Pressure pipe stresses and wall thickness

1. Use gauge pressure

Stresses depend on the pressure difference between inside and outside. Absolute pressure should not be used directly when the outside is also at atmospheric pressure.

Practical tip: always write p = pin − pout explicitly in the calculation note.

2. Obtain both membrane stresses

For a closed pipe, pressure creates hoop stress σθ and longitudinal stress σz. In the thin-wall model, σθ is twice σz.

Radial stress is neglected in this approximation, which is no longer valid for a thick wall.

3. Check the thin-wall assumption

The Dm/t ratio is a quick check. There is no universal threshold independent of required accuracy, but a high ratio better supports the model.

If the wall becomes thick, use Lamé equations or a method compliant with the applicable code.

4. Move from net to real thickness

A thickness from a simple formula should not be ordered directly. Add corrosion, erosion or machining allowance, account for negative product tolerance and select an actually available wall.

Nominal thickness may also be governed by stiffness, handling, welding or code minimum rules.

5. What the pre-calculation does not cover

Industrial piping is a system, not only a straight pipe.

• External pressure and buckling.
• Weight, thermal expansion, support reactions and flexibility.
• Branches, bends, reducers, welds and stress intensification.
• Flanges, bolts, gaskets and leakage.
• Fatigue, water hammer, vibration and occasional loads.
• Allowable stresses depending on material and temperature.