Corrosion allowances in B31.1 calculation 1

[carletes]

Dear all,

Anyone knows why B31.1 when calculating sustained or expansion load cases uses nominal thickness without considering corrosion allowances? And on the contrary, when calculating the required thickness due to pressure such allowances are taken into account? Moreover, taking into account that B31.1 doesn't specify corrosion allowances (or at a least a maximum value) there is a risk of someone taking very high allowances.

Best regards and thanks

 

[smckennz]

Hi carletes

B31.1 tries to take a worst case scenario. Transmitted loads will be highest when the pipe is new and the wall thickness is a maximum. Hoop stresses will be at a maximum when the pipe is corroded and the wall thickness is at a minimum.
B31.1 cannot specify a corrosion allowance because it does not know the process conditions (corrosion rate) or the design life of any particular application and materials choice. This is up to the engineer and client to agree.

Cheers

Steve

 

[bvi]

The following are the B31.1 requirements

Pressure design - subtract corrosion allowance and mill tolerance from the thickness.

Sustained load design - nominal thickness is used. This appears to me to be unconservative, but it is how it is. One argument put forth by some B31.1 folks is that the piping material should be selected such that corrosion will not be significant. Other codes do different things; for example, B31.3 requires you to subtract the corrosion/erosion/mechanical allowances from the wall thickness when calculating longitudinal stress (but not mill tolerance).

Expansion stresses - These are always calculated based on nominal wall thickness. The stress in the pipe is essentially independent of wall thickness for stresses due to thermal expansion.

 

[KevinNZ]

In our work we consider two design cases.

1. Full wall thickness corrosion = 0. Which also gives the correct support loads.

2. Reduced wall thickness. Assumes the pipe has corroded at an even rate everywhere. Which is the case on our projects.

We find the calculating forces from full wall and stresses from reduced wall too conservative.

Does anyone else use this approach?