Chapter 8: Cryogenic Liquids and Compressed Gases
Cryogenic liquids are materials with a boiling point of less than – 100 °F (-73 °C); common examples include liquid nitrogen, helium, and argon, and dry ice/alcohol slurries. Cryogenic liquids undergo large volume expansion upon transition to the gas phase, for example, one volume of liquid nitrogen vaporizes to 694 volumes of nitrogen gas. Consequently, the warming of a cryogenic liquid in a sealed container produces high pressure, which can rupture the container.
Hazards of cryogenic liquids include fire (in the case of flammable or oxidizing materials), pressure buildup, explosion, as well as severe frostbite (on contact with skin) and asphyxiation (due to depletion of available oxygen). Additionally, cryogenic liquids such as liquid nitrogen have boiling points below that of oxygen and are capable of condensing atmospheric oxygen, resulting in a localized, oxygen-enriched environment through formation of liquid oxygen. Liquid oxygen in combination with many organic (oxidizable) materials can result in a violent reaction. Systems that incorporate liquid nitrogen traps must never be opened to the atmosphere until the trap is removed from the coolant.
Safe Handling Practices
- The contents of cryogenic dewars and tanks must be clearly labeled to indicate the contents.
- Large tanks of liquid nitrogen and other cryogenic liquids should be secured to prevent sliding or rolling during an earthquake.
- Always wear appropriate personal protective equipment to prevent skin and eye contact. Heavy, loose-fitting gloves (special cryogenic gloves are available), safety glasses and face shield, and lab apron are recommended.
- Handle objects that are in contact with cryogenic liquids with tongs or proper gloves.
- Keep liquid oxygen away from organic materials and ignition sources.
- Only work with cryogenic liquids in well-ventilated areas to avoid localized oxygen depletion or buildup of flammable or toxic gas.
- Refrigerated rooms generally recycle room air and dangerous atmospheres can result from use of cryogenic liquids or dry ice in these rooms.
- Rooms containing cryogenic liquids in quantities sufficient to reduce atmospheric oxygen levels below 20% through leaks or catastrophic release should have a continuous oxygen monitor with alarm that will notify personnel inside and outside of the room of low oxygen levels.
- Transfers or pouring of cryogenic liquids should be done carefully to avoid splashing.
- Cryogenic liquid/dry ice baths should be open to the atmosphere to avoid pressure build up.
- Transfer of liquid hydrogen in an air atmosphere can condense oxygen in the liquid hydrogen, creating an explosion hazard.
- Containers and systems containing cryogenic liquids should have pressure relief mechanisms.
- Cryogenic liquid cylinders and other containers (such as Dewar flasks) should be filled no more than 80% of capacity to protect against thermal expansion.
- Shield or affix fiber tape around glass Dewars to minimize flying glass and fragments should an implosion occur.
Note: Plastic mesh will not stop small glass fragments
Compressed gases are considered more hazardous to handle than liquids or solids because of the high pressure involved and the ability of the gas to spread rapidly when released. Additionally, many compressed gases are flammable, toxic or corrosive.
Safe Handling Practices
- Each compressed gas cylinder must be clearly labeled to indicate the contents.
- Check connections and hoses regularly for leaks using a specific monitoring instrument or soapy water (or equivalent). When utilizing highly flammable or toxic gas, check the delivery system using an inert gas prior to introducing the hazardous gas.
- When using compressed acetylene: (i) do not exceed a working pressure of 15 psig, and (ii) do not use vessels, piping, or other materials that contain a significant amount of copper (usually considered to be more than 50% copper).
- Replace valve caps when cylinders are not in use.
- Remove damaged or defective cylinders from service (contact the cylinder vendor for assistance).
Transport of Compressed Gases
- Compressed gas cylinders must be transported using hand-trucks or other appropriate means. NEVER TRANSPORT UNSECURED COMPRESSED GAS CYLINDERS!
- Cylinders should be transported upright whenever possible (always transport and store acetylene in an upright (vertical) position).
- Before transporting any cylinder, remove the pressure regulator and ensure that a safety valve cap is in place.
- Elevators can be a confined space. Due to the potential for exposure to unsafe conditions in the event of a significant leak or catastrophic release, it is recommended that personnel do not ride in an elevator with compressed gas cylinders. Have one person send the elevator and another person receive the elevator.
Storage of Compressed Gasses
- Keep compressed gas cylinders secured to prevent falling or rolling. Restraint systems should be sufficiently robust so that cylinders cannot be moved significantly when wiggled by hand or during a small earthquake.
- Liquefied flammable compressed gas cylinders (e.g., acetylene) must always be stored in the upright position so that the pressure relief device is in direct communication with the vapor phase of the cylinder.
- Segregate and clearly mark full and empty cylinders.
- Store compressed gas cylinders away from heat sources, and flammable and highly combustible materials (such as oil and greases).
- Segregate according to hazard class and chemical compatibility.
- Separate flammable and oxidizing gases.
- Store flammable gases away from combustible material, ignition sources (including unprotected electrical connections), and oxygen gas cylinders, liquid oxygen, and other oxidizers (at least 20 feet if possible or separated by a fire-rated wall).
- Toxic gases (for example, NFPA health hazard rating of 3 or 4, or OSHA acute hazard rating of 1 or 2) should be stored and used in a ventilated hood or gas cabinet. An area monitor with alarm should be used if a ventilated enclosure is not used, and when the toxic gas has poor warning properties (such as carbon monoxide). Contact the University CHO for specific guidance.
Maximum Number of Gas Cylinders
As a State facility UNR is covered by the International Fire Code (IFC). The IFC specifies maximum allowable quantities of hazardous compressed gases per designated fire control area. Fire control areas generally consist of multiple laboratory rooms, making it difficult to assign a maximum allowable quantity for each room. NFPA 45, Standard on Fire Protection for Laboratories Using Chemicals, also applies in a regulatory sense to UNR. This standard provides guidance on the maximum number of compressed gas or liquefied gas cylinders per laboratory (500 ft2 or less) as shown below. Contact the University CHO for more specific guidance.
|Flammable or Oxidizing Gases||6||3|
|Liquefied Flammable Gases||3||2|
|Gases with Health Hazard Rating of 3 or 4||3||3|