Controlling Atmospheric Hazards in Confined Spaces
By: Douglas Niemtschk, Contributor
Understanding the principles of ventilation when controlling atmospheric hazards in a confined space is critical for any crew working inside that space. The use of proper, mechanical ventilation is a key control measure that helps ensure the air being breathed by the entrants is clean and safe. This article discusses the types of mechanical ventilation available and the practical aspects of using it in the field.
The lack of proper ventilation can create an asphyxiation hazard. According to the U.S. Department of Labor, asphyxiation is the leading cause of death in confined spaces. In general, this occurs when there is a lack of oxygen in the space or a build-up of airborne toxic substances. Toxic substances can include benzene, carbon monoxide, volatile organic compounds, welding fumes, dust and hydrogen sulfide. Ventilation helps ensure oxygen remains at the appropriate levels and the toxic substance are eliminated or kept below acceptable limits.
Selection & Use of Mechanical Ventilation
When I was a young industrial hygienist working on the ship channel in Houston, I once had a project that involved monitoring the outside of a crude oil tanker ship for hydrogen sulfide (H2S). The concentration inside the vessel’s hold was well above the Immediately Dangerous to Life and Health (IDLH) concentration for H2S of 100ppm. No one was allowed to enter that space, and my only purpose was to make sure the atmosphere outside the space was not dangerous for those working on the ship.
If the decision had been made to enter the space an expert, such as a Certified Marine Chemist (CMC) or Certified Industrial Hygienist (CIH), would have to be consulted and the space properly prepared before allowing anyone to enter. The selection and use of mechanical ventilation and the appropriate PPE for the entrants would have been critical. Although you may not work in the shipyard industry, the selection and use of the type of mechanical ventilation should be taken just as seriously.
Various types of mechanical ventilation exist. They fall into two different categories: dilution ventilation and local exhaust ventilation (LEV). Each type serves a different purpose with the goal of ensuring a safe breathing atmosphere for the workers inside the confined space. An example of dilution ventilation would be the natural flow of air in and out of the confined space (i.e., air changes) created by wind and air movements outside and inside the space. Although this “natural ventilation” can be appropriate in some instances, mechanical ventilation is the preferred method inside a confined space to ensure there are an adequate number of air changes occurring inside the space to reduce the concentration of toxic substances below recognized exposure limits.
Mechanical dilution ventilation, also known as “forced air ventilation,” is provided by either blowers or fans. Both devices are referred to as air movers, but they differ in the discharge and suction pressures they create and depend on different power sources to make them work. Both can be configured to either blow air into the space (positive pressure ventilation) or out of the space (negative pressure ventilation), depending on the configuration of the space; characteristics of the toxic gases and vapors potentially present in the space; and the location of the workers. An expert in choosing the appropriate device should be consulted when trying to achieve the needed amount of air changes per hour.
Local exhaust ventilation (LEV) is not used as frequently as forced air ventilation inside a confined space. It is typically used when the type of task being performed produces high concentrations of toxic gases, vapors or fumes inside the workers’ breathing zone before the dilution air is able to remove it. It depends on the placement of a ventilation hood at a location close enough to the source of emission that it captures and removes it away from the worker. (A good example of where LEV might be used is for welding inside a confined space.)
Ventilating a space should start before anyone is allowed to enter the space. This means you will have to start the forced air ventilation early enough to allow time for the toxic gases and vapors to be removed. In some instances, this may take a day or more, depending on the size of the space and the capacity of the air mover.
Monitoring the Air
Air monitoring is an important part of making sure the ventilation being provided is adequate. This should be done both before an entry occurs and as long as entrants are inside the confined space. The attendant, or “hole-watch,” should have an instrument that continuously monitors for 02 and LEL. Periodic monitoring for toxics should also be conducted. If the concentration of O2 decreases below 19.5%, or the concentration of toxics increases, the space should be evacuated immediately and the source of the problem identified and eliminated.
If the source of the exposure can’t be removed, the ventilation plan may have to be changed or the use of supplied air respirators, such as an SCBA, should be used. For more information about monitoring confined spaces, OSHA has a Fact Sheet entitled “Procedures for Atmospheric Testing in Confined Spaces” that provides a good overview.
The potential presence of flammable gases must also be considered when deciding what type of ventilation device to use. If a confined space contained a material such as crude oil, natural gas, methane or any other flammable material, intrinsically safe air movers should be used. These type of devices are designed in such a way that they do not become an ignition source capable of igniting the gas.
In summary, properly ventilating a confined space can be the difference between life and death for the workers inside the space. Understand the basics of ventilation and involve an expert when attempting to set up ventilation for the first time inside a space. It takes the right equipment and know-how to ensure atmospheric conditions are safe to breathe.
[Doug Niemtschk, CIH, CSP, is Health and Safety Manager, EHS, for Holly Energy Partners. He can be reached at Douglas.firstname.lastname@example.org.]