Vacuum Flask Safety
article by CR Scientific LLC
There have been vacuum implosion accidents in many different laboratories over the years. Many, but not all, involved vessels that weren't designed for vacuum. Some, however, had no identifiable cause. Anyone working around vacuum glassware should treat it as though it could implode at any second.
Because implosion can impart high velocity to the glass fragments, common sense would suggest putting something between you and the possible fragments.
1.) First, inspect your glassware. Most manufactured glass has small bubbles and very shallow (surface) scratches in it. These are not normally of concern; they're at least part of the reason filter flasks are made thicker than ordinary ones. However, if you see any deep scoring, star cracks, or any cracking at all in the flask, you should replace it immediately.
2.) Prior to use, reinforce the flask with criss-crosses of filament tape, and/or build an enclosure around the flask. A cardboard box reinforced with filament tape could be placed around the vacuum glassware, provided no heat sources or sparks were nearby. Alternatively, a plexiglas barrier could be placed between you and the flask; build it so that it will stand up on its own. Normally, the door of a fume cupboard is thick enough to provide protection... but it has to be pulled down to be of any use!
3.) Wear safety goggles - "chemical splash" type goggles are required whenever the vessel is under vacuum, and you should wear them anyway whenever handling lab glassware and/or chemicals. A full face shield is recommended for vacuum work. Wear thick clothing. Throat protection is also a good idea.
4.) Know the chemistry of what's going into that filter flask! A disastrous university accident in the 1980's involved some reactive compounds that should not have contacted each other, especially not inside a glass filter flask. The lab worker was lucky to survive but came away with serious injuries. Chemical reactions don't care where they're happening, so pay attention to what's going where.
5.) Do not jar, strike or drop a flask that's under vacuum pressure! Your flask should be anchored in place with a ringstand and clamp. Avoid unnecessary vibration (e.g., ultrasonic cleaner on same bench; loud music; bench grinder; floor waxer; etc.) when the flask is under vacuum.
6.) Release the vacuum before doing anything to the flask (removing funnel / stopper, adjusting hoses, etc). Do this by disconnecting the hose at the vacuum pump end of the setup, not at the flask end. If your hand pump has a pressure-release valve, use that. If using a water aspirator, turn off the faucet.
7.) Use smaller flasks. Avoid using any container larger than 1 liter (1,000 mL) for vacuum applications, unless it was specially engineered for the purpose. With other factors held equal, smaller containers are less likely to implode.
8.) Use Low-Intensity vacuum. It is not true that all vacuum is equally likely to implode a container. If a wall is subjected to a 2 psi overpressure from one side, while an identical wall is subjected to a 12 psi overpressure from one side, which wall is more likely to collapse?
A pressure differential of 6 inches Hg is simply not going to put as much force on the glass as would a differential of 28 inches Hg. Therefore, don't draw more vacuum than is needed to perform filtration. The goal is simply to help some liquid get through the pores of a filter paper, not perform high-vacuum physics. There is no reason you should ever have to draw more than 10 inches Hg when doing vacuum filtration; and if you do, it might be time to design a different procedure. We've done filtration successfully with as little as 5 inches Hg, using a non-engineered water jet pump made from pipe fittings and a ballpoint pen tube.
If using a water jet pump, turn your faucet on just enough to bring about filtration. Likewise, if using a hand pump, infrequent pumping should be sufficient.
The safety considerations listed above are really just good-sense lab hygiene. Any decent lab manual will recommend similar precautions for vacuum glassware.
When it comes to vacuum, an ounce of prevention is worth much more than a pound of cure.
DISCLAIMER: The safety warnings provided in this article are to serve as a courtesy reminder only. You are responsible for proper use of lab items, as well as for any & all consequences that may arise from using these items. The presence (or absence) of warnings does not in any way diminish that reponsibility.
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