4 Common Types of Pipe Flanges
author: www.qishine.com
2026-02-25
In industrial piping systems, flange connections serve as the most critical detachable joining method. Their proper selection directly impacts the safe operation and maintenance convenience of the entire facility. As a long-standing factory and supplier of pressure piping products, we provide a comprehensive analysis of the four most common types of flanges used in engineering — Welding Neck Flange, Slip-on Flange, Threaded Flange, and Socket Weld Flange — covering their structural characteristics, connection mechanisms, and applicable scenarios. We aim to assist with design, procurement selection, and field application.
I. Welding Neck Flange
Distinctive Features:
The most prominent feature of the welding neck flange is its tapered, high hub with a smooth transition between the hub and the flange disc. The flange sealing face connects to the larger end of the hub, while the smaller end aligns with the pipe for butt welding, creating an overall horn-like structure. This tapered hub design provides excellent reinforcement.
Distinctive Features:
The most prominent feature of the welding neck flange is its tapered, high hub with a smooth transition between the hub and the flange disc. The flange sealing face connects to the larger end of the hub, while the smaller end aligns with the pipe for butt welding, creating an overall horn-like structure. This tapered hub design provides excellent reinforcement.
Weld Neck Flange Product and Installation Demonstration
Connection Method:
Connection to the piping system is achieved through butt welding. The pipe end and the flange hub end are beveled for a full-penetration circumferential butt weld. The weld seam can be quality-controlled via radiographic inspection. After welding, the flange and pipe form an integrated continuous structure.
Connection to the piping system is achieved through butt welding. The pipe end and the flange hub end are beveled for a full-penetration circumferential butt weld. The weld seam can be quality-controlled via radiographic inspection. After welding, the flange and pipe form an integrated continuous structure.
Applicable Conditions:
Welding neck flanges are suitable for high-pressure, high-temperature, and severely fluctuating temperature/pressure conditions. Due to the hub's ability to effectively disperse stress concentrations, they are particularly ideal for piping systems handling flammable, explosive, or toxic/hazardous media. They are the preferred connection component for high-grade pipelines in petrochemical, thermal power, and nuclear industries. Typically used for pressures above PN 2.5 MPa, they are applicable across a wide temperature range, from cryogenic -196°C up to 800°C.
Welding neck flanges are suitable for high-pressure, high-temperature, and severely fluctuating temperature/pressure conditions. Due to the hub's ability to effectively disperse stress concentrations, they are particularly ideal for piping systems handling flammable, explosive, or toxic/hazardous media. They are the preferred connection component for high-grade pipelines in petrochemical, thermal power, and nuclear industries. Typically used for pressures above PN 2.5 MPa, they are applicable across a wide temperature range, from cryogenic -196°C up to 800°C.
II. Slip-on Flange
Distinctive Features:
Slip-on flanges have a flat, disc-like appearance with uniform thickness and lack the high hub structure of welding neck flanges. The inner diameter of the flange is slightly larger than the pipe's outer diameter, allowing the pipe to slip through. The overall structure is relatively less robust and rigid compared to welding neck flanges.
Distinctive Features:
Slip-on flanges have a flat, disc-like appearance with uniform thickness and lack the high hub structure of welding neck flanges. The inner diameter of the flange is slightly larger than the pipe's outer diameter, allowing the pipe to slip through. The overall structure is relatively less robust and rigid compared to welding neck flanges.
Slip-on Flange Product and Installation Demonstration
Connection Method:
Connection is achieved via fillet welding, typically requiring two welds: an inner fillet weld between the pipe end and the inner side of the flange disc, and an outer fillet weld between the outer side of the flange disc and the pipe's outer wall. Welding volume is small, operation is straightforward, and no special beveling is required.
Connection is achieved via fillet welding, typically requiring two welds: an inner fillet weld between the pipe end and the inner side of the flange disc, and an outer fillet weld between the outer side of the flange disc and the pipe's outer wall. Welding volume is small, operation is straightforward, and no special beveling is required.
Applicable Conditions:
Slip-on flanges are widely used in low to medium pressure conditions, typically for systems below PN 2.5 MPa and often in ambient temperatures or environments not exceeding 300°C. Suitable for water, air, inert gases, and some non-highly corrosive chemicals. They are extremely common in civil construction water supply/drainage, HVAC, and general industrial circulating water systems.
Slip-on flanges are widely used in low to medium pressure conditions, typically for systems below PN 2.5 MPa and often in ambient temperatures or environments not exceeding 300°C. Suitable for water, air, inert gases, and some non-highly corrosive chemicals. They are extremely common in civil construction water supply/drainage, HVAC, and general industrial circulating water systems.
III. Threaded Flange
Distinctive Features:
Threaded flanges are similar in appearance to slip-on flanges—flat disc-shaped, without a hub. Their key difference lies in the internally machined pipe threads (typically tapered NPT or parallel BSP threads). Sealing faces are usually flat or raised. The overall structure is compact with small axial dimensions.
Distinctive Features:
Threaded flanges are similar in appearance to slip-on flanges—flat disc-shaped, without a hub. Their key difference lies in the internally machined pipe threads (typically tapered NPT or parallel BSP threads). Sealing faces are usually flat or raised. The overall structure is compact with small axial dimensions.
Thread Flange Product and Installation Demonstration
Connection Method:
Threaded flanges connect to externally threaded pipes via screwing, eliminating the need for welding. Installation involves simply screwing the flange onto the pipe end, sometimes using sealant or PTFE tape to enhance sealing. In special cases, a combination of threading plus seal welding may be used.
Threaded flanges connect to externally threaded pipes via screwing, eliminating the need for welding. Installation involves simply screwing the flange onto the pipe end, sometimes using sealant or PTFE tape to enhance sealing. In special cases, a combination of threading plus seal welding may be used.
Applicable Conditions:
Threaded flanges are particularly suitable for locations where hot work is prohibited, such as flammable/explosive environments, existing plant renovation projects, and field construction sites. They are also widely used in high-pressure hydraulic systems, high-pressure gas lines, fire protection systems, and some temporary pipelines. Limited by thread machining accuracy, they are generally suitable for small diameters (DN150 and below) and pressures up to PN 25 MPa, but temperatures should not exceed 250°C to prevent thread creep and relaxation.
Threaded flanges are particularly suitable for locations where hot work is prohibited, such as flammable/explosive environments, existing plant renovation projects, and field construction sites. They are also widely used in high-pressure hydraulic systems, high-pressure gas lines, fire protection systems, and some temporary pipelines. Limited by thread machining accuracy, they are generally suitable for small diameters (DN150 and below) and pressures up to PN 25 MPa, but temperatures should not exceed 250°C to prevent thread creep and relaxation.
IV. Socket Weld Flange
Distinctive Features:
Socket weld flanges appear similar to slip-on flanges—flat disc-shaped. Their structural uniqueness lies in the flange bore: the section near the outer side of the flange is a straight hole slightly larger than the pipe's outer diameter; near the inner face of the flange, a stepped shoulder creates a socket recess. The flange sealing face is located on this inner end face.
Distinctive Features:
Socket weld flanges appear similar to slip-on flanges—flat disc-shaped. Their structural uniqueness lies in the flange bore: the section near the outer side of the flange is a straight hole slightly larger than the pipe's outer diameter; near the inner face of the flange, a stepped shoulder creates a socket recess. The flange sealing face is located on this inner end face.
Socket Weld Flange Product and Installation Demonstration
Connection Method:
During connection, the pipe is inserted into the socket until it contacts the bottom, then retracted approximately 1.5-2 mm to allow for welding thermal expansion. A single fillet weld is applied at the junction of the flange's outer end face and the pipe's outer wall. The weld metal fills the annular space between the pipe's outer wall and the flange bore; no beveling is required.
During connection, the pipe is inserted into the socket until it contacts the bottom, then retracted approximately 1.5-2 mm to allow for welding thermal expansion. A single fillet weld is applied at the junction of the flange's outer end face and the pipe's outer wall. The weld metal fills the annular space between the pipe's outer wall and the flange bore; no beveling is required.
Applicable Conditions:
Socket weld flanges are primarily used for small-bore, high-pressure piping systems, typically for pipe diameters DN50 and below, with pressure ratings up to PN 42 MPa. Commonly applied in high-pressure chemical, pharmaceutical refining, food-grade piping, and high-temperature/pressure steam systems. The socket structure provides good alignment and additional support, making it particularly suitable for clean piping systems requiring high medium purity and avoidance of stagnant fluid dead zones.
Socket weld flanges are primarily used for small-bore, high-pressure piping systems, typically for pipe diameters DN50 and below, with pressure ratings up to PN 42 MPa. Commonly applied in high-pressure chemical, pharmaceutical refining, food-grade piping, and high-temperature/pressure steam systems. The socket structure provides good alignment and additional support, making it particularly suitable for clean piping systems requiring high medium purity and avoidance of stagnant fluid dead zones.
Technical Commentary: Socket weld flanges combine the high strength of welding neck flanges with the easy installation features of threaded flanges. Their fillet weld is located on the pipe's outer surface, resulting in a smooth inner bore free of weld residue—especially suitable for applications demanding high internal cleanliness. However, the gap inherent in the socket structure may lead to crevice corrosion, requiring careful consideration in highly corrosive media.
Comprehensive Comparison and Selection Recommendations
In practical engineering, each of the four flange types has its irreplaceable application niche. Selection decisions should follow these principles:
In practical engineering, each of the four flange types has its irreplaceable application niche. Selection decisions should follow these principles:
Diameter & Pressure Dimension: For large diameters and high pressures, welding neck flanges are preferred; for small-to-medium diameters and moderate pressures, slip-on flanges are suitable; for small diameters and high pressures, socket weld flanges are appropriate; for small diameters and hot-work prohibited conditions, threaded flanges are applicable.
Medium Characteristic Dimension: For flammable/explosive, highly toxic, or radioactive media, welding neck flanges are mandatory; for generally harmless media, slip-on flanges are acceptable; for clean media, socket weld flanges are preferred; for corrosive media, the flange material must be considered alongside the type selection.
Construction Condition Dimension: When on-site hot work is prohibited or for field operations, threaded flanges offer irreplaceability; under good shop prefabrication conditions, the quality of welding neck and socket weld flanges is easier to control.
It is crucial to emphasize that flange connection is a systemic issue. Beyond the flange itself, appropriate gaskets, fasteners, and installation procedures must be selected in conjunction. Negligence in any aspect can compromise the entire sealing system's reliability.
Long-term engineering practice often holds that flange selection is "30% selection, 70% installation." Even with optimal design-stage choices, non-compliant field installation and bolting procedures can hardly guarantee sealing performance. Therefore, it is recommended to employ torque-controlled tightening techniques for critical services and establish a full lifecycle management file for flange connections.
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