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Cryogenic Liquids and Gases
Cryogenic liquids and their boil-off vapors can rapidly freeze human tissue and may cause materials to crack or fracture under stress.
All cryogenic liquids produce large volumes of gas when they vaporize (at ratios of 600:1 to 1440:1, gas: liquid) and may create oxygen-deficient conditions. Examples of common cryogenic liquids used in the labs are nitrogen oxygen, and helium. Only properly trained individuals should handle cryogenic liquids.
Recommended handling practices for cryogenics
- Use appropriate personal protective equipment (PPE) goggles a face shield and closed toe shoes during any transfer of cryogenic liquid.
- In the event of skin contact with a cryogenic liquid, do not rub skin: place the affected part of the body in a warm water bath. (Do not exceed 40°C [105°F])
- Use only equipment, valves and containers designed for the intended product , pressure, and temperature.
- Inspect containers for loss of insulating vacuum. If the outside jacket on a container is cold or has frost spots, some vacuum has been lost. Empty the contents into another cryogenic container and remove the damaged unit from service. The manufacturer should make any repairs.
- Transfer operations involving open cryogenic containers such as dewars must be conducted slowly to minimize boiling and splashing of the cryogenic fluid.
- Ice or other foreign matter should not be allowed to accumulate beneath the vaporizer or the tank. Ice buildup could result in the discharge of excessively cold gas or structural damage to the cryogenic container or surroundings.
- All cryogenic systems including piping must be equipped with pressure relief devices to prevent excessive pressure build-up. Pressure relief's must be directed to a safe location.
- Do not tamper with pressure relief valves or the settings for the valves.
- Hot air, steam or hot water should be used to thaw frozen equipment. DO NOT USE water to thaw liquid helium equipment.
Liquid Nitrogen Dewar Gas Release
If the container is leaking Nitrogen gas via the pressure release valve do not be alarmed, this is normal operation. All of the liquid will eventually boil off and the leaking will stop. If it is leaking from any other connection close the gas valve using cryogenic gloves and notify an emergency contact listed below.
The cylinders are designed to provide high pressure Nitrogen gas. The pressures will range from 250 to 350 psig.
What has most likely happened is that:
- The vacuum jacket between the inner and outer walls of the cylinder has been reduced. The vacuum acts as an insulator, higher vacuum levels result in better insulation and less gas loss.
- The pressure release valve has frozen open.
- The pressure building valve may be open.
The proper corrective actions are to:
- Not open the liquid valve and discharge liquid, cryogenic burns and asphyxiation can result.
- Verify that the pressure building valve is closed.
- If the dewar is connected to the piping system verify that the gas valve and regulator valves are open. This will relieve the excess pressure if there is an adequate gas demand from the lab.
- Close the door to the gas room to vent the excess gas from the building. Two exhaust fans vent this room continuously.