Cryogen Explosions (top)

    Lab Explosion Due to Liquid Nitrogen, University of Houstin,
    Environmental and Physical Safety

    Glass Flask Ruptures, Possible Overpressurization by
    Liquid Nitrogen (top)

Key Instruction Points:

1.  Consider shielding for operations involving vacuum or pressurization.

2.  Be aware of the potential for pressurization when working with cryogenic liquids.

A 250 ml glass flask became overpressurized and burst, spraying two laboratory workers with shards of glass.  Approximately 10 grams of styrene and a minute quantity of a drying agent were immersed in liquid nitrogen to keep the contents frozen.  The laboratory workers then attached the flask to a vacuum pump to evacuate the flask, without success.  Thinking the flask might have developed a crack, the laboratory worker removed the flask from the vacuum line and was defrosting it under warm water in the sink, holding it and examining it, when the flask ruptured.

The best guess as to the cause of the rupture is that a small leak, perhaps a pinhole in the flask, developed wile it was being frozen and that some liquid nitrogen entered the flask.  When the flask was warmed, the liquid nitrogen vaporized (expansion ration 696:1), overpressurizing the flask and leading to the explosion.

The laboratory worker holding the flask suffered from several lacerations to the face, hands, chest and abdomen.  The other worker, who was standing across the room, received lacerations to the abdomen.  The worker holding the flask noted shards of glass embedded in his prescription safety glasses.

The procedure was re-written such that under the same conditions, the stopcock will be unscrewed and the flask set in a catch-bucket in the hood to allow the contents to warm up and vaporize, if volatile.

Appropriate eye protection helped to avoid a potentially serious eye injury.  Consider shielding for operations involving vacuum or pressurization.   Be aware of the potential for pressurization when working with liquid nitrogen.

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   Investigator Exposed to Infectious Material in Cryotube
    Explosion (top)

Key Instruction Points:

1.  Be aware of the potential for pressurization when working with cryogenic liquids.

A researcher at a university reported that a vial of potentially infectious materials "exploded" when she removed it from liquid nitrogen.

As you may have guessed, the "explosion" occurred when the liquid N2 that has leaked into a vial expands when removed from the cold.  This used to be a fairly common problem with heat-sealed glass ampules, because it was difficult to obtain perfectly fused glass with no microscopic holes.  This problem was largely resolved with laboratories began using plastic cryovials with a silicone seal.  Nunc* makes a sleeve called CryoFlex that slips over the vial and then is heat-sealed to keep the liquid out.  However, even with this type of product an explosion infrequently occurs.

There are several ways to prevent this from happening:

1.  Cryogenic storage vials are designed for VAPOR PHASE STORAGE in liquid nitrogen freezers.  This means that they are designed to sit in the cloud of extremely cold nitrogen gas that sits just above a small reservoir of liquid nitrogen in the bottom of the freezer.  Leakage of liquid nitrogen into the vial occurs with the freezer is overfilled and the vials are immersed in liquid nitrogen.   This problem can be avoided by not overfilling the freezers with liquid nitrogen.

2.  Visually check each cryovial prior to filling to ensure there are no defects around the rim.  Cryovials should never be re-used.

3.  When removing samples, pause for a moment in the neck of the dewar before bringing them into the room atmosphere - if one is going to pop, it will usually do so early in the warm-up process.

The importance of gloves and face shield can not be overemphasized.   Tubes stored in liquid phase dewars, where the ampules are in canes is especially hazardous.  Since nitrogen freezers tend to be located separate from the labs, full face shields and gloves should be available near the nitrogen freezers so no one is tempted to pull a vial without protection because they forgot to bring a shield with them.

Information about Nunc products is at:  http://nunc.nalgenunc.com/products/catalog/handling/index.html

   Eye Injuries (top)

    Letter From A Nobel Prize Laureate on The Loss of    
    Sight In One Eye Due To A Cryogen Accident (top)

Dear Colleagues,

Many of you know that I was blinded in one eye during a lab accident in the year X, shortly after I arrived at the University of Y as an assistant professor.  I always wore safety glasses whenever I was at my bench and, while I felt that I was conscientious about observing safety measures, my experience proves that you can’t be too cautious about wearing glasses.

As I prepared to go home from the lab during the early hours of the morning of the accident, I checked in to see what my co-workers were doing and then returned to my bench, removed my safety glasses, and put on my parka.  As I was walking to the door, I passed the bay where a first-year graduate student was flame-sealing an NMR tube.  I asked how it was going and he replied, “Good.  I’ve got it sealed.”

He was sealing off the tube at atmospheric pressure in a big nitrogen bath, a procedure the two of us had discussed though neither had ever performed it.  I stopped by his bench, picked up the tube from the bath and held it to the light.  The tube immediately frosted over and, as I wiped it to better see the contents, I noticed that the solvent level was exceedingly high.  Suddenly the solvent dropped to a normal level and, though I instantly realized condensed oxygen had been sealed in the NMR tube, I was quite literally unable to move a muscle before it exploded.  Glass fragments shredded the cornea, penetrated the iris, and caused the partial collapse of one eye.  My only other injuries were superficial facial cuts.

My first ten days at the hospital were spent totally immobilized and with both eyes bandaged.  The pain was terrific, but my fear was even greater:  I had been warned that when my eyes were uncovered there was a small chance I might be blind in both eyes due to “sympathetic ophthalmia”.  Because eyes are walled off from the rest of the body in utero, eye protein driven into the blood stream can raise an immune response that leads to the “killing” of an uninjured eye.  My disappointment at having no functional vision in my injured eye was, needless to say, surpassed by my joy at retaining full vision in my good eye.

The lesson to be learned from my experience is straightforward: there’s simply never an adequate excuse for not wearing safety glasses in the laboratory at all times.   
                          
From Professor X

    Researcher Blinded in One Eye from Cryotube Explosion
    (top)

Key Instruction Points:

1.  Consider shielding for operations involving vacuum or pressurization.

2.  Be aware of the potential for pressurization when working with cryogenic liquids.

3.  Use appropriate personal protective equipment.

A University of X investigator was blinded in one eye when a cryotube exploded while being thawed.  The probable cause was the rapid expansion of liquid nitrogen that had entered the tube through a small crack during storage.   Suitable personal protective equipment for thawing cryotubes and handling cryogenic liquids consists of a face shield, heavy gloves, a buttoned lab coat and pants or a long skirt.  Cryotubes should be kept in a heavy, walled container or behind a safety shield while warming.

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Revision Date:  6/22/2004
url: http://www2.umdnj.edu/eohssweb/aiha/accidents/cryogens.htm