Confined Space - Gas Test
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| O2 detector |
What gases are tested in a confined space?
Multi-gas detector measured the standard gases are:
- Oxygen (deficiency or enrichment)
- Hydrogen Sulfide (toxic atmosphere)
- Carbon monoxide and (toxic atmosphere)
- lower explosive limit (LEL) (flammable atmosphere)
Oxygen
Oxygen deficiency is the most hazardous of the confined space and the biggest cause of death. The plant separated gases through a variety of refinement process involving pressure differentials and filtering through a dry medium. Nitrogen is the main byproduct of the process, as the major component of the air we breathe.
Nitrogen is an inert gas. O2 and N2 are the two largest components of the air that we breathe. When the concentration of N2 in the air increase, the oxygen level decrease, too much nitrogen can lead a worker to lose consciousness or adverse due to asphyxiation.
Do you know how many time you breathe at the last minute?
No one can ever answer precisely, only can guess " ten to 12 times" and that is the point. We never worry about the respiratory system. We take in a variety of gases, separate out the oxygen and infuse it into our blood and exhale waste product carbon dioxide.
Difficulty Breathing
According to the OSHA safe level of oxygen for entry into confined space is 19.5% to 23.5%. Low oxygen level leads to impair judgement and coordination. Extremely low level cause nausea, vomiting and loss of consciousness.
For example, hot work activity such as welding, cutting, grinding is going on inside confined space, it could potentially become oxygen deficient because the worker is also breathing oxygen and hot work activity is also consuming O2. The oxygen concentration could potentially drop below those required for human survival.
Concentration Kills
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| oxygen effect on the human body |
Hydrogen Sulfide
It is poisonous, corrosive and flammable. It is also known as a sewer or sour gas. A small amount of h2s occur in crude petroleum but natural gas contain up to 30%.
Toxicity
Hydrogen sulfide is toxic, smell like rotten eggs. It damages the olfactory gland. So the victim can not smell the presence of h2s.
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| h2s effect |
The short term lower exposure concentration (2 ppm) can result in eye irritation, nausea, shortness of breath and fluid in the lungs.
Long term low-level exposure (2 ppm) can result in fatigue, loss of appetite, headache and dizziness.
Short term high-level exposure can result in collapse, loss of breathing and a high likelihood of death due to respiratory paralysis.
Read the SDS of H2S
Carbon monoxide
Carbon monoxide is an odourless, colourless and poisonous gas that can kill you. CO is found in fuel in the car, coal-burning appliance, fireplace or furnace and incomplete burning of organic matter. The symptoms are dizziness, vomiting, headache and confusion etc.
Breath in a lot of CO can cause death.
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| CO Concentration Effect |
WHO (World Health Organization Recommendations for CO exposure:
- 9 ppm for 8 hour
- 26 ppm for 1 hour SDS
- 52 ppm for 30 minute
- 87 ppm for 15 minute
LEL (lower explosive limit)
The lowest concentration of flammable gases, dust, vapours that are capable of the flash of fire in presence of an ignition source is called LEL.
Zero per cent lower explosive limit is denoted combustible gas, vapour, dust-free atmosphere.
Example
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| lower explosive limit 100% |
The presence of 5% Methane concentration in the air means 100% LEL, 2.5% Methane concentration in the air means 50% LEL
(50% LEL methane * 5% = 2.5% volume
According to OSHA guideline for Hot Work
It means (10% LEL methane * 5% = 0.5% volume) acceptable for hot work but prefer 0%. .
Conducting a gas test in a confined space?
- Stay outside of confined space,
- At least 3 levels (top, middle, bottom) should be checked,
- Maintain the frequency of gas testing,
- Stop ventilation 15 minutes before the gas test
- Record the gas test,
- Trained, approved and certified gas tester,
- Gas testing device must be calibrated and charged
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| gas detector reading |
TWA, PEL and STEL?
TWA (time-weighted average)
Average exposure to hazardous chemical substances.
PEL ( permissible exposure limit)- OSHA
Average exposure to a chemical substances or physical agent over a specified period, usually a nominal 8 hours.
STEL ( short term exposure limit )
A short term exposure limit means the average exposure to hazardous chemical substances 15 minutes.
Summary:
- Know the OSHA standard
- Do not use your own sense, do not consider the space is safe.
- Gas testing only by approved gas tester
- Know the confined space hazards. emergency call point
- Maintain the frequency of gas test,
- If the condition change inside a confined space, leave the space immediately.
