Article
A flange joins two lengths of pipe, or a pipe to a valve, pump or vessel, with a bolted joint that can be taken apart. The flanges on both sides of a joint must match in size, class and facing, but the way each flange is attached to its pipe can differ. That attachment is what the flange "type" describes.
Weld neck flange
A weld neck flange has a long tapered hub that is butt welded to the pipe. The taper carries stress from the flange into the pipe wall gradually, which makes it the strongest type and the best for high pressure, high or fluctuating temperature, and cyclic loading. The butt weld can be fully radiographed. It is the default in refinery and petrochemical process piping. When ordering, the bore must match the schedule of the pipe it will be welded to.
Slip-on flange
A slip-on flange slides over the pipe and is fillet welded inside and out. It is cheaper to buy and easier to align than a weld neck flange, but its strength under internal pressure is roughly two-thirds of a weld neck, and its fatigue life is much lower. It suits low-pressure utility lines, water and cooling systems.
Blind flange
A blind flange is a solid plate with no bore. It closes the end of a pipe, a nozzle or a valve, and allows the line to be opened later for inspection or extension. Because it takes the full pressure load across its face, a blind flange is subject to the highest stress of any type in its class.
Socket weld flange
The pipe sits in a socket in the flange and is fillet welded on the outside. Used for small-bore, high-pressure lines, usually 2 inches and below. A small gap is left at the bottom of the socket before welding to allow for expansion. Not recommended where crevice corrosion is a concern, because of the gap left behind the weld.
Threaded flange
Screwed onto a threaded pipe, with no welding. Useful where welding is not allowed, such as in explosive atmospheres, and for small-bore utility lines. Threads are a leak path and a weak point, so threaded flanges are not used for high temperature, high pressure or cyclic service.
Lap joint flange
A loose backing flange that sits over a stub end welded to the pipe. The flange never touches the process fluid, so it can be made of cheaper carbon steel while the stub end matches an expensive pipe material. It also rotates freely, which makes bolt-hole alignment easy. Used in stainless and alloy systems that are dismantled often.
Compared
| Type | Attachment | Strength | Typical use |
|---|---|---|---|
| Weld neck | Butt weld | Highest | High pressure and temperature, cyclic service |
| Slip-on | Two fillet welds | Moderate | Low-pressure utilities |
| Blind | Bolted, no pipe | Takes full pressure | Closing lines and nozzles |
| Socket weld | One fillet weld | Good in small bore | Small-bore high pressure |
| Threaded | Screwed | Lowest | Where welding is not allowed |
| Lap joint | Loose, over a stub end | Moderate | Alloy systems, frequent dismantling |
What to put on the enquiry
Type, size, class, facing (RF, FF or RTJ), material grade, and for weld neck flanges the pipe schedule. If the flange is going to a non-standard vessel or pump nozzle, include the drawing.
See weld neck flanges, slip-on flanges and the full flange range.
Frequently asked questions
Which flange type is the strongest?
The weld neck flange, thanks to its tapered hub and butt-weld joint.
What is the cheapest flange type?
The slip-on flange is usually the most economical in buy price for low-pressure service.
Which standard covers flanges up to 24 inch?
ASME B16.5 covers NPS 1/2" to 24"; ASME B16.47 covers larger sizes.
Key takeaways
- Weld neck for severe service; slip-on for economy; blind to close lines.
- Socket weld and threaded for small bores; lap joint for frequent dismantling.
- Specify type, size, class, facing, bore, material and standard.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.