NPT Threaded Flanges
NPT threaded flanges are ASME B16.5 pipe flanges with a tapered NPT (ASME B1.20.1) thread in the bore, so they screw straight onto threaded pipe without welding. India works mainly to BSP threads, so stockists here rarely keep NPT flanges. We manufacture them to order and supply them just as a stockist would: B16.5 dimensions, gauged NPT threads, marked, with mill test certificates, ready to fit.
Also called: Screwed flange · THD flange · Female NPT (FNPT) flange · ANSI threaded flange · NPT pipe flange
What it is
A threaded (screwed) flange has the same outside diameter, bolt circle and facing as a B16.5 slip-on or weld-neck flange of the same size and pressure class, so it bolts to any matching B16.5 flange, valve or nozzle. The difference is the bore: it carries a female NPT thread with a 1:16 taper that seals on the thread itself, using PTFE tape or thread sealant. Where the service demands it, the joint can also be seal-welded.
NPT threaded flanges turn up on imported skids and machines, on US and Middle East project specifications, in fire-fighting and utility lines, and wherever welding on site is not allowed. Indian piping mostly uses BSP threads (IS 554 / ISO 7) and welded flanges, so stockists keep BSP and slip-on or weld-neck stock. Buyers who need an NPT flange usually end up offered the wrong thread, or waiting weeks for an import.
We make them the way the standard expects: forged blanks in ASTM A105 or A182 (B16.5 flanges must be forgings, not plate), heat treated to the material specification, CNC turned to B16.5 dimensions with a serrated raised-face finish, bolt holes drilled on CNC, and the NPT thread cut to taper on CNC. Every thread is checked with an L1 plug gauge for correct hand-tight engagement.
Each flange is marked to MSS SP-25 practice (size, class, material grade, NPT, B16.5 and heat number), supplied with an EN 10204 3.1 mill test certificate, rust-protected and face-capped. In short, it arrives exactly as it would from a stockist's shelf, ready to fit.
Typical applications
- Imported equipment and skids built to US standards
- Oil, gas, petrochemical and refinery utility lines
- Fire-fighting and sprinkler systems
- Compressed air, water, steam-condensate and instrument lines
- Sites where hot work (welding) is not permitted
- Replacement flanges for existing NPT piping
- OEM machines exported to the USA and Middle East
Materials & grades
- Carbon steel ASTM A105 / A105N
- Low-temperature carbon steel ASTM A350 LF2
- Stainless steel ASTM A182 F304 / F304L / F316 / F316L
- Alloy steel A182 F11 / F22 and duplex F51 / F53 on request
- Hot-dip galvanised or zinc-plated carbon steel on request
Points to consider
- Confirm NPT and not BSPT: they look alike but have different thread angles and, in several sizes, different pitches, so they will not seal together.
- Give size, pressure class, facing and material grade, e.g. 2" 150# RF THD NPT A105.
- Threaded flanges suit moderate pressure and temperature. For high-temperature cyclic service or heavy vibration, use weld-neck or specify a seal weld.
- Keep flange and pipe materials compatible (e.g. A105 on carbon steel pipe, F316L on SS 316L pipe) to avoid galvanic corrosion and galling.
- For galvanised flanges, threads are chased after galvanising so the fit is not lost to the zinc layer.
- Order matching gaskets and stud bolts with the flanges so the joint arrives complete.
Class 150 NPT threaded flange dimensions
Mating dimensions per ASME B16.5. Outside diameter, bolt circle and bolting are the same as slip-on and weld-neck flanges of the same size and class.
| Size | NPT threads / inch | Flange OD mm (in) | Bolt circle mm (in) | Bolts |
|---|---|---|---|---|
| 1/2" | 14 | 88.9 (3.50) | 60.3 (2.38) | 4 × 1/2" |
| 3/4" | 14 | 98.4 (3.88) | 69.9 (2.75) | 4 × 1/2" |
| 1" | 11.5 | 108.0 (4.25) | 79.4 (3.12) | 4 × 1/2" |
| 1 1/4" | 11.5 | 117.3 (4.62) | 88.9 (3.50) | 4 × 1/2" |
| 1 1/2" | 11.5 | 127.0 (5.00) | 98.4 (3.88) | 4 × 1/2" |
| 2" | 11.5 | 152.4 (6.00) | 120.7 (4.75) | 4 × 5/8" |
| 2 1/2" | 8 | 177.8 (7.00) | 139.7 (5.50) | 4 × 5/8" |
| 3" | 8 | 190.5 (7.50) | 152.4 (6.00) | 4 × 5/8" |
| 4" | 8 | 228.6 (9.00) | 190.5 (7.50) | 8 × 5/8" |
| 6" | 8 | 279.4 (11.00) | 241.3 (9.50) | 8 × 3/4" |
Class 300 NPT threaded flange dimensions
| Size | NPT threads / inch | Flange OD mm (in) | Bolt circle mm (in) | Bolts |
|---|---|---|---|---|
| 1/2" | 14 | 95.3 (3.75) | 66.7 (2.62) | 4 × 1/2" |
| 3/4" | 14 | 117.3 (4.62) | 82.6 (3.25) | 4 × 5/8" |
| 1" | 11.5 | 124.0 (4.88) | 88.9 (3.50) | 4 × 5/8" |
| 1 1/4" | 11.5 | 133.4 (5.25) | 98.4 (3.88) | 4 × 5/8" |
| 1 1/2" | 11.5 | 155.4 (6.12) | 114.3 (4.50) | 4 × 3/4" |
| 2" | 11.5 | 165.1 (6.50) | 127.0 (5.00) | 8 × 5/8" |
| 2 1/2" | 8 | 190.5 (7.50) | 149.4 (5.88) | 8 × 3/4" |
| 3" | 8 | 209.6 (8.25) | 168.3 (6.62) | 8 × 3/4" |
| 4" | 8 | 254.0 (10.00) | 200.2 (7.88) | 8 × 3/4" |
| 6" | 8 | 317.5 (12.50) | 269.9 (10.62) | 12 × 3/4" |
Nominal values for reference. Flange thickness, hub and thread length are per ASME B16.5; always confirm against the current standard and your piping specification.
NPT vs BSPT vs BSP: why Indian stock doesn't fit
| NPT | BSPT (R / Rc) | BSP parallel (G) | |
|---|---|---|---|
| Standard | ASME B1.20.1 | ISO 7-1 / IS 554 | ISO 228-1 |
| Thread angle | 60° | 55° | 55° |
| Taper | 1:16 | 1:16 | None (parallel) |
| Seals on | Thread + sealant | Thread + sealant | Gasket / O-ring face |
| Threads per inch, 1/2" | 14 | 14 | 14 |
| Threads per inch, 1" and 2" | 11.5 | 11 | 11 |
| Threads per inch, 3" | 8 | 11 | 11 |
| Common in | USA, Middle East, imported equipment | India, UK, Europe, Asia | Hydraulics, instruments |
Even where the pitch matches (1/2" and 3/4"), the 60° and 55° thread forms do not seal together. Full thread data for every size is in our free Thread Finder.
How to specify an NPT threaded flange
| Detail | Example |
|---|---|
| Size | 2" (DN 50) |
| Pressure class | 150# (Class 150) |
| Facing | RF (raised face) |
| Type and thread | Threaded, female NPT to ASME B1.20.1 |
| Material | ASTM A105 (or A182 F316L) |
| Standard | ASME B16.5 |
| Documents | EN 10204 3.1 MTC |
| Written in full | 2" 150# RF THD NPT FLANGE, ASTM A105, ASME B16.5, MTC 3.1, qty 20 |
Frequently asked questions
Why are NPT threaded flanges hard to find in India?
Indian piping mostly uses BSP threads (IS 554) and welded flanges, so stockists keep BSP and slip-on or weld-neck stock. NPT threaded flanges are mainly needed for imported equipment and US-standard projects, so they are usually made to order.
Are your flanges the same as stockist flanges?
Yes. They are machined from forged blanks to ASME B16.5 dimensions with NPT threads to ASME B1.20.1, gauged, marked with size, class, grade and heat number, and supplied with a 3.1 mill test certificate, ready to fit.
How quickly can you supply?
Small quantities of common sizes in A105 or SS 316 are typically ready in one to two weeks, depending on size, class and material availability. Tell us your required date when you enquire.
Is the thread on a threaded flange male or female?
Female (FNPT). The flange screws onto male-threaded (MNPT or MPT) pipe or nipples. If you need a flange with a male NPT spigot, for example as an adaptor, we can make it to your drawing.
What pressure can a Class 150 threaded flange take?
For A105 carbon steel, Class 150 is rated about 19.6 bar (285 psi) at ambient temperature, and Class 300 about 51 bar (740 psi). Ratings fall as temperature rises, per the ASME B16.5 pressure-temperature tables.
Can NPT and BSPT threads be connected?
No. They have different thread angles (60° vs 55°) and, in several sizes, different pitches, so the joint will leak. Use the right flange or a proper adaptor.
Do threaded flanges bolt to normal flanges?
Yes. A B16.5 threaded flange has the same OD, bolt circle and bolting as slip-on and weld-neck flanges of the same size and class, so it mates with any B16.5 flange, valve or equipment nozzle.
Can you also supply BSPT or BSP threaded flanges?
Yes. The same flange can be made with a BSPT (R/Rc) or parallel BSP (G) thread, or to DIN, EN 1092-1 or IS 6392 drilling, when your specification calls for it.