16.10.2 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.