Specific Guidance for Liquid Helium Users

Only experienced and properly instructed people should handle liquid helium.

Using Liquid Helium - Special Precautions

To know what precautions to take is to recognize that at 4 Kelvin, all other gases solidify. Therefore, helium systems and dewars must prevent back flow of air as this constitutes a major safety hazard.

Small volumes of liquid evaporate into large volumes of gas and must be allowed to vent safely, therefore:

• Always connect to helium recovery line and open appropriate vent valves;
• Never leave a dewar open to atmosphere;
• Use liquid helium only in well ventilated areas;

Dewars open to atmosphere for prolonged periods can cause "ice plugs" causing pressure build-up which can lead to over-pressurization and potential catastrophic failure (explosion).

Liquid Helium - Safe Handling and Usage

• Minimum protection recommended is:
  


• Cryogenic gloves
• Face visor or safety glasses


• Always inspect the dewar
  High pressure in the dewar will be indicated by:
  - an inflated rubber bladder when the connection valve is opened
  
  OR
  - a high pressure reading on the pressure gauge on dewar (if fitted)
  Vent any gas slowly through the helium recovery line if found in this condition.


• Never drop objects into the liquid
  
  Beware of cold gas and rapidly boiling helium when lowering equipment at ambient temperature into the dewar. This operation must be carried out slowly to minimize boil off and potential cold vapour burn.


• Never accompany a dewar in a lift or car (Moving confined space)
  A sudden release of vapour in a confined space could be fatal!


• Always use correct syphon and fittings
  
  A vacuum insulated syphon is the only method of transferring helium from a dewar. It consists of a vacuum shielded tube, which dips below the liquid level in the dewar. The dewar is then lightly pressurized which forces the liquid up and out through the syphon tube.
  
  It is important to follow correct order of assembly of a dewar head siphon, to prevent leaks.
  
  Remove pressure from dewar and connecting tubes. Remove brass plug and lock nut used to seal dewar when not in use. Insert syphon 'slowly' and tighten brass flange with the lock nut to seal the "o" ring.
  
  ENSURE ALL PRESSURE HAS BEEN REMOVED BEFORE PERFORMING THIS OPERATION


• Always transfer liquid slowly: To prevent thermal shock and to avoid high pressure build-up (back pressure). This also uses the helium most efficiently.


• Never pressurize the dewar with gas other than helium
  
  Pressurization with a bladder is sufficient for most purposes and this is obtained by squeezing the bladder to create a slight over-pressure.
  
  Do not use external regulated supplies unless competent to do so


• Pre-cooling equipment
  
  Any liquid nitrogen used to pre-cool liquid helium space in cryostats must be fully removed prior to adding liquid helium.


• Purging
  
  Purging of syphons and cryogenic equipment for liquid helium service should only be done with dry liquid helium gas.


• Always thaw equipment with hot air
  
  This is by far the quickest and safest method

• Syphon and cryostat: Ensure good vacuum is maintained in both
  
  
• Syphon and cryostat cool-down: Complete this operation slowly to prevent thermal shock or high back-pressure


• Boil-off due to pressurization gas (external supplies only): Slowly apply the helium pressurization gas as it is hot compared to the liquid


• Depressurization loss: Do not pressurize the dewar more than is necessary to perform the transfer and always try to fill in one operation.

1. Probe the inside of the dewar with helium dipstick to establish if it is clear and able to vent.


2. Report the event to your Supervisor,


3. If the dewar is blocked or partially blocked:
   
   
• clear laboratory of all personnel
  
  
• inform your immediate Supervisor