, the sealing effect largely depends on the sealing performance of the gasket.
1. Material Requirements: Corrosion resistant, does not contaminate the product, has a certain degree of elasticity, appropriate mechanical strength and flexibility, and does not easily deteriorate, harden, or soften with temperature changes.
2. Classification:
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Standards for Pressure Vessel Flanges and Pipe Flanges
Vessel flanges are only used for connections between vessel shells, such as connections between sections, or between sections and heads. Pipe flanges are only used for connections between pipes.
Special Note: The two are not interchangeable!!!
Pressure Vessel Flanges: JB/T4700~4707 "Pressure Vessel Flanges"
Pipe Flanges:
HG20592~20635 "Steel Pipe Flanges, Gaskets, and Fasteners"
GB/T9112~9125 "Steel Pipe Flanges"
JB/T74~90 "Steel Pipe Flanges, Gaskets, and Fasteners"
Three Important Parameters in Flange Standards
1. Nominal Diameter DN:
The nominal diameter of a flange refers to the nominal diameter of the container or pipe it mates with. The nominal diameter of a container refers to its inner diameter (except for containers with pipes as the cylindrical body). The nominal diameter of a pipe refers to its nominal diameter, which is a value between the inner and outer diameters of the pipe, with most values close to the inner diameter. Steel pipes with the same nominal diameter have the same outer diameter, but because the thickness varies, the inner diameter also varies. See Table 14-1.
2. Nominal Pressure PN:
Nominal pressure is a pressure rating used to establish standards. See Table 14-2.
3. Maximum Allowable Working Pressure:
The nominal pressure in pressure vessel flange standards is determined based on flange material 16Mn (or 16MnR) and a design temperature of 200°C. When the flange material and temperature change, the maximum allowable working pressure of the flange will increase or decrease. For example, the maximum allowable working pressure of the long neck welding flange in Table 14-3.
How to Select a Pressure Vessel Flange
Select a flange for the shell or head of a vessel with an inner diameter of Di, a design pressure of p, and a design temperature of t.
1. Based on the nominal diameter DN and design pressure p of the vessel, refer to Table 1 of JB/T4700 to initially determine the flange's structural type (Type A flat-face welding flange, Type B flat-face welding flange, or long neck welding flange).
2. Based on the vessel's design pressure p, design temperature t, and the flange material to be used, determine the PN of the selected flange according to Table 6 or Table 7 of JB/T4700 (Maximum Allowable Working Pressure Table).
3. Based on PN and DN, refer to Table 1 of JB/T4700 to verify whether the initially determined flange type meets the requirements. If it does not, the flange type or material needs to be changed, and PN needs to be re-determined.
4. Determine the flange dimensions according to JB/T4701 (Type A), 4702 (Type B), or 4703 (long neck welding).
