Aluminium Demineralized Water Storage Tank Leak

[Bambie]

thread238-153684
A 50 ft dia x 34 ft high aluminum demineralized water storage tank located outdoors began leaking from the floor at a single location on the perimeter in March.

After draining, internal inspection revealed a 1/2"dia hole in the floor plate approximately 6 inches inboard from the shell/floor corner weld.

The tank shell sits on a 6 foot deep cylindrical concrete slab that has a 6 inch wide and 4 inch high curb around the perimeter that was coated with tar.

The floor plates are supported by a 4 inch deep layer of sand that is retained by the curb.

The tank has a single 17AWG copper ground cable bolted to a welded lug on the exterior shell surface and lag bolted to the concrete curb.

The tank is heated in winter by a steam coil and is connected to the Turbine Building via a 20"dia buried pipe.

The hole is located diametrically across from the ground cable.

My question is whether the ground continuity was lost and that resulted in a single hole from either galvanic corrosion or a lightning strike.

Will measuring ground resistance then disassembling, cleaning and re-assembling the ground cable connections followed by ground resistance measurement point to the cause?

 

[Bambie]

Ground cable and hole pictures are attached.

 

[FacEngrPE]

This could be concentrated galvanic corrosion. Aluminum anodes are often used to protect other materials. Galvanic corrosion is a possibility if the corrosion was from the outside of the tank. Does the foundation include anything that could concentrate corrosion in a small area? Have you performed a tank floor thickness survey (part of an API Tank inspection)?

Galvanic corrosion is unlikely on the inside of the tank if the conductivity is as low as demineralized water usually is. However demineralized water can be corrosive to aluminum for other reasons, the literature is mixed on this issue. Do you know the pH of the stored water?

 

[Bambie]

FacEngrPE

I have attached a photo of the floor plate hole (it seems you can only upload one at a time). It is circular and appears to have straight sides all the way through the thickness. A borescope video and a replica of the hole is planned.

The floor is 1/2" thick ALCAN 5083-H12A, tank is fabricated to USA STD B96-1-1967.

A floor plate thickness survey is also planned, especially 6 inches inboard from the shell where the floor is in contact with the concrete curb.

I have no information about pH, just very low levels of silica, sodium, organic carbon, chloride, sulphate and .006 mS/m specific conductivity at 25 degC.

If it was a lightning strike discharging through the floor plate, would the hole have a characteristic appearance?

 

[Bambie]

Another Picture of the hole is attached.

 

[Kiribanda]

All corrosion prevention methods should protect the bottom plate of any surface installed tank.
I donot know whether the bottom plate of this tank is protected by a scarificial anode or
an impressed current system. In both cases the current has to be collected at the tank bottom
so that tank bottom will be a cathode. Having said that, I wonder whether the 20" underground
steam line is carrying any DC current and subsequently discharging through the tank bottom thereby
making the bottom plate ANODIC for a long time and created a hole at a weak spot.
Any thoughts?

 

[Bambie]

Kiribanda,

Cathodic protection is not shown on drawings and after 50 years of service I suspect a sacrificial anode may be depleted, so I agree that galvanic corrosion is likely the cause if galvanic corrosion also disabled the grounding cable.

The curious thing about this corrosion pit is its unusually large size and clean appearance.

I am hoping the borescope examination will reveal a 1/2" diameter holiday in the tar coating on the concrete curb that formed a constant diameter hole straight up through the 1/2" wall until the remaining thickness collapsed under the 34 foot hydrostatic head.

Otherwise, wouldn't galvanic corrosion stop once a small leak began?

I also think a lightning strike and discharge through the floor would leave a very messy crater.

I have attached a photo of the leak with approximately 30 feet of head in the tank.

 

[Kiribanda]

[highlight #EF2929]I also think a lightning strike and discharge through the floor would leave a very messy crater.[/highlight]
I donot think it can happen because this tank being a fixed roof one any lightning discharge current
will be grounded through the shell. But in a floating roof tank it can happen because during a lightning
dischagre to a remote place, the trap charges on the floating roof has no way to go other than through
the internals of the tank.

 

[Compositepro]

Thin skinned aluminum aircraft get struck by lighting regularly without damage. The high thermal and electrical conductivity of aluminum spreads the current and heat. Welding electrodes work by keeping the the arc tightly confined to a small area. This could happen in a tank under certain circumstances.

 

[Bambie]

In the last photo I posted there is a dark area on the aluminium shell directly above the leak, which could be explained by maintenance activity or it could be related to the 'event'.

 

[FacEngrPE]

In the pictures the inside surface of the tank floor is very smooth, except at the hole. The absence of pitting on the inside of the tank makes me think corrosion related to the tank interacting with the demineralized water must be insignificant.

If the edge of the hole is sharp edged I would suspect corrosion the hole is a corrosion pit, anodic corrosion is a likely component. There are some small dark marks near the corrosion hole that may be additional corrosion holes. You could explore these possible holes with a ice pick.

A similar leak in a steel waste water tank resulted in replacing the tank floor. We used the opportunity to install a vapor barrier and a drain under the sand bed. The theory is that keeping the sand bed dry reduces the conductivity of the sand, and will extend service life of the tank floor.

 

[Bambie]

FacEngPE,

The examination report did not mention any other surface pits, therefore I am inclined to think that these must be dirt particles.

If more detailed UT scans of the perimeter floor plate do not detect pits on the underside I will advise a fillet welded patch per API 653 and refurbishment of the copper grounding cable.

 

[FacEngrPE]

Bambie I agree - if examination of the tank floor indicates this is the only corrosion pit, or if it is desired to make a repair to get some more operating time, while organizing a more extensive repair, a patch per the API specification is appropriate.

 

[TugboatEng]

One pit? Aluminum is subject to crevice corrosion. It's possible a foreign object sitting in the bottom of the tank could create conditions to support crevice corrosion.

Has gauging been performed on the floor? It's possible the exterior of the floor is wasted and this is only the first pit to come through. An aluminum plate sitting on a sand floor is a bad situation if the sand ever becomes wet. Again, this is due to crevice corrosion.

 

[dpc]

Fifty years old? I think this tank does not owe you anything!

 

[LittleInch]

"The tank shell sits on a 6 foot deep cylindrical concrete slab that has a 6 inch wide and 4 inch high curb around the perimeter that was coated with tar.

The floor plates are supported by a 4 inch deep layer of sand that is retained by the curb."

Can you draw or sketch this arrangement and where the hole is in relation to it please.

Six feet deep foundation?? What's a "cylindrical slab"?

I know corrosion can take many forms, but that looks like some sort of inclusion in the parent metal to me which gave way finally.

The other option is if something like exposed rebar at that point or something else is there.

Only way to find out I think is cut out a say 12" coupon centred on the hole and have a look at what's underneath, then just weld a plate back on.

You may have got more traction in the corrosion engineering forum or the tanks forum....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Bambie]

LittleInch

Sketch is attached as requested.

Heating steam, condensate and Demin supply pipes cannot be disturbed, which is why a 6 foot deep somewhat cylindrical slab or foundation was required to prevent frost heaving.

 

[TugboatEng]

I strongly suggest you audio gauge the bottom of the tank before attempting repair. This is what the backside of a corrosion pit looks like when it breaks through. The underside of the bottom of your tank is likely corroded and gauging must be done to determine the extent of wastage.

 

[LittleInch]

Ah ok, frost heave issues...

I think like a few others though that you need to figure out what is going on underneath the floor and there's only one way to find out and that's cut part of the floor away and see what you can see and try and figure out if there is any reason why that hole is where it is.

I assume though you have checked that the earthing point still works as an earth?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

Since there's heating pipes snaking around under the floor could it have been a lighting strike that was a flashover to a heating pipe that happened to be the closest point to the tank bottom? The hole might look less like it was lightning-made since it was under water and prevented from having a sooty look. It could be the ground strap or more likely the ground point what-ever-it-is was not up to the task by itself.

I think removing a 12" coupon is still required to allow inspection of the piping directly below the hole. It should be done with a drill-press style drill so virtually no cutting teeth project below the bottom.

Keith Cress
kcress -

http://www.flaminsystems.com