how do i measure pipe flange sizes for putting on blanks?

Answer 1

Amended
There are two kinds of blanks. One is a blank flange which is put on the end of a pipe. It will have the same dimensions as a standard flange for that service, except it will be solid.
The other is the thin blank inserted between two flanges in the run if the piping and used for isolation. The blank thickness will depend on the pipe size and the pressure.

You will want the insertion blank to fit inside the bolts so that you do not have to drill holes. For each size of standard pipe flange there will be standard dimension drawings. Use those.
You will find that most steel flanges have a raised face. The diameter of the raised face is the same as the diameter of the gasket and will fit the same need for sizing a blank. Be sure to also have part of the blank ( a tab) extending between and past two bolt holes and a few inches past the flange OD. The blank will be installed so this tab may be easily seen, usually on the top. This tab will indicate to others that there is a blank in place between those flanges.
And make sure the gasket goes on the pressurized side of the blank.

Answer 2

Pipe Blanks

Answer 3

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RE:
how do i measure pipe flange sizes for putting on blanks?

Answer 4

If u know the pipe size than the flange size is the same
but discription of pipe is dia 4" sch 80 smls A-106 Gr B where as discription of flange will be dia 4" 3000# A-105 WN RF (Neck as per sch 80).

For material and rating depends on services and working condition check out any specification sheet