If you buy valves across borders, you meet two size systems and two pressure systems on the same datasheet: DN and inch for size, PN and Class for pressure. They describe the same physical envelope in different languages, and mixing them is a common cause of a valve that will not bolt up. The tables below give the conversions used in everyday valve enquiry work, followed by how to apply them.
DN to Inch Reference Table
DN is the nominal metric size and the inch figure is the nominal bore (NPS). Both are labels for a pipe size, not exact measurements, and the tables below cover the range most butterfly valve enquiries use.
| DN | Inch (nominal) | ASME Class (common) | PN (common) |
|---|---|---|---|
| DN25 | 1" | Class 150 | PN10 / PN16 |
| DN32 | 1¼" | Class 150 | PN10 / PN16 |
| DN40 | 1½" | Class 150 | PN10 / PN16 |
| DN50 | 2" | Class 150 | PN10 / PN16 |
| DN65 | 2½" | Class 150 | PN10 / PN16 |
| DN80 | 3" | Class 150 | PN10 / PN16 |
| DN100 | 4" | Class 150 | PN10 / PN16 |
| DN125 | 5" | Class 150 | PN10 / PN16 |
| DN150 | 6" | Class 150 | PN10 / PN16 |
| DN200 | 8" | Class 150 | PN10 / PN16 |
| DN250 | 10" | Class 150 | PN10 / PN16 |
| DN300 | 12" | Class 150 | PN10 / PN16 |
| DN350 | 14" | Class 150 | PN10 / PN16 |
| DN400 | 16" | Class 150 | PN10 / PN16 |
| DN450 | 18" | Class 150 | PN10 / PN16 |
| DN500 | 20" | Class 150 | PN10 / PN16 |
| DN600 | 24" | Class 150 | PN10 / PN16 |
The drilling standard is the point most often missed. EN 1092-1 and ASME B16.5 use different bolt circles and bolt counts for nominally similar pressures, so check the valve against the pipe flange drawing before ordering.
Two cautions also apply. DN and inch are nominal designations, so DN100 and 4" describe the same pipe size but are not exact dimensional equivalents — the actual bore depends on wall thickness and schedule. The pressure columns also show ratings commonly used for general butterfly valve service, not the only ratings available. Reguvale supplies butterfly valves from DN50 to DN1200 (2" to 48") with PN10 and PN16 ratings, in ductile iron, carbon steel and stainless steel.
PN and Class: Different Systems, Not Interchangeable
PN is a metric pressure designation in bar; Class is an imperial designation in pounds per square inch. They are defined by different standards — EN 1092 for PN, ASME B16.5 and B16.47 for Class — and they produce different flange dimensions, bolt hole counts and bolt sizes. The pairs below are commonly encountered together, but the pairing is approximate and applies in the ordinary temperature range only.
| PN | Approx. Class equivalent | Ambient pressure reference |
|---|---|---|
| PN6 | No direct ASME equivalent | 6 bar |
| PN10 | Class 150 (well below its capacity) | 10 bar |
| PN16 | Class 150, approximately | 16 bar |
| PN25 | Class 300, approximately | 25 bar |
| PN40 | Class 300, approximately | 40 bar |
| Class 150 | — | about 19.6 bar at ambient, carbon steel group |
| Class 300 | — | about 51 bar at ambient, carbon steel group |
Read the two ratings separately and never substitute one for the other in a purchase order. A PN16 specification and a Class 150 offer are not the same valve: the flanges may not match, and the allowable pressure at temperature follows a different curve in each system. Note also that ASME B16.47 replaces B16.5 for large sizes, and that Class 150 and PN16 flange dimensions diverge noticeably above roughly DN250.
How to Read a Size Designation on an RFQ
Valve size designations are compact, and the order of the fields carries meaning. "DN200 PN16 wafer butterfly valve" means a nominal bore of 200 mm, a PN16 rating, and a wafer body clamped between flanges. "8 inch Class 150 lug butterfly valve" means an 8-inch nominal bore, a Class 150 rating, and a lug body bolted to a single flange. Stating both size systems, the rating and the flange standard removes ambiguity.
Using the Chart in Five Steps
The tables are only useful if they are applied in the right order. A practical sequence:
- Identify the line size from the piping specification, in DN or inch.
- Convert with the first table to confirm the equivalent nominal size in the other system.
- Identify the pressure rating and the flange standard the pipe is drilled to, such as EN 1092-1 or ASME B16.5.
- Check the face-to-face dimension series, because a valve with the right bore and drilling can still be the wrong length.
- Confirm bolt hole count and bolt size for the rating, particularly above DN250, where PN and Class flanges differ most.
Choosing the Size, Not Just Converting It
Converting a size is not the same as selecting one. Line size follows the required flow rate and the velocity you are willing to accept. For a given flow, velocity rises as the bore falls, which increases pressure drop, noise and erosion. Bore area grows with the square of the diameter, so a nominal 6-inch line has roughly 2.25 times the area of a nominal 4-inch line and carries the same flow at less than half the velocity. Where a line runs near capacity, state the design flow rate in the enquiry.
What to Send With the Size on an RFQ
A size alone rarely settles a valve selection. Include the following, and the offer can be checked field by field:
- Size in DN and inch, plus the pipe schedule.
- Pressure rating as PN or Class, with the flange standard and drilling.
- End connection: wafer, lug or double flanged.
- Medium, temperature and whether the duty is isolation or throttling.
- Face-to-face series required, if the project specifies one.
- Actuation type and fail position.
For a fuller list of the fields an enquiry should carry, see the butterfly valve RFQ checklist. The general butterfly valve range and the wafer butterfly valve page are useful starting points for the body styles referred to above.
Frequently Asked Questions
Is DN100 exactly 4 inches?
No. DN100 and 4" are nominal designations for the same pipe size, but neither is an exact dimension — the real bore depends on wall thickness and schedule. Use the conversion to identify the equivalent size, then confirm the dimensions against the piping standard.
Can I use a PN16 valve on a Class 150 line?
Not automatically. PN16 and Class 150 come from different standards, so the outside diameter, bolt circle, hole count and bolt size can differ even though the pressure capability is broadly comparable at ambient. Compare the valve drilling with the pipe flange drilling before ordering.
Why does the same DN have different face-to-face lengths?
Because more than one dimension series exists. API 609 and EN 558 define different face-to-face series for butterfly valves, and a valve built to one may not fit an installation designed for another, so state the series when the piping design specifies one.
How do I choose between DN and inch when specifying?
Follow the convention of the piping specification that governs the line, then state the equivalent in the other system for clarity. If you order in DN, include the flange standard and drilling so a metric or imperial flange face is not assumed.
Does a larger nominal size always mean lower pressure drop?
For the same flow rate, yes in general, because velocity falls as bore area increases. But the pressure drop of a butterfly valve depends strongly on disc position and valve design, and a partly open valve of any size creates significant loss, so confirm the duty against the design flow.
If the size in your specification is ambiguous, or the piping standard and the valve standard do not obviously line up, send the line details and let us check them. Reguvale can confirm the equivalent size, the flange drilling and the face-to-face series, then quote against them. Use request a quotation or contact the sales team with the piping specification attached.