Slip-On vs Threaded Flange
Dimensionally the same flange in B16.5. The choice is entirely about the joint: two fillet welds, or an NPT thread and no hot work at all.
- Dimensions
- Identical — same B16.5 table
- Joint
- Two fillet welds vs NPT thread
- Hot work needed
- Slip-on yes, threaded no
- Thread standard
- ASME B1.20.1 NPT
Short answer. Dimensionally these are the same flange. ASME B16.5 gives slip-on and threaded flanges one shared set of dimensions, including the same length through hub. The only difference is the bore: one is smooth and gets fillet-welded to the pipe, the other is tapped NPT and screws on. Choose threaded when welding is impossible or forbidden; choose slip-on everywhere else.
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 slip-on and a threaded 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.
Same table, different bore
B16.5 tabulates length through hub Y once for slip-on, threaded and socket weld flanges together. That is not an approximation in this site's data — it is how the standard publishes it. So the table below is simultaneously the slip-on table and the threaded table.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Cl 150 OD | Cl 150 hub | Cl 150 bolting | Cl 300 OD | Cl 300 hub | Cl 300 bolting |
|---|---|---|---|---|---|---|
| NPS 1/2 | 3.5088.90 mm | 0.6215.75 mm | 4 × 1/2" | 3.7595.25 mm | 0.8822.35 mm | 4 × 1/2" |
| NPS 3/4 | 3.8898.55 mm | 0.6215.75 mm | 4 × 1/2" | 4.62117.35 mm | 1.0025.40 mm | 4 × 5/8" |
| NPS 1 | 4.25107.95 mm | 0.6917.53 mm | 4 × 1/2" | 4.88123.95 mm | 1.0626.92 mm | 4 × 5/8" |
| NPS 1 1/4 | 4.62117.35 mm | 0.8120.57 mm | 4 × 1/2" | 5.25133.35 mm | 1.0626.92 mm | 4 × 5/8" |
| NPS 1 1/2 | 5.00127.00 mm | 0.8822.35 mm | 4 × 1/2" | 6.12155.45 mm | 1.1930.23 mm | 4 × 3/4" |
| NPS 2 | 6.00152.40 mm | 1.0025.40 mm | 4 × 5/8" | 6.50165.10 mm | 1.3133.27 mm | 8 × 5/8" |
| NPS 2 1/2 | 7.00177.80 mm | 1.1228.45 mm | 4 × 5/8" | 7.50190.50 mm | 1.5038.10 mm | 8 × 3/4" |
| NPS 3 | 7.50190.50 mm | 1.1930.23 mm | 4 × 5/8" | 8.25209.55 mm | 1.6942.93 mm | 8 × 3/4" |
| NPS 3 1/2 | 8.50215.90 mm | 1.2531.75 mm | 8 × 5/8" | 9.00228.60 mm | 1.7544.45 mm | 8 × 3/4" |
| NPS 4 | 9.00228.60 mm | 1.3133.27 mm | 8 × 5/8" | 10.00254.00 mm | 1.8847.75 mm | 8 × 3/4" |
| NPS 5 | 10.00254.00 mm | 1.4436.58 mm | 8 × 3/4" | 11.00279.40 mm | 2.0050.80 mm | 8 × 3/4" |
| NPS 6 | 11.00279.40 mm | 1.5639.62 mm | 8 × 3/4" | 12.50317.50 mm | 2.0652.32 mm | 12 × 3/4" |
| NPS 8 | 13.50342.90 mm | 1.7544.45 mm | 8 × 3/4" | 15.00381.00 mm | 2.4461.98 mm | 12 × 7/8" |
| NPS 10 | 16.00406.40 mm | 1.9449.28 mm | 12 × 7/8" | 17.50444.50 mm | 2.6266.55 mm | 16 × 1" |
| NPS 12 | 19.00482.60 mm | 2.1955.63 mm | 12 × 7/8" | 20.50520.70 mm | 2.8873.15 mm | 16 × 1 1/8" |
| NPS 14 | 21.00533.40 mm | 3.1279.25 mm | 12 × 1" | 23.00584.20 mm | 3.0076.20 mm | 20 × 1 1/8" |
| NPS 16 | 23.50596.90 mm | 3.4487.38 mm | 16 × 1" | 25.50647.70 mm | 3.2582.55 mm | 20 × 1 1/4" |
| NPS 18 | 25.00635.00 mm | 3.8196.77 mm | 16 × 1 1/8" | 28.00711.20 mm | 3.5088.90 mm | 24 × 1 1/4" |
| NPS 20 | 27.50698.50 mm | 4.06103.12 mm | 20 × 1 1/8" | 30.50774.70 mm | 3.7595.25 mm | 24 × 1 1/4" |
| NPS 24 | 32.00812.80 mm | 4.38111.25 mm | 20 × 1 1/4" | 36.00914.40 mm | 4.19106.43 mm | 24 × 1 1/2" |
Length through hub Y is published once for slip-on, threaded and socket weld flanges.
Where threaded flanges earn their place
No hot work. In an operating plant, a live tank farm, or a classified area, getting a welding permit can take longer than the job — and sometimes it will not be granted at all. A threaded flange turns a welding job into a wrench job. That is the whole argument, and it is often decisive.
The cost is durability. An NPT thread seals on interference between tapered flanks, and it loosens under vibration and thermal cycling. The thread root also cuts into the pipe wall, so threaded connections need a heavier schedule — Schedule 80 as a practical minimum in most specs, and many codes prohibit threading thin-wall pipe outright.
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
- Two fillet welds instead of one butt weld; harder to inspect
- Roughly a third of the fatigue life of a weld neck
Threaded
- No welding, so usable in flammable or explosive areas
- Fast field installation with hand tools
- Not for cyclic, high-temperature or vibrating service — the thread loosens
- Thread root reduces the effective wall; needs thick-wall pipe
What the codes say
ASME B31.3 restricts threaded joints in several ways: they are not permitted where severe cyclic conditions apply, they are limited in size and pressure in many fluid services, and threaded joints in Category M fluid service require seal welding. Check the governing code before assuming a threaded flange is acceptable.
Dimensions: slip-on flanges · threaded flanges · socket weld vs threaded · end connections.
Common questions
Are slip-on and threaded flanges the same size?
Yes. ASME B16.5 publishes one set of dimensions covering slip-on, threaded and socket weld flanges, including the same length through hub. Only the bore differs.
When should I use a threaded flange instead of a slip-on?
When welding is not possible: hazardous areas without a hot work permit, live plant, galvanised pipe that must not be burned, or remote sites without qualified welders.
What pipe schedule does a threaded flange need?
Heavy enough that the thread root leaves adequate wall. Schedule 80 is the practical minimum in most specifications, and thin-wall pipe such as Schedule 10 cannot be threaded at all.
Can a threaded flange be seal welded?
Yes, and some codes require it — ASME B31.3 requires seal welding of threaded joints in Category M fluid service. Seal welding makes the joint non-removable and defeats the reason for choosing threaded in the first place.