Water Storage Tank Inspection and Repair

I don't quite understand this description. The tank circumference is just under 30m, which likely would be fillet welded to the base plate. Where is the annular ring? Inside or outside the cylinder? Top or some intermediate position? If welded continuously, that too would be just under 30m of weld.

As to man-hours, consider this: If one man can dig a post hole in 60 seconds, how long will it take 60 men to dig a post hole? (Courtesy Ambrose Bierce)

Outer Annular ring. Haven't inspected the internals yet but as anular ring is all gone, so we can assume the situation inside.

Hmm? Is there some source of external corrosion? If the base plate projects outside the cylinder, what is the condition of the base plate, so far as you can see?

That's what I'm trying to figure out. During next shut, we'll go in with full preparation of whole base plate replacement, if required so we need to have a contingency plan. Also, do we cut the annular ring at the corner of tank shell as I dont wana change the height of tank? I saw somewhere a suggestion to start cutting tank lower shell 1 meter from the base. Is it necessary?

That's a large and potentially dangerous tanks and you should be consulting specialists for advice specific to the actual tank that you have.

What counry are you in??

In Australia there are a couple of contractors who specialise in tank inspections (Even for potable water while the tank is still full) and they give good condition assessment details. A tank that size would take them less than a day on site to inspect the inside and outside, and even to remove sediment from the tank floor using the available scour lines or else a syphon over the top edge.

Wherever you are, there should be similar services available.

From your description, the tank is in very poor external condition but you do not know the internal situation. If there is sediment and it has excluded oxygen from getting to the metal, you might find that the floor is in better condition than you suspect.

The best advice might turn out to be to scrap it and replace it with a plastic one.

First of all do thickness testing of the shell plate of the tank. If the thickness is found OK with respect to the minimum required thicknesses, then you should not worry about internal/internal parts corrosion. You can plan for bottom plate replacement. If not then plan for replacement of whole tank (CS or plastic as suggested earlier) If the shell thickness is found OK and you plan for the replacement of the bottom plate, replace the bottom plate with the annular plate (if i assume you are talking of the bottom annular plate that is outside of the shell). You are not required to procure the annular plate separately as that can be fabricated along with the bottom plate/ as a part of bottom plate. Refer the drawing you have to understand the bottom plate layout. As for manpower nobody can tell you the time required as there is very little idea of scope of work/resources/competency/conditions available/inspection requirements. We had replaced the bottom plate of 6m dia. CS tank in around one month. This might give you some idea. Also please check the condition of the bottom plate (thickness) whenever the tank is available for internal inspection. If the plate is OK then plan for annular plate replacement only.

The annular plate is preferred to be thicker than the bottom plate, please refer to your regional code- and we prefer to use butt-welded joints at that annular plates, where we use lap-joint for rest of bottom plates.

To repair and replace of corroded bottom plates of storage tank including its annular plates, you have to follow such procedure to avoid collapse of tank shell. The attached file is very interesting method for repairing and replacing: Niigata_-_Replacing_Bottom_Plate.

Mahesh said it took 1 month to replace the floor when the tank size was 6 m. In our case, its 9.5m, so it will take more than that. It means we will be required atleast 2 welders and whole base plate replacement around 1.5 month. Do we order base plate in the same sections as mentioned in the drawing? Is it necessary to calculate existing base plate thickness? How we will insert the plate in, cant use manhole.

To assess this, one would need a drawing showing the tank and what surrounds it, to see what size of plates could be slid under, around, beside, inside, etc.

As stated earlier there are many factors affecting the duration of this type job. Thus generalization to 1.5 months is not justified. There are many people who do this type of bottom plate replacements. This is normally done with hydraulic lifts/supports for the tank to make space for bottom fabrication. You can enquire about them and ask them about the required time duration according to the site conditions, resources available.