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Unformatted text preview: olved. Most flammable vapors have a density greater than that of air
and will settle on a bench top or floor where they may diffuse to a distant burner or
other ignition source and “flash back.” That is, if the flammable vapors are not controlled—within a hood, for example—they can evolve, expanding up and out over
the containing vessel rim. Then they can travel undetected at floor level over astonishingly long distances. If there is a source of ignition at that distant point, the train
of vapor will instantly flash back all the way to the container and either ignite the
liquid in the container or cause the vapors near the container to explode.
Use a laboratory hood when working with a system under reduced pressure (which
may implode). Close the sash of the hood to provide a shield. Note that unless
designed and built for the purpose, hoods are not to be relied on for protection in case
of an explosion. Equipment Use
Laboratory hoods control exposures to toxic, offensive, or flammable vapors. They protect users from implosions but not from explosions. If it is necessary to perform a procedure that could result in an explosion, conduct such work behind sturdy barriers that
are designed and built for the purpose. Ordinary laboratory hoods are not strong
enough to withstand the forces released in any but the mildest of explosions.
Before each use, be sure that the hood is working properly. If you have questions, ask
your instructor. Do not rely on a monitoring device such as a strip of tissue paper held
inside the hood to flutter in the breeze. At best, such a device can only indicate that
the hood fan is pulling some air into the duct. A properly operating laboratory hood
requires both an adequate airflow and the absence of excessive turbulence.
Never block, even partially, exhaust ports or slots in the rear wall and ceiling of the
hood; do not change the size of the vent openings in the rear and ceiling of a hood.
Never alter the supply air vents to the room...
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This document was uploaded on 02/27/2014 for the course PHYS 1B at UCSD.
- Spring '07