Replacing a valve in an existing line looks like the easiest procurement job there is. The size is known, the class is known, the flanges are already there. Then the valve arrives and does not fit, and the line is down while a spool piece gets made. Almost every time, the cause is face-to-face.
What face-to-face means
Face-to-face is the distance between the two flange faces of the valve, or end-to-end for butt weld ends. ASME B16.10 tabulates it by valve type, size and class, so a Class 150 DN100 flanged gate valve has a defined length that every manufacturer building to that standard will hold. This is what makes valves interchangeable at all.
Where the surprises come from
The first surprise is valve type. B16.10 gives different lengths for different types. A gate valve, a globe valve and a ball valve of the same size and class are not the same length, and a butterfly valve is dramatically shorter than all of them. If someone decided to replace a gate valve with a ball valve because that was what the store had, the gap in the pipe is now wrong.
The second is class. Length is listed per class, and higher classes are generally longer for the same size. A Class 300 valve going into a gap sized for a Class 150 valve will not fit, even though the flange bolt circles look similar at a glance on small sizes.
The third is end type. A flanged valve and a butt weld valve of the same size and class have different tabulated lengths, and a wafer body has essentially none at all in the flanged sense. Swapping between them changes the geometry of the joint as well as its length.
The fourth is the short pattern. Some valve types are available in both a full and a short face-to-face, and API 6D and B16.10 both recognise more than one length for certain configurations. A valve can be entirely correct to standard and still be the wrong one of the two.
The fifth is that the original valve may never have been to standard in the first place. Older plant sometimes carries a valve built to a manufacturer-specific length, or a valve that was installed with a spacer nobody documented. Once that is in the line, the tabulated dimension is irrelevant and only a measurement helps.
What to measure before you order
Measure the gap between the mating flange faces with the old valve removed, or measure the old valve itself face to face. Record the flange standard and rating stamped on the mating flanges, the facing type, and the bolt circle diameter and hole count. If the line has a spacer or a spool already fitted, note its length separately rather than including it in the measurement.
What we do with those numbers
Send the measured face-to-face, the flange rating and standard, and the valve type you need. We will confirm whether a standard length valve drops straight in, whether a short pattern exists for that combination, or whether the honest answer is that a spool piece has to be made. Knowing which of the three it is before the shutdown starts is worth far more than the valve itself.