Lay-up method for newly constructed storage tanks before entering into operation. 4

[rikywere]

Dears,
I would appreciate your advice on following matter…
We are constructing some API 650 storage tanks close to seaside. These tanks are designed to store hydrocarbons once in operation so are not internally coated and are made of CS. There are 11 of them in different sizes from 11m diameter x 11 m height to 20m x 20m. All of them have fixed dome roof and most of them are not designed to withstand any internal/external pressure, are provided with open vents.
These tanks are going to be waiting for entering into operation around 1.5 years from end of construction activities.

My question is: What would be the correct preservation method for these tanks?

Some info to keep in mind:
- Tanks are made of CS.
- Tanks are not internally coated.
- Vents cannot be sealed since we do not have vacuum breakers to install.
- Inert gas would not work; it would escape thru the open vents.
- Desiccant would not work, the amount would be huge and we would have to replace it continuously because it would continuously absorb moisture from open vent.
-For application purposes, there is no access to all metal surfaces, some tanks are around 20m height and there is no internal scaffolding. Product can be blown, fogged of left on bottom of the tank but “paint” of all metal surfaces is not accessible. Basically, we cannot reach the roof.
- Some internals will be installed at some point, so we cannot block access the interior of the tank forever. (Like in wet lay-up).
- We are in Middle East weather.

There are only two options I can think of:
1) Not to do anything, ambient lay-up and check the tank internal surfaces frequently to make sure there is not severe corrosion. This was our first option but rejected by client.
2) Apply oil to internal surfaces. It may work but there are several cons like;
·How to apply the oil (20m height scaffolding inside the tank would not be allowed by safety)?
·How to install the internals if all is wet of oil?
·All around the tank is a construction area with many works ongoing, wouldn´t it be a hazard? Atmosphere inside the tank would be an air-oil mixture, hazard of explosion?
·Even if I can apply it, like fogging. How to remove it before commissioning?


Thanks and regards,

 

[EdStainless]

I have seen people use a rust preventative oil, oil with special corrosion inhibitors in it.
If you are welding internals at some point you would have to blast and wash those areas anyway, so the oil won't matter.
The inhibitors are volatile, but the oil isn't very, so risk of fire is not an issue.
It is soluble in common hydrocarbons so there is no need to remove it. You will have to flush the tanks anyway to remove debris and such.

Collected water from condensation is a serious issues in such tanks.
And I don't know how persistent the RP oils are, they may require re-application.
Reach out to a company that make corrosion inhibitors.

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P.E. Metallurgy, Plymouth Tube

 

[rikywere]

Dear EdStainless,
I already consulted VCI suppliers; we commonly use them in different formats for pressure vessels, packages, and many other equipments. The problem with our tanks is that due to the volume and metal surface of the tanks, it is nearly unaffordable. If something is required, it is required, whatever it costs. But, is it really required?
Tanks are clean and dry, we plan to inspect them every other week to make sure there is no sand, debris or condensation water accumulation and plates are already passivated by natural CS rust thin layer. is it worth it to spend thousands of hundreds to prevent 0.1 mm of metal thickness loss per year?
Thanks for the reply

 

[kingnero]

The thin rust layer doesn't actually protect the CS from further degrading.
I guess it depends on what was mentioned in the order specifications.
Without a doubt, applying corrosion protecting oils is costly. Perhaps the alternatives are even more so.

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[EdStainless]

Condensate will collect in the tanks, and being near the coast it will contain chlorides.
You will have serious pitting of tank bottoms without some action.
I am very surprised that the bottom and lower wall sections are not coated. Over time the small amounts of moisture that will collect in service will lead to corrosion of the tank bottoms.

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P.E. Metallurgy, Plymouth Tube

 

[rikywere]

We have to keep in mind that we are not in operation or mothballing, we are still in construction phase and it will not take longer than one year to the tanks to enter into operation.

Thin rust layer does not protect the tanks but penetration rate will not be more than 5 mpy, corrosion allowance that we certainly have. That is why I do not really think it is worth it to spend that money to prevent 5 mili inches .

Certainly, some of the tanks have the bottom and 0.6 m of the first shell course painted, but others are designed to work at high temperature or tank product is free of water and not compatible with paint, both of them will be protected with nitrogen blanketing during operation so there will not be any air moisture or water soruce, but now we do not have that nitrogen system installed yet. For teh remaining, bottom and first 0.6 meters are painted.

 

[EdStainless]

How will you nitrogen blanket with open vents?
Everyday as the tanks heat and cool at night they will draw in fresh moist air, and some water will condense. This is what you need to address.
If these were totally open (no tops) then you would just get superficial surface rusting. But standing salt water in the bottoms will not be good.

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P.E. Metallurgy, Plymouth Tube

 

[MJCronin]

I agree with EdStainless ...

After completion, the tanks floors should immediately be coated with an approved crude oil coating system. The coating should extend 4-6 feet up the tank shell.

The remainder of the tank interior should be spray coated with a petroleum based preservative product. You should be prepared to repeat this interior coating periodically.

Have you reviewed your original design method to periodically drain the condensate from the tank interior ?

What was the original design ?

Can it be implemented if the new tanks are layed up for a long time ???

MJCronin
Sr. Process Engineer

 

[rikywere]

EdStainless,
Not to write an even longer first post I generalized some details. Out of 11:
Seven are designed to store hydrocarbons at ambient temperature, these are provided with permanent open vents and bottom plates are internally coated.
Two are designed to store hydrocarbons at high temperature; these are not internally coated because high temperature HC will not allow water condensation.
Remaining two are methanol tanks so no coating is indicated for this service.
The last four tanks, high temperature HC and methanol, will be provided with nitrogen blanketing but right now, we are in construction phase and that system is not constructed yet, so currently, I only have one nitrogen inlet and outlet nozzle without any pipe attached to them. These last four tanks will not have open vents in operation but right now I have to leave, at least, one nozzle open as temporary tank vent in order to prevent any mechanical damage due to the lack of required breathing during temperature changes along the day.

MJCronin
In reply to EdStainless you have answer to why four tanks does not have internal coatining on bottom plates.
Regarding interior spray coating with a petroleum based preservative product; why? What will happen if not? Obviously, this coating will preserve the tanks but again, it is worth it to provide that resources to avoid 3 mpy? Tanks will not be longer a year in this status, so… all that work to preserve 0.08 mm?
Regarding design method, that is another business. We are now erecting the tanks with a closed approved design already provided. Design part is completely out of our current scope. But to answer your question, yes, all tanks are provided with drain sumps. I assume that final owner will use them to periodically drain water form these tanks.
Thanks for the answers,

 

[LittleInch]

One option maybe is to initially blow super dry warm air through the tank with the main vents / manholes all open to get the RH of the tank below dewpoint at the lowest tank metal temperature ( prob 10C in the ME winter?)

Then continue to blow dry air through the tank at a flow rate to get a small continuous outlet on the tank vents.

The real issue with the corrosion rate is that whilst the general corrosion rate will be very low as you state, the issue is pitting corrosion in discrete locations, especially tank floor . wall joints and low spots within the tank floor.

The walls might get internal rust steaks which will look bad, but are not significant.

however small pools of water, especially if it can get chlorides, can eat significant holes in a tank floor. However if you are able to get in there on weekly basis to inspect / mop up any condensed water, then even the dry air bleed thing might be a bit excessive.

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

 

[bimr]

Temporarily block the tank connections and use a dehumidifier to dry the air.

 

[metengr]

bimr has it correct.

 

[rikywere]

Gents,
I considered the continuous dray air flow, it sounds good. When it comes to desiccant, theoretically it is ok but practically not sure, vent cannot be sealed so continuous moisture coming form vent nozzle will collapse the desiccant capacity very soon. Also amount of desiccant for a storage tank is huge. If dehumidifier is provided by another means but not desiccant, them it may also be good option, an air heater to keep internal air above dew point all times?
Regards,

 

[bimr]

A desiccant alone will not work. A desiccant wheel, yes.

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