Handling Liquid Cryogens Safely

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Handling Liquid Cryogens Safely

• Tony KentSchool of Physics and Astronomy

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Hazard

• Describes the potential of an agent to cause
  harm to persons or property. For cryogenic
  liquids the main hazards are
• Extreme cold
• Asphyxiation
• High pressures
• Fire and explosion

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Risk

• A measure of the likelihood that harm from a
  particular hazard is realised the severity of the
  consequences
• Risk is situation specific, i.e. depends on
  application, quantity and frequency of use.
• Local risk assessment to identify risks and put
  in place measures to minimise them

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Asphyxiation

• Nitrogen (helium) gas is heavier (lighter) than
  air and can displace oxygen leading to
  asphyxiation.
• Never accompany liquid cryogens in a lift. Use
  liquid cryogens only in well ventilated places.
• If quantities are large or you are not sure about
  the ventilation, fit oxygen depletion alarms set
  to sound at 19.5 Oxygen.
• Do not enter an enclosed area where you suspect
  there has been a large spillage of liquid
  cryogen. Seek expert assistance.

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ASPHYXIATION

• oxygen
• 21 Normal Level
• lt19 Danger Level
• lt14 Risk of permanent effects and death
• There is no sensation of breathlessness to warn
  you that you are being asphyxiated

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4 Stages of Breathlessness

• 1st Stage
• Oxygen below 20 Pulse rate increases, breathing
  rate increases, clear thinking starts to become
  difficult and co-ordination reduces
• 2nd Stage
• Oxygen below 14 Judgement becomes poor, become
  bad tempered, loss of sense of pain and has rapid
  fatigue.

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• 3rd Stage
• Oxygen below 10 Loses ability to perform
  vigorous movements, suffers nausea and vomiting,
  loss of caring, unable to stand or walk,
  permanent brain damage possible even if
  resuscitated
• 4th Stage
• Oxygen below 6 Breathing comes in gasps with
  long periods between, convulsive movements,
  breathing stops and then after a few moments, the
  heart.
• BREATHING PURE NITROGEN OR HELIUM WILL PRODUCE
  IMMEDIATE LOSS OF CONSCIOUSNESS AND ALMOST
  IMMEDIATE DEATH

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Extreme cold

• Liquid nitrogen 77 K (-196 C)
• Liquid helium 4.2 K (-269 C)
• Avoid direct contact with cryogen and objects at
  low temperature, e g pipes carrying the liquid
• Wear eye protection
• Wear loose fitting gloves
• Wear sensible shoes/boots (not sandals) with
  trousers covering top to ensure spilt cryogens do
  not run inside.
• Prevent spillage on electric cables and plastic
  floor

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High Pressures

• 1 litre of liquid nitrogen (helium) expands to
  0.7 (0.75) cubic metres of gas at room
  temperature.
• Can cause build up of high pressure if gas is
  trapped, i.e. your piece of equipment becomes a
  bomb! Bungs and stoppers become projectiles.
• Ensure all vents are clear and checked regularly.
• Tie stoppers to the top of the container with
  short length of string to act as a retainer.
• Use non-return and pressure relief valves.

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Fire and Explosion

• Liquid oxygen has a higher boiling point than
  liquid nitrogen. The liquid seen dripping from
  cold pipes carrying liquid nitrogen is mostly
  liquid oxygen.
• Liquid oxygen is a fire and explosion hazard
• Insulate pipes carrying liquid nitrogen.
• Do not allow condensate to drip onto clothing or
  come into contact with grease/oil.
• Keep away from naked flames or incandescent
  material.

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Emergency Procedures

• Cold burns - Treat as a normal burn' hold under
  tepid running water (not hot water!) for at least
  15 minutes. If serious seek medical assistance.
• Spillage - Evacuate area in which spill has taken
  place leaving doors open for ventilation.
• Overpressure in container - Evacuate area and
  call emergency services.
• Asphyxiation - Seek immediate medical assistance.