Minimum wall thickness or the maximum allowable working pressure of ferrous boiler tubing, up to and including 127 mm O.D., the following formulae, as given in PG-27.2.1, are used:

The symbols in the formulae to be used in this module are found in Paragraph PG-27.3 and are defined as follows. It is extremely important that the correct units be applied when performing the calculations:

t = minimum required thickness (millimeters, mm). (Also see PG-27.4, Note 7)

P = maximum allowable working pressure (megapascals, MPa). (Note - this refers to gauge pressure)

D = outside diameter of cylinder (millimeters, mm)

R = inside radius of cylinder (millimeters, mm)

E = efficiency of longitudinal welded joints or of ligaments between openings, whichever is lower. The values allowed for ‘E’ are listed in PG-27.4, Note 1. This is a factor that has no units, (for example, the value of ‘E’ for seamless cylinders is 1.00)

S = maximum allowable stress value, at the operating temperature of the metal, as listed in the Table PG-23.1, (megapascals, MPa). See PG-27.4, Note 2. The tables are located in an Appendix near the back of the 1983 Code Extract. (For example, the max. allow. working stress for SA-192, at 400°C, is 73 MPa)

C = minimum allowance for threading and structural stability, (millimeters, mm). See PG-27.4, Note 3

e = thickness factor for expanded tube ends (millimeters, mm). See PG-27.4, Note 4

y = a temperature coefficient: This factor has no units and has a value between 0.4 and 0.7. The values allowed for y are listed in PG-27.4, Note 6, (for example, for ferritic steel at 550°C, the value of ‘y’ is 0.7)