Weld Neck vs Slip-On Flange
The two most-specified B16.5 flange types: identical bolting, completely different joints. What each costs, where each is allowed, and how much length the hub adds.
- Weld type
- Butt weld vs two fillet welds
- Bore
- Matches schedule vs matches pipe OD
- Extra length, Class 150
- 1.71 in average
- Fatigue life
- Weld neck roughly 3× slip-on
Short answer. A weld neck is butt-welded to the pipe through a tapered hub, so its bore matches the pipe bore and the joint is a single radiographable full-penetration weld. A slip-on slides over the pipe OD and takes two fillet welds. The weld neck is stronger in fatigue by roughly a factor of three and is the correct choice for pressure, temperature and cyclic service; the slip-on is cheaper, shorter and easier to fit, and belongs in low-pressure utility work.
ASME B16.5 tabulates one set of outside diameters, thicknesses, bolt circles and bolt patterns per class per size, and every flange type in that class uses them. A weld neck and a slip-on of the same class and size have identical bolting and are fully interchangeable at the joint — the difference is entirely in how each attaches to the pipe.
The bore is the real difference
A weld neck flange is bored to a specific schedule. Order a weld neck for NPS 6 Schedule 40 and its bore is the Schedule 40 bore, so the inside of the joint is smooth and continuous — no step, no crevice, no turbulence. That is why weld necks are mandatory in most specs for erosive, cyclic and hydrogen service.
A slip-on is bored to clear the pipe outside diameter, so one slip-on fits every schedule in that NPS. Convenient for stocking, but the pipe end stops short of the flange face and leaves an internal step, and the inner fillet weld sits right in the flow.
Length through hub
The weld neck hub adds length. Across the sizes B16.5 tabulates, a Class 150 weld neck runs about 1.71 in longer through the hub than a slip-on in the same size — enough to matter in a congested rack, and enough to change a spool dimension if the type is substituted late.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Cl 150 WN hub | Cl 150 SO hub | Difference | Cl 300 WN hub | Cl 300 SO hub | Difference |
|---|---|---|---|---|---|---|
| NPS 1/2 | 1.8847.75 mm | 0.6215.75 mm | +1.26 in | 2.0652.32 mm | 0.8822.35 mm | +1.18 in |
| NPS 3/4 | 2.0652.32 mm | 0.6215.75 mm | +1.44 in | 2.2557.15 mm | 1.0025.40 mm | +1.25 in |
| NPS 1 | 2.1955.63 mm | 0.6917.53 mm | +1.50 in | 2.4461.98 mm | 1.0626.92 mm | +1.38 in |
| NPS 1 1/4 | 2.2557.15 mm | 0.8120.57 mm | +1.44 in | 2.5665.02 mm | 1.0626.92 mm | +1.50 in |
| NPS 1 1/2 | 2.4461.98 mm | 0.8822.35 mm | +1.56 in | 2.6968.33 mm | 1.1930.23 mm | +1.50 in |
| NPS 2 | 2.5063.50 mm | 1.0025.40 mm | +1.50 in | 2.7569.85 mm | 1.3133.27 mm | +1.44 in |
| NPS 2 1/2 | 2.7569.85 mm | 1.1228.45 mm | +1.63 in | 3.0076.20 mm | 1.5038.10 mm | +1.50 in |
| NPS 3 | 2.7569.85 mm | 1.1930.23 mm | +1.56 in | 3.1279.25 mm | 1.6942.93 mm | +1.43 in |
| NPS 3 1/2 | 2.8171.37 mm | 1.2531.75 mm | +1.56 in | 3.1981.03 mm | 1.7544.45 mm | +1.44 in |
| NPS 4 | 3.0076.20 mm | 1.3133.27 mm | +1.69 in | 3.3885.85 mm | 1.8847.75 mm | +1.50 in |
| NPS 5 | 3.5088.90 mm | 1.4436.58 mm | +2.06 in | 3.8898.55 mm | 2.0050.80 mm | +1.88 in |
| NPS 6 | 3.5088.90 mm | 1.5639.62 mm | +1.94 in | 3.8898.55 mm | 2.0652.32 mm | +1.82 in |
| NPS 8 | 4.00101.60 mm | 1.7544.45 mm | +2.25 in | 4.38111.25 mm | 2.4461.98 mm | +1.94 in |
| NPS 10 | 4.00101.60 mm | 1.9449.28 mm | +2.06 in | 4.62117.35 mm | 2.6266.55 mm | +2.00 in |
| NPS 12 | 4.50114.30 mm | 2.1955.63 mm | +2.31 in | 5.12130.05 mm | 2.8873.15 mm | +2.24 in |
| NPS 14 | 5.00127.00 mm | 3.1279.25 mm | +1.88 in | 5.62142.75 mm | 3.0076.20 mm | +2.62 in |
| NPS 16 | 5.00127.00 mm | 3.4487.38 mm | +1.56 in | 5.75146.05 mm | 3.2582.55 mm | +2.50 in |
| NPS 18 | 5.50139.70 mm | 3.8196.77 mm | +1.69 in | 6.25158.75 mm | 3.5088.90 mm | +2.75 in |
| NPS 20 | 5.69144.53 mm | 4.06103.12 mm | +1.63 in | 6.38162.05 mm | 3.7595.25 mm | +2.63 in |
| NPS 24 | 6.00152.40 mm | 4.38111.25 mm | +1.62 in | 6.62168.15 mm | 4.19106.43 mm | +2.43 in |
B16.5 tabulates length through hub for Classes 150 and 300 only; above Class 300 the hub follows the bore and is given per bore in the standard.
Weld neck
- Bore matches the pipe, so no turbulence or crevice at the joint
- Full-penetration butt weld is radiographable
- Best fatigue and thermal-shock performance of any B16.5 flange
- High pressure, high temperature, cyclic loading, hydrogen and lethal service.
Slip-on
- Cheaper than weld neck and tolerant of small alignment errors
- One bore fits every schedule in the size
- Shorter than a weld neck, useful in tight spaces
- Low-pressure, non-cyclic utility and water service.
Cost, honestly
The flange itself is the smaller part of the difference. A weld neck costs more to buy, but the joint needs one weld instead of two, and that weld can be radiographed rather than requiring magnetic particle inspection on two fillets. On a line with NDE requirements the installed cost gap narrows considerably, and on a line without them the slip-on usually wins.
Dimensions: weld neck flanges · slip-on flanges · lap joint vs slip-on · bolt torque.
Common questions
Which is stronger, a weld neck or a slip-on flange?
The weld neck, decisively in fatigue — roughly three times the cycle life. In static pressure the difference is smaller, but the weld neck still carries load better because the tapered hub moves the stress concentration away from the flange face.
Can I use a slip-on flange on high-pressure service?
B16.5 rates slip-on flanges to the same class as weld necks, so the flange is not the limit. Most piping specs still restrict slip-ons to Class 300 and below, and exclude them from cyclic, hydrogen and lethal service, because the two fillet welds are harder to inspect.
Does a weld neck flange need a schedule specified?
Yes. The bore is machined to a schedule, so a weld neck ordered without one cannot be made. A slip-on needs only the NPS.
Are the bolt holes the same on both types?
Identical. Bolt circle, bolt count and bolt diameter are a function of class and size, not of flange type, so the two bolt together without issue.
Which is shorter?
The slip-on, by about 1.71 in through the hub in Class 150. That is often the deciding factor where space is tight.