Cryogen Safety



Liquid nitrogen and liquid helium are the two cryogens used at Connecticut College, primarily in the chemistry department for the Nuclear Magnetic Resonance (NMR) machine. Small quantities of liquid nitrogen are also used for medical purposes in the Warnshuis Student Health Center.

Definitions

Cryogen: Cryogenic liquids are liquefied gases that are kept in their liquid state at very low temperatures. Cryogenic liquids have boiling points below -150°C (- 238°F). All cryogenic liquids are gases at normal temperatures and pressures. These gases must be cooled below room temperature before an increase in pressure can liquefy them. Different cryogens become liquids under different conditions of temperature and pressure, but all have two properties in common: they are extremely cold, and small amounts of liquid can expand into very large volumes of gas. The most commonly used cryogens at Connecticut College are liquid nitrogen and liquid helium

Dewar: An insulated container used to store and transport liquified gases. It is insulated by use of a vacuum between the container's inner and outer walls and is equipped with a pressure release device.

Dry Ice: Frozen carbon dioxide. Dry ice sublimates from a solid to a gas at room temperature.

Pressure-relief devices: Devices on cryogenic systems, intended to relieve pressure build up. These devices may be (1) pressure release valves, (2) bursting discs, that are designed to fail at a certain pressure or (3) loose fitting dewar lids.

Hazards Associated with Cryogens and Dry Ice

  • Asphyxiation
  • Freeze Injuries
  • Fire and Explosion Hazards
  • Vacuum System and Over-pressurization Hazards

Low Oxygen Alarm

There is a low oxygen monitor installed in the basement hallway of Hale Laboratory, with sensors both in the hallway and NMR Room. If the alarm should go off, immediately evacuate the building. The oxygen level must be checked before re-entering.

Personal Protective Equipment (PPE)

Store and use cryogens and dry ice in an open, well-ventilated location, especially when filling dewars. If you are working in a small space, open a door to increase ventilation.

  • Always wear eye and/or face protection when working with cryogens. Wear other personal protective equipment as required by the operation. Remove clothing or jewelry which could trap cryogenic liquids against the skin.
  • Be sure that all cryogen containers are clearly labeld with a cryogen warning, and the cryogen's name.Use only insulated cryogen gloves; do not use rags to touch uninsulated equipment surfaces.
  • Examine dewars and pressure relief valves for signs of defects. If any defect is suspected, notify the Director of Environmental Health & Safety, and the cryogen supplier immediately.
  • Keep equipment and systems clean and free of oil, grease, or other materials which may create hazardous conditions upon contact with the cryogenic fluids, or with condensed oxygen.
  • Do not overfill the dewar. This may cause liquid nitrogen to leak into the cryotubes stored in the dewar. Upon removal from the dewar, cryotubes may explode when the liquid nitrogen in the cryotube warms and expands. Store sample vials in the gaseous phase above the liquid nitrogen.
  • When transferring a cryogen into a secondary container, do not fill the secondary container to more than 80% of capacity. If the possibility exists that the secondary container might be might be exposed to temperatatures above 30 degrees C (86 degrees F), do not fill it to more than 60% of its capacity.
  • Wherever possible, when cooling objects, lower them into the liquid. Always hang the component on a suitable wire or string to submerge it - never lower a part in by hand. Use a lid on the tank to reduce contact between the cryogenic liquid surface and the atmosphere. If a fog develops over the liquid surface, do not blow on it or try to "sweep" it off with the hand, because more fog will be created. The fog will eventually boil off.