CO2 snuffing 2

[suraiya]

Hi all,

Can someone help me on CO2 snuffing for offshore platform vent stack. I mean, does anyone know any guidelines or standards that could guide in calculations of number of CO2 bottles required for the snuffing operations.

 

[Montemayor]

suraiya:

I don't know whether you are aware of it or not, but CO2 is conventionally stored in the liquefied state - either as a high pressure liquid at 1,100 psig & 85 oF or as a low pressure liquid at 265 psig & -8 oF. If you opt for the LP storage, then you will require refrigeration - normally of the mechanical type.

Another thing you may not know is that when you expand the liquid (whether HP or LP), you will create a 2-phase mixture of "snow" + cold CO2 gas. In order to be effective as a "snuffing" medium against flames or combustibles, you must inject the products of the liquid expansion; to try to inject solely gas from either the HP or LP sources will not be effective because you will be restrained by choke flow of the gas medium. Additionally, you will deplete your gas source very quickly by using this method because the contained liquid becomes denser and more refrigerated as you deplete the vapor pressure above it and it ultimately reverts to the solid ("dry ice") state - which is a very poor or nil source of gas unless you apply heat of sublimation. You don't want to do that; nobody in his right mind would design around that process. Therefore, you are stuck with expanding the liquid and its resultant 2-phase product. This is how your conventional CO2 fire extinguisher works. The "horn" is used to eliminate the clogging or plugging of the discharge with sticky dry ice snow. The dry ice snow exists at -109 oF, so you have to design your system for quick and efficient sublimation by the heat source in order not to weaken your steel equipment and related structural components by low temperature embrittlement. You certainly don't want (or need) this in an offshore platform.

If you have little or no process design experience in this area, you should have the work done by an experienced design engineer who understands the process and it's trade-offs.

I hope this helps you get started. Although I don't know your heat load and snuffing capacity required, I rather doubt that you will be able to do the snuffing simply by using conventional 50-lb steel cylinders manifolded together. I would guess you're talking about a ton or two of LCO2.

Art Montemayor
Spring, TX