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Reducing weld necks are an elegant solution to a common problem. Something has a particular flanged outlet but you want a smaller size pipe attached to it. The problem is that, for most commercial flanges whose dimensions are described by ANSI B16.5, reducing weld necks are not addressed. Section 3.3 of B16.5 mentions reducing flanges and refers to Table 7 which explains about reducing threaded flanges but not weld necks. That doesn’t mean that you can’t make them. Here is how it works:
Now let’s look at the following tables from ANSI B16.5 to see what flange you would get. The outside diameter of the flange, O, the body thickness, Tr, raised face dimension, R (specified on Table 4 for all flanges), and bolt pattern will all be for a 4” 150# flange.
O = 9.000”
Tr = .88” + .062” for the raised face height = .940”
R = 6.190”
Drilling = 8 - .750” holes on a 7.500” Bolt Circle
Next we incorporate the 2” reducing hub to the 4” body. The hub diameter, X, the beginning diameter of the chamfer, Ah, and the bore size for a 2” schedule 40 pipe, B, are all taken from the 2” 150# line of the table:
X = 3.060”
Ah = 2.380”
B = 2.067”
The only dimension left to determine is the length through the hub, also called the over-all length or AOL. This is important because some engineer probably wants to make this assembly part of a larger system of a certain length. To do this we need to do a little math (oh, no!). We are going to figure out the height of the hub only for a 2” 150# weld neck and add it to the body thickness of the 4” flange. In our example the 2” weld neck has a length through hub of 2.440” and a body thickness of .690” which, when subtracted (hey, I warned you there was going to be math involved) gives us a hub height of 1.750”. Now let’s add that to the body thickness of the 4” 150# flange, Tr, from above of .940” to get the reducing hub length, Y, of 2.690”. That is a good bit shorter than the 3.000” of a standard 4” 150# weld neck.
Check out the shop print for our finished reducing flange. We hope that this helps you understand reducing weld necks a little better. Reducing slip-on and reducing threaded flanges are much simpler … but that is a topic for another time.