Pressure design of flanges is a complex task, requiring consideration of the configuration and materials of the flange, bolts, and gasket. Potential causes of failure are bending stresses in the flange, localized stress concentrations in the hub, yielding of the bolts, or unloading of the gasket, causing leakage. Design of flanges is covered in detail in Section VIII of the ASME Boiler an Pressure Vessel Code; however, due to the complexity, it is rarely done by the pipe stress engineer. Instead, the most common piping codes endorse the use of flanges conforming to recognized standards such as ANSI B16.5 "Pipe Flanges and Flanged Fittings". This standard designates standard pressure classes of flanges, which are recognized by the codes to be acceptable for the following combinations of pressure and temperature:
A more detailed discussion of flange analysis, with specific regard to determination of leakage under load, is provided in Section 3 (Modeling And Analysis Of The Piping System) of these Piping Stress notes.
Blanks are designed based upon formulas for the calculation of bending stresses for plates under pressure loading. The minimum thickness for a blank is calculated as:
dg = inside diameter of gasket for raised or flat face flanges, or gasket pitch diameter for ring joint and fully retained gasketed flanges, in