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One of the many standard procedures when preparing to enter and occupy a confined space is to check for the gases with a Confined Space Gas Detector. Whether or not the readings come back clean, the work being performed within the tank, vault or pit may generate gases (welding for instance), or may disturb the contents in such a way as to release previously trapped gases. The answer is both continuous and effective ventilation.
Mechanical ventilation, such as is provided by a fan and ducting, must supply sufficient volume to replace contaminated air with clean, breathable air. This may seem straightforward, but experienced rescuers will tell you that this is not always the case. The volume, shape and depth of the confined space may present obstacles to getting a complete purge.
Supply ventilation, or pushing air into the space, is generally felt to be the most effective method. The contaminated air will exit by the least restrictive path. This must be taken into consideration, and it goes without question that every situation will present it's own unique challenges. The so-called 'Nomograph' located below is used to obtain a rough estimate of the amount of time it takes a given fan (capacity) to change the air in a given space (volume). We say estimate, because a deep space with heavier than air contaminants (for instance), may take more volume to clear than a shallow tank with multiple openings.
We generally recommend electrically powered fans for confined space work. There are gasoline powered fans available, but the possibility of introducing carbon monoxide from the exhaust has caused us to shy away from these. Air powered ventilation devices, sometimes called air ejectors are another. These are noisy, but have the advantage of no moving parts, and with proper grounding may be useful in explosive atmospheres.
Speaking of explosive atmospheres...If your confined space gas detector indicates that your proposed work area contains methane or another combustible gas, you may want to consider choosing a hazardous location fan. These devices have specially constructed motors, and conductive ducting, to insure that dangerous sparks or static electricity won't set off an explosion. Our Euramco RamFan Hazardous Location blowers are ATEX rated. Getting approved is rigorous, and recognized as the global standard when referring to this type of blower.
Using the air movement volume figures shown with each fan or blower assembly, refer to the nomograph below to estimate the amount of time needed to purge your confined space. You may note from the chart that Allegro recommends changing the air 20 times per hour (once every 3 minutes). Using any particular figure out of context is a little controversial. Rescue professionals with years of experience argue this point. Your training and company rules (if applicable) are the best guidelines.