Storage Tank Bottom Replacement 5

[Inchtain]

Hello,

Does anyone have a procedure or advise on replacing all corroded bottom plates in a tank? and do you think it is necessary to cut one shell plate to take the new plates inside and replace all the bottom media underneath the plates, as the tank was leaking under?

Thanks for the support

 

[LittleInch]

Depends on what the level of corrosion / leaking is.

You can replace all, part or none of the floor. We don't have a clue.

Sometimes you can patch it and then seal the floor with a spray type fibre glass coating a few mm thick.

sometimes you need to replace large sections and cutting a hole in the tank is the only way to practically get the plates inside unless you can lift the roof off.

Is this a FR tank or fixed roof?

Size?

Any columns?

Give us rather more than one line....



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Also: If you get a response it's polite to respond to it.

 

[MJCronin]

Call in an experienced expert .... Find someone who has fixed many tanks with the same type of corrosion as your situation

On-the-job-training for Storage Tank Bottom replacement is risky business..

Get a variety of quotes from a number of contractors .....

Do you have experience doing this work yourself ?

MJCronin
Sr. Process Engineer

 

[HTURKAK]

This subject has been discussed in past with several threads. I will suggest to look to the previous threads. The common practice is adding new bottom rather than changing the existing one. The common practice is shell is slotted completely 100-150mm above the bottom .
The new bottom is placed on spacer material ( sand or concrete ). A doorsheet is opened on the shell to take new plates etc. inside.

If the tank is ambient temperature tank, fiberglass internal lining of the existing bottom could be an option.

I will suggest you to listen Mr CRONIN. He always outsmarts and gives reasonable advises to junior engineers with excellent English.

 

[Inchtain]

Thanks for your input in the matter.

Actually, I've followed a tank construction project before.

However, maintenance is different.

Our Tank is 30 m diameter and in service since 2004. The bottom plates have about 80% reduction in thickness (8 mm original thickness)and is leaking. The annular plates are still good. The tank has sacrificial anodes inside and no external CP system was installed.
The Tank is fixed Roof Tank.

What do you think was the underlying reason for corrosion? and what is the best option here?

In case replacement of the bottom, does it necessary to replace the foundation underneath and do these activities will not disturb the tank settlement and it can left to rest on the annular plates?

What we have in stock is plates of 7mm (EN S275JR), can we use them for the bottom replacement?

Any other advices are welcomed in regard to the best foundation (sand, sand-oil, sand-bit, etc) and the underside painting of the bottom plates?!

Thanks a lot

 

[IFRs]

Is the corrosion mostly on the underside or topside of the bottom plate?

 

[Inchtain]

The corrosion is mostly from the exterior side

 

[IFRs]

What were the inside anodes protecting the bottom from? Were they consumed? How were they attached to the bottom? Was the corrosion near these anodes?

I presume when you said "external" you meant "underside" as opposed to the surface inside the tank. Is the underside of the bottom exposed? Is the bottom on a sand pad, concrete slab, engineered fill, crushed shells, etc?

Underside corrosion can present in several ways and be caused by several conditions. Was the underside corrosion fairly uniform or was it very local (lots of individual perforations but between the holes pretty good)? If very local it could be as simple as stones under the bottom instead of a nice dry sand pad. Since the annular plates are good and the bottom was not, I'd look for differences in the foundation for the annular plates vs the rest of the bottom.

Lots of things to look at, very little infomation to go on...

 

[LittleInch]

Inchtain,

You are asking some sensible questions, but when we don't have any information we can't answer it.

So at them moment things we know we don't know include:

What did the tank store? - flammable or hazardous fluids leaking into the sub base can be problematic
What is the tank floor made of under the tank
What is the percent area that is corroded to <50% original thickness
Have you considered simply coating the floor and 1m up the walls?
Is the corrosion wide spread or localised?

As said before, tank repairs are a special subset of tank construction and experienced vendors can provide more advise as they know what the total cost will be.

If there is substantial metal replacement and not just patches required then cutting a hole in the side of the tank will pay huge dividends compared to only access through a 30" or 36" manway

This could be anything from a few welded patches to complete replacement or laying a second floor over the old one.

If you don't have good design data or don't trust what was actually done, then take some sections of floor out and have a look. But do it using cold type grinders / coring bits and not flames or hot work unless you're trying to kill people.



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

 

[Inchtain]

Greetings,

It's always pleasure to hear from you.

Here are the required information:

The Tank is a self-supported fixed roof for Sweet Crude Oil at Atm. pressure and around 90 degrees Celsius.

The diameter is 37.2 meter and the height is 14 meter.

The Tank foundation is composed of a concrete ring, with gravel and sand top layer inside the ring.

The Sacrificial anodes are aluminum and are mounted vertically via male threads to the bottom-welded female threads. They were quietly consumed at around 10% only, and around some of them there were pitting and corrosion.

The bottom and the shell up to 2 meters are coated and the coating was fairly good at the un-attacked areas.

Many holes were seen on the bottom plates and we do not exactly know the zero-hole or the start direction of the holes. Any clues to look at?

Apart from that, 80% of the bottom plates' thickness was found half the original which is 8mm.

Widely spread internal pitting can be seen and the thickness around them is badly reduced.

Approximately half of the annular plates width at the side of the bottom plates were moderately attacked by corrosion and they were found very good (10 mm original thickness) when you scan the other half of the plates which are near the shell.

The Shell plates were in very good condition.

What do you think the root cause of such severe corrosion? and which are the appropriate recommendations?

Thanks for your feedback.

 

[IFRs]

90C will drive moisture and accellerate underside corrosion. Pretty common for asphalt tanks and other heated tanks. The sand pad may have been wet initially, and it may gather water depending on the local rainfall, bottom slope, foundation type and sealing between the chime and the foundation. Is the bottom cone up or cone down? Does the top of the ringwall slope down to the inside? Is the chime to ringwall sealed? Is the tank chime underwater during heavy rains? Is the water table close to the surface?

 

[JGard1985]

Agree with IFR, just finished working on a project where asphalt apron and grading around tank directed water under the chime rather than away

 

[LittleInch]

"Sweet Crude" at 90C??

Why?

Looks like you have a coating failure to me. - "the coating was fairly good at the un-attacked areas." Maybe it wasn't suitable for the temperature?

but if you get a break in the coating - what is it? - then you can get tracking of the water under it and at 90C get some microbial action going on maybe just plain steel corrosion that will be quite rapid.

Cone up or cone down will be interesting to see. Also what level of water drain off occurred?

Vertical anodes sound very odd as normally the water layer is quite thin at the base and that is what your anode needs to be in to be effective.

Any drawings or photos would help us answer your question for sure.



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

 

[HTURKAK]

Dear INCHTIAN,

You are asking the root cause but you do not provide necessary info.. Mr IFR has some valid points .. I assume you are hesitating to share the details so provided very little info. to go on..

If this is the case, let me start the guessing ;

- The tank is ambient temperature tank rather than elevated temperature tank and the temp . defined is 90 Fahrenheit so 32 Celcius..
- The CP is provided only for interior tank surface ..
- The tank bottom is compacted sand layer without bitumen or asphalt layer
- The foundation is prone to surface flow and GWL is high enough and fluctuates and can raise with capillary action .. so the tank bottom is prone to corrosion..

...IMO, you need a new tank bottom . I will suggest a new bottom 150 mm above the existing one. The common method is ; shell is slotted (complete severing of the tank wall ) and new bottom is installed on spacer material sand or concrete ..( my suggestion is the use of 150 mm concrete spacer so you will not need C.P. for external corrosion. )..

Pls look API 653 9.10.2.1.1 through 9.10.2.1.6...

 

[LittleInch]

hturk - the OP clearly states "90 Celcius" (!!), though he or she could have made a mistake as it seems very high for sweet Crude oil, but not if this is actually HFO or some other bottom of the barrel stuff.

I agree that the correct action is to install a new floor in entirety.

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

 

[HTURKAK]

Yes..the OP states 90 Celcius .. He also states dia 30 m and then dia. is 37.2 meter and the height is 14 meters. and the content is SWEET CRUDE.. If 90 degr.Celcius is correct, the content could be HFO . In this case, approximately 15000 cu-m tank is heated to 90 degr ???.. If sweet crude oil is correct , then the tank shall be ambient temp..

So i started a guessing ... You may continue with your guess so on..

Regards..

 

[bimr]

This subject has been discussed in past with several threads.

https://www.eng-tips.com/viewthread.cfm?qid=471348

https://www.eng-tips.com/viewthread.cfm?qid=390865

https://www.eng-tips.com/viewthread.cfm?qid=465945

 

[Inchtain]

Hello everyone, and thanks for your informative input.

Actually, I'm trying my best to provide any data requested about the tank in question.

With regards to the temperature: the design temperature is 90 oC, while the operating temperature is around 60 oC.

The contained fluid is sweet crude oil and is often heated before it is being sent to the Tank.

The bottom slopes down towards the Tank wall (Cone Up) and it has one sump off at about 1m deep.

The tank sets on sand which has an underneath gravel supporting layer, and on a concrete ring slap that slopes down.
Altought the bottom-to-concrete external seal was quietly damaged, no clear evidence of water entrapment, and that was further confirmed by the good condition of the chime and the half part of the internal annular plates.

The water table is quite deep in the area (around 12000m).

What else, there was about 30% of the internal coating failure in the forms of hairy cracks, blisters, and detachments.

I can answer any raised queries to come up with a clearer picture.

Additionally, could you inform me about the benefits of raising the bottom by slotting the shell over the new bottom insertion.

Best regards,

 

[bimr]

Additionally, could you inform me about the benefits of raising the bottom by slotting the shell over the new bottom insertion.

The tank fabricators will generally cut an access hole into the side of the tank to allow access for the replacement steel.

There are several methods to replace the bottom. Some tank fabricators will weld "C" clips on the base of the bottom tank sidewalls to hold the tank up and allow the bottom plates to be slid under the tank wall. After the "C" clips are installed, a torch runs on a track to cut away the bottom.

Another method used by tank fabricators is to jack the tank up.

The bottom ring will probably be replaced in sections rather than replacing the entire bottom ring at one time.

Link to tank bottom replacement

Link to tank bottom replacement

 

[Inchtain]

Dear colleagues,

Many thanks for your feedback.

Could you possibly see and comment on the attached MFL scan result for the Tank bottom?

Furthermore, do you think that Jotun Tankguard HB classic is a suitable choice for the painting of the interior of the Tank (shell 1.8 m + bottom)?

Another issue is that we found the floating collector (suction arm) pittied at the upper side of the float and the main pipe? what does that mean? and as long as it was made of Aluminum, is it the right material? why it is not carbon steel or stainless steel? I heard that the operators faced many problems with the floating suction unit when the Tank was in service; does that mean a problem in the design? is there any recommended operational acts? or is there better alternatives?

Aditionally, when I reviewed the Tank documents carefully, I saw that the tank bottom was consists of a layer of sand and on top of it was the gravel layer? is that a best practice?; and the erector at that time claim that the soil resistivity measured (666 to 714 ohm meter) will not cause any corrosion and that's why they recommended not to install ICCP system and to disconnect the Tank's earthing from the Plant Earthing Grid and install insulation kits? so what do you think about that?

Finally, the first horizontal welding of the shell was at 2100 mm. do you think is suitable to open the door sheet at that height? otherwise, could you inform me about any typical dimensions for the door sheet?

Thank you very much again