It's typically D to 1/2 D, more or less. But just look at the standard reducers offered by the fitting manufacturers product lists. They usually don't build standard items that are outside recommended anything. Otherwise, how much pressure drop can you stand?
I'm not sure there is one other than pure practicalities of reducer manufacture and common sense. If the flow is low enough there should be no reason why you can't go from say 12" down to 1" in one go, but as they are at the same pressure, the issue over wall thickness difference between the two ends becomes an issue (122 would need to be much thicker) and it becomes uncommon to do that. Better off to use a weldolet and a cap end.
The rate of change sounds ok to me in terms of length. After all to go from 12" to1" would need two or maybe three standard B16.5 reducers back to back, will end up being a pretty similar rate of change.
By straight do you mean a reducer made from plate and turned into a cone?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Thanks both.
LittleInch : yes, for big diameters, till 60". They are for water transport, made according to AWWA C 208 but with ref. to ASME. Here, in Europe, we have no specific standards and we always refer to USA's ones, but, as You can understand, we are not so familiar
If i were you i would look at the manufactures e.g. Trouvay & Cauvin that makes an excellent _book_ (yep the paper thingies) with a lot of typical fitting. Well they max out at 40"+48" here then length is 711 mm for a std excentric ANSI B16-9 (1978) reducer - since D is 1219 mm (OD) thats about 1/2D - but its also close to what you intialyy mentione (L=4(D-d)
