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Posted By NeilTonge
Can anyone tell me the minimum and maximum safe % content of oxygen to allow confined space entry without BA?
I have been through L101 and all it gives me is that oxygen levels below 16% can lead to unconsciousness and death. What margin for safety above 16% do the rest of you use when testing oxygen levels prior to confined space entry and what is that based on?
At what concentration is oxygen an explosion risk?
Would appreciate references to support answers please.
Thanks in advance,
Neil
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Posted By Dave Merchant
Generally anything below 19% is a BA job, and anything over 23.5% carries a flash risk. Oxygen doesn't explode per se, but the speed of combustion in enriched atmospheres can be extreme and conventional extinguishants can be ineffective. OSHA defines 'hazardous' as below 19.5% or above 23.5%
The lower limit is a bit variable as you could suffer at 19% if you were exerting yourself, or be relatively OK at 15% if you just walked in and out. It depends how much oxygen you need, rather than precisely how much is available - which is why climbing mountains is harder than sitting in a plane pressurized to the same partial pressure of O2 (ppO2). A percentage number isn't really all that useful to define down to the decimal places, as it's the ppO2 which counts, and that depends on atmospheric pressure. "19%" at sea level is a whole lot safer than "19%" on a mountain.
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Posted By D H
You could also look at it from this point of view;
If the air space contains 20.9% oxygen - normal atmospheric conditions - and then drops to 19% - what has taken up the other 1.9%? Is it flammable, H2S etc?
Do the assessment and play on the safe side - and have a rescue plan ready and tested.
Dave
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Posted By Helen C
To quote an email I was sent recently.....
"Oxygen depletion:
21 - 19% fit for respiration, as long as no other contaminants are present.
I'm sure you know all the details of the associated health effects for reducing levels (eg, 16% dizziness, 10% nausea etc)
Normal oxygen content in air is almost 21% and that air is deemed to be adequate, provided the oxygen content in the body does not fall below 19%. Gas monitors will usually have the oxygen level alarm set to 19% when provided by manufacturers. The authority for the 19% value for oxygen in air comes from Section 55 of the Mines and Quarries Act, where mine air is deemed not to be suitable if oxygen content falls below 19%. This is where the 19% figure can be found in statute, as it is not explicit in the Confined Space Regulations. References to oxygen percentage levels in the L101 ACoP/Guidance to the Confined Space Regs can be found under para 27."
Let me know if you need anything else.
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Posted By Nicholas Ross-Clunis
To enter a confined space the atmosphere has to be free of all hazards. Oxygen levels should be between 19.5 and 23.5%. Beyond these limits the oxygen levels are considered hazardous. Under the OSHA regulation 1910.146 entry is not permitted when the oxygen level is not within this range.
The question arises can you enter with a BA set when the oxygen level is below 19.5%?
Where work is the objective this should be undertaken by specially trained personnel wearing full life support systems. Just using a simple BA set is insufficient.
Entering a confined space with a known lethal atmosphere (e.g. zero oxygen) should only be undertaken with a BA set for the purposes of rescue.
Effect of Oxygen Deficiency
Oxygen volume %
14 – 16 Breathing rate is increased
Pulse Accelerated
Diminished clear thinking
10 – 14 Loss of sense of pain
Rapid fatigue
Faulty judgement
Very emotional
Spasmodic breathing
6 – 10 Loss of consciousness followed by death
I hope this helps.
Nicholas Ross-Clunis
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Posted By D H
Nicholas - good info there in your posting - but what I was trying to point out was what had taken up the "air space"
If H2S, depending on the space available over 10 ppm of H2S - this is unacceptable - even with and oxygen concentration of 20%
Any gas present even with 20.9% air content can be fatal!
Dave
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Posted By Nicholas Ross-Clunis
Dave you are quite right and I did like the neat way you set it out, after all there is still another 79% to make up the total quantity.
Nicholas Ross-Clunis
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Posted By ScotsAM
Also bear in mind that the oxygen content may vary with height in the confined space due to the differing weights of gases that make up the air.
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Posted By garyh
I now work as a consultant. I have previously worked for 20 years in the chemical industry, mostly on higher tier COMAH sites, including 10 years as a SHE manager in a safety excellent organisation.
Here is my advice - DO NOT ALLOW ENTRY IF THE OXYGEN LEVEL IS ABOVE OR BELOW 20.9%. Period.
The only time I have seen an entry at potentially low oxygen levels done was with site fire brigade stood by, and all rescue equipment (eg life lines, BA) worn by the entrants.
No arguments; follow the ACOP re the required safe system of work, manage the hazards. However.........
I repeat, my advice, do not allow such an entry.
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Posted By Dave Merchant
"Entering a confined space with a known lethal atmosphere (e.g. zero oxygen) should only be undertaken with a BA set for the purposes of rescue."
People use full working BA for entry into non-respirable atmospheres all the time - that's what it's for. A last resort certainly, but not an absolute prohibition, and INDG258 explains that.
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Posted By mike morrell
I would agree with the observation that if the Oxygen content within the confined space is different to that outside there needs to be a clear understanding of why this may be. It is also true that on occasions too much reliance is placed solely on the oxygen levels as an indicator danger. As has been pointed out the toxicity of many gases is such that in percentage terms their presence will make little difference to the oxygen content. Many uninformed people don't appreciate the huge difference when considering PPMs and percentages
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Posted By emiliodettore
Deck compression chambers should be located on the support vessel or installation in a position that affords maximum safety for the divers occupying the chambers, e.g. away from overhead cranes and parts of the installation that may be particularly susceptible to fire or explosion hazards. They should be sited so that effective external fire fighting equipment can be mobilised in the event of a fire on the support vessel or installation.
During therapeutic treatments, the deck decompression chamber’s atmosphere shall be constantly analyzed (oxygen percentage exceeding 23.5% must be avoided). When necessary, the hyperbaric environment shall be flowed with air, even if the oxygen circuit is provided with an exhaust flow-out system. In order to avoid dangerous oxygen concentration near the chamber, the outflow of BIBS shall be conveyed in an appropriate area in case the chamber is located in a poorly ventilated environment.
1.6.3. Oxygen Safety
a) Equipment used with oxygen or oxygen mixtures greater than 22 percent per volume must be designed for such use; and
b) oxygen systems with pressure greater than 860 kPa (125 psig) must have slow-opening shut-off valves except for pressure boundary shut-off valves made of ball valves
c) All hp oxygen supply in use is to be equipped with a pressure reducer at the quad/cylinder outlet.
d) All the oxygen circuits shall be maintained free of any oil or grease contamination and only oxygen compatible lubricants shall be used (such as cristo lube).
e) The oxygen storage should be located in a well ventilated area, free of any flame risk.
f) Oxygen shall always be handled with care, smoking is forbidden near oxygen lines or storage.
g) If any doubt exist that an oxygen line, a pressure reducer or any other component of the circuit could have been polluted with a different gas the item shall always be purified with approved products (avoid toxic products such as trichloroethylene for breathing gas hoses).
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Posted By Merv Newman
I think that no attention has been paid to the second part of Neil's post : "At what concentration is oxygen an explosion risk?"
And there is no answer to that. Or rather the answer is "it all depends"
It all depends on what else is present and the concentration of that other substance. Not getting too technical (wot at my age) there are "Higher Explosive Limits" (HEL) and Lower Explosive Limits" LEL
A substance mixed with oxygen at lower than it's LEL will not explode. Not enough of the substance. The same substance mixed with oxygen at a concentration higher than it's HEL will not explode. Not enough oxygen.
The difference between LEL and HEL for a given substance is it's explosive range. Which will vary from substance to substance. From memory, hydrogen has an extremely low LEL (O.1% ?) and an extremely high HEL (98% ?)so the explosive range of hydrogen is very broad. Methane, to continue with an example, has a higher LEL and a lower HEL than hydrogen, thus a narrower explosive range. Still makes a good bang if you get it right though.
On a slightly (just a bit) sideways note some materials will not burn at normal O2 concentration (20.9%) but will in higher concentrations. And vice-versa.
On the other half of this thread, I am very very conservative when it comes to confined spaces. I've been in quite a few and I hate it. Too much can go wrong. (and it's not claustrophobia. I'll race you to the bottom of a deep mine any day)(though these days you would probably win)
I did always rely on the oxygen detector, set at 19.5/21.5 so that was my "safe" level if you like.
But anything I (and that is a Capital "I") defined as a confined space and it was the full bells and whistles. And standby person, lifting gear, siren, walkie-talkie and, whenever possible, the plant manager authorising the entry. (while standing next to the hole. Not in the office) Exercise : complete the foregoing list. (hint : at least six items are missing)
Have a nice penetration
Merv
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Posted By Phil
In hyperbaric chambers as mentioned above, O2 levels are closely monitored and controlled due to problems arising from partial pressures.
As well as the increased risk of fire propogation, breathing O2 in a hyperbaric enviroment has its own unique problems.
Breathing 100% O2 at deeper than an 18msw treatment table may result in an O2 fit, as the gas is toxic due to partial pressures at the se depths. For deeper treatment tables, we overcome this by introducing helium into the breathing gas to reduce the partial pressure of O2.
Slightly off topic i know, thought it may be of interest!!
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Posted By Jeffrey Watt
The 19% O2 level as quoted by Helen C from Section 55 of the Mines and Quarries Act piqued my interest.
I had Oxygen detectors on the brewhouse floor set for 19.5%, which was not a confined space but there was a risk of high CO2 levels from the brewing process and lone-workers walking through to take samples were at risk.
Why 19.5%?
The 15 min STEL for CO2 was 15000ppm or in other words 1.5 parts per hundred or 1.5%.
So if the O2 meter was reading 19.5% it was fair to say that the STEL for CO2 was being breached or close to it.
So folks depending on the contaminants you may find your workers in confined spaces suffering ill health effects before the 19% Oxygen level is breached.
Just echoing Gary H's advice, with some practical experience from the brewing industry.
Kind regards
Jeff
"All things are poison and nothing is without poison, only the dose permits something not to be poisonous."- Paracelsus 1493-1541
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