Flange types: weld neck, slip-on, blind, socket weld, lap joint and more
Deskely Completions team Singapore
A flange joins two lengths of pipe, or a pipe to a valve, vessel or pump, with a gasket between two faces and bolts pulling them together. Flanged joints can be taken apart, which is why they are used wherever equipment needs to be removed, isolated or tested.
This guide covers the main pipe flange types, flange faces, ASME pressure classes, gaskets and bolting, and how flanged joints are managed through construction, testing and commissioning.
- Weld neck flanges are the strongest and the default for high pressure, high temperature and cyclic service.
- Slip-on, socket weld, threaded and lap joint flanges trade strength for cost or convenience.
- Raised face (RF), flat face (FF) and ring type joint (RTJ) faces need matching gaskets.
- ASME B16.5 covers classes 150 to 2500 up to NPS 24; B16.47 covers larger sizes.
Flange types at a glance
| Flange | How it attaches | Strengths | Typical use |
|---|---|---|---|
| Weld neck (WN) | Butt welded to the pipe through a tapered hub | Strongest, good in fatigue, weld can be radiographed | High pressure, high temperature, cyclic and critical service |
| Slip-on (SO) | Slides over the pipe, fillet welded inside and out | Cheap, easy to align | Lower pressure utility and water lines |
| Socket weld (SW) | Pipe inserted into a socket, fillet welded | Smooth bore, simple for small sizes | Small bore, typically NPS 2 and below |
| Threaded (screwed) | Screwed onto threaded pipe | No welding | Small, low-pressure lines where welding is not allowed |
| Lap joint (LJ) | Loose flange over a welded stub end | Rotates for bolt alignment, flange can be cheaper material | Stainless or alloy lines, joints opened often |
| Blind (BL) | Bolted over the end, no bore | Closes a line or nozzle | Line ends, test points, future connections |
| Orifice flange | Weld neck pair with pressure tappings | Holds an orifice plate for flow measurement | Flow metering runs |
Weld neck flanges
A weld neck flange has a long tapered hub that is butt welded to the pipe. The taper spreads the stress from the flange into the pipe wall, so weld necks handle high pressure, temperature swings and vibration best. The butt weld can be radiographed. The bore of the flange is matched to the pipe schedule, so it must be ordered to the same wall thickness as the pipe; see the pipe schedule chart.
Long weld neck flanges extend the hub into a nozzle and are used on vessels and columns.
Slip-on, socket weld and threaded flanges
Slip-on flanges slide over the pipe and are fillet welded on both sides. They cost less and are easier to fit than weld necks, but are weaker under pressure and much weaker in fatigue, so specifications usually limit them to lower classes and non-cyclic service.
Socket weld flanges take the pipe into a socket and are fillet welded at the back. A small gap, typically about 1.6 mm (1/16 inch), is left at the bottom of the socket before welding so the weld does not crack as it cools. They are used on small bore piping.
Threaded flanges screw onto the pipe and need no welding, which makes them useful where hot work is not allowed. They are limited to small sizes and lower pressures, and are often seal welded or avoided in hydrocarbon service.
Lap joint, blind and other flanges
Lap joint flanges sit loose behind a stub end that is butt welded to the pipe. Because the flange never touches the fluid, it can be carbon steel on a stainless line, and it can rotate to line up bolt holes. Good where joints are opened often.
Blind flanges close the end of a pipe, valve or nozzle. They carry the full pressure on their face and are used at line ends, for future tie-ins and to isolate sections for a pressure test.
Spectacle blinds, spades and spacers (ASME B16.48) fit between two flanges to isolate a line positively. On a project they are listed in the test pack and tracked until they are back in their operating position.
Reducing flanges join pipes of two sizes, and swivel ring flanges (common in subsea and offshore) ease alignment.
Flange faces
| Face | Description | Gasket | Typical use |
|---|---|---|---|
| Raised face (RF) | A raised ring around the bore, inside the bolts: 1.6 mm high in classes 150 and 300, 6.4 mm in class 400 and above | Spiral wound, sheet | Most process piping |
| Flat face (FF) | Flat across the whole face | Full-face sheet gasket | Cast iron, plastic and low-pressure equipment flanges |
| Ring type joint (RTJ) | A groove machined into the face for a metal ring | R, RX or BX metal ring | High pressure and high temperature, wellheads |
| Tongue and groove, male and female | Matching raised and recessed faces | Confined flat gasket | Where the gasket must be held in place |
Never bolt a raised face flange to a flat face cast iron flange without changing one of them: tightening rocks the faces and can crack the cast flange.
Flange pressure classes (ASME B16.5)
Flanges to ASME B16.5 come in seven pressure classes: 150, 300, 400, 600, 900, 1500 and 2500, for sizes NPS 1/2 to NPS 24. Larger flanges, NPS 26 to 60, are covered by ASME B16.47 Series A and Series B, which do not bolt to each other. In Europe, EN 1092-1 flanges use PN ratings instead.
The class is not a pressure. The allowable pressure depends on the material and the temperature, and falls as temperature rises. For carbon steel (material group 1.1) at ambient temperature the ratings are:
| Class | Maximum pressure at -29 to 38 °C (group 1.1) |
|---|---|
| 150 | 19.6 bar (285 psi) |
| 300 | 51.1 bar (740 psi) |
| 600 | 102.1 bar (1,480 psi) |
| 900 | 153.2 bar (2,220 psi) |
| 1500 | 255.3 bar (3,705 psi) |
| 2500 | 425.5 bar (6,170 psi) |
Always check the pressure-temperature table in ASME B16.5 for the actual material and design temperature.
Gaskets and bolting
- Spiral wound gaskets (ASME B16.20): metal windings with a graphite or PTFE filler and inner and outer rings, the usual gasket for raised face flanges.
- Ring joint gaskets (ASME B16.20): solid metal rings for RTJ flanges, softer than the groove so they deform into it.
- Non-metallic sheet gaskets (ASME B16.21): for flat face and low-pressure joints.
- Kammprofile and other metal-backed gaskets: for exchangers and demanding services.
- Stud bolts and nuts: commonly ASTM A193 B7 studs with A194 2H nuts in carbon steel service, with other grades for low temperature, high temperature and corrosive duties.
Bolted joint assembly (ASME PCC-1)
Most flange leaks come from how the joint was made up, not from the flange. ASME PCC-1, the guideline for pressure boundary bolted flange joint assembly, covers cleaning and inspecting the faces, the right gasket, lubricating the studs, tightening to a target bolt load in several passes using a star (cross) pattern or an alternative pattern, and checking the result. It also sets out training and qualification for joint assemblers.
The target torque or bolt tension comes from the project's bolting procedure for each size, class, gasket and bolt material. Critical joints are often tightened by hydraulic tensioning or torque tools and recorded joint by joint.
Flange management on projects
Flanged joints are broken and remade many times between construction and start-up: for flushing, for the pressure test, to fit spades and blinds, to remove instruments and PSVs. Each time is a chance for a joint to be left loose or with the wrong gasket.
Projects manage this with a flange (joint) register: every joint gets a tag, and each make-up records the gasket, the bolting, the tightening method and value, who did it and who checked it. Joints opened for a test are listed in the test pack and must be reinstated and signed off before the pack is closed.
In Deskely Testpack, the joints, spades and blinds broken for a pressure test are listed in the pack and tracked until every one is back in place and signed off.
The Deskely products for this
Frequently asked questions
What are the main types of flanges?
Weld neck, slip-on, socket weld, threaded, lap joint and blind flanges, plus orifice, reducing and long weld neck flanges for special uses.
What is the difference between a weld neck and a slip-on flange?
A weld neck is butt welded through a tapered hub and is the strongest flange, suitable for high pressure and cyclic service. A slip-on slides over the pipe and is fillet welded; it is cheaper but weaker, especially in fatigue.
What is a blind flange used for?
To close the end of a pipe, valve or nozzle, for line ends, future tie-ins and isolating a section for a pressure test.
What does RF mean on a flange?
Raised face: a raised ring on the flange face where the gasket sits. It is the most common face type in process piping.
What is a class 150 flange rated for?
It depends on material and temperature. For carbon steel (group 1.1) at ambient temperature, class 150 is rated 19.6 bar (285 psi), falling as temperature rises.
What standard covers pipe flanges?
ASME B16.5 for NPS 1/2 to 24 in classes 150 to 2500, ASME B16.47 for NPS 26 to 60, and EN 1092-1 for PN-rated flanges in Europe.
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