16.10 Cryogenic Material Safety16.10 Cryogenic Material Safety
According to the Compressed Gas Association, a cryogenic fluid is a material that has a boiling point of less than -130°F (-90°C). Examples of cryogenic materials include the liquids nitrogen, argon, and helium, and solid carbon dioxide (dry ice). Hazards associated with cryogenic fluids include:
Extreme Low Temperature - These liquids and their boiled off vapors are extremely cold and can cause severe cold contact burns. They also make many materials brittle, such as the epoxy or phenolic resin that laboratory benchtops and sinks are made of.
Asphyxiation - When liquid and solid cryogenic materials vaporize they greatly expand, some by a factor of ~700:1. This displaces oxygen and creates an oxygen depletion hazard. This is especially dangerous when confined in poorly ventilated space like an elevator.
Oxygen makes up 20.9% of the air we breathe. The environment is considered to be oxygen deficient below 19.5%. Two breaths of air with no oxygen can be enough to render a person unconscious.
Typical background levels of carbon dioxide are about 400 ppm. Vaporization of even small quantities of Dry Ice in an unventilated area can exceed the permissible exposure limit of 5000 ppm.
Exposure to oxygen-deficient atmospheres produce dizziness, nausea, vomiting, loss of consciousness, and death. Such symptoms may occur in seconds without warning.
Death may result from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. Working with cryogenic substances in confined spaces, such as walk-in coolers, can be especially hazardous. Where cryogenic materials are used, a hazard assessment is required to determine the potential for an oxygen-deficient condition. Controls such as ventilation and/or gas detection systems may be required to safeguard employees. Asphyxiation and chemical toxicity are hazards encountered when entering an area that has been used to store cryogenic liquids if proper ventilation/purging techniques are not employed.
Toxicity - Many of the commonly used cryogenic gases are considered to be of low toxicity, but still pose a hazard from asphyxiation. Check the properties of the gases you are using because some gases are toxic, for example, Carbon monoxide, Fluorine, and Nitrous oxide.
Flammability and Explosion - Fire or explosion may result from the evaporation and vapor buildup of flammable gases such as hydrogen, carbon monoxide, or methane. Liquid oxygen, while not itself a flammable gas, can combine with combustible materials and greatly accelerate combustion.
Oxygen clings to clothing and cloth items, and presents an acute fire hazard.
High Pressure - In cryogenic systems, high pressures are obtained by gas compression during refrigeration. Warm temperatures surrounding containers cause a constant expansion of the liquid as it turns back into gas. Sudden release through a rupture or break in a line may be violent. Over- pressurization of cryogenic equipment can occur due to improper venting and expansion during the phase change from liquid to gas. All cryogenic fluids produce large volumes of gas when they vaporize.
Materials and Construction Hazards - The selection of materials calls for consideration of the effects of low temperatures on the properties of those materials. Some materials become brittle at super low temperatures. Brittle materials fracture easily and can result in almost instantaneous material failure.
Low temperature equipment can also fail due to thermal stresses caused by differential thermal contraction of the materials. Over-pressurization of cryogenic equipment can occur due to the phase change from liquid to gas if not vented properly.
16.10.1 Cryogenic Safety Guidelines16.10.1 Cryogenic Safety Guidelines
Personnel who are responsible for any cryogenic equipment must conduct a safety review prior to the commencement of operation of the equipment. Supplementary safety reviews must follow any system modification to ensure that no potentially hazardous condition is overlooked or created, and that updated operational and safety procedures remain adequate.
Clothing and Personal Protective Equipment
Wear the appropriate clothing and PPE when working and transferring cryogenic materials.
- Safety glasses are a minimum to be worn during the transfer and normal handling of cryogenic fluids. Face shields must be worn over safety glasses if there is the potential for splashing in the face, such as filling dewars.
- Loose fitting, heavy leather or other insulating protective gloves must be worn when handling cryogenic fluids.
- Shirt sleeves should be rolled down and buttoned over glove cuffs, or equivalent protection, such as a lab coat, should be worn in order to prevent liquid from spraying or spilling inside the gloves. Do not wear short sleeves when working with cryogenic liquids.
- Long pants without cuffs and that don’t expose skin must be worn.
- Shoes that cover the feet are required. These must not allow cryogenic materials to come into contact with skin. Shoes must not allow a cryogenic liquid to become trapped in the material or soak in liquid in the event of a spill. Leather or other non-woven material is recommended.
Cryogenic fluids must be handled and stored only in containers and piping systems specifically designed to withstand the extremely cold temperatures, and in accordance with applicable standards, procedures, and proven safe practices, and be based on the specific cryogen.
Transfer operations involving open cryogenic containers such as dewars must be conducted slowly to minimize boiling and splashing of the cryogenic fluid. Transfer of cryogenic fluids from open containers must occur below chest level of the person pouring the liquid and on a steady surface.
Only conduct such operations in well-ventilated areas to prevent possible gas or vapor accumulation that may produce an oxygen-deficient atmosphere and lead to asphyxiation. If this is not possible, an oxygen monitor must be installed.
Small spaces, environmental chambers, and cold rooms often do not have sufficient exhaust ventilation to support storage and use of cryogenic materials. Contact EHS with any questions and to conduct a risk assessment.
All cryogenic systems, including piping, must be equipped with pressure relief devices to prevent excessive pressure build-up. If a container vents continuously, shows signs of blockage by not venting at all, or there is frost buildup on the outside, which indicates that there is loss of vacuum, do the following:
Do not attempt to remove a blockage;
Move the vessel to a remote location or notify others of the problem;
Contact the supplier for assistance with the vessel.
- The caps of liquid nitrogen dewars are designed to fit snugly to contain the liquid nitrogen, but also allow the periodic venting that will occur to prevent over pressurization of the vessel. Do not ever attempt to seal the caps of liquid nitrogen dewars. Doing so can present a significant hazard of over pressurization that could rupture the container and cause splashes of liquid nitrogen. Depending on the amount of liquid nitrogen that may be spilled, this could cause an oxygen deficient atmosphere within a laboratory due to the sudden release and vaporization of the cryogenic liquid.
- In the event of a spill, oxygen deficiency or a flammable atmosphere will exist beyond any visible fog cloud.
Emergency treatment if skin or eyes come into contact with cryogenic liquid or vapor is as follows:
- If the cryogenic fluid comes in contact with the skin or eyes, flush the affected area with generous quantities of cold water. Never use dry heat. Splashes on bare skin cause a stinging sensation, but in general are not harmful.
- If clothing becomes soaked with liquid, it should be removed as quickly as possible and the affected area should be flooded with water as above.
- Where clothing has frozen to the underlying skin, cold water should be poured on the area, but no attempt should be made to remove the clothing until it is completely free.
- If inhalation of the cold vapors has occurred, move the person to warm, fresh air. The person may be suffering from frostbite tissue in their throat and lungs, and also asphyxia.
- Do NOT rub frostbitten skin as tissue damage may occur. Place in a warm bath that is not above 105°F (40°C).
- For emergencies: Call 607-255-1111 from a cell phone or 911 from a campus phone.
- Complete an Injury/Illness Report
16.10.2 Cryogenic Chemical Specific Information16.10.2 Cryogenic Chemical Specific Information
Liquid helium must be transferred via helium pressurization in properly designed transfer lines. A major safety hazard may occur if liquid helium comes in contact with air. Air solidifies in contact with liquid helium, and precautions must be taken when transferring liquid helium from one vessel to another or when venting. Over-pressurization and rupture of the container may result. All liquid helium containers must be equipped with a pressure-relief device. The latent heat of vaporization of liquid helium is extremely low (20.5 J/gm); therefore, small heat leaks can cause rapid pressure rises.
Since the boiling point of liquid nitrogen is below that of liquid oxygen, it is possible for oxygen to condense on any surface cooled by liquid nitrogen. If the system is subsequently closed and the liquid nitrogen removed, the evaporation of the condensed oxygen may over-pressurize the equipment or cause a chemical explosion if exposed to combustible materials, e.g., the oil in a rotary vacuum pump. In addition, if the mixture is exposed to radiation ozone is formed, which freezes into ice and is very unstable. An explosion can result if this ice is disturbed. For this reason, air should not be admitted to enclosed equipment that is below the boiling point of oxygen unless specifically required by a written procedure.
Any transfer operations involving open containers such as wide-mouth dewars must be conducted slowly to minimize boiling and splashing of liquid nitrogen. The transfer of liquid nitrogen from open containers must occur below chest level of the person pouring the liquid.
If liquid nitrogen or helium traps are used to remove condensable gas impurities from a vacuum system that may be closed off by valves, the condensed gases will be released when the trap warms up.
Adequate means for relieving resultant build-up of pressure must be provided.
Any proposal for the use of liquid hydrogen must obtain prior approval and undergo an EHS risk assessment and engineering code review.
- Because of its wide flammability range and ease of ignition, special safety measures must be invoked when using liquid hydrogen.
- Liquid hydrogen must be transferred by helium pressurization in properly designed transfer lines to avoid contact with air. Properly constructed and certified vacuum insulated transfer lines should be used.
- Rooms where flammable gas is used must be positively pressurized.
- Only trained personnel familiar with liquid hydrogen properties, equipment, and operating procedures are permitted to perform transfer operations. Transfer lines in liquid hydrogen service must be purged with helium or gaseous hydrogen, with proper precautions, before using.
The safety philosophy in the use of liquid hydrogen can be summarized as the following:
- Isolation of the experiment. Provision of adequate ventilation.
- Exclusion of ignition sources plus system grounding/bonding to prevent static charge build-up. Containment in helium purged vessels.
- Efficient monitoring for hydrogen leakage.
- Limiting the amount of hydrogen cryo-pumped in the vacuum system.
Any proposal to use liquid oxygen must obtain prior approval and undergo an EHS risk assessment and engineering code review.
- Liquid oxygen does not burn; however, it greatly accelerates fire. Adequate ventilation to avoid oxygen enrichment of the air is necessary.
- A room with combustible materials, oils and grease, electrical equipment, an ignition source, or flammable liquids and vapors should be avoided.
- OSHA defines an oxygen enriched atmosphere as being more than 22% oxygen. Oxygen enriched atmospheres present a significant fire and explosion risk.
- In the event of saturation of clothing with liquid oxygen, stay away from ignition sources for at least 30 minutes or change clothing.
16.10.3 Handling Cryogenic Materials on Elevators16.10.3 Handling Cryogenic Materials on Elevators
There is typically little or no fresh air supplied inside of elevators. Cryogenic materials displace air. To avoid a situation in which there is insufficient breathable air, the following are expected in Cornell buildings:
- Quantities up to 5 liters of a cryogenic material are acceptable to travel in an elevator accompanied by people.
- Quantities greater than 5 liters of cryogenic material must be transported on an elevator unaccompanied as it travels between floors. Do NOT transport a pressurized container in an elevator with any person/s in the elevator car.
- Transport in a freight elevator, if possible.
- Post a sign on the vessel in the elevator reading “DO NOT ENTER – CRYOGEN ASPHYXIATION HAZARD” to warn potential passengers.
- Have another person available on the receiving floor to take the container off the elevator at its destination.
- Beware of tipping the vessel while moving on or off the elevator. This is especially important of uncapped dewars of liquid nitrogen.
- Wear insulated cryogen safety gloves, safety goggles, long shirt, and long pants without cuffs. Wear footwear that will not soak in the liquid in case of a spill (e.g. leather instead of woven mesh).
- In case of a spill of any quantity on an elevator:
- Press the button to stop at the nearest floor;
- Block the elevator door open so that it doesn't move to the next floor;
- Call 607-255-1111 from a cell phone or 911 from a campus phone.