Large storage tanks - leak testing of tank bottoms

[jonesey]

Not sure if i'm in the right forum here (it's not a pressure vessel type question) but i'll ask the question anyway. We have a number of large atmospheric storage tanks (50-80m dia) that we inspect on a 15 -20yr basis. After repairing the tank bottoms all welds are vacuum tested before coating to ensure they are free from cracks. Normally once the tank bottom has been vacuum tested, driving of vehicles/lifts is forbidden as they can cause cracking of welds as the tank plates flex. Our current contractor wants us to paint the tank bottom after vaccum testing and then afterwards drive a 2000kg lift around the tank bottom as we set the tank roof up. We are worried that this lift can now cause cracking of the welds under the coating that cannot be detected with a vacuum test. Any thoughts/ideas would be welcome.

 

[BigInch]

Wouldn't you want to set the tank roof up and generally finish the tank work in its entirety BEFORE doing a leak test?

Going the Big Inch!

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[jonesey]

That is what we normally do but we are soon approaching winter and the plan for coating after test would put us into winter with subsequent problems with minimum coating temperatures and longer curing times. Therefore the sub-contractor has requested to coat the tank bottom now while we have temperatures well over freezing.

 

[zdas04]

Can you put down swamp mats for the lift to drive on? These things can spread a very heavy load over a broad surface--they're used to allow track hoes and side booms to drive on very soft surfaces.

Regardless of how you protect the tank bottom, I would lengthen the time of the vacuum test probably by a factor of two to allow time for the paint coating to fail over a crack.



David Simpson, PE
MuleShoe Engineering

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[jonesey]

The coating is some form of epoxy paint. I'm not sure that a lengthened vacuum test would be enough to make the paint fail ? In accordance with API 650 we are talking about a partial vacuum of 3-5psi.