REPAIRING A FIRE-FIGHTING WATER TANK (CAP. 35000 BLS) 4

[vargaslu]

Because of a failure in the filling control system of the tank, the roof and the last shell course were plastically deformed, as you can see in the pictures of the attached file. Our Construction crew want to weld a thinner shell over deformed shell, but I am not agree with this solution. My proposal is, cut the deformed foil sections and replace with new ones, following a proper job procedure. The main difficulty is that the tank can not be empty, because the plant is in continuous operation and can not be stopped. My questions are: any of you had a similar experience? what would be your recommendation based on API STD 653?
The tank has a height of 40 ft and a diameter of 81 ft, five shells of steel ASTM A36. Thickness: 13mm, 10mm, 8mm, 8mm, 8mm.
I would appreciate your comments about the case.

 

[Duwe6]

I can't see any creases, just smooth deformation. It's ugly, but not debilitating. I'd fix the paint and leave it alone.
on the other hand, with some strategic temporary stiffening, what would be the problem with doing a repair while 80-90% full of water?

 

[MJCronin]

Ok.....as you described it....this seems to be a problem because of overfilling the tank, subsequent roof deformation etc.

My understanding of tank design is that a properly sized overflow should prevent this accident ....????!!?

Quick question: Does the tank have a functioning overflow ? What is the sized of the overflow and the rate of fill during the incident

 

[vargaslu]

MJCronin..
Answering your question:
Does the tank have a functioning overflow? yes
Size of the overflow = Ø4" designed for a rate of filling of 6 L/s
Rate of fill during the incident = aprox 50 L/s

 

[LittleInch]

I thought the damage was rather odd, your tank has clearly been over pressurised as the overflow was overwhelmed and the first thing you need to fix is the overflow to stop this happening again.

I tend to agree with duwey6, It desn't look great but does not structually affect the tank so I would leave it alone until you can either build another one or have a complete shutdown when you can drain the tank and replace the top sections.

Maybe reduce your max fill height by a few hundred mm and clean up the bent / flaking bits and the safety barrier.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[hokie66]

I agree that the tank itself should still be adequate for water storage, but the support for the roof has been compromised. How is the roof framed? Do you have high snow loads where this is located?

 

[MRH1335]

The repair can be done by adding new plates but the tank roof and wall are buckled and at yield. The future Internal and external stresses won’t be uniform. There is still some inelastic capacity left. Try to add plates where material is at yied and use Girts (horizontal Plates) perpendicular to the tank wall to reinforce it.

MRH

 

[JStephen]

If you do the repairs proposed by welding plates over it, it will be cosmetic only, and won't really accomplish anything structurally. I would just wait until the tank can be taken out of service and properly repaired- even if that's a year or two down from now.

 

[Duwe6]

It's full of WATER. With proper bracing around the deformed areas, they can be cut out and replaced with this tank 80-90% full. Welding is safer with water in the tank than on "empty" tanks. Many folks have been killed or maimed working on empty tanks.


If 90% full is deemed too little water for safe operation of the unit(s) this tank provides fire-water for, you need to build another tank as soon as possible. You are operating too close to the limit to be prudent.

 

[vargaslu]

hokie66:
Thanks for your answers.
The tank has a central column which supports the roof and circumferential and radial beams.
Never snows around here. There is an attached drawing.

 

[JStephen]

If you have rafters dislodged or something like that, it's a minor structural problem.
Otherwise, it's mainly a corrosion problem waiting to happen.
If you weld stuff on the outside to "fix" it, you'll still have that problem on the inside.
Quite likely, if you do repairs while it's got water in it, you'll still have issues.
And if you do nothing, you have issues.
So it needs to be properly repaired, but that is not an immediate need.