Storage Tank Corrosion Allowance

This will depend on various conditions:
What fluid is stored in the tank?
Is the tank painted or otherwise coated or treated?
Is it buried? If above ground, is it in a wet or coastal climate?
Does API 653 address the matter? If so, does it mandate a procedure; or does it only say that you must consider it?
Do you have original drawings or other way to be sure of the original thickness?

The tanks are above ground. Inside is not coated.

But the main subject is; Do we need to consider C.A. or not? Because all the tanks are existing tanks and used for 30 years. All tanks are corroded. For the future use do I need to add C.A. to my calculation?

Thanks

Unless your local code addresses the question specifically, I'm not sure there is an absolute correct answer.

The tank may have had a C.A. initially. For the sake of discussion, imagine that it was 2.0 mm. Now suppose that you measure the thickness and find that 1.5 mm has been lost to corrosion in the 30 years. In 10 more years, you could expect to lose another 0.5 mm. If so, the total loss 10 years from now would be 2.0 mm, which is already accounted for.

OTOH, if none of this is known, you should determine the thickness needed, add to it the amount of corrosion that could be anticipated, and verify that the remaining thickness exceeds that total. In a dry climate, you might need little or no C.A., in a wet climate more C.A., or in a coastal climate even more still.

Whatever may be the case, be sure of what your local regulations require.

No, you do not need to add the CA to your wall thickness corrosion...

At 30+ years of tank life, you are in the process of consuming the corrosion allowance.

Try to get some examples of the API wall thickness calculations. It is explained in the method used to calculate remaining tank life.

Consider, if you will, restricting the maximum liquid level permitted in the tank to suit the remaining wall thickness,

Also, please tell the MBAs that they have gotten 30 years service from a group of tanks and, sooner or later, everything has to be replaced.

Be Respectful....Come back and tell us your final resolution....

Hire a qualified engineering firm to carry out the study.

Asking an anonymous forum to provide expert advice indicates that your company is not qualified for this project.

"lyn" I know what to do. Yours is not an answer...

Then why ask? The original post suggests otherwise.

You obviously DO NOT know what to do, or else you would not be asking an anonymous forum how to do it!

Does your client know that you have solicited total strangers to do your work for you?

Have you done a corrosion survey? There are stated minimum wall thickness in the code.

We have measured the thicknesses of all coarses and we have the data sheets of the tanks which shows the shell thickness and heights. But the datas do not contain the C.A.

Thanks Tornado and MJCronin for your approaches. I will share for final resolution...

The purpose of the forum is to ask questions. I take it you woke up on the wrong side of the bed this morning.? Please be nice or be gone. ;)

When you measured wall thickness during your tank survey, did you remember to check the bottom of the tanks? Where the tank sits (usually on some sort of concrete pad, are the pads raised above local grade? Are the bottoms lying on a cushion of some sort? If so, can the cushion wick moisture? Or are the bottoms of the tanks sealed. This in my humble opinion is where corrosion is likely to be the worst in many instances, simply because it is a local zone that is not carefully controlled.

Another question: did the original specs call for cathodic protection (impressed or sacrificial anode)? The success of application of cp may well relate to the size of the tank and number of anode points.

If internal corrosion is the only concern, that would depend on maintaining a dry state with respect to water/electrolytes inside the tank(s).

Hi, what you need to do is consider the rate of corrosion that you had on the tank during the 30 years and forecast for next ten years, check that with the thickness remaining and if it is enough you are in good shape to continue using the tank, otherwise you will need to restrict the maximum level of fill in your tank.