A weld-neck (WN) flange under ASME B16.5 has a tapered hub butt-welded to the pipe for a full-bore, full-pressure joint used in process and cyclic-load service. A slip-on (SO) flange has a straight bore that the pipe slides into and is fillet-welded both sides; it costs 30 to 40 percent less, derates 5 to 10 percent below WN and suits utility duty.
Weld-neck vs slip-on flange — side-by-side buyer table
| Attribute | Weld-Neck (WN) | Slip-On (SO) |
|---|---|---|
| Geometry | Tapered hub matched to pipe bore | Straight bore slightly larger than pipe OD |
| Weld type | Single full-penetration butt weld | Two fillet welds (outer + inner) |
| Bevel prep | Required on hub and pipe end | Not required — pipe slides in |
| Bore | Full bore — no flow disturbance | Pipe ID continues into flange bore |
| Pressure rating | Matches parent pipe (full B16.5 class) | ~5-10 % derate vs WN at same class |
| Cyclic-fatigue resistance | High — no abrupt geometry change | Lower — stress concentration at inner weld toe |
| High-temperature service | Recommended — full-strength joint | Acceptable for moderate temperatures only |
| Cost (4" Class 150 CS) | ~Rs 1,300-1,500 | ~Rs 850-1,000 (30-40 % cheaper) |
| Welding labour | Slower — bevel prep, fit-up, root pass, cap | Faster — slide-in, two fillet welds |
| Inspection method | RT / UT typically required on critical | Dye-penetrant — RT not effective on fillets |
| Primary applications | Process, refinery, high-T, cyclic service | Utility water, fire mains, low-pressure plant air |
When to choose weld-neck
For any process-side line at a refinery, petrochemical plant or power station, weld-neck is the default because the full-bore, full-pressure joint matches what ASME B31.3 expects on a process header. For high-temperature service above 200 °C continuous, WN is the safer pick because the single butt weld behaves predictably under thermal cycling. For any line that sees pressure pulsation, vibration or cyclic fatigue (compressor discharge, pump suction headers, reciprocating-equipment piping), WN is required because the SO fillet-weld stress concentration fails early under fatigue.
When to choose slip-on
For utility-water mains, fire-water headers, cooling-tower piping, low-pressure plant air, drainage and other non-critical Class 150 or Class 300 service where the design conditions sit comfortably inside the SO derated allowable and the line is essentially static, slip-on saves real money. Field installation is faster because alignment is easier and bevel prep is not required, and the dye-penetrant inspection regime is simpler. Many plant piping classes explicitly permit SO for Class 150/300 utility service and require WN for everything else.
Welding requirements differ
A WN flange weld is a single full-penetration butt weld at the joint between the flange hub and the pipe — the bevel angles must match (typically 37.5 degrees), the root gap is held to 2 to 3 mm, and the weld is built up in root, hot-pass, fill and cap layers. A SO flange weld is two fillet welds: one between the pipe OD and the flange face at the front, and one between the pipe end and the flange bore at the back. The double fillet is faster to lay but doubles the inspection points, and only the front fillet is exposed for visual inspection without disassembly.
Lifecycle and maintenance trade-offs
WN flange joints are inspectable to radiographic standards and behave predictably for decades under steady process service. SO flange joints save capital but the inner fillet weld is a known fatigue-failure site under thermal cycling — many SO failures in older Indian plants trace back to incorrect inner-fillet sizing, missing inner weld, or thermal-shock loading that the original specifier did not anticipate. For lines that will see cycling, vibration or future-uprating in scope, spend the WN premium up front. For static utility service, the SO saving compounds across the project.
FAQ — weld-neck vs slip-on flange
What is the difference between a weld-neck and slip-on flange?
Is a weld-neck flange always stronger than a slip-on?
How much cheaper is a slip-on flange than a weld-neck?
When is a slip-on flange acceptable in plant piping?
Do I need radiography on weld-neck or slip-on flange joints?
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