Cryogenic Liquids

I. Introduction

Cryogenic liquids are used for accessing low temperatures. They are extremely cold, with boiling points below 222K [or - 60°F (-51°C)]. Carbon dioxide and nitrous oxide, which have slightly higher boiling points, are sometimes included in this category.
Cryogens have high expansion ratios, which average ~700:1. When they are heated (i. e., exposed to room temperature), they vaporize (turn into a gas) very rapidly. If the volume cannot be expanded (no outlet), the pressure will increase approximately 700-fold or until it blows something out.

Table 1: Physical Properties of Common Cryogenic Liquids

Cryogen	Boiling point (1 atm) °C (°F)	Critical pressure psiga	Liquid density, g/L	Gas density (27°C), g/L	Liquid-to-gas expansion ratio	Type of gas
Ar	-186(-303)	710	1402	1.63	860	Inert
He	-269(-452)	34	125	0.16	780	Inert
H2	-253(-423)	188	71	0.082	865	Flammable
N2	-196(-321)	492	808	2.25	710	Inert
O2	-183(-297)	736	1410	1.4	875	Oxygenb
CH4	-161 (-256)	673	425	0.72	650	Flammable

^a pound per square inch gauge; ^b Although oxygen does not burn, it will support combustion. Oxygen-enriched atmospheres may lead to violent reactions, such as rapid combustion or explosions, with incompatible materials.

The typical container used to store and handle cryogenic fluids is the dewar. The dewar is multi-walled designed with a vacuum jacket for insulation and pressure relief valves to protect against over-pressurization. Cryogens normally are stored at low pressures.

Liquid nitrogen dewars have one pressure relief valve set at 22 psig. Liquid helium dewars need two over- pressure relief valves. They are initiated at 0.5 psig and 10 psig it room temperature, respectively. The 0.5 psig relief value can be used to perform liquid transfers.

All cryogen dewars should be clearly labeled and operated in accordance with the manufacturer's instructions. The pressure relief devices should be periodically inspected for ice formation. Proper personal protective equipment must be worn whenever handling cryogenic liquids.

II. SAFE HANDLING AND USAGE

If not handled properly, cryogenic liquids can be hazardous to personnel. By reviewing this fact sheet you will become aware of the conditions that increase the risk of accidents and injuries that can occur when working with cryogenic liquids. You will learn what can make your work place safe.

1. Hazards Associated with Cryogenic Liquids 1.1 Skin and Eye Hazards

Cryogens are extremely cold and can cause instant, severe frostbite. A jet of cryogen vapors can freeze the skin or eyes faster than liquid contact, and even faster than metal contact. Also, direct contact with cryogenic liquids, uninsulated cryogenic pipes or equipment can cause freeze burns and tissue damage. The eye’s fluids will freeze in contact with a cryogen, causing permanent eye damage. If the funnel used to transfer cryogens (which you should not be using) freezes, it will discharge the cryogen upward into your face (for funnel alternative, see below).

1.2 Cold Embrittlement

At cryogenic temperatures many materials, such as rubber, plastic and carbon steel can become so brittle that very little stress can break the material. Avoid using these materials at cryogenic temperatures. Do not dispose of cryogenic liquids down the drain! Polyvinyl chloride (PVC) piping in laboratory sinks may not be able to resist cryogenic temperatures. Let cryogenic liquids evaporate in a fume hood.

1.3 Oxygen Enrichment

When transferring liquid nitrogen through uninsulated metal pipes, the air surrounding a cryogen containment system can condense. Nitrogen, which has a lower boiling point than oxygen, will evaporate first. This can leave an oxygen-enriched condensate on the surface that can increase the flammability (combustibility) of materials near the system, creating potentially explosive conditions.Equipment containing cryogenic fluids must be kept clear of combustible materials in order to minimize the fire hazard potential.

1.4 Asphyxiation

The air consists of 78% nitrogen, 21% oxygen, and trace gases making up the remaining 1%. If sufficient liquid nitrogen is vaporized so as to reduce the oxygen percentage to below 19.5%, there is a risk of oxygen deficiency. In confined or poorly ventilated areas (such as a laboratory, transport elevator or storage area), the expanding gas will displace oxygen, presenting an asphyxiation hazard to personnel working in the area.

Simple asphyxiants such as nitrogen do not have good warning properties! One might not feel "light-headed," but may simply pass out without any warning whatsoever, and then die without regaining consciousness. To prevent asphyxiation hazards, make sure that the room is well ventilated when using cryogens indoors. If large quantities of cryogenic vapors are released, the areas suspected of being oxygen deficient shall be evacuated immediately.

1.5 Pressure Buildup and Explosions

Without adequate venting or pressure-relief devices on the containers, enormous pressures can build up on cryogen evaporation. Cryogens boil as they sit in their storage vessels by absorbing heat energy from the (much warmer) surroundings. The gas boiling out of the liquid must either expand or the pressure will increase. Users must make certain that cryogenic liquids are never contained in a closed system. Use a pressure relief vessel or a venting lid to protect against pressure build-up.

Liquid dewar flasks are non-pressurized, vacuum-jacketed vessels, somewhat like a "Thermos bottle". Dewars are designed with either loose-fitting caps or pressure relief valves, that prevents air and moisture from entering, yet allows excess pressure to vent. Do not connect the tank and the dewar tightly to avoid pressure build up in the dewar. Do not use any stopper or other device that would interfere with venting of gas.

1.6 Working with Liquid Oxygen or Hydrogen

An oxygen (chemically reactive) or a hydrogen (highly flammable) gas-enriched atmosphere could greatly increase the risk of fire or explosion. Organic materials (oil, grease, kerosene, cloth, wood, paint, tar and dirt that may contain oil or grease) can react explosively with liquid oxygen.

2. Personal Protection

1. Do not transfer liquefied gases from one tank to another the first time without supervision.
   
   
2. Use care when filling portable dewars and do not overfill them.
   
   
3. Transfer or pour cryogens slowly to minimize boiling and splashing. Use a phase separator or special filling funnel (the top of the funnel should be partly covered to reduce splashing). If the liquid cannot be poured, use a cryogenic liquid withdrawal device for the transfer (be sure to follow all instructions provided with the device).
   
   
4. You are advised to remove metal jewelry/watches on your hands and wrists before working with cryogens. If exposed to cryogenic liquids or boil-off gases, the jewelry can freeze to the skin.
   
   
5. Protect your eyes by wearing safety goggles whenever working with cryogen fluids. Full face shields shall be used in the following situations: i) when a cryogen is poured; ii) for open transfers; iii) if fluid in an open container is likely to bubble.
   
   
6. Wear a cryogen apron when working with cryogen liquids.
   
   
7. Try to cover all exposed skin by wearing long sleeve shirts, long pants (skirts), a long sleeve lab coat, well-fitted leather shoes (no sneakers) and gloves. Gloves should be loose-fitting, lightweight, flexible, and insulated so that they can be quickly removed if cryogenic fluids are spilled on them.
   
   
8. When hand-carrying cryogen-containing dewar, ensure the dewar is your only load (no books, coffee or other items). Watch carefully for people who may run into you, and ensure that the dewar is carried with both hands and as far away from your face as comfortably possible.
9. Ensure dewars are properly labeled with the identity of the cryogen. Do not mix different cryogens dewars.
10. To avoid asphyxiation, an oxygen monitor in a good working order is recommended if you are working with a cryogen in a confined space.
11. Do not permit smoking or open flame in any area where oxygen or hydrogen is stored, handled or used.
    
    
12. Do not lubricate oxygen equipment with oil, grease or unapproved lubricants.
    
    

III. TREATMENT

1. First Aid Procedures in Case of an Accident

You should obtain medical assistance as soon as possible when cryogens contact your skin. Immediately upon exposure, the frozen skin appears waxy and yellow and the burn usually is not painful. Then, it painfully swells and blisters while the skin defrosts. Immediately follow these first aid procedures until you get medical assistance.

1. Take the victim away from the cryogen hazard.
   
   
2. Any clothing that may interfere with the circulation of blood to the frozen tissues should be removed in a slow, careful manner to prevent salvageable skin from being pulled off.
   
   
3. Do not rub or massage the affected parts of the body Rubbing may further damage the tissue.
   
   
4. Immerse the affected area in a warm water bath not to exceed 105 °F or exposure to warm air
   
   
5. The rewarming, or thawing, of affected area(s) should be done gradually. It may take up to 60 minutes to thaw the affected area(s) and bring back the natural color of the skin.
   
   
6. If the eyes are affected, flush them with warm water for at least 15 minutes.
   
   

2. Who to Call in Emergency

Call OEHS at (617) 373-2769 (day time) for advice and assistance as needed. Public safety at (617) 373-3333 must be contacted in the event of a major spill or other emergency.

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