Proper handling of Cryogenic Liquids

Key Takeaways

• Cryogenic Liquids can cause instant severe frostbite: hand and eye protection are Essential
• Cryogenic liquids can cause asphyxiation 
• Cover all skin when working with cryogenic liquids

Introduction

Cryogenic liquids are used for accessing low temperatures. They. Carbon dioxide and nitrous oxide, which have slightly higher boiling points, are sometimes included in this category. Due to the health hazards to researchers, this factsheet aims to raise awareness about safe work practices.  

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Cryogens have high expansion ratios, which average ~700:1. They are normally stored at low pressures.  When they are heated (e.g.., exposed to room temperature), they vaporize (turn into a gas) very rapidly. If the volume cannot be expanded (e.g. if there is no outlet), the pressure will increase approximately 700-fold or until there is an opening. 

The typical container used to store and handle cryogenic fluids is called a dewar.  A dewar hass a multi-walled design with a vacuum jacket for insulation and pressure relief valves to protect against over-pressurization.  

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 in room temperature. 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 visually inspected prior to use for ice formation. Pressure relief valves should be inspected quarterly for proper operation. Proper personal protective equipment must be worn whenever handling cryogenic liquids. 

Safe Handling and Usage

As mentioned, cryogenic liquids can be hazardous to personnel when handled improperly.  

Hazards 

Skin and Eye Hazards 

• Direct contact with cryogenic liquids, uninsulated cryogenic pipes, or equipment can cause freeze burns and tissue damage.  
• Cryogens 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. 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.

Cold Embrittlement

At cryogenic temperatures, many materials – – such as rubber, plastic and carbon steel  – – can become so brittle that even small amounts of stress can break the material. Avoid using these materials at cryogenic temperatures. 

Disposal

Do not dispose of cryogenic liquids down the drain. Polyvinyl chloride (PVC) piping in laboratory sinks cannot resist cryogenic temperatures. Alternatively, let cryogenic liquids evaporate in a fume hood. 

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 could, therefore, 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. 

Asphyxiation 

The air consists of 78% nitrogen, 21% oxygen, and trace gases making up the remaining 1%. If sufficient liquid nitrogen is vaporized, resulting in an oxygen percentage  below 19.5%, there would be 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 asphyxiates such as nitrogen do not have good warning properties. One might not feel “light-headed” or “shortness of breath” and faint without any warning, resulting in a fatality without regaining consciousness. To prevent asphyxiation hazards, make sure 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. 

Pressure Buildup and Explosions 

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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 need to make sure that cryogenic liquids are never contained in a closed system. To protect against pressure build-up, use a pressure relief vessel or a venting lid. 

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.   

Note: 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. 

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. 

Personal Protection

• Do not transfer liquefied gases from one tank to another for the first time without supervision.
• Use care when filling portable dewars;  .
• Do not permit smoking or open flame in any area where oxygen or hydrogen is stored, handled, or used.
• Do not lubricate oxygen equipment with oil, grease or unapproved lubricants.

• 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).
• Remove metal jewelry/watches from your hands and wrists before working with cryogens. If the jewelry is exposed to cryogenic liquids or boil-off gases, it can freeze to the skin.
• Protect your eyes by wearing safety goggles whenever working with cryogen fluids. Full face shields should 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.
• Wear a cryogen apron when working with cryogen liquids.
• 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.
• When hand-carrying cryogen-containing dewar, ensure the dewar is your only load (no books, coffee or other items). Ensure that the dewar is carried with both hands and as far away from your face as comfortably as possible.  Check your route to prevent bumping into anyone.
• Ensure dewars are properly labeled with the identity of the cryogen. Do not mix different cryogens dewars.
• To avoid asphyxiation, an oxygen monitor in a good working order is recommended when working with cryogens in a confined space. 

Treatment

First Aid Procedures in Case of an Exposure Incident 

You should obtain medical assistance as soon as possible if 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. Remove 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. Flush eyes with warm water for at least 15 minutes if exposed.

Emergency and Questions

Contact NU Public safety at (617) 373- or via SafeZone in the event of a major spill or other emergency.  Contact our office for general questions or non-emergencies at (617) 373-. 

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In cryogen systems, dewars are the containers that hold liquid cryogens at extremely cold temperatures. At a basic level, they work like garden variety thermos bottles; the liquid nitrogen or other cryogen is stored in an inner container with a space between it and the outside wall. The central container, thus insulated in a vacuum, is able to maintain the liquid’s temperature for long periods of time.

The original design, invented by James Dewar in , when research on liquid cryogens was still in its infancy, was the standard for many decades. His double-walled container had a reflective silver exterior, which helped further retain low-temperatures.

The nitrogen dewars used in modern cryogen labs are based off of Dewar’s original design (as are the thermos bottles we use to keep ice water cold or coffee hot).

Cryogen Dewars Are Used in Many Fields

Chemistry and physics research labs around the world need cryogen dewars to conduct work at extremely low temperatures. From astronomy to healthcare to the food industry, more and more disciplines are relying on cryogens. The semiconductor industry relies heavily on cryogenic cooling to remove heat from manufacturing processes and Quantum computing requires cooling to millikelvin range.

Safe Usage of Liquid Nitrogen Dewars

When filling a dewar with liquid nitrogen or other cryogenic liquid, always wear protective gear. A lab coat or long apron is important. Extreme caution is necessary; coming into contact with liquid cryogens can cause frostbite.

Ice can build up on the connections exposed to atmosphere, so cryogenic gloves are a must. A face shield and safety glasses will protect your eyes from splashing; sturdy, closed-toed shoes are recommended for the same reason. Cuffed pants are a bad idea as the cuffs can catch the cryogen and hold it close to your skin.

Storing liquid nitrogen tanks in a well-ventilated area is crucial. In small spaces or spaces without proper ventilation, nitrogen can displace oxygen, causing an asphyxiation risk. Oxygen depletion alarms should be considered if in doubt.

Vents keep dangerous pressure from building up. Nitrogen expands quickly when shifting from liquid to gas. Rapid temperature changes or vent plugs can cause an explosion if safety release devices on tanks are altered, removed, or improperly used.  

Customizing Dewars to Meet Your Specifications

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