Fires and explosions Flashcards
What are the essential elements required to cause a fire or explosion
- Activation energy
- Combustible agent (fuel)
- Oxygen (or other oxidising agent)
—-> reaction products + Energy
What is ‘activation energy’?
It is the energy required to initiate a fire or explosion
Activation energy provides heat –> increased temperature –> T above certain level –> reaction becomes self sustaining as the energy produced is then sufficient to supply the activation energy to propogate the reaction.
Define The Stoichiometric concentration
The stoichiometric concentration of any combustible vapour and oxidising agent is the concentration at which all combustible vapour and oxidising agent are completely used up.
E.g. Spark –> 2 volumes cyclopropane + nine volumes of O2 (18% cyclopropane in O2) then ALL of the cyclopropane and O2 are used up and transformed into product molecules, carbo dioxide and water. So 18% is the stoichiometric concentration
Why is the stoichiometric concentration relevant
The most violent reactions take place in stoichiometric mixtures.
If proportions differ in a mixture –> reaction less violent
When one component is greatly in excess of the other then the mixture cannot be ignited at all.
What are the flammability limits. Give an example
The limits outside which the mixture will not burn are known as the flammability limits
E.g. Lower and upper Flammability limits for cyclopropane in O2 are 2.5% and 65% respectively.
Within these limits is the narrower range within which explosions can occur
Why is cyclopropane no longer used in anaesthetic practice
In normal anaesthetic practice, cyclopropane burns but does not explode in air as the stoichiometric concentration and flammability range is significantly different vs in O2.
Stoichiometric concentration of cyclopropane in O2 = 18%
MAC of cyclopropane is 17.5%
What does the speed with which a reaction spreads through a mixture depend on
The proportions of fuel and O2 present. Fire rather than explosion is likely to occur when the proportions are nearer the flammability limits
What is cyclopropane’s stoichiometric concentration and flammability limits in air
Stoichiometric concentration = 4.3%
Flammability limits: 2.5% - 10%
Compare the conditions present in a fire vs an explosion
FIRE
- Pressure - 1 bar
- Temp - 200 - 500 deg C
- spread - 60 kmph
EXPLOSION
- Pressure 25 bar
- Temp 3000
- Spread Mach 8 ( 8 times speed of sound)
Why is the stoichiometric concentration and flammability limits different for cyclopropane in air vs O2
Inert Nitrogen molecules absorb some of the energy but also do not take part in the reaction. Hence the reaction is much less violent
Compare the stoichiometric concentrations of cyclopropane in Air and O2 with regard to ratios
AIR
2 cyclopropane : [9 O2 + 36 N2]
= 2 cyclopropane : 45 Molecules of air (4 N for every Oxygen)
= 4.3 %
O2
2 cyclopropane : 9 O2
= 18%
Is N2O an oxidising agent?
Yes. Nitrous rapidly breaks down into N2 and O2 in the presence of the heat of a fire or explosion, giving a 33% O2 mixture and releasing further energy
Why are explosions more violent with N2O
More heat produced when fuel burns with N2O vs O2
Apart from cyclopropane, what other volatile anaesthetic agent is flammable. Provide the stoichiometric concentration and flammability limits of this agent in air and O2
Ether
O2
- Stoichiometric concetration = 14%
- Flammability limits = 2 - 82 %
Air
- Stoichiometric concentration = 3.4%
- Flammability limits = 2 - 34%
Can ether cause explosion in theater
if Administered in 100% O2, yes as its Stoichiometric concentration inO2 is 14 %
but less likely than cyclopropane because Ether’s MAC is 2%
What are cool flames and what is the risk in theatre
Cool flames are invisible and are at much lower temperature than normal flames. They can be produced by Spilt ether, which ignites near the upper limit of its flammability limit near the floor in theatre. Carbon dioxide are normally formed but in the case of cool flames, acetic acid and acetaldehyde are formed instead as the oxidation is only partial.
The danger of cool flames is that they transmit the risk of ignition to the anaesthetic breathing system
Describe another entity regularly in theatre which carries a high fire risk.
Ethyl alcohol or surgical spirit
SVP is 6% and this is within its flammability limits in air (risk much higher if N2O or O2 near it
What flammable entities might be present in the patient
Flammable agents:
- Liquid chlorhexidine cleaning solution
- Methane / Hydrogen gas within bowel
- Ethyl chloride
- Ether to degrease skin
All may be ignited by electrosurgical apparatus
Combustible agents
- Paper, plastic drapes
- Dressings
NB
Compressed O2 supplies in wall –> very high risk –> oil and grease must be avoided in any part of the compressed O2 supplies
Special precautions at O2 manifolds and liquid O2 VIE storage units
What is the ignition temperature
A mixture of combustible agent and O2 are normally kept apart by repulsive forces. In order to burn the molecules must be brought close enough together to react. This is accomplished by increasing the speed at which molecules collide –> done by increasing the temperature locally, wither by spark or some source of heat.
The degree to which the temperature must be raised is known as the IGNITION TEMPERATURE which varies with the nature and proportions of the components
Why are sparks effective ignition entities
In contrast to hot wire or naked flame, sparks concentrate the energy into a very small volume. Smaller volume, less energy needed to raise it to the ignition temperature
Energy of the spark depends on______ ?
The potential difference of the source
Laryngoscope –> Low V battery –> not high enough energy
Electrosurgical equipment has high voltages and may cause sparks in the region of airway or breathing circuit.
What prevents the build up of static electricity in theatre
Antistatic, conductive floors.
Too low resistance isundesriable as it increases electrocution risk
To high resistance undesirable as it leads to accumulation of static electricity.
What does antistatic rubber contain to make it conductive
Carbon
Why are ETT antistatic
Not made of antistatic material but the moisture of the humidified gases flowing through them conducts static away
