RISK
ASSESSMENT: USE OF REDUCED PRESSURE OR VACUUM
Glassware
Hazards
Implosion and flying glass leading to cuts and lacerations. Any piece
of glassware under vacuum e.g. rotary evaporators, vacuum desiccators,
Schlenk lines and storage bulbs on vacuum lines has the potential to do
harm following implosion.
The energy imparted to flying fragments is directly proportional to the
volume of the glass vessel evacuated. It follows that the potential to
do harm is also directly proportional to the volume of the glass vessel
and a rotary evaporator with its associated flasks is a greater hazard
than a small Schlenk tube.
It is a common misconception that so called "high vacuum" (typically
10-3 mbars or better) systems present a significantly greater hazard than
everyday vacuums produced by e.g. a water pump (around 30 mbars). These
may differ by four orders of magnitude but the forces to which the glassware
is subjected is essentially the same i.e.
- High Vacuum, 99.999% of atmospheric pressure.
- Water Pump, 97% of atmospheric pressure.
Risks and Who is likely to be injured?
In the event of glassware implosion, both the immediate user and near
co-workers are likely to be injured with damage likely to be moderate
(small cuts) to severe (major lacerations or eye damage).
Precautions
- Lab coats and glasses should be worn. In certain circumstances e.g.
when introducing liquid nitrogen or other cryogenic material or when
warming storage tubes from low temperature, a face mask and appropriate
gloves should be worn.
- Only suitable glassware should be used: conical flasks, except the
heavy walled Buchner type flasks should never be subjected to a vacuum.
- Glassware should be free from chips, cracks or flaws that would make
it unsafe to use. Particular care should be taken to spot any star cracks.
- Volumes of 1 litre or larger must be enclosed in tape or plastic mesh
to restrain fragments in case of implosion. This will normally apply
to rotary evaporators, vacuum desiccators and storage bulbs on glass
lines. Schlenk lines and tubes are generally of small volume and are
quite robust in nature and do not require extra protection in the shape
of tape or plastic mesh.
- Glass dewars should be fully wound in tape or preferably enclosed
in a metal container.
Metal Vacuum System
Hazards
There are fewer hazards in handling metal vacuum systems due to the very
unlikely risk of implosion.
Pumps
Hazards
Vacuum pumps are of various kinds. The most common are oil rotary pumps
and oil (or more rarely mercury) diffusion pumps of glass or metal. Turbomolecular
pumps are also used but apart from being electrical equipment, these present
little danger being totally enclosed.
- Vacuum pumps are electrically powered apparatus.
- Belt driven rotary pumps present danger of entrapment in the moving
belt and pulley wheels.
- The exhaust of rotary pumps may be contaminated chemically but will
also contain an oil mist from the pump itself.
- There is a danger of explosion if the exhausts of rotary pumps that
are pumping large volumes of air or other gas are blocked or obstructed.
- Diffusion pumps are heated to boil the pumping liquid and so present
a risk of burns.
- Glass diffusion pumps are vulnerable to breakage and if these contain
mercury the danger of mercury contamination is great.
Precautions
- The usual precautions must be taken when using electrical equipment.
- Rotary pumps must have belt guards to prevent entrapment.
- A trap (either a cold trap or molecular sieve) should be used between
system and pump to prevent contaminants reaching the pump oil or being
exhausted into the laboratory.
- The exhausts of rotary pumps must be free from obstruction.
- Exhaust lines must be vented to a fume hood by tubing of large enough
cross section not to cause obstruction.
- Where possible mercury diffusion pumps should be replaced by oil versions.
Mercury pumps must have secondary containment.
- The boilers of diffusion pumps must be shielded to prevent burns by
contact.
- Diffusion pump fluids may be subject to a COSHH Assessment.
Pump Maintenance, Changing Oil
Pump maintenance including oil changes may be carried out by users themselves
or by a member of the technical staff assigned that duty.
Hazards
Pump oil possibly contaminated with solvents, mercury, corrosive or obnoxious
substances.
Precautions
- As far as possible, pump oil should be drained with the pump in a
fume hood.
- Appropriate gloves and a lab coat must be worn.
- If there is any suspicion of contamination, the oil must be treated
as hazardous waste.
- Waste oil should normally be taken to the technician in charge of
pump maintenance for proper disposal.
- Pumps left for service by technical staff should bear a warning about
possible oil contaminants.
Pressure Gauges
Hazards
Vacuum pressure gauges are mainly of two kinds i.e. the manometer or
McLeod Gauge type which are made of glass and contain mercury or some
other liquid, and electrical devices which measure pressure dependent
properties such as thermal conductivity or ionisation current.
- Danger from glass apparatus and possibly mercury.
- Electrical equipment.
Precautions
- Glassware gauges should be treated as indicated above under "Glassware".
- Secondary containment must be used around systems that contain mercury.
- Where possible, mercury should be replaced by some other less hazardous
fluid.
- Manometer fluids may be subject to a COSHH Assessment.
- The usual precautions must be taken when using electrical equipment.
Training
The use of glassware under vacuum or reduced pressure is part of undergraduate
chemistry training. For more advanced vacuum systems, users must be instructed
by a person competent and experienced in their use.
Remaining Risk
This is slight if the precautions outlined above are followed. However,
glass systems remain more dangerous than metal systems because of the
possibility of implosion.
Emergency Procedures
In the event of injury or fire follow the procedures outlined under "What
to do if" and under "Basic First Aid"
Back to Completed Risk Assessment Forms
Adapted
with permission from School of Chemistry, University of Bristol
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