Cracks in large storage tank wall 1

[corrosionman]

We have an open topped circular tank and the base is a steep sided cone. It is a settlement tank where the heavy solids sink and get drawn off at the base and clarified liquid overflows into a channel at rim level. Tank dia is 25 feet and total height approx 30 feet. Temp ranges from ambient
to 60 C. Tank is 6 mm mild steel. Contents vary daily from slightly acid to slightly alkali. Tank has some protective lining but it is in poor condition. There is no turbulence or vibration - - - it is a very gentle flow .
A small support bridge was WELDED from side to side at rim level. Soon after we noticed cracks down the wall (approx at 45 degree) going out from the new welded points.

The vessel is 12 years in present location , but it had prior use in an unknown situation.
i.e. A second hand tank !

What can be cause of these cracks.
Any comments much appreciated.

 

[sshep]

My bet is stresses caused by some sort of differential thermal expansion cause by the bridge which has nothing to do with the tank service. -sshep

 

[JStephen]

If the bridge is welded solid, and the tank tries to expand as the volume varies, or with temperature, you would get some stresses there. As you load the bridge, it would rotate at the ends, and that could be part of the problem. You normally wouldn't expect cracks like that, though.

Not knowing the past history of the tank, not much telling what is going on. Could be a welding problem. Could be from previous service in caustic or other service, too. Better detailing (some kind of expansion joint in the bridge) might help.

 

[unclesyd]

Looks like you have a vibration problem. This is very common on attachments to thin wall vessels/tanks. It is probably a low and slow oscillation of the bridge.
A possible remedy is to put a pad, no sharp corners, on the vessel walls and attach the bridge to it.

 

[JStephen]

You mentioned no vibration (in the flow). Does the bridge support some sort of mixer? "Vibration" wouldn't necessarily be the problem, but fatigue from continued movement could be.

 

[corrosionman]

The bridge supports the central feed pipe and an open ended
6 foot dia cylinder which compels the incoming feed to go down into the body of the tank liquor so it does not just drift across the surface and flow over into the outlet launder
at the tank rim. Upon reflection I guess there must be some undulating load from this .
Today someone suggestd Stress Corrosion Cracking - - - is this
something that can occur with Mild Steel at trivial stress levels and relatively weak acid ?????
Please keep the comments coming. DW.

 

[unclesyd]

Not to say that SCC or more than likely HIC can't occur I believe that you have the undulating load as you state.
A quick way to check this is petri dish filled with water set on the bridge. I'll bet you will have a standing wave.

One other possibility is the possible differential expansion between the bridge and the tank wall.

 

[sshep]

Can you clarify that the bridge is welded to the tank rim on both sides rather than resting on some bearing surface so as to allow some independent motion of the bridge?

Hydrogen embrittlement is the most documented form of stress corrosion cracking in carbon steels. Hardness testing is a typical way to check for problems in steel which has been in this type service. I wouldn't think slightly acid wastewater (pH>5.5?) service would be a likely service to cause SCC. In any event you can't have stress cracking without stress. I believe mechanical explanations for stress and/or fatigue will ultimately prove to be the real culprit. -sshep