Recent enquiry asking for an assessment of risk concerning the provision and use of a 2 Kg CO2 fire extinguisher within an underground switch room of circa 15 m 3.

Have provided an answer but would be interested in other opinions.

exclosed is my new word ;)

Worth asking, whats the ventilation in this switch room like?

You don't want good ventilation as it would allow the discharged CO2 to escape thus rendering it useless.

I would advise, isolate the power if possible, then use of the said extinguisher as a first attack on the fire, from a position near the door to the room with your back to the doorway, door open, with one foot out of the doorway so the door cannot close and lock you in. Once the extinguisher has been discharged leave the room quickly closing the door behind you. Call the fire brigade and leave the rest up to them. Follow normal evacuation procedure. Do not return in to the room and leave the door shut.

I would agree with ChrisBurns description of dealing with the incident. In the scenario posted there would be no asphyxiation risk, but due to the increased concentration of CO2 in the room there is a risk due to carbon dioxide toxicity. If the exposure was of short duration i.e.immediate evacuation of area after discharging the extinguisher, the risk will be mitigated.

I am not sure if there is a significant risk of ashyxiation or not, but would think that the approach that has been suggested seems reasonable in order not only to tackle the fire but to manage the risk of asphyxiation!

I wonder if as you already have the answer, whether you could share it with us?!

Hi Bleve, who asked you for the assessment?
Is that the answer you gave?
Was he/she baffled by the science?

I just used my previous fire fighting knowledge and experience when I gave my answer.

I don't think even now that fire officers consider the chemical formulae before sending fire fighters in. They just use what they have on board the machine and work to set procedures. Some firefighters nowadays haven't a clue. If it works that's all right - if it doesn't they just send for some more machines :-)

And that is my considerd opinion.

And while I was typing mine the previous two happened - in the words of Tommy Cooper "Just like that" :-)

I agree that the asphyxiation risk is small, although I wonder whether most of the CO2 would remain at low level in the room. 7% is a toxic concentration of CO2 however and the room would need ventilation if the device had been fully discharged. (Obviously if there had been a fire, the products of combustion are also toxic!).

Bleve, is this an assessment of risk to persons or property/business?

From a life safety perspective I wonder whether a portable extinguisher is necessary in an underground, generally unoccupied switchroom, assuming the fire separation is of good quality. Reliable detection and auto shut-off may be a better option.

Just as a by-line to this interesting topic and responses - we were all taught that you should not discharge CO2 extinguishers in confined spaces - right? When they told us that, the reasons why were made obvious, but no aternatives were ever given. Maybe it would be a reasonable requirement to provide additional training at basic level fire safety courses to enable people to calculate the likeliihood of axphixiation etc. i.e. - actually defining what a confined space is in this context. Also this would make it far more relevant a factor when it comes to carrying out fire risk assessment.


PS
I am in complete agreement with the post which recommended a good spull chucker on the site !!!!!

Let's not forget that you won't be hanging around once you've discharged the extinguisher. You'll be leaving to raise the alarm as quickly as possible.

Bleve and I have been having an interesting side discussion regarding the method used to assess the risk of asphyxiation and CO2 toxicity.

The formula BLEVE used (see post 8) is as follows:

Volume of Room = 15 cu.m.
Percentage of oxygen = 21%

Therefore the volume of O2 in confined space = 0.21 x 15 cu.m.
2 kg Carbon dioxide @ 1.87 kg/cu.m. = 1.069 cu.m.

0.21 x (15 cu.m (air)-1.069 cu.m (CO2))
=0.21*(13.93 cu.m.)
= 2.92 cu.m. O2

2.92 cu.m. O2 /15 cu.m. = 19.5% O2
1.069 cu.m CO2/15 cu.m. = 7.2% CO2

This assumes that the Carbon dioxide displaces an equal amount of air.

The formula I use (based on the assumption that the discharge is quick and that the room is small (15 cu.m.) is as follows:

Volume of Room = 15 cu.m.
Percentage of oxygen = 21%
The density of air is 1.19 kg/cu.m.

Therefore the mass of air in room is 15 cu.m. x 1.19 kg/cu.m. = 17.85 kg

2 kg of Carbon dioxide is added to the room and as the room is small and unventilated the Carbon dioxide does not displace the air, but increases the mass of gas in the room to 19.85 kg. Please note that the room is not gas tight but that the pressure does have not have time dissipate.

0.21 x 17.85 kg O2 = 3.75 kg O2
3.75 kg O2 / 19.85 kg = 19.9% O2
2 kg CO2 /19.85 kg = 10.6% CO2

Both agree that Oxygen displacement is not a problem. However, the difference between 7.2% CO2 and 10.6% CO2 is the difference betweeen having a headache or being dead; the lowest lethal concentration of CO2 is 9% after 5 minutes. (SAX Dangerous properties of industrial substances 9th edn). I accept that both methods are not perfect in that some of the Carbon dioxide would leave the room with the displaced air and that the room is not gas tight so the figure I use would be an overestimate.

Any thoughts or comments?

Regards Adrian

I think the demonstration of the correct methodolgy and result is the support that the British Compressed Gas Association calculations equalt that of my own.

VIZ

The following is taken from the BCGA Guidance and is as stated by them based on an instant catastrophic release of CO2 into a workplace.


BCGA GUIDANCE NOTE GN11
Use of Gases in the Workplace
The management of risks associated with reduced oxygen atmospheres

Calculations
Calculations 1b below assume all the gas is instantly and uniformly released into the workplace. Calculation 1b should be performed to establish the extent of risk of reduced oxygen atmosphere in the workplace resulting from the sudden release of all available gases e.g. all of the gases from within the room or being piped in from an external source e.g. an outside bulk storage tank or gas store.


Calculation 1B : Example

One 6.35kg carbon dioxide cylinder being used in a workplace with a free air volume of 75 m3.
VR = 75 m3

VO= 0.21(75-((6.35X535)/1000)) =0.21(75-3.4) =15.0 m3
Resulting oxygen concentration (Cox) = (100X15)/75 = 20%

My calculation method results in the same result:

0.21(75-(6.35/1.87))=0.21(75-3.4) = 15.0 m3


This oxygen concentration is above the minimum recommended by the HSE. However carbon dioxide is mildly toxic and therefore the HSE have defined an occupational exposure limit of 0.5% averaged over 8 hours, with a maximum exposure of 1.5% for short periods of 15 minutes.

The volume of carbon dioxide from this 6.35kg cylinder could produce a concentration of 4.5% in case of complete loss via, for example, a bursting disc failure. This would produce a dangerous atmosphere and preventive measures are necessary.

i.e. Conc CO2 (100X3.4m3)/75 m3 = 4.53%

Calculation 1B : Example (Adrians method)

One 6.35kg carbon dioxide cylinder being used in a workplace with a free air volume of 75 m3.
VR = 75 m3

The density of air is 1.19 kg/cu.m.

Therefore the mass of air in room is 75 cu.m. x 1.19 kg/cu.m. = 89.25 kg

6.35 kg of Carbon dioxide is added to the room and as the room is small and unventilated the Carbon dioxide does not displace the air, but increases the mass of gas in the room to 95.6 kg. Please note that the room is not gas tight but that the pressure does have not have time dissipate.

0.21 x 89.25 kg = 18.74 kg O2
18.74 kg O2 / 95.6 kg = 19.6% O2
6.35 kg CO2 /95.6 kg = 6.64% CO2


Calculation 1B : Example (My method)

One 6.35kg carbon dioxide cylinder being used in a workplace with a free air volume of 75 m3.
VR = 75 m3

0.21(75-(6.35kg/1.87kg/m3))
= 0.21(75m^3 - 3.4 m^3) = 15.0 m^3 Oxygen

oxygen concentration = (100*15m^3)/75m^3 = 20%


Concentration of CO2 in room

(100* 3.4m^3)/75 m^3 = 4.53%

Bleve,

Here's a new one: I was wrong! The formula is correct but it solves for mass not volume - which was my mistake.

Appologies Adrian

Bleve, I think you're being a little pedantic here, besides it is possible that a given emergency situation might lead you to extinguish a fire first and raise the alarm later.

On the topic of the volumes, I'd like to point out that concentration values can be represented in many different ways, in this case % is used but it should be noted that the persentage can be either "by Volume" or "by Weight". I think your calculations may be mixing the two. For my part I have done some calcs and this is what I found.

% by Volume (after discharge of the CO2)
Nitrogen: 73.6
Oxygen: 19.5
Carbon Dioxide: 6.9

Perhaps I'm being pedantic about your calcs!?! Either way, the use of fire extinguishers is primarily to preserve life. Once discharged the next step is to leave the room and go to a safe area. Better to suffer high concentrations of CO2 for a short time than to perish in a fire.

Mick

Mick

Surely 2 kg co2 would be placed outside room where hopefully a MCP may be located. In event of small fire developing in switchroom, employee (obviously trained) would tackle fire (after raising alarm) by open door reducing the risk that has been mentioned.
If fire develops further, shut door and make way to assembly point.

All this after your assessment has also confirmed the room has 1 hour fire passive protection and AFD, so you could even train the guy to just leave, shut the door and raise alarm knowing in comfort you have 1 hour for FRS to extinguish fire.