Pipe Wall Thickness Calculator (Internal Pressure)

How is pipe wall thickness calculated?

Internal pressure pulls the pipe wall in the hoop direction. The simplest relation is Barlow's formula: t = P × D / (2 × S), where P is the internal pressure, D the outside diameter and S the allowable stress of the material. For process piping, ASME B31.3 writes the same idea in a little more detail:

t = P × D / (2 × (S × E × W + P × Y))

E is the quality factor of the longitudinal weld, W the weld joint strength reduction factor at elevated temperature and Y a coefficient that depends on material and temperature.

Allowances and tolerance

The formula gives only the thickness that resists pressure. Corrosion, erosion and thread allowances are added to it: tm = t + c. Because pipe may be made thinner than nominal, mill tolerance is taken into account too: Tmin = tm / (1 − tolerance). Seamless pipe standards usually set the tolerance at 12.5 percent.

Example

A pipe of 168.3 mm outside diameter, 40 bar design pressure, 138 MPa allowable stress, seamless (E = 1) and Y = 0.4. The pressure design thickness is 4 × 168.3 / (2 × 139.6) = 2.41 mm. With a 1.5 mm corrosion allowance it becomes 3.91 mm, and with 12.5 percent tolerance the minimum nominal wall is 4.47 mm. The next thickness above that is chosen from the standard table.

Limits

This calculator covers straight pipe under internal pressure only. You enter the allowable stress from the code table. Bending, thermal expansion, external pressure, bends and branches and flange ratings are checked separately. Final design is the job of a qualified engineer.