Valve Body Flange Thickness Without ASME B16.5

You could use a vessel code such as PD 5500 or ASME VIII, or use an API flange.

Depending on the size and pressure this might be a lot more of an exercise than a person would first think.

Flange manufacturers have computer programs that calculate the dimensions in accordance with various codes, taking into account the bending, end load and torsion that might be exerted by adjacent piping. If you are dealing with flammable or toxic liquids or gases you need to get it right. If it is something like very low pressure water or low pressure air you might be able to cut a flange from 1/4" plate.

Why not use a standard flange? These are relatively low cost and you save the trouble of designing it.

If you think you need additional details, send a note to the forum and I (or just as likely someone else) should be able to assist. You will need to confirm maximum pressure, fluid in the valve, any end loading (including bending moments), temperature whether it is indoor, outdoor, buried, subsea etc. and the valve or pipe body material. The type of gasket (Ring joint or flat) would also help although this is often decided by the service conditions.