Flashcards in Natural Gas and CO: Policy 804 Deck (41)
How is natural gas distributed?
Through a system of high pressure (85 bar) pipelines to local distribution zones medium pressure (25 bar) which then goes to smaller distribution sites where the supply is staged down to lower pressure for domestic use. It is at this point that the stenching agent ethyl mercaptan is added so leaks are easily detectable.
Who has legal responsibility for natural gas leaks including CO and natural gas safety?
Gas Authority (GA)
What are the hazards of a 25-85 bar pressure jet of gas?
* Physical mechanical damage to surroundings and people. Loud jet like noise.
* Large fireball/jet flame causing oxygen starvation and asphyxiation.
* Significant radiated heat.
* Large unignited release could present explosion risk in open air if confined or congested.
What is an easy way of identifying a natural gas distribution site?
Large gas holders.
How long can it take to turn off the high pressure system at a natural gas distribution site?
Up to an hour as the natural gas has to be staged down to prevent a rupture from sudden shock of the valves being closed.
What are the hazards of low pressure gas leaks?
* Even though lighter than air if it built up in confined compartments such as hidden air pockets it can reach flammable/explosive mixtures.
* In high concentrations can become an asphyxiant but only likely in confined spaces or in very close proximity to a very large gas release.
Is the smell of ethyl mercaptan confirmation of a gas leak?
No only potential. It is heavier than air whereas gas is lighter so may have dispersed. In addition, maintenance work on national grid infrastructure may mean deliberately vented.
What is the only true way of confirming the presence of natural gas?
DIM (Detection, Identification and Monitoring) equipment.
What are the hazards of ethyl mercaptan when stored in high levels such as at distribution centres?
* Explosive range of 2.8-18.2%.
* Extremely harmful to the environment and not be allowed to enter water course or drains.
* Colourless to yellow liquid but may be dyed.
What is CO?
Carbon Monoxide. A by product of burning of natural gas and other fuels. Very toxic in small doses.
How does CO’s density compare with air?
Same density so will mix in the room and adjoining rooms have potential to be contaminated.
Information on natural gas risks on the ground should be gathered from?
* local knowledge.
* On site specialist/responsible person.
* Other specialist advice.
How do you quote wind directions? Is quoting wind direction always crucial?
Where the wind is coming from.
Eg ‘the wind is coming from the north’
Don’t place to much emphasis on wind direction in heavily built up areas as tall buildings create turbulence and unpredictable patterns.
What time or weather conditions will effect risk?
If no wind there may be limited or no dispersion above COMAH sites. Could be localised gas/vapour clouds.
This will be worse at night or during foggy days when vertical dispersion could be less effective.
Where should an initial appliance be sited for a low pressure gas leak?
Min 25m away unless incident or Pre-planning dictate otherwise. Not in line of site or front or rear of building. Greater than 45 degrees from front or rear.
How can you immediately tell a high pressure gas pipeline has been ruptured?
The loud jet like noise.
Hazard zone for a high pressure rupture?
What is minimum PPE for crews working within the hazard zone on a high pressure rupture?
Full PPE and BA.
What is suggested hazard zone for congested conditions?
600m but consider 1000m/advice of HMEPO.
What must the IC do for a ruptured high pressure main?
Request national grid to shut down supply via control. However, be aware that it may take a while.
What should crews not do at a ruptured high pressure main?
* Extinguish Any Fire where leak has ignited (surrounding risks may be protected if not putting crews at any undue risk).
* attempt to Control Any leak.
Hazard zone when a controlled leak from mass storage site?
1000m and await advice from HMEPO.
Hazard zone for low pressure and domestic network?
25m and advice of GA.
Can crews be committed to a building with a gas leak?
Yes if not at risk of gas explosion and wearing Full PPE and BA. Only when risk to life or critical infrastructure. Safety of gas supply rests with GA.
Considerations prior to entering a building suspected of having a natural gas leak?
* Isolate electrical supply externally.
* Shutting off gas externally.
* Ventilating property prior to entry.
* PPE and BA.
* Use of DIM.
* Intrinsically Safe kit.
* BARIE no hand held radio.
What should crews do when a gas leak has ignited?
Not extinguish flames, evacuate area and lash jets in place to protect surroundings.
Does LFB have power of entry at gas leaks to secure gas supply?
No. GA do. Crews should not make entry on behalf of GA unless exceptional public safety issue.
Following any fire what may be a consideration if the IC related to the gas supply?
Radiated heat and high temp may affect gas apparatus and integrity causing a leak. If any reason to suspect this is the case the gas authority must be requested.
Can you commit BA for a CO leak?
Yes. If persons involved then rescue number one priority.