Sealing Tank After Field Erection?

[ReliaEng2008]

Hello,

I am working for a green field project and one of the contract companies just finished erecting some carbon steel API 650 tanks. They will not commissioned for the next 12 months at least. To avoid atmospheric corrosion of the insides of these tanks (internal surface not coated) we are planning to seal them (make them airtight). We would be using blind flanges on all nozzles/manways with no pressure/vacuum relief installed. Those accessories would be installed later when they are ready to be commissioned.


I have a few questions:
(1)Is sealing a tank after fabrication a common preservation practice?

(2)We are concerned about the tank heating up in the sun increasing the pressure inside the tank above its design pressure. The design pressure is +69.20 in H2O (2.5 psig) / -8.31 in H2O (-0.3 psig). I did some high level thermodynamic calculations. Considering that the tank was sealed with air at 70 degrees F inside, the upper design pressure would not be exceeded until the air inside the tank reaches 160 degrees F (which is very unlikely considering that the tank is painted white). But the negative design pressure would be exceeded if the temperature drops 11 degrees F (from 70 to 59).

I am about to recommend we don't seal these tanks. Anyone have any comments of this?

 

[MJCronin]

https://www.eng-tips.com/viewthread.cfm?qid=419058

MJCronin
Sr. Process Engineer

 

[HTURKAK]

It is hard to comment ( at least for me ) without knowing the design liquid content, size and roof type of the tank..Some options could be ;

- Dry Nitrogen purging if the tank is small,
- Crude oil spray to the interior surface if the tank is for oil storage.
- The use of air dryer..
etc
..

 

[LittleInch]

First off - Are you really sure those figures are the design pressures for your tank? They are normally the maximum for API650, but I've rarely seen them in use.

For these tanks, like I said in the other post MJC linked to, I would fit pressure and vacuum valves and then pump in cold. dry air until your humidity levels are very low (<10% RH)

Then seal it off with your P/V valves. And check on humidity every now and then and pump some more cold dry air in. Or you could inert with Nitrogen and maintain a minimum pressure of say 1 psig and let it vent off if more than that. once you purge a few bottles will do. And fit a vacuum valve incase it runs out.

But vacuum is your biggest enemy for sure.

How big are these?

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

 

[Geoff13]

As noted by LittleInch, I've never seen tanks with the API maximum pressure and vacuum. Are you sure?

Accidentally sealing a tank overnight can result in this type of vacuum damage. Can't imagine what a year would do.

 

[bimr]

Consider blowing dehumidified air into the tank. Steel corrosion increases exponentially above 50%RH and accelerates the corrosion process.

Link

 

[HTURKAK]

I found the following picture yesterday when i was searching the web..

Seems 45% is the optimum..So , keep the relative humidity around 45% and get free from chemical corrosion, viruses, bacteria ..etc. etc..

 

[ReliaEng2008]

The tank has indeed design pressure of +69.20 in H2O (2.5 psig) / -8.31 in H2O (-0.3 psig). All documentation and calculations shows that. The tank is 20 ft diameter by 32 ft high.

To anyone that gave the advise of using dry air, I don't see how that would prevent the tank from collapsing on itself if the temperature of the air inside the tank drops (in case of a hail storm for example - very common here in the Gulf Coast). We could use a pressure/vacuum breaker, but that would just keep introducing humid air inside the tank.

 

[LittleInch]

Well at that design pressure you can afford to keep a nitrogen bottle attached and regulate to say 1 psi. Any sudden temp drop and a bit of humid air will be small compared to the low humidity bulk air in there at the moment.

Or flush it through with dry air say every month?

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

 

[TiCl4]

Any reason you cannot purge the tank initially, then seal it with the exception of a breather vent that pulls through a desiccant pot? This seems like a low-cost way of maintaining low humidity inside the tank without sealing it completely.

 

[ReliaEng2008]

I guess the reason would be my lack of familiarity with these equipment. I don't know about breather vent. I will google them soon. If you have any info about them you can send me, it is very appreciated.

 

[TiCl4]

Below is a link to an example (just found from random internet search). These are commonly called conservation vents, and the vacuum protection side can be piped to a desiccant pot (that has its own conservation vent). The vents can be specified to lift at X inches of water column, normally. You’d just need to check the vents and desiccant pot (get color-indicating beads) on some intermittent basis. Just an idea.

https://www.protectoseal.com/produc...tions/pressure-and-vacuum-vents/series18540d/