Questions about floating roof and N2 blanketing

[jrmc]

Hello,

If i want to build/reconstruc a tank to operate with hydrocarbons i need to know if i need to install:

1) External floating roof, internal floating roof with conic roof, or simply a conic roof. We should choose in function of the need to reduce emissions
There is any code that can guide this choices?? i know it is connected with vapor pressure of the product, but i dont find this informations in any API or EN 14015 they assume the choice is already done.

2) N2 blankting system is relationed with security issues
what is the code that guide us in this choice??

to resume:
if i have to design a tank
what i should installl relationed with emissions and blankting?
What documents that i have to use as guidelines?

 

[SNORGY]

The decision, in my mind, has more to do with what you want to do site-wide with the tank emissions than what's in the tank(s).

If the tanks are off-gassing and you want to recapture those vapours either for recycle to the front end or as wet fuel gas makeup, then a fixed roof tank design with a VRU might be the way to go. If you have a continuously lit flare or incinerator and the fugitive tank emissions are not economically worth going after, then you might put in a fixed roof tank design with fuel gas blanket. If you put in a nitrogen blanket, you might want to recapture, clean and recycle as much of that nitrogen as is economically justifiable.

Any blanketing system does come with some increase in potential for emissions and odors due to the operating pressures involved (ounces positive at the tanks). You also do need to look at your tank loading / off-loading system to see what you want to do with vapours returned from the rail car / truck / boat etc.

Floating roof tanks automatically keep the vapour space effectively out of the flammability range (air exclusion) but there can be emissions due to passing around the seals and (if present) slotted stilling wells for level transmitters, etc. You also need to control the filling and evacuation rates to align with the maximum vertical rise and fall rates of the roof within the tank to avoid having the roof hang up.

 

[jrmc]

Snorgy,

in my opinion:
Connecting tanks to flare or incinerator
you can have O2 inside the tank, and can achieve a dangerous situation. your explosion triangle have 2 points it only needs one more (air)
VRU
We have a VRU and it´s a expensive equipment to buy and maintain
recapture N2
cost lot of money and i think payback is very high
Blankting increases emissions
i think it´s the oposite, please explain what you mean

Floating roofs
but you should install in what products, we follow the vapor tension?? exist any code that can guide me??

 

[SNORGY]

I think you can connect tank (at least small tank) vapors to flare or incinerator making judicious use and placement of purge gas, knockouts and / or flame arrestors between the tank and the combustion equipment. I hope so, anyway...
You can also connect tank vapours to de-odorizer pots that basically contain a de-odorizing agent and are, in turn, vented to atmosphere - again, making use of flame arrestors as appropriate.
Agreed on VRU and N2 systems; they are expensive but they might be the most effective fugitive emission capture facilities.
Positive pressure imposed by blanket gas setups, if not designed carefully, may increase the potential for outbreathing through thief hatches, gauge board seals, EPRVs (if equipped) and other roof appurtenances - you can mitigate these risks through correct sizing and specification of set points.

The US EPA Emissions Standards Division (or another country's equivalent) might have some guidelines with respect to tank selection. See link.