Handling cryogenic liquids

Please refer to the safety information contained in your laboratory safety folder.

Anyone needing to use cryogenic liquids must undertake appropriate hazard training before commencing work. Contact Chris Vickers (room C9a, phone 24281) to arrange a training session. In addition to this, supervisors should also provide training to new workers on the handling of cryogenic liquids for the specific procedures used in their laboratory.

The most common hazards relating to cryogenic liquids are:

• Cold burns
  
  
• Asphyxiation
  
  
• Ice blockages
  

Cold burns

The temperature difference between the skin and hot tea is only about 40°C, yet a serious scald can be caused by a splash of hot liquid. The damage that would be caused by a cryogenic liquid, where the temperature difference is 160°C or greater, is severe.

Asphyxiation

Most cryogenic liquids expand to gas at about 800 times their liquid volume when allowed to evaporate, and the gases (with the exception of oxygen) will not support life. The contents of a 100 litre dewar evaporates to about 80,000 litres, sufficient to fill a small storeroom (eg 5m x 5m x 3m). Even a small spill of 10 litres or less could create a serious hazard. It is very important to control the amount of potential spillage in small rooms, and to have adequate ventilation and/or alarm systems.

Ice blockages

If the top cap or gas vent valve of a cryogenic vessel is left open, air can be sucked into the neck of the dewar. Moisture contained in the air freezes inside the neck at a point where the temperature is 0oC. If left in this condition the soft snow ice can build up and solidify to create a complete neck blockage. As the liquid in the dewar continues to warm pressure builds up, and if the over-pressure is not removed by unblocking the neck or by another pressure relief system the dewar may explode. The hazard with liquid helium dewars is particularly severe, as solid air as well as ice blockages may form, and pressure build-up in a blocked vessel can be much more rapid due to helium's low latent heat of evaporation.

Golden rules

• Always use and store cryogenic liquids in a well-ventilated area. Where this is not possible, atmospheric monitoring must be installed, maintained, tested and calibrated regularly according to the manufacturer's instructions.
  
  
• Always wear eye protection and leather gloves.
  
  
• Always use the rigid syphon and gas phase separator.
  
  
• Always use vessels designed for cryogenic materials.
  
  
• Never use plastic, glass or rubber with cryogenic materials.
  
  
• Ensure that receiving vessels are secure.
  
  
• Open valves slowly and be aware of gas noise from gas phase separator or pressure relief device.
  
  
• Avoid splashing and use minimum quantity required.
  
  
• Never touch un-insulated pipes or vessels.
  
  
• Never put water into polystyrene cryogenic buckets.
  
  
• When filling other vessels, use appropriate metal funnels and always fill slowly.
  
  
• Never fill detectors, traps, etc with a liquid nitrogen bucket (or similar) at or above head height.
  
  
• If moving large cryogenic liquid tanks, always use the correct trolley and take care during transportation. Make sure you use the correct technique as specified in the risk assessment.
  
  
• If you are in any doubt about what to do, or are untrained in cryogenic liquid procedures, ALWAYS seek the advice of a competent person.