A factory in Vietnam runs a production line built in Japan, a utility system installed by a local contractor to ASME, and a packaged unit from Europe. The maintenance store holds “100 mm” flanges, “4 inch” gaskets and “DN 100” valves on the same shelf. Sooner or later someone takes the wrong one, and the bolts do not line up, or worse, they nearly do.
One plant can contain all three standard families. Each is internally consistent, and parts to each can be sourced in Asia. The trouble starts at the joints between them. This article sets out where JIS, ASME and EN differ for pipe, flanges, fittings, threads and valves, why “equivalent” is a dangerous word in an order, and how to buy spares for a plant built to mixed standards.
💬 Running a plant with Japanese, American and European piping side by side? Send us the spare-parts list and we will mark which standard each line belongs to before anyone quotes.
Three systems, three ways of naming the same size
ASME uses NPS, JIS uses A (millimetre) or B (inch) designations, and EN uses DN; the nominal sizes correspond, but nominal size alone never defines the part.
The nominal size is a label. It does not tell you the outside diameter, the wall thickness, the flange drilling or the thread. Those come from the standard for each component, and the standards were developed separately.
| Component | ASME (US) | JIS (Japan) | EN / ISO (Europe) |
|---|---|---|---|
| Size designation | NPS (e.g. NPS 4) | A or B (e.g. 100A / 4B) | DN (e.g. DN 100) |
| Steel pipe | B36.10M (wrought steel), B36.19 (stainless) | JIS G 3454 (carbon steel, pressure piping), JIS G 3459 (stainless piping) | EN steel tube standards for the product form |
| Butt-weld fittings | B16.9 | JIS B 2312 (for piping), JIS B 2311 (ordinary use) | EN fitting standards |
| Flanges | B16.5 (to NPS 24), B16.47 (NPS 26 to 60) | JIS B 2220 | EN 1092-1 |
| Pressure designation of flanges | Class (150, 300 …) | K (5K, 10K …) | PN (PN 16, PN 40 …) |
| Taper pipe threads | NPT to B1.20.1 | JIS B 0203 | ISO 7-1 |
| Valve face-to-face | B16.10 | JIS or maker’s table | EN or maker’s table |
Pipe: the same nominal size is not always the same tube
Before mixing JIS, ASME and EN pipe and fittings, compare the outside diameter and wall thickness in each standard; for some sizes and schedules they differ enough to affect welding and fit-up.
ASME publishes B36.10M (Welded and Seamless Wrought Steel Pipe; ASME lists the 2018 edition, now on stabilised maintenance, and it is the edition referenced in ASME’s Boiler and Pressure Vessel Code tables) and B36.19 (Welded and Seamless Wrought Stainless Steel Pipe, issued in May 2022 as the revision of B36.19M-2018). Stainless pipe under B36.19 uses schedule designations with an “S” suffix. JIS has its own pipe standards — JIS G 3454 for carbon steel pressure piping and JIS G 3459 for stainless steel piping — with their own diameter and schedule tables.
We do not reproduce the dimension tables here; they belong to the standards and must be read from the current edition. The practical rule is simple. If a JIS pipe is to be welded to an ASME fitting, or the other way round, compare the outside diameter and the wall thickness at the weld end for that size and schedule. A small difference in outside diameter shows up as a step at the weld; a difference in wall thickness changes the bevel and the weld procedure. Where the two do not match, the joint needs a transition piece designed for it, not a hammer.
Flanges: never bolt across systems
JIS, ASME and EN flanges of the same nominal size are dimensionally different; do not join them unless a designed adaptor is shown on the drawing.
ASME B16.5 rates flanges by Class; ASME’s catalogue lists Classes 150 to 2500. JIS B 2220 uses nominal pressures such as 5K, 10K, 16K and 20K. EN 1092-1:2018 uses PN designations from PN 2,5 to PN 400. These are three separate rating systems with separate dimension tables — flange outside diameter, bolt circle, number and size of bolts, and facing all come from the standard in use.
The danger is the near miss. Some combinations of size and rating across systems come close enough that bolts can be forced through, or a gasket can be squeezed into place. A joint assembled that way may hold at a low test pressure and leak later. If a European package unit must connect to a JIS line, the connection should be a designed adaptor flange or a spool with one end to each standard, shown on the drawing and ordered as a special item. How to write an order line for each system is in pipe fittings and flanges from Asia.
| Mistake | What happens | How to prevent it |
|---|---|---|
| JIS 10K flange ordered as “Class 150” | Bolt holes do not line up, or nearly do | Order to the standard on the existing flange |
| ASME gasket on a JIS or EN flange | Gasket inside or outside diameter is wrong | Order gaskets to the flange standard |
| EN PN flange mated to JIS flange | Drilling and face differ | Use a designed adaptor |
| ”Equivalent” valve with different face-to-face | Valve does not fit the gap in the line | Check face-to-face before ordering |
| Class converted to bar in an email | Wrong rating at operating temperature | Read rating from the standard for the material and temperature |
💬 We source pipe, fittings, flanges and valves to ASME, JIS and EN across Asia and check the standard on each piece before shipment. See pipe fittings, valves and couplings.
Threads: R, Rc, Rp and NPT do not mix
ISO 7-1 and JIS B 0203 taper pipe threads and NPT threads to ASME B1.20.1 are different thread forms; a joint between them may seem to tighten and still leak.
ISO 7-1:1994 (Pipe threads where pressure-tight joints are made on the threads — Part 1: Dimensions, tolerances and designation, with a 2007 corrigendum) specifies taper external, parallel internal and taper internal threads in sizes 1/16 to 6, and states that a jointing medium is used on the thread to make the joint pressure-tight. JIS B 0203 is the Japanese taper pipe thread standard used on Japanese equipment. ASME B1.20.1 (Pipe Threads, General Purpose, Inch) is the base standard for NPT and related American threads; ASME lists the 2013 edition, reaffirmed in 2018.
ISO/JIS taper threads and NPT threads are defined by different standards with different thread profiles, and they are not designed to mate. A male NPT fitting may screw a few turns into an Rc port and stop, or seem to tighten with extra sealant. That joint is not reliable. Check instrument, hydraulic and pneumatic connections on mixed equipment first, because small threaded ports are where the systems meet. Record the thread type on every spare-parts line (“R 1/2”, “Rc 1/2”, “1/2 NPT”) and never write “1/2 inch thread”.
Hydraulic and pneumatic hose ends add their own families of thread and seal forms; see industrial hoses and couplings.
Materials: “equivalent grades” need a documented basis
Treat a JIS, ASTM or EN material grade as a substitute for another only when the substitution is approved by the designer or the end user, with the basis written down.
Cross-reference tables between JIS, ASTM and EN grades are easy to find and useful as a starting point. They are not a specification. Chemistry limits, mechanical properties, heat treatment and testing requirements differ between nominally similar grades, and the pressure-temperature rating of a flange or valve depends on the material group in the standard it is built to. A fitting in a JIS stainless grade is not automatically an ASTM-graded fitting, even where the chemistry overlaps.
Where a substitution is necessary, record which grade was specified, which is offered, the basis for accepting it, and who approved it. Ask for an inspection certificate that reports the tests required by the specification actually named on the order; see mill test certificates under EN 10204. Some products are certified to more than one specification at once, which is legitimate when the certificate shows that each specification’s requirements are met.
Valves: same size, same rating, different length
Before replacing a valve with one built to another standard, compare face-to-face dimension, end connection and rating basis.
ASME B16.10 (Face-to-Face and End-to-End Dimensions of Valves; ASME lists the 2022 edition) sets face-to-face dimensions for ASME valves. JIS valves and European valves follow their own tables or the maker’s. A replacement valve of the same nominal size and rating can be longer or shorter, and its flanges will follow its own standard. On a line built to JIS, the replacement should normally be a JIS valve; where it cannot be, the change needs a spool piece and a drawing revision. For valve specification and test records, see sourcing industrial valves.
Pressure ratings and the rules of the country of use
Class, K and PN ratings are not convertible by a factor; the permitted pressure comes from each standard’s tables for the material and temperature, and national pressure-equipment rules apply on top.
When a plant contains equipment from several systems, the design basis for each section should be on the drawing. If a section is modified, the modified part must meet the rating of the system it sits in. In the EU, Directive 2014/68/EU covers the design, manufacture and conformity assessment of stationary pressure equipment with a maximum allowable pressure greater than 0,5 bar, according to the European Commission. For plants in Vietnam, Japan and elsewhere, confirm the current rule in the country of use with the end user and its inspector before changing a pressure system.
Ordering spares for a mixed plant
Tag each line in the spare-parts list with its standard family, and order every replacement to the standard of the piece it replaces.
A workable routine:
- Walk the plant by system and record, for each line, the standard family (JIS, ASME, EN) from the drawings or from the markings on existing flanges and valves.
- Put the standard on every spare-parts line: “JIS B 2220 10K 100A RF” rather than “flange 100”.
- Store spares by standard family, not by nominal size.
- Where two systems meet, record the adaptor or transition piece as its own part number.
- When the original maker’s part is unavailable, order a replacement to the same standard and confirm face-to-face, drilling and thread before accepting it.
Where a plant in Vietnam or Thailand was built with Japanese equipment, it will need JIS parts for as long as that equipment runs, and Japanese standard parts can also be sourced from Japan itself — see genuine Japanese industrial parts and, for the wider product range, industrial sourcing.
💬 Need JIS, ASME and EN spares sourced and consolidated for one plant? Tell us what the plant runs.
Frequently asked questions
1. Is 100A the same as NPS 4 and DN 100?
They are the same nominal size in three systems, but the parts made to each standard can differ in diameter, wall thickness, drilling and thread. Order to the standard of the existing piece.
2. Can I join a JIS 10K flange to an ASME Class 150 flange?
Not directly. The dimensions differ. Use a designed adaptor shown on the drawing.
3. Will an NPT fitting seal in a JIS or ISO taper thread port?
It should not be relied on. The thread forms differ, and a joint that seems tight may leak. Use matching threads or an adaptor made for the purpose.
4. Are JIS stainless grades equivalent to ASTM grades?
Similar in chemistry in many cases, but not automatically interchangeable. Any substitution needs approval and a documented basis.
5. Which standard should I use for new work in an existing plant?
Usually the standard of the system you are connecting to. Where the project specification says otherwise, follow it and design the transitions.
Sources: ASME catalogue and revision pages for B36.10M, B36.19-2022, B16.5-2025, B16.9-2024, B16.10-2022, B16.47-2025 and B1.20.1; ASME BPVC referenced-standards table (B36.10M-2018); EN 1092-1:2018 as listed by SIS; ISO 7-1:1994 and Cor 1:2007 on iso.org; JISC database titles for JIS B 0203, B 2220, B 2311, B 2312, G 3454 and G 3459; European Commission page on Directive 2014/68/EU. Checked on 27 October 2026.