Handling Liquid Cryogens Safely Tony Kent School of Physics and Astronomy

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

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

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.

ASPHYXIATION oxygen 21%Normal Level <19%Danger Level <14%Risk of permanent effects and death There is no sensation of breathlessness to warn you that you are being asphyxiated

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

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

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

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.

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.

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.