How To: Calculate The Maximum Allowable Working Pressure Of Piping

While preparing for your 3A2 exam you’ll be required to complete ASME code calculations involving how to calculate the maximum allowable working pressure of piping. If you read through the material below. Complete the practice questions provided. You’ll have a much better understanding of the information covered within:

All page numbers referenced below are from the 2007 ASME Boiler & Pressure Vessel Code.

Formula

To calculate the maximum allowable working pressure of piping during different operating conditions you can use the formula contained within ASME Boiler & Pressure Vessel Code PG-27 Cylindrical Components Under Internal Pressure page 10. Specifically, PG-27.2.2 page 10

PG-27.2.2 Piping, Drums, and Headers. (based on strength of weakest course) It is important to remember any cylinder with an outside diameter less then 125mm is to be considered tubing and the equation provided in PG-27.2.1 page 8 should be used.

PG-27.2.2 Formula to calculate the maximum allowable working pressure for piping:

Outside Diameter Equation

Inside Radius Equation

Formula Variables

The symbols used within the PG-27 formulas are contained within paragraph PG-27.3 page 10 and are defined as follows:

C = Minimum allowance for threading and structural stability (mm) (PG-27.4, note 3) page 11

D = Or O.D. is Outside diameter of cylinder (mm) “In this case piping”

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) page 11

e = Thickness factor for expanded tube ends (mm) (see PG-27.4, note 4) page 11

P = Maximum allowable working pressure “Gauge Pressure” (MPa) (see PG-21, refers to gauge pressure)

R = Inside radius of cylinder (mm) “In this case piping”

S = Maximum allowable stress value at the operating temperature of the metal (Section II, Part D, Table 1A. See PG-27.4, note 2) page 11. It is important when determining the maximum allowable stress value to check (PG-6 plate materials) page 4 and (PG-9 boiler tube materials) page 5 before starting calculations as this information will determine the correct stress table to use by indicating whether the material is carbon steel or an alloy steel.

t = minimum required thickness (mm) (see PG-27.4, note 7) page 12

y = temperature coefficient (see PG-27.4, note 6) page 11