Guide to-atmospheric-testing-in-confined-spaces-usqc
- 1. GUIDE TO ATMOSPHERIC TESTING IN CONFINED SPACES This application note is intended to provide general information and to act as a reminder of the dangers associated with atmospheric hazards in a confined space. It outlines the following: • What is a confined space? • Atmospheric hazards found in a confined space. WHAT IS A CONFINED SPACE? The confined space entry standard was established by OSHA 29CFR 1910.146 in April of 1993. The standard was developed to provide a defined work plan for confined space entry. Confined space entries are part of a daily routine throughout the industrial workplace. A Confined Space Is Defined As a Space That: • Is large enough for an employee to enter and perform work. • Has limited or restricted means for entry or exit. • Is not designed for continuous human occupancy. A Permit-Required Confined Space Is Defined As: A confined space, plus one of the following: • Contains, or has a known potential to contain, a hazardous atmosphere. • Contains material with the potential for engulfment. • Has an internal design that could entrap or asphyxiate the entrant. • Contains any recognized safety or health hazard. Examples of confined spaces: • Storage tanks and vessels • Sewers and manholes • Underground utility vaults • Agriculture silos • Railcar tanks • Marine vessel tanks • Tunnels • Grain elevators ATMOSPHERIC HAZARDS IN CONFINED SPACES Atmospheric hazards in a confined space are those that expose entrants to a risk such as death, entrapment, injury, or acute illness from one or more of the following causes: Oxygen An atmospheric oxygen concentration below 19.5% (oxygen deficiency), or above 23.5% (oxygen enrichment). 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 vary, due to an individual’s state of health and physical activity. Combustible Gases
- 2. A flammable gas or vapor in excess of 10% of its lower explosive limit (LEL) yet still remaining below the upper explosive limit UEL). Lower Explosive Limit (LEL) Vs. Upper Explosive Limit (UEL) • 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. THE FIRE TETRAHEDRON For combustion to occur, there must be four elements: 1. Fuel 2. Oxygen to support combustion 3. Heat or a source of ignition 4. Chain reaction, i.e., the above three must be present in such proportions as to allow a flame to propagate. This is called the fire tetrahedron (formerly known as the fire triangle). If any of the four elements are missing, combustion cannot occur. The fourth element, chain reaction, means that not any mixture of fuel and oxygen can sustain a flame if heated. The proportions must allow a flame to propagate. For normal air, this means that the fuel concentration must be between the LEL and the UEL. COMBUSTIBLE GAS - PERCENT BY VOLUME Surveying monitors read out in both % LEL and % by volume. For example the LEL of methane is 5% by volume, and the UEL is 15% by volume. When a confined space reaches 2.5% methane by volume this would be equal to 50% LEL. (5% methane by volume would be 100% LEL.) Between 5 to 15% by volume, a spark could cause an explosion. Different gases have different % by volume concentrations to reach 100 % LEL. Some examples are: Propane’s LEL is 2.1% by volume; Pentane’s LEL is 1.5% by volume; Hexane’s LEL is 1.1% by volume and gasoline’s LEL is 1.3% by volume. TOXIC GASES An atmospheric concentration of any toxic compound above the permissible exposure limit established by OSHA, NIOSH or ACGIH. Here are examples of common toxic gases found in a confined space. Toxic Gas TWA STEL Ceiling IDL Ammonia 25 ppm 35 ppm -- 500 ppm Carbon monoxide 25 ppm -- 200 ppm 1 ,500 ppm Chlorine 0.5 ppm 1 ppm -- 30 ppm Hydrogen cyanide -- -- 4.7 ppm 50 ppm Hydrogen sulfide 10 ppm 15 ppm -- 300 ppm Nitric oxide 25 ppm -- -- 100 ppm Sulphur dioxide 2 ppm 5 ppm -- 100 ppm Life-Threatening Effects: CO and H2S Effects Of Carbon Monoxide Exposure ppm Time Effects & Symptoms 35 8 hours Permissable Exposure Level 200 3 hours Slight headache, discomfort 400 2 hours Headache, discomfort 600 1 hours Headache, discomfort 1000 to 2000 2 hours Confusion, discomfort 1000 to 2000 ½ to 1 hour Tendency to stagger 1000 to 2000 30 Slight heart palpitations 2000 to 2500 30 Unconsciousness 4000 > 1 hour Fatal Effects Of Hydrogen Sulfide Exposure
- 3. ppm Time Effects & Symptoms 10 8 hour Permissable exposure level 50 to 100 1 hour Mild eye and respiratory irritation 200 to 300 1 hour Marked eye and respiratory irritation 500 to 700 ½ - 1 hour Unconsciousness, death > 1000 Minutes Unconsciousness, death 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 should be done in this order: • Oxygen. Ensure that proper oxygen levels are present. • Combustible gases. Ensure that combustible gases are not present. • Toxic Gases. Ensure that toxic gases are below the OSHA permissible exposure limit. Common toxic gases in a confined space could be hydrogen sulfide (H2S) and carbon monoxide (CO), but other toxic compounds could be present. In a confined space, it is important to take samples at the top, middle, and bottom to locate varying concentrations of gases and vapors. Highly concentrated gases can accumulate at the top or bottom of a confined space depending on whether they are less or more dense than air. Dilute gases and vapors in the ppm range distribute evenly throughout a confined space. It is especially important to sample at a distance from the opening because air intrusion near the entrance can give a false sense of adequate oxygen presence. As the remote air monitoring is completed and the area is safe for entry, confined space entry permits should be completed and followed. After the initial entry, monitoring the air in the confined space should be continuous. A confined space entry attendant or hole watch should carry out the continuous monitoring. Conditions in a confined space can change without warning, due to leakage, toxic vapor release, or disturbing the contents of the space. Before entering the confined space, take samples at several levels, and after entering, monitor the space continuously because conditions can change. wide variety of interchangeable toxic sensors supplementable with a PID for applications such as confined space entry. ADDITIONAL INFORMATION Federal Register 29 CFR 1910.146 Permit Required Confined Spaces for General Industry http://cos.gdb.org OSHA U.S. Department of Labor http://www.osha- slc.gov OSHA Regulations (Standards - 29 CFR) - 1910.146 - Permit-required confined spaces. www.osha- slc.gov/OshStd_data/1910_0146.html OSHA Regulations (Standards - 29 CFR) - 1910.146 App C - Examples of Permit-required Confined Space Programs. www.osha-slc.gov/OshStd_data/1910_0146_APP_C.html OSHA Regulations (Standards - 29 CFR) - 1910.146 App A - Permit-required Confined Space Decision Flow Chart. www.osha- slc.gov/OshStd_data/1910_0146_APP_A.html Disclaimer: This application note contains only a general description of atmospheric testing in a confined space and equipment used to monitor a confined space. Under no circumstances should a confined space be entered or monitoring equipment used except by qualified and trained personnel, and after all instructions have been carefully read and understood and all precautions followed.