How to size a confined space purge
Manholes, tanks and pits hold whatever atmosphere they last collected, so the contaminated air has to be pushed out before anyone enters. How long that takes depends on the volume of the space, the airflow of the blower, and how many air changes it takes to bring the starting concentration down to the target.
The dilution equation is t = K × (volume ÷ airflow) × ln(initial ÷ target). Dropping a concentration to one tenth needs about 2.3 air changes and to one hundredth about 4.6. K corrects for the fact that real spaces never mix perfectly; tight corners and dead pockets call for a larger factor.
OSHA 1910.146 treats an atmosphere as hazardous when oxygen falls below 19.5 percent or rises above 23.5 percent. The upper limit matters as much as the lower one, because an oxygen enriched space turns a small spark into a rapidly spreading fire.
A calculated purge time is a plan, not a clearance. The atmosphere has to be tested after ventilation and monitored during the work, and the entry permit, attendant, communication and rescue provisions of the permit-required confined space program all still apply. Have a qualified safety professional review the entry plan before work begins.
Frequently asked questions
Use the dilution equation t = K × (volume ÷ airflow) × ln(initial ÷ target). Cutting a concentration to one tenth takes about 2.3 air changes; one hundredth takes about 4.6.
OSHA 1910.146 defines a hazardous atmosphere as oxygen below 19.5 percent or above 23.5 percent, so an acceptable entry atmosphere sits between those two figures.
No. Atmosphere must be tested after ventilation and monitored during entry, with a permit, an attendant outside and rescue arrangements in place before anyone goes in.