The type of flange you choose affects how a piping system is installed, how well it handles pressure and temperature, and how easy it is to maintain. With dozens of flange types available across multiple standards, selecting the right one requires understanding each design's strengths and limitations.
This guide covers the most common flange types used in industrial piping systems, along with specialty designs for demanding or unusual applications.
Common Flange Types
Weld Neck Flanges
The weld neck flange is the most widely used type in critical piping systems. It features a long tapered hub that transitions gradually from the flange thickness to the pipe wall thickness, distributing mechanical stress across a larger area. The butt-weld connection provides full penetration, making weld neck flanges suitable for high-pressure, high-temperature, and cyclic-load applications.
Best for: Refineries, power plants, high-pressure steam lines, and any service where integrity and long service life are priorities.
Slip-On Flanges
Slip-on flanges slide over the pipe and are welded on both the inside bore and outside face. They cost less than weld neck flanges and are easier to align during installation. The trade-off is lower fatigue resistance — the double fillet weld is not as strong as a butt weld, so slip-on flanges are limited to lower-pressure, lower-criticality services.
Best for: Water systems, utility piping, HVAC, and general process lines rated at Class 150 or 300.
Blind Flanges
A blind flange is a solid disc with no bore. Its purpose is to close off the end of a pipe, nozzle, or vessel opening. Despite being a simple design, blind flanges experience significant bending stress from pressure acting across the full face, so they are sized carefully for higher pressure classes.
Best for: Pressure testing, isolating unused process connections, and blocking off nozzles on vessels and heat exchangers.
Socket Weld Flanges
Socket weld flanges have a bore slightly larger than the pipe OD, allowing the pipe to insert into a recessed socket. A single fillet weld is applied on the outside. A small gap (approximately 1/16") is intentionally left between the pipe end and the socket bottom to accommodate thermal expansion during welding.
Best for: Small-bore, high-pressure piping (typically 2" and under) in non-corrosive services. Not recommended for corrosive or lethal fluids where crevice corrosion in the socket gap is a concern.
Threaded Flanges
Threaded flanges connect to the pipe via NPT threads rather than welding. This makes installation fast and eliminates the need for hot work in areas where welding is prohibited. However, threads can loosen under cyclic loading and leak if not properly sealed with thread compound or PTFE tape.
Best for: Low-pressure air, water, and utility services; also used in hazardous areas where welding permits are difficult to obtain.
Lap Joint Flanges
Lap joint flanges are used exclusively with a stub end fitting. The flange slides over the pipe and the stub end is butt-welded to the pipe. Because the flange is loose on the pipe, it rotates freely, which makes bolt-hole alignment easy during assembly. The flange itself does not contact the process fluid, so it can be made of lower-cost carbon steel even when the pipe and stub end are stainless or alloy.
Best for: Systems that require frequent disassembly and where expensive alloy material can be saved by using a carbon steel backing flange.
Specialty Flange Types
Orifice Flanges
Orifice flanges are weld neck or slip-on flanges with tapped ports on either side of the bore for differential pressure measurement across an orifice plate. They are supplied in matched pairs and include jack screws to separate the flanges when removing the plate. Orifice flanges conform to ASME B16.36.
Spectacle Blinds
A spectacle blind (also called a spectacle plate) is a figure-8 shaped fitting consisting of a solid blind disc and an open ring connected by a web. Installed between two flanges, the spectacle blind can be flipped to either isolate or open the line — providing visible positive isolation without removing any piping.
Long Weld Neck (LWN) Flanges
Long weld neck flanges have an extended hub that functions as a nozzle extension. They are commonly used on pressure vessel nozzles and heat exchanger heads where the flange must extend through insulation or vessel wall thickness.
Reducing Flanges
Reducing flanges allow a pipe to connect to a larger or smaller flange without installing a separate reducer fitting. The flange has two different bore sizes — one to match the pipe and one to match the mating flange. This reduces fitting count and eliminates potential leak points.
Studding Outlets
Studding outlets (also called instrument flanges or pad flanges) are welded directly to vessel or pipe walls to create branch connections. They eliminate the need for a separate nozzle weld and provide a flanged connection point for instruments, valves, or bypass lines.
Flange Face Types
In addition to the flange body type, the seating surface (face) type matters for gasket selection and sealing performance:
- Raised Face (RF): The most common face type. A raised ring contacts the gasket, concentrating seating force. Used in Classes 150–2500.
- Flat Face (FF): The entire flange face is flat. Required when mating to cast iron or other brittle flanges to avoid cracking from bending stress.
- Ring Type Joint (RTJ): A machined groove holds a solid metal ring gasket. Used in high-pressure, high-temperature services where a soft gasket would be unsuitable.
- Tongue and Groove (T&G): Matching raised and recessed faces lock the gasket in place. Common in heat exchanger applications.
- Male and Female (M&F): Similar to tongue and groove, one flange has a raised face that fits into a recessed face on the mating flange.
Standards That Govern Flange Types
Most industrial piping flanges are manufactured to one of these primary standards:
- ASME B16.5: Pipe flanges and flanged fittings, NPS ½ through NPS 24, Classes 150–2500.
- ASME B16.47: Large diameter steel flanges, NPS 26 through NPS 60, Series A (MSS SP-44) and Series B (API 605).
- ASME B16.36: Orifice flanges.
- MSS SP-44: Steel pipeline flanges, typically used in pipeline applications.
- API 6A: Wellhead and Christmas tree equipment flanges for oil and gas production.
- DIN EN 1092-1: European metric flanges, common in international projects.
Frequently Asked Questions
Which flange type is strongest? Weld neck flanges offer the best strength and fatigue resistance because the butt weld provides full penetration and the tapered hub distributes stress. For critical, high-pressure services, weld neck is the standard choice.
Can I mix flange types in a piping system? Yes, as long as the mating flanges have the same pressure class, face type, and bolt circle dimensions per the applicable standard. Mixing is common — for example, a weld neck flange on one side of a valve and a blind flange on an isolation port.
What is the difference between Series A and Series B in ASME B16.47? Series A derives from MSS SP-44 (pipeline flanges) and Series B derives from API 605 (pressure vessel flanges). They have different bolt patterns and outside diameters for the same NPS and pressure class, and flanges from the two series are not interchangeable.
Conclusion
Choosing the right flange type means matching the design to the service conditions — pressure, temperature, fluid corrosivity, inspection requirements, and maintenance frequency all factor into the decision. When in doubt, the conservative choice is a weld neck flange to the applicable ASME standard.
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