Q&A: ISO TANK CORROSION ALLOWANCE

Q: “I inspected a 1987 Consani tank.  I have a question about minimum thickness on IM-101 tank barrels.  The data plate said the thickness was 4.8mm (equivalent to 6.35mm steel).  I did not see a note that any corrosion allowance was built in.  What is the minimum thickness of this tank?  I took some readings and want to make sure the barrel is thick enough.  Any other items I should note about this 1987 Consani model?”

A:  In general, tank containers have very little corrosion allowance built-in.  What the data plate is telling you is that the tank was built with shell material that was 4.8mm actual thickness (before forming), equating to a Mild Steel Equivalence of 6.35mm.  The head material will undoubtedly have a greater nominal thickness as this is likely a 4 bar MAWP tank, and the heads would have to be thicker to maintain the same MSE, not to mention to handle the 4 bar pressure rating, due to the thinning that occurs during head forming.

The latter number of 6.35mm is important because the IMDG regulations specified what MSE thickness was required for carriage of different products (for example 6mm MSE, 8mm MSE, etc., depending on how hazardous the product is).  Most of the tanks of that vintage had a 6.35mm MSE and were constructed of 3/16″ shell material Type 316L or equivalent, with 1/4″ thick heads (again, nominal thickness before forming).

1987 Consani tanks were well-built in general but considered heavy in terms of their tare weight as compared to other (newer) tank containers.  We have sold a number of these units, and our guess is that you are looking at a 23000-liter tank with a tare weight of approximately 10,000 lbs.  If the shell thickness readings are comparable to the original values and the tank passes a hydro test at its full Test Pressure of 6 bar, the only thing that I would be concerned about (if it is important to you) is having a working steam system in a tank that is 22 years old.