Cyogenic Material Safety: Liquid Nitrogen and Helium
Areas of concern
1.
2.
3.
4.
Freezing
Oxygen enrichment
Asphyxiation
Pressure-related explosions
1. Freezing
While the Lindenfrost effect offers, a measure of protection freeze damage from liquid nitrogen
is still a major concern. Brief contact with Liquid Nitrogen, while to be avoided will not
necessarily cause harm, as the evaporating gas causes a protective buffer. Harm usually occurs
due to prolong contact, when the liquid Nitrogen soaks into clothing.
Do Not wear clothing that can capture liquid nitrogen run-offs or spills, such as cuffed pants.
Always wear protective gloves that can easily be removed in the event of a spill should be worn
when handling liquid nitrogen. If the liquid Nitrogen soaks the gloves, they should be removed
immediately.
Always wear safety goggles—LN can boil up and into your face at any moment, quickly
blinding you.
Never pour LN above your head, and do not kneel next to someone pouring LN.
Do Not hold containers of LN with one hand, or in any other precarious way.
Have a clear exit path in case something goes wrong—if you drop a whole container of LN,
you and everyone else around you needs to get away from it quickly.
Some Do’s and Don’ts. Do wear goggles or a face shield. Do Not Wear
clothes that can trap liquid Nitrogen- like canvas shoes. Do Not use
unprotected hands to hold onto things that have been cooled by
liquid Nitrogen.
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2. Oxygen enrichment
Although not flammable, oxygen when present in higher concentrations can significantly increase the
chance of fire or an explosion. The boiling point of oxygen is above those of nitrogen and helium. In
closed systems (such as cold traps cooled with liquid nitrogen), these liquids can cause oxygen to
condense on their surface (resulting in a bluish liquid on the surface). This can lead to the ignition of
normally non-combustible materials, and the flammability limits of flammable gases and vapors are
widened. Oil and grease may spontaneously ignite and as such should not be use where oxygen
enrichment may occur.
In the Sykes lab we should dispose of any standing liquid Nitrogen at the end of each work day or
when it is no longer required.
3. Asphyxiation
A small amount of liquid nitrogen turns into a large amount of nitrogen gas—LN expands by 700
times when it vaporizes. If you use a lot of LN in a closed space, it will displace the oxygen and
suffocate you before you know what’s happening. This is the largest cause of death from
LN. The danger is very, very real.
You can’t tell that you are suffocating on nitrogen. The panicky feeling you get from choking or
staying under the water too long doesn’t come from lack of oxygen— it comes from the buildup
of CO2 in your blood. Your lungs can get rid of CO2 just fine in a pure nitrogen environment, so
your body doesn’t send those helpful distress signals. Also, breathing pure nitrogen is much
worse than just holding your breath. Breathing nitrogen actually sucks the existing oxygen out of
your bloodstream. In a room with 10% Oxygen just two breaths will kill you.
• Do Not use LN in an unventilated area.
• Never go into an elevator with LN. If a dewar breaks in the elevator, you’re dead.
• Never carry LN in a vehicle’s passenger compartment. You get into an accident, the LN spills
and vaporizes, you suffocate instantly.
• If you see someone passed out near an LN tank, do not try to help. Many people who die from
nitrogen suffocation were trying to help co-workers. If the victim has passed out they are
probably already dead. Call campus police x66911. 617 627 6911
In the Sykes lab the door to the quiet room should remain open whenever the room is
occupied.
4. Pressure-related explosions
Here are two examples of LN Dewar explosion. First is a blow-out of a pressure release plate of
a properly maintained Dewar. Second is a major explosion of an incorrectly modified Dewar.
This first Dewar is a similar design to that used in the Sykes lab. It was used by a restaurant that
cooked food with LN. In their own words:
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While moving our 160 liter LN dewar from the 4th floor to the second floor of our building the
dewar got knocked over—don’t ask how. One of our interns pulled a Hercules and righted the
many-hundred pound monster single handedly.
Seconds later, the dewar vented with a thundering
ka-boom, blowing the cap off the top, and
punching out a ceiling tile. The hallway filled with
nitrogen. We were okay because we followed the
rules. Everyone cleared the area. I opened the
window and monitored oxygen levels on our
oxygen meter. We put the dewar in the elevator
without anyone in it (that is one of our standard
safety rules), and took it down and outside where
Pressure release plate, post release
we chained it up.
Imagine if we didn’t have the “no people riding
in the elevator with the dewar” rule. If the
dewar had vented 20 seconds later and it was in
an elevator with people, they most likely would
have been asphyxiated. Some of the safety rules
we follow with liquid nitrogen seem to guard
against far-fetched scenarios that will never
happen.
Ceiling tile hit by pressure release plate
Looks like they can happen. tay safe.
Second incident due to modifications of dewar safety systems
Liquid nitrogen tanks are usually equipped with pressure
relief fittings, since nitrogen does tend to want to be a gas,
and gases do tend to want to expand quite a bit. This tank,
though, which seems to have been kicking around since
1980, had been retrofitted by a real buckaroo. Both the
pressure relief and rupture disks had failed for some reason
in the past, so they'd been removed and sealed off with metal
plugs. You may commence shivering now.
Why it didn't blow long ago is a real stumper, but
presumably people were taking nitrogen out of it quickly
enough to keep things together. Not this time, though: at
around 3 AM, things came to a head as the internal tank (the
dewars are double-walled) expanded until it pressed against
the outer one. That kept it from expanding anywhere else
except on the ends, and as fate would have it, the bottom
blew out first. The engineer's best guess is that this took
place at around a 1200 psi load. I'll let the engineer's report
Dewar compressed by force of striking the
ceiling. The bottom of these Dewar failed
first.
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take it from here:
The cylinder had been standing at one end of a ~20' x 40' laboratory on the second floor of the
chemistry building. It was on a tile covered 4-6" thick concrete floor, directly over a reinforced
concrete beam. The explosion blew all of the tile off of the floor for a 5' radius around the tank
turning the tile into quarter sized pieces of shrapnel that embedded themselves in the walls and
doors of the lab. The blast cracked the floor but due to the presence of the supporting beam,
which shattered, the floor held. Since the floor held the force of the explosion was directed
upward and propelled the cylinder, sans bottom, through the concrete ceiling of the lab into the
mechanical room above. It struck two 3 inch water mains and drove them and the electrical
wiring above them into the concrete roof of the building, cracking it. The cylinder came to rest
on the third floor leaving a neat 20" diameter hole in its wake. The entrance door and wall of the
lab were blown out into the hallway, all of the remaining walls of the lab were blown 4-8" off of
their foundations. All of the windows, save one that was open, were blown out into the courtyard.
In the Sykes Lab we should be aware of the dangers inherent with both N2(L) and He(l) tanks, both of
which are used in the lab. We should be aware of ice forming on any of the instruments causing a
blockage, which could lead to an increase in pressure. The LT-STM dewars should not be allowed to
run dry of cryogen, unless planned. He(g) should be used, when appropriate, to flush out the dewars
before adding cryogen.
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