Blanketing of Storage Tanks 1

[Nova N]

Hi Fellow Engineers,

We have several floating roof storage tanks at our refinery that we are currently using LP Nitrogen for blanketing. The reason being for process safety/fire hazard concerns. We have been using LP N2 for around 20 years with success. We are considering moving to NG blanketing due mainly to cost, as our N2 usage & demand are increasing. What are the benefits/drawbacks for switching to NG?

 

[LittleInch]

Internal floating roof??

Do you just vent the n2 direct to the air when you need to or?

Methane is a serious green house gas and I can't see any regulator being happy at having methane emissions increase.

And N2 doesn't burn or explode.




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[1503-44]

Quite an unorthodox use of methane, is it not?
Do you know of anywhere that methane is used for blanketing purposes?
How did you even come up with that idea?

 

[LittleInch]

I've seen Fuel Gas used for this before, but they then need to send the exhaust gas up the flare to burn it off and make sure there are no leaks anywhere in the tank.

I would first investigate where all your N2 is going first before embarking on anything like this.

You may well find that installing lots of large diameter virtually no pressure lines from the tanks to the flare header is many more times more expensive than some Nitrogen...

If your demand is getting high are you bringing this in or why not install a Nitrogen plant? 78% of what is all around you is your raw material and it's free!

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

 

[Nova N]

Hi All,

I am gonna try responding to all in the one thread here.

The N2 will vent to atm in the event of overpressure through PVRV on the tanks.
The plant used NG as the blanketing medium initially for +/- 10 years before switching to N2. This was during the mid-late 90's.
We are looking at all our regulators to check for leaks and seals on the floating roofs.

Regards
N

 

[LittleInch]

My guess was that in the mid 90's environmental emissions regulations / laws came into play which required the reduction / elimination of methane gas venting, but you might need to dig deeper to find out why this change occurred.

But surely you can't mean blanketing under an External Floating roof?? It won't be floating for long if you did that....

Do you mean the space above an internal floating roof but under a fixed roof??

A drawing / sketch would help.

Also "gonna" and "wanna" are not proper English terms. Please avoid slang in these forums. /rant over

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

 

[1503-44]

If going to atmos, the whole point of blanketing is to replace as much hydrocarbon gas as you can with N2, thus so far this makes no sense. Methane will probably be worse than what you have now.

Any significant amount of any kind of gas will sink a floater. Hope that's not what you're talking about doing here. Why do I think it is?

 

[LittleInch]

The main aim, if this is filling the void between an IFR and the tank roof is to drive out the oxygen to avoid an explosive atmosphere developing.

But we don't really have the information to know.

Only that in this location it looks like FG / NG is a bad idea and probably wouldn't be allowed by the relevant laws / regulations.

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

 

[Compositepro]

There are inert gas generators that will burn natural gas to produce a gas that is safe to use for inert atmospheres.

 

[1503-44]

Really? With no blanketing and vented to atmos, which many tanks are, there can be an explosive mixture inside, but without sources of ignition it won't happen. Not that dangerous. Hence I think it is not the main idea, pollution control and loss of product by evaporation.

Blanketing keeps the void space full of a non-polluting, preferably inert, gas, so that when you filled the tank, you would expel that gas to atmos instead of putting out HC vapours (hence the undesirablity of using methane), which helps control pollution and product loss by evaporation, and, when you emptied the tank, you fill the void space with non-polluting gas instead of intaking atmospheric air that has O2, creating that explosive mix, H2O vapours, and even dust, that condense and rain inside the tank and cause corrosion and gum on the tank seals and bottoms and messes up product quality. So non-explosive mixture can be the result of blanketing, but only if you avoid using O2.

EDIT
This made me curious enough to search "What is the purpose of tank blanketing". I WAS WRONG.
Only one on the first page of results mentioned corrosion and none listed pollution control, so I will agree and defer to LI's opinion. It is primarily used for explosive mixture control. This doesn't appear until page 3 or 4

https://www.eng-tips.com/viewthread.cfm?qid=390690

At least it was on somebody's list other than mine.

I also found this ABS guide for tank blanketing.

https://ww2.eagle.org/content/dam/e...t gas systems/Hydrocarbon_Blanket_Guide_e.pdf

 

[Nova N]

Hi All,

I have read through all the comments and valuable insights regarding the use of both nitrogen and NG/FG. I appreciate all the valuable information based on both your experience and judgement. I will definitely be digging deeper into the feasibility of using NG, based on all the comments.

Note: We have also fixed cone roofs (seven), that we would also be evaluating. Regarding the 10+ floating roofs we have - I will continue with N2 blanketing.

Thanks Again Fellow Engineers, keep well & safe.

N