# ASME 31.3 Allowable Pressure Calculator

## Calculate ASME 31.3 allowable pressure

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)*

### 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 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) (16000 psi) (0.8) / ((4.5 in) - 2 (0.237) 0.4)*

* = 1408 psi *

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

*t - thickness of pipe (in)*

* S - allowable tensile stress (psi)*

* E - quality factor for piping material*

* D - outside pipe diameter (in)*

* Y - wall thickness coefficient*

### 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**:

Coefficient Y | ||||||
---|---|---|---|---|---|---|

Material | Temperature (^{o}F) | |||||

< 900 | 950 | 1000 | 1050 | 1100 | > 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 |