Content area
Full Text
How do you set up ventilation equipment for worker protection where access is difficult, such as manholes?
Steve Wierzchowski, RLS Solutions Inc Proper ventilation of a confined space, such as a manhole, is a critical component of any Confined Space Entry Program. Almost all manholes, whether they be for sewer, telecommunications, water, or gas services, fit the definition of a permit-required confined space, as defined by the Occupational Safety and Health Administration (see 29 CFR 1910.146 and 1910.268(o)(2)). When fatal confined space accidents occur, more than one element of the safety system has typically failed, including, most often, the accurate monitoring of the atmosphere. OSHA estimates that 85% of permit space accidents would be eliminated by entry personnel reviewing atmospheric testing before entry. While these tests and controls are critical, there are many other possible hazards in a confined space. Ventilation requirements hinge on accurate monitoring.
Once the atmospheric hazards of a space have been identified through testing and site evaluation, the next step is to implement controls, such as isolation and ventilation, to mitigate the hazards. Isolation can be accomplished in manholes by blocking or plugging entry points of toxic, flammable, or oxygen depleting/displacing gases. However, in some situations, not all hazard sources can be blocked, and proper ventilation is thus critical. The following are some key points to ventilating manholes.
? Ventilation equipment must be properly sized
Properly sizing manhole ventilation equipment is a fairly simple process. The average manhole, at 4 ft (1.2 m) diameter x 10 ft (3 m) depth, contains only about 125 ft3 of atmosphere. A standard portable blower produces about 600 CFM of air at the end of a 15 ft (4.5 m) x 8 in. (0.2 m) duct. Using this equipment effectively changes the atmosphere in such a manhole over 200 times per hour, greatly exceeding the minimum recommendation of twenty.
Additionally, it is recommended to allow at least seven air changes to sufficiently purge a structure, which in this case would take about two minutes. When dealing with large structures, the calculations become more critical but most portable ventilation equipment is suitable for manholes less than 15 ft (4.5 m) deep.
? Ventilation must draw from a source of safe supply air
Ensuring a clean...