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ASME 31.3 Piping Materials - Allowable Pressure Calculator

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The ASME code B31.3 recommends allowable tensile stress levels in pipe materials. The equations used in B31.3 are

t = P D / (2 (S E + P Y)                    (1)

where

t = thickness of pipe (in)

P = internal pressure in pipe (psi)

D = outside diameter of pipe (in)

S = allowable tensile stress (psi)

E = quality factor for the piping according ASME 31.3

Y = wall thickness coefficient according ASME 31.3 (y = 0.5 for thin pipes and y < 0.5 for thick pipes)

or

t = P (d + 2 c) / (2 (S E - P (1 - Y))                    (2)

where

d = inside diameter of pipe (in)

c = sum of the mechanical allowances (thread or groove depth) plus corrosion and erosion allowances (in) - typical 0.02 in where tolerances are not specified

Eq. (1) can be rearranged to express allowable internal piping pressure as

p = 2 t S E / (D - 2 t Y)                      (1b)

Eq. (1b) can be modified to compensate for thinner pipes due to the thickness tolerances used by the industry.

p = 2 t (1 - α / 100) S E / (D - 2 t Y)                      (1c)

where

α = thickness tolerance (%)

For ASTM A53, A106 and other codes the accepted thickness tolerances are 12.5%. 

Example - Allowable Pressure in a Steel Pipe

The allowable pressure in a 4 inch schedule 40 carbon steel pipe with outside diameter 4.5 in, wall thickness 0.237 in, thickness tolerance 12.5% and allowable stress 16000 psi - can with quality factor E for steel 0.8 and wall thickness coefficient 0.4 be calculated as

p = 2 (0.237 in) (1 - 12.5 / 100) (16000 psi) (0.8) / ((4.5 in) - 2 (0.237) 0.4)

   = 1232 psi      

ASME 31.3 Allowable Pressure Calculator (from eq. 1b)


Quality Factors E

Some quality factors according ASME 31.3:

  • furnace butt weld joint, continuous weld joint, straight seam: 0.6
  • electrical resistance weld joint, straight or spiral seam: 0.85  
  • double butt weld joint, straight or spiral seam, 100% radiographed: 1.0

Thickness Coefficients Y

The values of coefficient Y depends on type of material, temperature and thickness limits.

Thickness coefficients when t > D / 6:

Y = (d + 2 c) / (D + d + 2 c)                  (3)

Thickness coefficients when t < D / 6:

ASME 31.3 - Wall Thickness Coefficients Y
MaterialTemperature (oF)
< 900950100010501100> 1150
Ferritic steels 0.4 0.5 0.7 0.7 0.7 0.7
Autenitic steel 0.4 0.4 0.4 0.4 0.5 0.7
Other ductile materials 0.4 0.4 0.4 0.4 0.4
Cast iron 0.0
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