The purchase order said six inch Class 150 ball valve, full port, flanged, raised face. The valve that arrived was exactly that in every particular, and it was 38 millimetres too short. The existing valve was a long pattern body measured at 305 millimetres face to face. The new one was a short pattern body at 267 millimetres, which is also a legal six inch Class 150 ball valve under the same standard. The spool on the upstream side was welded in place with no slip joint and no flange to adjust, the downstream pump was on a fixed baseplate, and the plant had a two day outage booked to install a valve that could not be installed. The replacement was correct and useless.
This is one of the most common and least discussed failure modes in valve procurement, because it is invisible on every document until the valve is on the pipe rack. Size, class, material, bore, pressure test and certification can all be perfect while the valve does not fit, and the reason is that "same size and same class" describes a pressure boundary, not a shape. Face to face dimension, flange drilling and facing, and the actuator mounting pad are three separate interfaces, each governed by its own standard, and each one has to be confirmed separately. This article sets out where those numbers come from, where the millimetres disappear, and what to measure before the order goes out.
1. Same size and same class is not a dimension
A valve is defined for procurement purposes by its pressure boundary: nominal size, pressure class, body material, end connection, bore and trim. Every one of those can be quoted and verified, and a valve can be fully compliant with all of them and still not fit the gap it was bought for. The gap is a separate specification, and in a flanged system it is fixed by the pipe, the gaskets and the adjacent equipment. In a welded system it is fixed by the spool pieces, which are usually already fabricated.
The reason the gap can vary is that the dimensional standard, ASME B16.10, contains more than one valid answer for the same size and class. For Class 150 and Class 300 ball valves in the smaller sizes it tabulates both a short pattern and a long pattern, and both are compliant. A manufacturer chooses the one that suits its casting range or its biggest customer and lists it in the catalogue, and nobody is doing anything wrong. The buyer, meanwhile, is comparing a catalogue dimension against a pipe that was built twenty years ago to a different edition of the same standard.
2. The standards stack: who defines which number
ASME B16.10, face to face and end to end dimensions of valves, is the document that fixes the length. It is organized by valve type, then by pressure class, then by nominal size, and it gives dimensions in inches with a tolerance. API 6D, which is the specification for pipeline and manifold valves, does not invent its own lengths; it refers back to B16.10 for flanged valves and to the purchaser's drawings for welded ones. ANSI B16.5 fixes the flange, which is the second interface: outside diameter, bolt circle, bolt hole count and diameter, facing type and facing height. ASME B16.25 fixes the weld end preparation for butt welded valves, and the 37.5 degree bevel is part of the overall length rather than an addition to it.
In Europe the equivalent stack is EN 558 for face to face and centre to face dimensions, EN 1092-1 for flanges up to PN 40, and EN 12627 for butt weld ends. EN 558 is organized into basic series rather than by valve type, and the series that matter for ball valves are series 1 (the short pattern used with PN 10 and PN 16 flanges), series 2 (the long pattern used with the same flanges) and the series aligned to ASME class patterns. The practical consequence is that a European short pattern valve and an American short pattern valve are usually close but not always identical, and the word usually is doing a lot of work in that sentence.
ISO 5752 sits above both as the international basic series document, and it is the reason a valve can be described as series 1, series 2 or series 3 without saying which regional standard it came from. When a purchase order says only the size and the class, the supplier is free to choose any compliant series, and the buyer has no basis for complaint afterwards.
3. Short pattern and long pattern: two legal valves that do not interchange
The classic case is the six inch Class 150 ball valve, where the short pattern body is 267 millimetres and the long pattern body is 305 millimetres, a difference of 38 millimetres. Both are listed, both are sold, and neither will replace the other without pipe modification. Two inch is similar in kind if different in magnitude: about 178 millimetres short against a longer body, a difference large enough to matter and small enough that someone will try to pull it up with flange bolts.
The long pattern exists for historical reasons and it is still useful. Gate valves in the same size and class are long, and a plant that replaces a gate valve with a ball valve in the same gap needs a ball valve of the same length. Long pattern bodies also give more room for a trunnion support, for a larger ball, and for insulation on a hot line. Short pattern bodies are cheaper, lighter and use less space on a skid, which is why they dominate new construction and why they dominate the stock of a distributor who is not asked the question.
The European comparison is worse, because the nominal sizes are not the same sizes. A DN100 valve to EN 558 series 1 is about 190 millimetres face to face, while a four inch Class 150 ball valve to B16.10 is 229 millimetres. That is a difference of roughly 40 millimetres between two valves that a non specialist would describe as the same thing. The DN 100 and the four inch are not the same size, they are not the same pressure rating, and they are not the same length, and yet the catalogue pages look identical at a glance.
4. The flange face is the second half of the interface
Even when the length is right, the flanges may not mate. ASME B16.5 and EN 1092-1 describe flanges that look similar and are not interchangeable: the bolt circles differ by a few millimetres, the bolt hole diameters differ by two to four millimetres, the bolt counts differ at some sizes, and the outside diameters differ enough to foul a socket. The table below gives the numbers for the sizes where the confusion is most common, and it is worth reading the difference column carefully, because 1.3 millimetres is the most dangerous number in the table.
| Nominal | PN 16 flange (EN 1092-1) | Class 150 flange (B16.5) | Practical consequence |
|---|---|---|---|
| DN50 / 2 inch | OD 165, BC 125, 4 holes 18 mm | OD 152.4, BC 120.7, 4 holes 19 mm | Bolt circle 4.3 mm apart, will not enter |
| DN80 / 3 inch | OD 200, BC 160, 8 holes 18 mm | OD 190.5, BC 152.4, 4 holes 19 mm | Different bolt count and circle, no fit |
| DN100 / 4 inch | OD 220, BC 180, 8 holes 18 mm | OD 228.6, BC 190.5, 8 holes 22 mm | 10.5 mm circle offset, 4 mm hole difference |
| DN150 / 6 inch | OD 285, BC 240, 8 holes 22 mm | OD 279.4, BC 241.3, 8 holes 22 mm | 1.3 mm offset, looks right, will not fit |
| DN200 / 8 inch | OD 340, BC 295, 12 holes 22 mm | OD 342.9, BC 298.5, 8 holes 22 mm | 12 bolts against 8, no fit at all |
| DN250 / 10 inch | OD 395, BC 355, 12 holes 22 mm | OD 406.4, BC 362, 12 holes 25 mm | 7 mm circle offset, 3 mm hole difference |
| DN300 / 12 inch | OD 445, BC 410, 12 holes 22 mm | OD 482.6, BC 431.8, 12 holes 25 mm | 22 mm circle offset, obvious on sight |
The DN150 case deserves a sentence of its own because it is the one that gets forced. The bolt circles differ by 1.3 millimetres, the hole diameters are both nominally 22 millimetres, and the outside diameters are within 6 millimetres. A fitter with a podger and a hammer will get four bolts through, tighten them, and create a joint that is loaded asymmetrically, that will not compress the gasket evenly, and that will leak on the first thermal cycle. Reaming holes to make a joint fit is prohibited by every standard that applies and is done on site every week.
Facing type is the third variable. A raised face is 2 millimetres high on Class 150 and 300 flanges and the gasket sits inside it. A ring joint facing is a groove about 6 millimetres deep, and a ring joint valve will not mate with a raised face flange at all without a transition spool. A flat face is used against cast iron and against some equipment nozzles and will not mate correctly with a raised face. The surface finish matters too: a spiral wound gasket needs a finish in the range of 3.2 to 6.3 micrometres Ra, and a valve delivered with a mirror polished face will not hold it.
5. Where the last few millimetres go
Even with the right length and the right drilling, a joint that measures correctly on the bench can fail to close on the pipe, and the reasons are always the same four. Gasket compression takes 1 to 1.5 millimetres per joint, so a valve that is dimensionally exact in a gap that is dimensionally exact will be short by two to three millimetres across the pair of joints, which the bolts will try to take out of the pipe rather than the gasket. Thermal growth moves things: a six metre carbon steel spool between ambient and 200 degrees C grows about 13 millimetres, and if both ends are anchored that growth has to go somewhere, usually into a flange that was already tight.
Weld shrinkage moves things the other way. A butt weld on a six inch pipe closes by 1.5 to 3 millimetres as it cools, and a spool fabricated between two fixed flanges will be short by the sum of its welds. Pipe stress moves things in whichever direction the analysis did not predict, and a nozzle that was aligned cold will not be aligned hot. The practical conclusion is that a replacement valve should be ordered with a length tolerance that is understood, and that a gap should be measured hot and cold, in more than one place, before the order is placed.
The fourth variable is the actuator, and it is the one that is forgotten because it is not part of the valve. ISO 5211 defines the mounting pad: flange size F10, F12, F14, a square or keyed drive of a defined size, a defined stem height and a defined bolt circle. Two valves of the same size and class from different manufacturers will frequently have different pad sizes, different drive squares and different stem heights, which means the actuator, the bracket and the coupler from the old valve will not transfer. A replacement that fits the pipe perfectly can still require a new bracket, a new coupler and a re-commissioning of the positioner.
6. The seven measurements to take before the order goes out
All of this is avoidable with a tape measure, a caliper and twenty minutes at the valve. Take the measurements with the line cold and, if the line operates above 150 degrees C, again at operating temperature or from the isometric with the growth calculated. Record them on the requisition rather than in a notebook, because the requisition is the document that survives.
Face to face length, measured between the flange faces at four points around the circumference, with the gaskets in place and the bolts snug. Record the spread as well as the average.
Flange outside diameter and bolt circle, measured across the flange and across two opposite bolt holes. A 1 millimetre difference is enough to reject a flange.
Bolt hole count and diameter, measured with a caliper. A 22 millimetre hole passes a 3/4 inch stud and an 18 millimetre hole does not.
Facing type and height, checked with a straight edge. Raised face, flat face, ring joint groove, or a tongue and groove.
Pipe schedule and bore, because a full port valve on schedule 80 pipe has a different bore than the same nominal size on schedule 40 and the reducer or the weld prep changes.
Actuator interface, meaning the ISO 5211 pad size, the drive square or key dimensions, the stem height above the pad and the bolt circle.
Obstructions: the distance to the nearest structural steel, the insulation thickness, the clearance for the actuator to be lifted off, and whether the valve can be stroked with the actuator fitted.
7. What to do when it does not fit
There are four legitimate answers and one illegitimate one. The legitimate answers are a spool piece, a transition flange, a change of body pattern, and a change of valve type. The illegitimate answer is to force the joint, which is what happens when the outage is already running.
| Situation | Cause | Correct response | Do not do this |
|---|---|---|---|
| New valve 38 mm shorter than the gap | Short pattern supplied against a long pattern installed | Fabricate a spool to the measured gap, or exchange to long pattern | Pull the gap up with bolts through a gasket |
| Bolts will not enter the holes | PN flange against Class flange | Transition flange or a new valve with the correct drilling | Ream the holes or use smaller bolts |
| Six inch joint with 1.3 mm offset | DN150 PN16 against 6 inch Class 150 | Replace the mating flange or use an adapter | Hammer the bolts through and tighten |
| Gasket will not seat | Ring joint valve on a raised face flange | Correct facing or a transition spool | Stack two gaskets |
| Old actuator will not mount | Different ISO 5211 pad or stem height | New bracket and coupler, re-datum the positioner | Adapt the bracket with a torch and a drill |
| Leak appears at first heat up | Cold measured gap, no allowance for growth | Re-measure hot, add expansion loop or slip joint | Re-torque hot without a procedure |
| Valve will not stroke after installation | Actuator fouling insulation or steel | Relocate or extend the stem | Remove the insulation permanently |
8. Writing it into the purchase order
None of this is expensive to prevent and all of it is expensive to fix, and the prevention is a single sentence in the requisition. The specification should state the face to face dimension explicitly, in millimetres, with the standard and series named, rather than leaving it to be inferred from size and class. It should state the flange standard and the facing type, and it should require the dimensional drawing for approval before casting or machining begins. It should state the ISO 5211 pad size and the drive dimensions. And it should state that any deviation from the approved drawing is a non conformity, because the alternative is a valve that is technically compliant and physically unusable.
A model clause reads as follows. Flanged ball valves shall be full port, and shall have a face to face dimension in accordance with ASME B16.10 for the specified size and class, long pattern where the adjacent pipe is fixed, stated in millimetres on the approved drawing. Flanges shall be to ASME B16.5, Class as specified, raised face with a surface finish of 3.2 to 6.3 micrometres Ra, drilled to the bolt circle and hole diameter of that standard for the size and class, and shall not be drilled to any other standard. The valve shall be provided with an ISO 5211 mounting pad of the size stated, with a drive square and stem height as shown on the drawing. A dimensional drawing shall be submitted for approval before manufacture, and the finished valve shall be dimensionally inspected against it, with the face to face length, flange outside diameter, bolt circle, bolt hole diameter and pad dimensions recorded on the inspection report. Valves outside the drawing tolerance shall be rejected.
That last sentence is the one that matters, because the difficulty is never the standard, it is the assumption that the standard has only one answer. It does not, and the gap in the pipe has only one.
On the supply side, J-VALVES covers DN15 to DN1400, which is 1/2 inch through 56 inch, with pressure classes from ASME 150 up to 2500, in carbon steel, stainless, duplex and nickel alloys, over a temperature window of minus 196 to plus 650 degrees C. Ends are drilled to B16.5 or to EN 1092-1 as ordered, weld ends prepared to B16.25, and both body patterns are offered wherever B16.10 tabulates a short and a long dimension for the size. Shell testing runs at 1.5 times cold working pressure and seat testing at 1.1 times, and every valve leaves with a dimensional record taken against the drawing that was approved. Send a marked up sketch, or a photograph of the old nameplate, and a dimensioned and priced reply follows within three working hours.
9. Frequently Asked Questions
Why can two six inch Class 150 ball valves have different face to face dimensions?
Because ASME B16.10 tabulates more than one valid length for the size and class. For six inch Class 150 the short pattern body is 267 millimetres and the long pattern body is 305 millimetres, a difference of 38 millimetres, and both are compliant. The long pattern exists so that a ball valve can replace a gate valve of the same size in an existing gap. A catalogue will list whichever pattern the manufacturer builds, and a purchase order that states only size and class accepts either.
Will a DN100 PN16 valve fit a four inch Class 150 flange?
No. The nominal sizes are not equivalent and the flanges are drilled to different standards. A DN100 PN16 flange has an outside diameter of 220 millimetres, a bolt circle of 180 millimetres and eight 18 millimetre holes. A four inch Class 150 flange has a 228.6 millimetre outside diameter, a 190.5 millimetre bolt circle and eight 22 millimetre holes. The bolt circle is 10.5 millimetres out and the holes are 4 millimetres smaller, so the studs will not pass. Reaming the holes to make it fit is prohibited and creates an unevenly loaded joint.
What is the difference between face to face and end to end?
Face to face is the dimension between the gasket faces of a flanged valve, measured with the valve closed and with no gaskets fitted. End to end is the dimension between the weld end preparations of a butt welded valve, and because the bevel is part of the valve the weld gap is additional to it. Both are tabulated in ASME B16.10, and both are affected by the pattern, so a short pattern flanged valve and a short pattern welded valve of the same size are not the same length.
A replacement valve is 1.3 millimetres off on the bolt circle. Can it be fitted?
It can be forced and it should not be. The 1.3 millimetre case is the DN150 PN16 flange against the six inch Class 150 flange, and because the hole diameters are both nominally 22 millimetres the bolts will go through at an angle if they are hammered. The result is a joint with asymmetric bolt load, a gasket that is not evenly compressed, and a leak that appears at the first thermal cycle rather than at hydrotest. Use a transition flange, a spool piece with the correct drilling at each end, or a valve drilled to the mating standard.
Does the actuator have to be replaced when the valve is replaced?
Frequently, yes, even when the valve is dimensionally identical at the pipe. ISO 5211 defines the mounting pad by flange size, drive square, stem height and bolt circle, and two manufacturers building to the same size and class will often use different pads. Check the pad size, the drive dimensions and the stem height on the old valve before ordering, and specify them, otherwise the bracket and coupler will have to be remade and the positioner re-datumed during the outage. It is the cheapest item on the list to confirm and the most annoying to discover on the day.
