A flange spec sheet often lists a type — slip-on, weld-neck, blind — as if it were a minor detail alongside nominal bore and pressure class. It isn't. The flange type determines how the fitting attaches to the pipe and how it behaves under pressure and cyclic load, and getting it wrong is expensive to fix once a line is already welded and pressure-tested.
What a flange actually does
A flange is a connective fitting that joins two lengths of pipe, or a pipe to a valve or piece of equipment, using a bolted joint rather than a permanent weld across the whole run. That bolted joint lets a section be opened for maintenance or inspection without cutting the pipe. The flange type describes how the flange itself is attached to the pipe end, and that attachment method is what governs the pressure and stress it can safely handle.
Slip-On Flanges
A slip-on flange slides over the outside of the pipe end and is then fillet-welded at both the inside and the outside edge. It's simpler and generally faster to fit than a weld-neck flange, and easier to align during installation since the pipe doesn't need to be cut to an exact length beforehand. The trade-off is strength: because the weld doesn't carry a smooth, tapered load path into the pipe wall, a slip-on flange is rated for lower-stress service than a weld-neck flange of the same nominal size and standard. It's a common choice for lower-pressure lines and general piping where ease of fitting matters more than maximum pressure rating.
Weld-Neck Flanges
A weld-neck flange has a tapered hub that is butt-welded directly to the pipe end, so the wall thickness transitions gradually from the pipe into the flange body. That gradual transition spreads stress more evenly across the joint, which is why weld-neck flanges are the standard choice for high-pressure piping, cyclic-load service, and critical process lines where a joint failure would be a serious problem. They cost more to fit than slip-on flanges and require the pipe to be prepared and aligned accurately before welding, but for demanding service that extra step is generally worth it.
Blind Flanges
A blind flange is a solid disc with no bore through the centre, bolted onto the open end of a pipe run, a valve outlet, or a vessel nozzle to seal it off completely. Unlike slip-on and weld-neck flanges, it isn't joining two pieces of pipe — it's closing one off. Common uses include capping a line during hydrostatic pressure testing, blanking off a spare connection left for future expansion, and isolating a section of piping during shutdown or maintenance so work can be done safely on the rest of the system.
| Property | Slip-On | Weld-Neck | Blind |
|---|---|---|---|
| Attachment | Slides over pipe end, fillet-welded inside and out | Tapered hub, butt-welded to pipe end | Bolted to an open end — not welded to a pipe run |
| Bore | Through-bore matching pipe ID | Through-bore matching pipe ID | No bore — solid disc |
| Stress transition | Less smooth — lower rated service | Smooth, tapered — higher rated service | Not applicable |
| Typical use | General and lower-pressure piping | High-pressure, cyclic-load, critical process lines | Sealing a line end, pressure testing, future connection points |
| Fitting effort | Simpler, faster to install | Requires precise pipe prep and alignment | Bolted on — no welding to the pipe run itself |
Buyer's rule of thumb: specify weld-neck where pressure, cyclic loading or safety margin matters most; slip-on where the line is lower-pressure and ease of fitting is the priority; and blind wherever a line end needs to be sealed rather than connected onward. The flange type is one input alongside standard, nominal bore and pressure class — all four should be confirmed together, not decided separately.
Specifying flanges for export orders
Flange type alone doesn't fully specify an order. The standard the piping system is built to — ANSI/ASME, DIN, BS or another national standard — along with nominal bore and pressure class need to be confirmed in writing, since flanges built to different standards are generally not interchangeable even at the same nominal size. On our end, incoming material and dimensions are checked against the agreed specification before production or dispatch, and finished flanges are checked again before packing, in line with the same verification process we apply across every product category. Specific certifications vary by product and should be confirmed with us before you commit to an order, rather than assumed from a general spec sheet.
Specifying flanges for an order?
Send your flange type, standard, nominal bore and pressure class and we'll confirm the right specification before you commit.