Flanges connect pipes, valves, pumps and vessels through a bolted joint instead of a permanent weld, letting engineers assemble and dismantle piping systems without cutting metal. A gasket between two flange faces seals the joint against the process fluid. Choosing the right option among the many types of flanges depends on pressure, temperature, pipe diameter and how often the joint must come apart for maintenance. This guide covers six designs: weld neck, slip-on, blind, socket weld, threaded and lap joint.
What Is a Pipe Flange?
A pipe flange is a fitting attached to a pipe, valve or piece of equipment so two components bolt together instead of being welded directly. Bolts pass through matching holes on each face, drawing the faces together under compressive load. A gasket compresses between them, closing the small gaps that would otherwise let fluid escape. This is most important where equipment needs periodic removal, since unbolting a flange takes far less time than rewelding a joint.
6 Common Types of Flanges
Six flange designs cover most industrial piping requirements, each built around a different connection method and installation effort.
Weld Neck Flanges
Weld neck flanges attach through a single butt weld at a tapered hub, where the thickness reduces gradually to the pipe wall thickness. That taper spreads stress across a wider area instead of concentrating it at one point, so this design appears in high-pressure steam lines. The trade-off is alignment: the pipe bore must match the flange bore closely before welding, or the mismatch creates a flow restriction.
Slip-On Flanges
A slip-on flange slides over the pipe end and is fillet welded on the inside and outside faces. Because the pipe does not need to match the bore exactly, alignment is less demanding than with a weld neck design. Fabricators use this type widely in general piping where pressure and temperature stay moderate, though the fillet welds carry less strength than a single butt weld.
Blind Flanges
A blind flange has no pipe bore. It is a solid disc bolted onto a pipe end, valve outlet or equipment nozzle to seal it completely. Plants use blind flanges to close unused branches, isolate sections during maintenance or leave a connection point ready for future expansion. Since the flange carries the full pressure load across its face, its thickness and rating need close attention on larger diameters.
Socket Weld Flanges
A socket weld flange has a recessed socket into which the pipe is inserted, sealed with a fillet weld around the outside. This suits smaller-diameter piping, generally up to 4 inches, where a compact joint and smooth bore matter more than large sizes. A small gap between the pipe end and socket base allows for thermal expansion.
Threaded Flanges
Threaded flanges connect through matching external threads, with no welding on the joint itself. This suits situations where welding is unsafe, such as hazardous atmospheres or repairs without hot work permits. The connection relies on thread engagement and usually a sealant, so it fits low- to moderate-pressure services only.
Lap Joint Flanges
A lap joint flange works with a separate stub end, which is the part actually welded to the pipe. The flange slips over the stub end and rotates freely, so bolt holes align with the mating flange without rotating the pipe. This suits systems needing frequent dismantling and cases where the stub end uses a corrosion-resistant alloy while the flange uses lower-cost carbon steel.
Types of Flanges Comparison
The table below summarises connection method, main advantage and typical use for each flange type as a quick reference.
Types of Flanges Comparison
The table below summarises connection method, main advantage and typical use for each flange type as a quick reference.
| Flange Type | Connection Method | Main Advantage | Typical Applications |
| Weld Neck | Butt weld | Strong, stress-distributing joint | High-pressure and critical piping |
| Slip-On | Fillet weld | Simple installation | General industrial piping |
| Blind | Bolted closure | Seals ends and outlets | Line ends, nozzles, isolation |
| Socket Weld | Socket with fillet weld | Compact joint, clean bore | Smaller-diameter piping |
| Threaded | Threaded engagement | No on-site welding | Selected low/moderate-duty systems |
| Lap Joint | Stub end plus bolting | Easy alignment and dismantling | Maintenance-intensive systems |
How Do Flange Types Differ?
The six designs differ mainly in how they attach to the pipe and how much stress that attachment carries. Weld neck and socket weld types rely on welding, which holds up better under pressure and temperature extremes than the fillet welds on slip-on flanges or the threads on threaded flanges. Threaded and slip-on flanges install faster, while weld neck flanges demand precise alignment first. Maintenance frequency sets lap joint flanges apart, since their rotating design allows bolt-hole alignment without disturbing the pipe.
How to Choose the Right Flange Type?
Check pressure and temperature. Match the design and pressure class to service conditions.
Consider pipe size and system design. Dimensions and connection type must match the piping system.
Determine whether welding is possible. Site access or hot work restrictions can rule out welded connections.
Consider maintenance requirements. Frequent dismantling often favors a lap joint arrangement.
Evaluate the service environment. Corrosion and material compatibility affect long-term performance.
Verify applicable standards. Confirm ASME, ANSI or project-specific requirements before finalizing flange selection.
Common Applications of Different Flange Types
Oil and gas piping often uses weld-neck flanges on high-pressure lines and blind flanges to isolate sections during turnarounds. Petrochemical plants use socket weld flanges on smaller instrumentation lines. Power generation piping relies on weld neck flanges in steam lines, while marine systems favour lap joint flanges in sections that need regular dismantling.
Common Flange Selection Mistakes to Avoid
Choosing a flange on price alone, without checking pressure and temperature ratings, is a frequent cause of early failure. Overlooking material compatibility between the flange, pipe and gasket can lead to corrosion at the joint. Selecting a bolt pattern that does not match the applicable standard creates fit-up problems.
FAQs
What are the six main types of flanges?
Weld neck, slip-on, blind, socket weld, threaded and lap joint flanges cover most industrial piping needs.
What is the difference between weld neck and slip-on flanges?
Weld neck flanges attach through a butt weld and suit higher-pressure service, while slip-on flanges attach with two fillet welds and install with less alignment effort.
When should a blind flange be used?
Use a blind flange to seal a pipe end, valve outlet or equipment nozzle or to leave a connection point for future expansion.
What is the purpose of a lap joint flange?
A lap joint flange rotates freely around a stub end, allowing bolt-hole alignment without disturbing the pipe.
How do I choose the right flange type for a piping system?
Match the flange to service pressure and temperature, confirm pipe size and welding feasibility, and verify the design against applicable standards.