A butterfly valve is not interchangeable just because two valves have the same nominal diameter. Four dimension families must match: nominal size, pressure rating, face-to-face length, and flange drilling or bolt pattern. If any one of them is wrong, the valve may not fit between the flanges, may not match the pipe bolts, or may not be rated for the operating pressure.
This guide explains DN, NPS, PN, Class, face-to-face standards, and bolt patterns for butterfly valves. Use it when selecting a new valve, replacing an existing unit, importing a butterfly valve, or preparing an RFQ.
A complete butterfly valve dimension package includes more than the bore diameter. The four dimensions that most often cause field problems are:
Nominal size and pressure rating are specification decisions. Face-to-face and bolt pattern are installation decisions. All four must be checked against the manufacturer drawing before ordering.
Butterfly valves are commonly designated in DN, the metric nominal diameter, or NPS, the North American nominal pipe size. Neither value is always the exact measured inside diameter. DN and NPS are standardized nominal values used to keep valves, flanges, and piping compatible.
Common DN to NPS conversions are shown below.
| DN | NPS | Typical Butterfly Valve Application |
|---|---|---|
| DN40 | 1 1/2" | Small process lines, equipment isolation |
| DN50 | 2" | General utility, HVAC, water supply |
| DN65 | 2 1/2" | Fire protection, commercial piping |
| DN80 | 3" | HVAC, water treatment, process isolation |
| DN100 | 4" | General industrial and building services |
| DN150 | 6" | Water distribution, cooling water, fire mains |
| DN200 | 8" | Municipal water, process, and HVAC mains |
| DN250 | 10" | Waterworks, industrial headers |
| DN300 | 12" | Water treatment, utility, and process lines |
| DN350 | 14" | Large water and industrial distribution |
| DN400 | 16" | Pumping stations, large HVAC systems |
| DN450 | 18" | Municipal mains and treatment plants |
| DN500 | 20" | Waterworks, power, and large process lines |
| DN600 | 24" | Municipal transmission and utility service |
If the existing pipe is imperial, give NPS. If the flange standard is EN or ISO, give DN. For imported valves, provide both DN and NPS where possible, plus the flange standard. That removes ambiguity when the manufacturer works in one system and the installation drawing uses the other.
Pressure rating is not the same as operating pressure. PN and Class are nominal rating systems, and the allowable pressure changes with material and temperature.
PN is used in ISO and EN standards. The number is the nominal pressure in bar at a reference temperature. For example, PN16 is nominally 16 bar. Common butterfly valve ratings are PN6, PN10, PN16, PN25, and PN40.
PN rating is convenient, but the actual working pressure still depends on the valve body, seat, disc, stem, and temperature. A PN16 flange does not automatically mean the complete butterfly valve is suitable for 16 bar at every temperature or with every seat material.
Class is used in ASME and North American piping. The number is a class designation, not a direct pressure limit. Class 150, for example, does not mean 150 psi or 150 bar at all operating temperatures. It is a temperature-dependent pressure rating.
Common butterfly valve classes are Class 125, Class 150, Class 300, and Class 600. Class 125 is typically associated with cast iron flanges, while Class 150 and higher are common for steel valves. The allowable pressure decreases as temperature rises.
| Common Butterfly Valve Group | PN | ASME Class | Reference Pressure at Ambient Temperature |
|---|---|---|---|
| General water and HVAC service | PN10 / PN16 | Class 125 / Class 150 | 10 to 16 bar; approximately 14 to 20 bar |
| Higher-pressure industrial service | PN25 / PN40 | Class 300 | 25 to 40 bar; approximately 51 bar |
| High-pressure or high-temperature service | PN63 / PN100 | Class 600 | 63 to 100 bar; approximately 102 bar |
For municipal waterworks butterfly valves, AWWA C504 uses pressure classes such as Class 75, Class 150, and Class 250, expressed in psi. These classes must be treated separately from ASME Class ratings. See the AWWA C504 butterfly valve guide.
Face-to-face is the axial distance between the two sealing reference faces of the valve. It controls how much room the valve needs between the pipe flanges and whether a replacement valve can be installed without changing the pipe spool.
Butterfly valves use several face-to-face systems depending on the body style and design standard:
A wafer and a lug valve of the same DN and pressure class are usually interchangeable in face-to-face length when both use a short-pattern standard. A double flange valve is normally longer and may not fit in the same space. A PTFE lined valve can also have a different measurement reference because the PTFE sealing lips may extend beyond the metal body. See the EPDM vs PTFE lined wafer butterfly valve face-to-face guide.
For replacement work, measure the installed face-to-face gap and compare it with the new valve drawing. Do not rely on DN alone.
Typical body style differences:
For the full body style comparison, see the wafer vs lug vs double flange butterfly valve guide.
The flange drilling is the last critical fit check. The valve must match the pipe flange standard, size, pressure class, bolt circle, hole count, hole diameter, and facing. A valve with the correct DN and face-to-face can still be impossible to bolt into the line if the drilling is wrong.
The most common flange standards are:
For every flanged butterfly valve, confirm these five bolt pattern values:
| Dimension | What It Defines | Example |
|---|---|---|
| Flange standard | The system and drilling table | EN 1092-1 PN16 or ASME B16.5 Class 150 |
| Bolt circle diameter | The pitch circle of all bolt holes | 180 mm for DN100 PN16; 7.5" for NPS 4 Class 150 |
| Number of bolts | Total holes around the flange | 8 holes for DN100 PN16; 8 holes for NPS 4 Class 150 |
| Bolt or stud size | The fastener diameter | M16 for DN100 PN16; 5/8" for NPS 4 Class 150 |
| Hole diameter | The clearance through the flange | 18 mm for M16; approximately 3/4" for 5/8" studs |
Metric and imperial flange systems sometimes have similar bolt circles, but the bolt size and hole diameter can differ. A valve drilled to EN 1092-1 may look close to an ASME flange and still not accept the correct imperial stud bolts. Confirm the facing as well: flat face, raised face, or grooved end. The facing affects the gasket, bolt torque, and whether the valve can seal against the mating flange.
For wafer valves, the bolt length must cover two pipe flanges, the valve body, two nuts, and washers. For lug valves, each side uses shorter independent bolts. For double flange valves, each side is a standard bolted flange joint. For grooved end valves, there is no bolt pattern to match, but the groove profile and coupling compatibility must be confirmed.
| Item | Wafer | Lug | Double Flange | Grooved End |
|---|---|---|---|---|
| Typical DN range | DN40 to DN1200 | DN50 to DN600 | DN200 to DN3000+ | DN50 to DN300 typical |
| Typical pressure | PN10 / PN16 / Class 150 | PN10 / PN16 / Class 150 | PN10 to PN40 / Class 150 or 300 | PN10 / PN16 / 300 psi grooved systems |
| Face-to-face | Short, EN 558 Series 20 or API 609 Category A | Short, EN 558 Series 20 or API 609 Category A | Long, EN 558 Series 13 or API 609 Category B | Manufacturer profile; shorter than flanged valves |
| Bolt interface | Through-bolts clamp the body | Independent bolts on each side | Standard bolted flange on each side | Grooved couplings, no flange bolts |
| Dead-end service | No | Yes, when rated | Yes | No |
| Self-centering | No | Limited | Yes | Coupling-controlled |
| Best use | Compact, economical isolation | Dead-end and maintenance sections | Large, critical, buried, or high-pressure service | HVAC, fire protection, quick installation |
Follow this sequence to avoid changing the valve after the first quote:
They are not exact equivalents. PN16 is a nominal 16 bar rating used in ISO and EN standards, while Class 150 is a temperature-dependent ASME rating. A PN16 and a Class 150 valve are commonly grouped for general service, but the flange drilling, materials, and pressure-temperature limits must be compared.
Use the standard nominal size table. For example, DN50 is NPS 2, DN80 is NPS 3, DN100 is NPS 4, DN150 is NPS 6, DN200 is NPS 8, DN300 is NPS 12, and DN600 is NPS 24. Conversion alone is not enough; confirm the flange standard and face-to-face dimension.
Often yes, because wafer and lug valves typically use the same short face-to-face standard such as EN 558 Series 20 or API 609 Category A. The bolt set changes from through-bolts to independent bolts on each side, and the dead-end rating must be confirmed.
Because face-to-face depends on the body style and design standard. A wafer or lug valve uses a short pattern, while a double flange valve uses a long pattern. PTFE lined valves may also be measured from the protruding liner faces instead of the metal body faces.
Include the flange standard, size, pressure class, bolt circle diameter, number of bolts, bolt or stud size, hole diameter, and flange facing. If the valve replaces an existing unit, include the measured face-to-face gap and a flange photograph where possible.
For on-off isolation, the valve is usually the same DN or NPS as the pipe. For throttling or control service, the valve may be smaller, but the selection must include flow coefficient, pressure drop, control range, and actuator sizing.
AWWA C504 pressure classes are expressed in psi, commonly Class 75, Class 150, and Class 250 for waterworks service. They are not the same as ASME Class 150 or Class 300, and the face-to-face and flange drilling must be checked separately.
For a dimensional check against your piping, contact Laux Valve with the DN or NPS, pressure class, body style, flange standard, and measured face-to-face gap. Browse the butterfly valve product range or download drawings from the catalog center.
Disclaimer: Dimension examples are for general reference. Final face-to-face, pressure-temperature, and bolt pattern data must be confirmed from the applicable standard and the manufacturer’s certified dimensional drawing.
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