Face-to-Face Dimensions: ASME B16.10 vs EN 558 for Carbon and Stainless Steel Valves

A plant orders a replacement DN100 gate valve for a line built to ASME B16.10. The store issues a DN100 PN16 valve, face-to-face to EN 558 Series 14. Both are 4 inch carbon steel gate valves, both are rated for the duty, both are correct to their own standard. The new valve is 39 mm shorter, and the line cannot be closed up without pulling the flanges together.

It is not a manufacturing error. It is what happens when a DN number is treated as a length.

The Length Is Set by the Standard, Not the Size

Face-to-face is the distance between the gasket faces of a flanged valve; end-to-end is the distance between weld bevels or thread ends. Both are fixed by a standard so that any compliant valve fits any spool built to the same table.

Two families dominate export orders. ASME B16.10 tabulates lengths by valve type, size and pressure class. EN 558 tabulates them by series number, where a series is a body pattern rather than a pressure class. A drawing that says "DN100" and nothing else has not specified a length.

The material does not change the number. A Class 150 DN100 gate valve is 229 mm whether the body is WCB or CF8M, because B16.10 fixes the envelope and lets wall thickness vary inside it. The class and the pattern decide the length; the grade decides whether the valve survives the fluid.

The ASME Side: The Class Changes the Length

Inside B16.10, the size and the type stay the same and the class moves the dimension. A flanged gate valve at NPS 2 is 178 mm in Class 150 and 216 mm in Class 300. At NPS 6 it goes from 267 mm to 403 mm, a difference of 136 mm on the same nominal bore.

The reason is physical: a higher class means thicker walls, a heavier bonnet joint and thicker flanges, so the casting grows. The bore is unchanged.

A spare valve therefore cannot be substituted across classes, even when the pressure margin looks generous: a Class 300 valve will not fit a spool cut for Class 150, and the reverse leaves a 136 mm gap at NPS 6.

The EN Side: The Series Changes the Length

EN 558 works differently. The same DN, at the same PN, has several legal lengths depending on which series the valve is built to.

At DN100, Series 14 gives 190 mm, Series 15 gives 300 mm and Series 1 gives 350 mm. All three are valid EN 558 dimensions for a DN100 flanged valve. Series 14 is the short pattern inherited from DIN 3202 F4 and is the usual choice for resilient-seated water and sewage gate valves. Series 15 is the longer pattern normally used for PN 25 and PN 40 gate valves. Series 1 is the basic series, the longest of the three, and appears in projects that specify it explicitly.

A European catalogue that lists a DN100 PN16 gate valve without a series is incomplete in the same way an ASME catalogue would be without a class. The series is the length.

Side by Side: Gate Valves, Flanged, in Millimetres

DN	NPS	EN 558 S14	EN 558 S15	EN 558 S1	B16.10 Class 150	B16.10 Class 300
50	2	150	250	230	178	216
65	2½	170	270	290	191	241
80	3	180	280	310	203	282
100	4	190	300	350	229	305
150	6	210	350	480	267	403
200	8	230	400	600	292	419
250	10	250	450	730	330	457
300	12	270	500	850	356	502

Read the table by column, not by row. Series 14 is the shortest everywhere: fine for buried water mains, the worst possible substitute for an ANSI spool. Series 1 is longer than the ASME Class 150 column at every size: 121 mm at DN100, 213 mm at DN150, 308 mm at DN200.

At DN50 the gap is 28 mm, which a fitter will try to close with a spanner. At DN300 it is 86 mm, and persuasion becomes a cutting torch.

Tolerances: What the Standards Actually Allow

ASME B16.10 permits ±1.5 mm up to NPS 10 and ±3.0 mm from NPS 12. Angle valves get half those figures, because the centre-to-face dimension is shorter and an error has more leverage.

Two compliant valves can still be 3 mm apart at NPS 12, and errors add in a spool with two valves in series.

Do not design to the nominal dimension with zero allowance. Leave one joint adjustable and check that the gasket can absorb what is left: a 3 mm gap closed by bolt tension leaves the flanges in bending, and it leaks at the first thermal cycle rather than at hydrotest.

EN 558 carries its own tolerance clause, and it is not a copy of the ASME one. A project that mixes the two systems must say which clause governs, because a valve accepted against one may be rejected against the other.

Face-to-Face Is Not End-to-End

A flanged valve is measured between gasket faces. A butt-weld valve of the same size, class and type is measured between weld bevels, and it is longer because each end carries a hub the flange face does not. Both numbers appear in the same standard, in different columns.

A spool dimensioned for a flanged valve cannot be shortened by swapping in a weld-end valve, and a weld-end valve quoted from a flanged catalogue row arrives at the wrong length. Threaded and socket-weld ends follow yet another table, usually a manufacturer's standard, so they should be dimensioned from a drawing.

The face type matters too: raised face, flat face and ring joint are measured to different surfaces, and mixing facings inside one joint is a leak path even when the length is right.

Length and Drilling Are Two Separate Checks

A valve can match the face-to-face dimension and still refuse to bolt up, because the flange drilling is governed by a different standard from the length.

At DN100, ASME B16.5 Class 150 has eight holes on a 190.5 mm bolt circle; EN 1092-1 PN16 has eight on 180 mm. Same count, 10.5 mm apart. At DN80 even the count differs: eight against four. A valve that satisfies the length table and not the drilling table goes back to the warehouse. The length decides whether the spool closes; the drilling decides whether the joint exists at all.

Why the Drawing and the Valve Disagree

Piping isometrics are modelled once, in one system. Procurement then buys against whatever the datasheet says, sometimes from a different system, sometimes from a catalogue that lists a different valve pattern in the same row.

The classic version is a ball valve length printed in a gate valve table. Long-pattern ball valves are much longer than gate valves of the same size, and a datasheet built by copying a column carries the wrong number until the spool is cut.

Three habits prevent most of it: order the valve before the spool is fabricated; put the standard, series and end type on the purchase order in words, not just the DN; and measure the first valve of a delivery, because a pattern 5 mm out is 5 mm out on every valve in the container.

What to Write on the Purchase Order

A dimensional clause that survives an international supply chain answers six things:

• The length standard and edition: ASME B16.10-2022, or EN 558:2017.

• The series or table, not only the size: EN 558 Series 14, or B16.10 Class 150 gate valve.

• The size in that standard's units: DN100 or NPS 4, both where systems mix.

• The pressure class or PN, because inside ASME the class changes the length.

• The end type and facing: flanged raised face, butt weld, threaded.

• The tolerance, and which clause governs it when two standards are quoted.

A usable sentence reads: "DN100 gate valve, face-to-face 229 mm per ASME B16.10 Class 150, flanged raised face to ASME B16.5, body WCB, trim 13Cr, tested to API 598." A valve built to EN 558 Series 14 would be 190 mm and would not fit.

The Rule That Survives

Face-to-face is a property of the standard, the pattern and the class, and only then of the size. A carbon steel and a stainless steel valve of the same class share the same length; a Class 300 valve does not share the length of a Class 150 valve; a Series 14 valve does not share the length of a Series 1 valve.

Before an enquiry goes out, write the standard and the series next to the DN. It takes one line, and it decides whether the valve fits.

Is DN100 the same as 4 inch?

Only as a nominal label. DN100 corresponds to NPS 4, but the systems set different bolt circles and, in some series, different lengths.

Can an EN 558 Series 14 valve replace an ASME B16.10 Class 150 valve?

No. At DN100 the Series 14 valve is 190 mm and the Class 150 valve 229 mm, and the gap grows to 86 mm at DN300. The flange drilling differs too, so a spacer would not fix it.

Does a higher pressure class always mean a longer valve?

Inside ASME B16.10, yes for the same type and size. Across standards, no: an EN 558 Series 14 PN16 valve is shorter than an ASME Class 150 valve of the same size.

What tolerance should I accept on face-to-face?

±1.5 mm up to NPS 10 and ±3.0 mm from NPS 12, half of that for angle valves. Accept the standard tolerance unless the design demands tighter, and never close a gap with bolt tension.