Gas Detection in a Confined Space

GAS DETECTION IN A CONFINED SPACE

What is a confined space?

A space, room or building is called a confined space when there is a possible presence of the following;

Flammable airborne contaminants
Toxic airborne contaminants
Engulfment
Hazardous level of Oxygen

Examples of a confined space include:

Storage tanks and vessels
Sewers and manholes
Agriculture silos
Marine vessel tanks
Tunnels

Monitoring confined spaces for Atmospheric Hazards

Monitoring the air inside a confined space is required prior to entering.

Testing a confined space for atmospheric hazards should be done remotely before entering and it is recommended that it is done in this order:
Remember – O.C.T- for Oxygen / Combustible Gases / Toxic Gases

OXYGEN

Ensure that proper oxygen levels are present.

Potential Effects of Oxygen Enriched and Deficient Atmospheres
Oxygen Content (% by Vol.)	Effects and Symptoms (At Atmospheric Pressure)
> 23.5 %	Oxygen enriched, extreme fire hazard
20.9 %	Oxygen concentration in normal air
19.5 %	Minimum permissible oxygen level
15 % to 19 %	Decreased ability to work strenuously; may impair coordination and may cause early symptoms for persons of coronary, pulmonary or circulatory problems
10 % to 12 %	Respiration further increases in rate and depth; poor judgment, blue lips
8 % to 10 %	Mental failure, fainting, unconsciousness, ashen face, nausea, and vomiting
6 % to 8 %	Recovery still possible after four to five minutes. 50% fatal after six minutes. Fatal after eight minutes.
4 % to 6 %	Coma in 40 seconds, convulsions, respiration ceases, death
These values are approximate and may vary with each individual

COMBUSTIBLE GAS

Ensure that combustible gases are not present.

The lowest concentration (air-fuel mixture) at which a gas can ignite is called lower explosive limit (LEL). Concentrations below this limit are too lean to burn.

The highest concentration that can be ignited is its upper explosive limit (UEL). Above that concentration, the mixture is too rich to burn.

Most confined space entry gas detectors measure in the lower explosive (LEL) range.

TOXIC GAS.

Ensure that toxic gases are below the permissible workplace exposure limits.

Common toxic gases in a confined space are hydrogen sulphide (H2S) and carbon monoxide (CO), but other toxic compounds could be present.
Check New Zealand’s workplace exposure standards www.dol.govt.nz/workplace/knowledgebase/item/1444

In a confined space it is important to take samples at the top, middle and bottom to locate varying concentrations of gases and vapours. Highly concentrated gases can accumulate at the top or bottom of a confined space depending on whether they are heavier or lighter than air.
Dilute gases and vapours in the ppm (parts per million) range distribute evenly throughout a confined space.

The ABC of Gases in Industry

Courtesy of Rae Systems Application Note AP-202
Talk to Teltherm about Rae systems wireless gas detection solutions

Ammonia: NH₃	*n-Butane: C₄H₁₀**
Colorless gas with a pungent suffocating odor Fertilizer Plants Fish & Meat Packing Plants Industrial Refrigeration and Cold Storage Semiconductor Industry Water and Wastewater Treatment Plants Munitions	Colorless gas with a gasoline-like odor Aerosol Propellant Filling Docks Feed stock for Chemical Processes Storage Tanks and Filling Docks
Benzene: C₆H₆	Carbon Dioxide: CO₂
Colorless liquid with aromatic odor Refineries Oil & Gas Distribution Feed Stock for Chemical Production Solvent Distribution Centers Gas Stations	Colorless, odorless gas Breweries and Wineries Carbonated Beverage Bottling Plants Dry Ice Plants, Food Processing Plants, Fruit Storage and Ripening Chambers Greenhouses, Indoor Air Quality Studies, and Ventilation Control Mushroom Farms Stack Gas Oil Well Injection
Carbon Monoxide: CO	Chlorine: Cl₂
Colorless, odorless gas - most abundant toxic gas Furnaces Gasoline Generators/Engines Grain Storage Silos Lumber Drying Kilns Mining and Metals Parking Garages	Green-yellow gas with a pungent, irritating odor Mining & Metals Industry Nuclear Reactors Pulp & Paper Mills PVC Plastics Manufacturing Semiconductor Water Etching Facilities Swimming Pool Chlorinization Plants Water Treatment Plants Chlorine Gas Manufacturing
Hydrogen Cyanide: HCN		Hydrogen Sulfide: H₂S
Colorless gas with a bitter, almond-like odor Plating and Mining Nylon Manufacturing		Colorless gas with a strong odor of rotten eggs Leather Tanneries and Paper Mills Mining and Metals Industry Oil Fields and Refineries Sewage Treatment Plants Sewer Maintenance
Methane: CH₄		Nitric Oxide: NO
Colorless, odorless gas – odorized with mercaptans Primary component of natural gas No exposure limits – Simple asphyxiant Oil & Gas Distribution & Refining Mining Industry		Colorless gas Semiconductor Plants Mining
Nitrogen Dioxide: NO₂		Oxygen Deficiency: O₂
Reddish-brown with a pungent odor Boilers and Furnaces Diesel Emissions Semiconductor Plants Mining Industry		Colorless, odorless gas Cargo Holds and Storage Tanks Grain Storage Silos (Inerted Atmospheres) Liquid Nitrogen Storage Liquid Nitrogen Cooled Laser Facilities Liquid Nitrogen Cooled Telescope Sewer Maintenance Sewer Treatment Facilities Underground Vaults (Utilities)

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