Standards · Standards worlds
Mixing standards worlds: what fits, what only appears to fit, what does not fit
In the warehouse a Class 150 flange lies next to a PN 16 flange, on site an NPT nozzle meets an R thread, in the bill of materials schedule 40 stands beside 3.6 mm. Much of it can be joined – and still does not hold. This page sorts the most common pairings into three verdicts: fits, appears to fit, does not fit.
- Verdicts
- fits · appears to fit (can be joined, does not hold) · does not fit
- Pairings
- flange · raised face · bolt · pipe · nominal size · thread · material · bevel
- Basis
- catalogue dimensions, stock data layer – no standard tables
- Rules of thumb
- DN is not NPS, PN is not Class · Two turns of thread do not seal
The pairings
| Part A | Part B | Verdict | Reason | More |
|---|---|---|---|---|
| Class 150 (ASME B16.5) | PN 16 (EN 1092-1) | appears to fit | Same nominal size, but outside diameter, bolt circle, number and size of bolts differ in most sizes – the rating curves differ too. "PN 20" is a trade legend, not a standard. | |
| raised face B16.5 (1.6 mm / 6.4 mm) | raised face form B EN 1092-1 | appears to fit | Both are raised faces, but height, diameter and surface finish (AARH in microinch vs Ra/Rz) follow different tables – gasket sizes do not match automatically. | |
| UNC stud bolt (A193 B7) | metric bolt (EN ISO 898, 8.8) | does not fit | Inch thread and metric thread do not mate – bolt hole diameters of Class flanges are laid out for inch bolts. | |
| NPS 2 (60.3 mm) Sch 40 (3.91 mm) | 60.3 × 3.6 (EN 10220 series) | appears to fit | The outside diameter is identical, so the pipes butt – but the wall thicknesses differ (3.91 vs 3.6 mm), so bore and weld preparation must be matched. | |
| NPS 3/4 (26.7 mm), NPS 1 (33.4), NPS 1 1/4 (42.2), NPS 2 1/2 (73.0), NPS 5 (141.3), NPS 12 (323.8) | 26.9 / 33.7 / 42.4 / 76.1 / 139.7 / 323.9 mm (EN 10220 series 1) | does not fit | Six sizes differ in outside diameter – by a few tenths of a millimetre at 3/4, 1, 1 1/4 and 12, by about 2 to 3 mm at 2 1/2 and 5 (73.0 and 141.3 mm exist in EN 10220 only as a further series) – enough for press and compression fittings to miss and for butt welds to need alignment. | |
| DN 50 | NPS 2 | fits | Both name the same pipe (60.3 mm) – up to NPS 12 the number is a name, from NPS 14 it equals the outside diameter in inches – DN stays a name throughout. | |
| NPT 1/2 (ASME B1.20.1) | R 1/2 (EN 10226-1) | appears to fit | Both are tapered 1:16 and have 14 threads per inch at 1/2 inch, so two turns screw in – but the flank angle (60° vs 55°) and the thread form differ: the joint is not tight. Other sizes differ in pitch as well. | |
| G 1/2 (ISO 228-1, parallel) | Rp 1/2 (EN 10226-1, parallel) | appears to fit | Same Whitworth profile and pitch, so they mate – but G seals on a face with a gasket, Rp in the thread against a tapered R: a G into Rp joint seals only with the right partner. | |
| A234 WPB | P265GH (EN 10253-2) | appears to fit | Comparable by strength and weldable to each other, but analysis bands, test scope and certificate differ – and under the PED only the EN grade carries the presumption of conformity. | |
| bevel 37.5° (ASME B16.25) | bevel 30° (EN 10253 / EN ISO 9692-1) | appears to fit | Both weld, but a joint between a 37.5° and a 30° bevel needs its own preparation and a welding procedure that covers the asymmetric groove. |
Why “appears to fit” is the most dangerous verdict
What does not fit shows at assembly: the metric bolt does not pass through the inch hole, the NPS 1 elbow does not sit on the 33.7 mm pipe. What appears to fit shows only in service. At DN 50 the Class 150 flange can be bolted to the PN 16 flange because bolt circle and number of bolts happen to coincide there – but the raised faces have different diameters and roughness, and the pressure-temperature curves of the two flanges are not the same. “PN 20” on the delivery note is the attempt to translate Class 150 into the PN world; there is no standard for it, and whoever looks for one finds the dimensions side by side on EN 1092-1 versus ASME B16.5.
The pipe is the second classic: NPS 2 and DN 50 both have 60.3 mm outside diameter, so the elbow fits the pipe – but schedule 40 has a 3.91 mm wall, the EN series 3.6 mm, and the weld preparation at 37.5° against 30° is not the same. At NPS 3/4, 1, 1 1/4, 2 1/2, 5 and 12 even the outside diameters differ. The standards translator names counterpart and deviation for every nominal size.
Threads: NPT versus R and G
NPT and R are both taper pipe threads with the same taper 1:16 and, in many sizes, almost the same number of threads per inch – and have different flank angles: 60° against 55°. The first turns can be screwed, then it jams, and whatever is tightened anyway does not seal in the thread but somewhere. G is parallel and does not seal in the thread at all but on a face with a gasket; Rp is parallel and seals against a taper R. The basics are explained on Pipe threads; the threads per inch per size are in the thread tables.
| Thread | Standard | Form | Flank angle | Taper | Sealing principle |
|---|---|---|---|---|---|
| R (taper external thread) | EN 10226-1 (ISO 7-1) | external, taper | 55° | 1:16 | sealing in the thread with sealant |
| Rp (parallel internal thread) | EN 10226-1 (ISO 7-1) | internal, parallel | 55° | – | sealing in the thread against a tapered R |
| Rc (taper internal thread) | EN 10226-2 (ISO 7-1) | internal, taper | 55° | 1:16 | sealing in the thread |
| G (parallel, not sealing in the thread) | ISO 228-1 | external and internal, parallel | 55° | – | sealing on a face with a gasket, not in the thread |
| NPT (National Pipe Taper) | ASME B1.20.1 | external and internal, taper | 60° | 1:16 | sealing in the thread with sealant (NPTF dry-seal without) |
Two turns of thread do not seal. An NPT external thread in an R or Rp internal thread engages the first turns because pitch and taper are similar. The flank angle is not – the flanks bear only on edges, the thread does not seal and strips when tightened. The same number of threads is no proof of the same thread; only flank angle and sealing principle decide.
Further reading
The standards-worlds topic: Why standards are replaced · How binding standards are · Standards worlds by region · EN and ASME compared · Standards translator · Materials between the worlds.
From our knowledge base: Standards guide · ASME and ASTM profiles · EN 1092-1 versus ASME B16.5 · ASME flange dimensions · Standards comparison · Inspection documents to EN 10204 · Reading markings · Identifying a component.
Frequently asked questions
Can I bolt a Class 150 flange to a PN 16 flange?
In individual nominal sizes it can be bolted because bolt circle and number of holes happen to coincide – that does not make the pairing tight or permissible: raised faces, bolt dimensions and pressure-temperature curves differ. The dimensions are shown on EN 1092-1 versus ASME B16.5.
What is PN 20?
A trade designation for Class 150 flanges meant to make them look as if they belonged to the PN series. No standard knows it; a “PN 20 flange” is a flange to ASME B16.5 Class 150 or to EN 1759-1 Class 150 – and should be ordered as such.
Does an ASME elbow fit an EN pipe?
At NPS 2 / DN 50 and most sizes from DN 65 yes in outside diameter; the wall thickness is usually different, the weld bevel too. At NPS 3/4, 1, 1 1/4, 2 1/2, 5 and 12 the outside diameters differ – for 2 1/2 and 5 the ASME diameter (73.0 and 141.3 mm) exists in EN 10220 only as a further series, not as series 1. The pairing table and the translator give the figures.
May I mix worlds in one line?
Technically, if every connection has been checked; contractually, if the specification allows it. Under the Pressure Equipment Directive every pressure-retaining part needs its material evidence – see Materials between the worlds. Which world applies is clarified on How binding standards are.
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As of 2 October 2026. Statements taken from forewords and scopes of standards, legal texts (Pressure Equipment Directive, German Betriebssicherheitsverordnung), standards bodies, inspection bodies and technical articles; regional list approved 30 September 2026. No standard text reproduced. Not legal advice.