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What does a flange call-out have to say?
A flange call-out has to say which published system it is written in before anything else on the line can be read. A number on its own is not a specification. PN 16 and Class 150 are designations from different documents, written by different bodies, and a drawing that carries one of them has told you the system as well as the number.
On a Gulf project both systems arrive on the same site, often in the same package. A European mechanical contractor's chilled-water schedule is written in PN and DN. An American oil and gas specification beside it is written in Class and NPS. Both are correct in their own document. What causes trouble is a requisition that has taken the number and lost the document it came from.
Flowsynergy Solutions Limited supplies industrial valves, pipeline fittings and flow-control equipment from Level 41, Emirates Towers, Sheikh Zayed Road, Dubai, for commercial, industrial, HVAC, water, power and oil and gas applications. This page sets out what each of the published documents says it covers, so that a call-out can be read before it is quoted.
What do PN, DN, NPS and Class designate?
All four are designation systems, and each belongs to a particular family of documents. PN and DN are the pair used by the EN 1092 flange standards, whose own titles read "PN designated": the record for BS EN 1092-1:2018 states its scope in "PN designations PN 2,5 to PN 400 and nominal sizes from DN 10 to DN 4000". In plain words, and this is our wording rather than the record's: PN is the nominal pressure designation and DN the nominal size designation. On the ASME side NPS is the nominal pipe size designation, and it is the one the flange titles carry: "NPS 1/2 through NPS 24" and "NPS 26 through NPS 60". Class is the pressure designation on that side, and ASME's own catalogue description of B16.47 shows it in use: the standard "covers pipe flanges sizes NPS 26 through NPS 60. Class 75, 150, 300, 400, 600, and 900 for forged flanges and cast, forged, or plate blind flanges".
None of the four can be read as a working pressure on its own. Pressure and temperature ratings are something the flange documents themselves tabulate: the BS EN 1092-1:2018 record lists "pressure/temperature ratings" among the things the standard specifies, and the BS EN 1092-2:2023 record does the same for cast iron. This page reproduces none of those tables. The terms are set out again in our glossary.
What does EN 1092-1 cover?
EN 1092-1 is the steel flange document in the PN system. The BSI catalogue record, held by the standards distributor Nimonik, for BS EN 1092-1:2018, Flanges and their joints. Circular flanges for pipes, valves, fittings and accessories, PN designated. Steel flanges, published by BSI and issued April 2018 at 152 pages, states that the standard "specifies requirements for circular steel flanges in PN designations PN 2,5 to PN 400 and nominal sizes from DN 10 to DN 4000".
The same BS EN 1092-1:2018 record sets out what the document contains. The standard "specifies the flange types and their facings, dimensions, tolerances, threading, bolt sizes, flange jointing face surface finish, marking, materials, pressure/temperature ratings and approximate flange masses". The record adds that for the purposes of the standard, "flanges" include also lapped ends and collars, and that non-gasketed pipe joints are outside its scope. The 2018 edition supersedes BS 4504-3.1.
One thing the record does not do is tell you which PN to choose. That decision belongs further up the line, with the design conditions, and it is the subject of our guide to valve selection by service condition.
What do ASME B16.5 and B16.47 cover?
The Class system splits its steel flanges by size across two documents. ASME's own catalogue lists B16.5, Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard, at the 2025 edition, and B16.47, Large Diameter Steel Flanges: NPS 26 through NPS 60, Metric/Inch Standard, at the 2025 edition.
On the Class side, a size settles which of the two documents applies, and the published titles say which. ASME shows a 2025 edition against each of them today. The edition belongs on the requisition next to the designation, because the designation is stable and the document behind it is not.
Where the valve itself is the item being bought rather than the flange, the rating of the valve body is a separate published question again, and we have taken it apart in ASME B16.34 pressure and temperature ratings.
| Document | Material it is written for | Range stated in its own record |
|---|---|---|
| BS EN 1092-1:2018 | Steel | PN 2,5 to PN 400, DN 10 to DN 4000 |
| BS EN 1092-2:2023 | Ductile, grey and malleable cast iron | DN 10 to DN 4000, PN 2,5 to PN 100 |
| ASME B16.5 | Pipe flanges and flanged fittings | NPS 1/2 through NPS 24, in its published title |
| ASME B16.47 | Large diameter steel flanges | NPS 26 through NPS 60, in its published title |
| ISO 7005-1:2011 | Steel flanges for industrial and general service piping | A base specification; the series is the purchaser's choice |
Who decides which flange series a project uses?
The purchaser does, and one of the standards' published records says so in as many words. The ISO catalogue record, held by the same distributor, for ISO 7005-1:2011, Pipe flanges, Part 1: Steel flanges for industrial and general service piping systems, states that the standard "establishes a base specification for pipe flanges suitable for general purpose and industrial applications including, but not limited to, chemical process industries, electric power generating industries, petroleum and natural gas industries", and that "it places responsibility for the selection of a flange series with the purchaser".
That sentence is worth quoting in a technical query, because it closes the most common argument on a mixed project. The record also says the standard is "applicable to flanges within facilities engaged in the processing or handling of a wide variety of fluids, including steam, pressurized water and chemical, petroleum, natural gas or related products", which covers most of what a Gulf plant or district cooling scheme carries.
There is a second reason the choice cannot be left implicit. BSI's own product record for the edition of BS EN 1092-1 identical to EN 1092-1:2001 states that "whilst the mating dimensions, the flange and facing types and designations are compatible with those given in ISO 7005-1, it is important to take account of the following differences which exist in EN 1092-1". It lists them: the pressure and temperature ratings "have been reduced in many cases"; the document "also includes PN 63 and PN 100 flanges" beyond "the range of PN 2,5 to PN 40 DIN origin flanges contained in the ISO standard"; and it "permits the use of those grades of ASTM steels which are specified in the ISO standard for use with the ANSI/ASME based flanges". Two documents can agree on dimensions and still differ on what the flange is rated to carry.
That BSI record is an earlier edition, and it is cited here for the comparison it draws rather than for any current figure. The current steel document is the 2018 edition. Confirm both the edition and the series before an order, not after it.
Where does cast iron sit?
Cast iron has its own document, and a line that says only "PN 16 flange" has not said whether it means that one. The publisher record for BS EN 1092-2:2023, Flanges and their joints. Circular flanges for pipes, valves, fittings and accessories, PN designated. Cast iron flanges, issued November 2023 at 64 pages, states that the document "specifies requirements for circular flanges made from ductile, grey and malleable cast iron for DN 10 to DN 4000 and PN 2,5 to PN 100".
The cast iron record's contents list runs parallel to the steel document without being identical: it "specifies the types of flanges and their facings, dimension and tolerances, bolt sizes, surface finish of jointing faces, marking, testing, quality assurance and materials together with associated pressure/temperature ratings". Testing and quality assurance appear here where the steel record lists threading and approximate flange masses.
The cast iron document runs to DN 4000, the same top size as the steel one, so a large line can sit in either. Our page on valves for HVAC and district cooling covers where each body material tends to be specified.
What does a flange standard not settle?
Naming the flange document settles the flange, not the joint. Five things still have to be stated, and none of them is implied by the standard and the number alone. Each row below is open until the requisition closes it, and each is a separate line to write.
| Still open | Why it is not implied |
|---|---|
| Facing type | EN 1092-1's own record lists the flange types and their facings as separate items it specifies |
| Jointing face surface finish | Listed separately again in the same scope sentence |
| Bolt sizes and the material of the bolting | Bolt sizes are in the flange document; the bolting material is a purchase order matter |
| Gasket | Chosen for the facing, the fluid and the duty, not by the flange designation |
| Flange material and its pressure and temperature rating | Materials and p/T ratings are inside the flange document, and the two systems do not set them the same way |
Will a flange to one system bolt up to a flange to the other?
That is decided on the two call-outs, not by a rule that can be published in advance. No record quoted on this page compares an EN 1092 flange with an ASME B16 flange at all. The one comparison any of them draws is BSI's, between the EN 1092-1:2001 edition and ISO 7005-1, and even there the ratings differ where the dimensions are compatible. So the answer turns on both documents, both editions, the facing and the design conditions. Send the two call-outs and the design conditions and it is answered against the documents rather than against a general rule. Our guide to valve selection by standard and service condition sets out where the end connection sits in that sequence.
What arrives with the goods is a separate discipline again. Valve documentation and material test certificates covers the records to ask for, and sourcing documentation covers the rest of the paperwork.
What should an enquiry say?
Six lines close a flange call-out. Send them with the line conditions and an offer can be prepared against the documents rather than against an assumption.
- The system: PN with DN, or Class with NPS. Never a number on its own.
- The standard and the edition: EN 1092-1, EN 1092-2, ASME B16.5 or ASME B16.47, with the year.
- The flange material, and whether the body is steel or cast iron.
- The facing type and the jointing face surface finish required.
- Bolting and gasket requirements, where the project specifies them.
- The design pressure and temperature, which is what the rating has to be read against.
Flowsynergy supplies industrial valves, pipeline fittings and flow-control equipment, with valve selection and technical support behind them. Call +971 4 313 2786 or write to enquiry@flowsynergy.ae, Monday to Friday, 8:30 am to 5:00 pm Dubai time. Choosing a valve supplier in Dubai sets out what else to ask before the purchase order.
Questions buyers ask about flange call-outs
These three are the questions a mixed-standard package raises. Each is answered from the published records quoted above, and where a question turns on a value rather than on a record, the answer says so and sends you to the standard.
Our drawing says PN 16 and the valve datasheet says Class 150. Which is right?
Both may be right in their own document, and that is the problem rather than the answer. PN designations belong to EN 1092-1 for steel and EN 1092-2 for cast iron; Class belongs to the ASME B16 flange standards, split by size between B16.5 for NPS 1/2 through NPS 24 and B16.47 for NPS 26 through NPS 60. Ask which document the project is being built to before the order, because the published record for ISO 7005-1 puts responsibility for selecting a flange series with the purchaser.
Does the edition year really matter if the standard number is the same?
Yes, and the records show why. BS EN 1092-1:2018 supersedes BS 4504-3.1, and BSI's record for the earlier EN 1092-1:2001 edition sets out differences from ISO 7005-1 in pressure and temperature ratings and in which PN designations are included. ASME shows a 2025 edition against both B16.5 and B16.47 today. Name the year on the requisition and the argument does not start.
The schedule says PN 16 ductile iron. Is EN 1092-1 the right standard to name?
Not for the flange itself. BS EN 1092-2:2023 is the document that specifies requirements for circular flanges made from ductile, grey and malleable cast iron for DN 10 to DN 4000 and PN 2,5 to PN 100. EN 1092-1 is the steel document. If the schedule names a cast iron body, the flange call-out should point at the cast iron standard, and the material should be stated separately from the PN number.
Sources: the published records quoted on this page
Read 14 October 2026. Every quotation above comes from one of these six records, and a saved copy of each is kept with this page's working papers.
- The BSI record for BS EN 1092-1:2018, held by the standards distributor Nimonik: the title, the April 2018 issue, 152 pages, the scope and what the standard specifies.
- The BSI record for BS EN 1092-2:2023, same distributor: the title, the November 2023 issue, 64 pages, the cast iron scope and contents.
- The ISO record for ISO 7005-1:2011, same distributor: the base specification scope, the industries named, and the purchaser's responsibility for selecting a series.
- ASME's own catalogue page for ASME B16.5: the published title with its size band, and the 2025 edition shown.
- ASME's own catalogue page for ASME B16.47: the published title with its size band, and the 2025 edition shown.
- BSI's own product record for the edition of BS EN 1092-1 identical to EN 1092-1:2001, cited only for the comparison it draws with ISO 7005-1.
This page summarises those records in our own words and quotes only short marked runs from them, each attributed to the record it comes from. It reproduces no dimension, bolt size, tolerance, mass or pressure and temperature rating from any standard, and it states no interchangeability rule. Buy the current editions from BSI, ISO and ASME for the requirements themselves, confirm every line against the maker's own drawing, and your purchase order governs.