Tank bottom with pile foundation system : Backing strips failure 1

[ali-gct-meca]

Dear mechanical engieers,

I have a 20 year old steel tank designed per API 650 with the following caracteristics:
- Stored fluid : Phosphoric acid
- Foundation : pile foundation system
- Coating : rubber inside coating
- A mixer is mounted on a steel bridge over the tank for continious stirring (6 rpm)

I have recently encountered a failure in the tank bottom throughout the backing strips (view photo)
PS: After inspection of the downside of tank bottom, I have noticed a degradation of the backing strips

Tank experts, could you please identify the origin of the failure and give me your recommendation on the repairs that must be executed

Thank you

 

[HTURKAK]

Phosphoric acid tank bottom plates are normally but welded joints and tank bottom is supported on grid and dwarf walls / columns.

Will you provide some pictures from the tank bottom showing the degradation of the backing strips ?

 

[ali-gct-meca]

Thank you for your quick response,

Please find attached a photo of the tank downside bottom showing the degradation of the backing strips

 

[HTURKAK]

I could not see the picture?...I am online now..

 

[ali-gct-meca]

sorry i am havin a problem uploading the photo

 

[ali-gct-meca]

Photo

 

[HTURKAK]

Dear ali-gct-meca (Industrial);

I looked to the photos. Phosphoric acid is highly corrosive for the CS. This is typical corrosion of weldment. Apparently, the rubber lining is leaking and the acid penetrated to butt weld and seems concrete raft slab also deteriorated . The gypsum build up under the tank bottom between dwarf walls implies this is an old event an it is strange nobody seen the leakage ? I could not see any berm around the tank..the last picture shows there is another similar size tank ..Is there a common dike ?

I will suggest ; the rubber lining at the bottom shall be removed (at least leaky zone ) and corroded bottom shall be checked. If the remaining thickness is sufficient, the weld shall be repaired from bottom and top acc to API 653.

The RC raft and dwarf wall surfaces shall be protected with suitable lining .

 

[HTURKAK]

Dear ali-gct-meca (Industrial);

I just want to remind that , it is polite to answer when you get a respond.

Rubber lined phosphoric acid tanks are not very common . When you decide upon your final solution for the repair , please share your solution for record and to help younger fellows and keep us involved..

Kind regards..

 

[Dhurjati Sen]

Normally two types of repair.

Temporary - you identify the leaky areas using holiday / spark test, patch those areas with self-cured rubber. No welding / hot job is permitted.

Permanent - Replace the entire bottom plate and maybe 1 feet of the first shell course from bottom. Apply rubber lining after hydro-test.

DHURJATI SEN
Kolkata, India

 

[HTURKAK]

DHURJATI SEN, the tank is not ground supported . The base plate is a structural element and with bending strength ( between the dwarf walls ) shall resist the weight of acid content .... Although we do not know the concentration, the S.G. should be around 1.6 .

Apparently, the bottom plate corroded and the butt weld is perforated. IMO, a corrosion assessment study should be conducted and the remaining strength should be verified before deciding a repair.

If O.P. shares his final solution for the repair and some details of the tank, he may get more valuable comments.

 

[ali-gct-meca]

Dear HTURKAK,
I would like to apologize for not responding, it was due to a familiy emergengy ( death of a sibiling),

You have mentionned in your comment the other tank, actually there is no such failure in that tank bottom and this can be explained by the fact that backing strips axis is perpendicular to the axis of the dwarf walls whereas in the tank being discussed the axis are paralel.

What would you say is a suitable lining for the RC raft and the dwarf wall syrfaces,

I would be glad to share the solutions with you all but it may take some time

 

[ali-gct-meca]

Dear Dhurjati Sen,

The solution you have suggested is very effective ... as it is expensive, As HTURKAK, have mentioned the bottom plates have already been subjected to bending stress due to phosphoric acid weight wich is indeed with specific gravity of 1.6 ...1.7 (good guess by the way)
I think we all agree that the rubber lining shall be removed as a start and juging by what will find ...will decide what we are going to do next,

By the way ... there are around 15 tanks containing phosphoric acid in the plant where I work

 

[HTURKAK]

- Please accept my condolences,

- The problem is the perforation of the butt weld at bottom plate joint(s) rather than the failure of backing strip .

- The direction of butt welded joints is correct for this tank.. the bottom plate butt welds SHALL BE ACCESSIBLE AND VISIBLE .The backing strips axis which perpendicular to the axis of the dwarf walls IS NOT CORRECT !.

- The lining for the drainage channels should be rubber + Carbon Brick. For pavement and dwarf wall surface, raft foundation surface could be epoxy, PU, GRP .. You may consult with suppliers.

 

[ali-gct-meca]

HTURKAK,
could you give a reference from API standart which explains the specifications for the design of storage tanks with dwarf walls foundation and backing strips, because now I have two different tanks ... each with its own bottom & foundation design

Kind regards

 

[HTURKAK]

-API 650 addresses the use of tanks Supported by Grillage for the leak detection and subgrade protection. The applicable details are explained in Appendix I. API 650 is the Standard for use in the storage of petroleum, petroleum products, and other liquid products. For your case , dwarf walls are provided for inspection and repair rather than only detection.

- the backing strips provided for butt welded bottom joints and the use of it, is not mandatory if welds made from both sides. Refer to 5.1.5.5 Butt-Welded Bottom Joints.

The use of dwarf walls for phosphoric acid reactors and storage tanks is a common practice and in general the rules are set by process licencing company. You may dig for the licencing company and for the design criterias for the subject plant.

Good luck..

 

[ali-gct-meca]

HTURKAK

That explains a great deal for the case i am encountering, the informations that you have provided have been very helpfull,
I shall post the results as soon as we elaborate them on site,

Thank you all