DRAFT Basic Guide to Handling Liquid Nitrogen (Small Unpressurized Quantities)
This guide is intended to cover only small quantities of unpressurized LN2 and is expressly limited to such cases. Small quantities are defined as those less than about 1 liter. Unpressurized means, of course, that the liquid is in a quiescent equilibrium state (or simply "saturated") at the prevailing ambient pressure.
This guide does not apply for pressurized or non-quiescent LN2 or for amounts greater than about 1 liter. In such cases, the procedures and guidelines related to that task must be followed. Regardless of quantities or pressures, the same precautions are always in order; however, the level of exposures to risk are different. This guide provides precautions for improved personnel safety risk reduction in handling small, unpressurized quantities of LN2.
The handling of small, unpressurized quantities of LN2 is a common practice in many areas of daily work. These areas include laboratories, doctors offices, clinics, animal farms, classrooms, culinary arts, electronic sensor calibration, and so forth. For example, a small amount, such a literal "cup full," of LN2 may be used to quench a material test specimen or provide cooling for the calibration of an electronic device. In classroom, laboratory, or industrial settings such small amounts of LN2 are used for a very wide range of purposes including for cooling, inerting, machining, quenching, surface treatments, etc.
General Summary of Hazards Associated with Handling LN2
(Ref. Argonne National Lab)
The hazards associated with the handling of cryogenic fluids include:
● Cold contact burns and freezing (contact with cold liquid, gas or surface): The potential for freezing by contact with the extreme cold of cryogens necessitates varying degrees of eye, hand and body protection.
- When a cryogenic fluid is spilled on a person, a thin gaseous layer apparently forms next to the skin. This layer protects tissue from freezing, provided the contact with the cryogen involves small quantities of liquid and brief exposures to dry skin. However, having moist skin, exposure to moving cryogens, or extended periods of time, can freeze tissue.
The most likely cause of frostbite to the hands and body is contact with cold metal surfaces. Since there is no protective layer of gas formed, frostbite will occur almost instantaneously, especially when the skin is moist. The damage from this freezing (frostbite) occurs as the tissue thaws. Intense hypothermia (abnormal accumulation of blood) usually takes place. Additionally, a blood clot may form along with the accumulation of body fluids, which decreases the local circulation of blood. Adequate protection and clothing is required at all times when handling, transferring or operating near cryogenic fluids.
● Asphyxiation (displacement of oxygen by inert gas): When liquid cryogens are expelled into the atmosphere at room temperature, they evaporate and expand on the order of 700 to 800 times their liquid volume. Whenever possible, handling of cryogenic fluids where release into the atmosphere is possible should be done in open, well ventilated areas.
● Explosion (excessive buildup of pressure in container of cryogenic fluid): Heat flux into the cryogen is unavoidable regardless of the quality of the insulation provided. Since cryogenic fluids have small latent heats and expand 700 to 800 times to room temperature, even a small heat input can create large pressure increases. Dewars must be moved carefully. Sloshing liquid into warmer regions of the container can cause sharp pressure rises. Pressure relief devices must be provided on each and every part of a cryogenic system. Satisfactory operation of these devices must be checked periodically and may not be defeated or modified at any time. Vents must be protected against icing and plugging. When all vents are closed, enough gas can boil off in a short time to cause an explosion. Vents must be maintained open at all times. Some materials may become brittle at low temperature and fail in the case of overpressure or mechanical shock. Only suitable materials may be used to store or transfer liquid cryogens.
● Fire/explosion (condensation of liquid oxygen): Liquid oxygen liquifies at a higher temperature than liquid helium or nitrogen. Consequently, liquid oxygen can condense on the exterior of cryogenic containers or transfer lines. An explosive situation may result if this oxygen-rich liquid is allowed to soak insulating or other materials which are not compatible with oxygen. Some oils can form an explosive mixture when combined with liquid oxygen. Surfaces where there exists a possibility of liquid oxygen condensation must be thoroughly cleaned and degreased.
Whenever handling or transfer of cryogenic fluids might result in exposure to the cold liquid, boil-off gas, or surface, protective clothing shall be worn. This PPE will include:
● face shield and/or protective eyewear
● safety gloves
● long-sleeved shirts, lab coats, aprons.
Eye protection is required at all times when working with cryogenic fluids. When pouring a cryogen, working with a wide mouth dewar, or, in particular, cooling down a warm object, the eyes must be protected. The use of a full face shield is recommended if pressurized fluids are involved or if splashing or large amounts of boil-off vapors are possible. Hand protection is required to guard against the hazard of touching cold surfaces. Loose insulating gloves can be used. (We are looking to find a balance between tactile control and insulation in a thin profile glove for the "light-duty" lab use that is so common. Most gloves are bulky and offer a level of protection that is overkill while prohibiting safe working practices for the small-scale, bench-top, routine, detail jobs. )
Supply of LN2
The small, unpressurized amount of LN2 usually comes from another larger source. This source could be pressurized or unpressurized. (The safety precautions relative to handling and dispensing of these larger quantities of LN2 must be followed as previously discussed.) Unpressurized sources include standard dewars that are commercially available in sizes from 2 liters to 30 liters; the 5-liter and 10-liter sizes are most commonly used. Pressurized sources include liquid cylinders or liquid storage tanks. Liquid cylinders are commonly provided in 160-liter or 240-liter sizes and are set to operate at approximately 22 psig or 75 psig. Liquid storage tanks are stationed outdoors and come in sizes from 300 gallons to 18,000 gallons or more and are designed to operate at pressures up to 250 psig.
The most typical case for dispensing a small amount of LN2 is to pour from a standard dewar (the 10-liter size is most common). These highly thermal efficient, vacuum-jacketed dewars have a foam cap/plug to keep the moist air out of the inner vessel but allow for constant venting that provides an effective purge against moisture intrusion. The liquid nitrogen is typically poured from the dewar into a number of different receptacle vessels. These receptacles small open-mouth dewars such as a "carafe" or "dog-dish," or a styrofoam cup or similar insulated dish.
Whatever the source of supply of LN2, the applicable standard safety procedures used for dispensing the desired small amount of LN2 shall be followed.
Handling Small Amounts of LN2
Once the open vessel of LN2 (about one liter or less) has been obtained by normal procedures as described above, the safety considerations for handling the small amount is outlined as follows.
- Minimum PPE: safety glasses (always), gloves (sometimes)
- Additional PPE: as needed per discretion in proper context
The goal here is safety; not safety attire protocols. No one would don a full electrical lineman's suit to check the voltage on a car battery. The PPE must match the need and contribute to safety; and not the other way around.
Special Notes for School Presentations and Other Similar Demonstrations
ALWAYS start off with an INTENSIVE safety talk. Safety of the children and teachers is always PARAMOUNT. I explain that I am the expert, I know what is going on (I’ve done this before, many times) and am in control of the demonstration (as compared to the student who doesn’t know what is going on).
Photos are always “dangerous” in that they can be perceived out of context of what is really happening. Situational awareness is a key aspect in safety along. Understanding the context is also a key piece of information.
There are certain things that PPE makes more dangerous for various reasons. The use of a faceshield, up and down, will impede spoken communication and can impede good visual contact with what you're doing. Gloves will reduce the tactile senses and can impede the proper operation of the articles of demonstration. Gloves also reduce the real-time feedback of the cold, will harbor the cold, and can lead to burns, in some cases, much easier than if no gloves were worn. In addition, the gloves are bulky and taking them on and off does take a constant level of care and concern so that items aren't knocked over, spilled, etc.
Below are links to a couple of videos that comment on and demonstrate some of the safety considerations to do with handling small amounts of LN2. The bottom line is that getting cold burns by casual contact with LN2 is actually quite hard to do.
Boiling LN2 and mixtures
Hand into LN2 and the Leidenfrost effect