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CRYOGENIC FLUIDS AND COMPRESSED GAS HAZARDS

Cryogens are fluids at extremely low temperatures e.g.

Liquid oxygen -182^oc

Liquid nitrogen –195^oc.

Cryogens are usually kept in a metallic thermos flask called Dewar, which are never sealed because if they are sealed, they will develop a pressure build up which could lead to an explosion.

Potential Hazards Of Cryogenics

There are three groups of health hazards associated with cyrogenic liquids: extreme cold, asphyxiation, and toxicity.

Extreme Cold Hazard

Cryogenic liquids and their associated cold vapours and gases can produce effects on the skin similar to a thermal burn , they also cause cold burns if they get contact with skin. Brief exposures that would not affect skin on the face or hands can damage delicate tissues such as the eyes. Prolonged exposure of the skin or contact with cold surfaces can cause frostbite. The skin appears waxy yellow. There is no initial pain, but there is intense pain when frozen tissue thaws.

Unprotected skin can stick to metal that is cooled by cryogenic liquids. The skin can then tear when pulled away. Even non-metallic materials are dangerous to touch at low temperatures. Prolonged breathing of extremely cold air may damage the lungs.

Asphyxiation Hazard

When cryogenic liquids form a gas, the gas is very cold and usually heavier than air. This cold, heavy gas does not disperse very well and can accumulate near the floor. Even if the gas is non-toxic, it displaces air. When there is not enough air or oxygen, asphyxiation and death can occur. Oxygen deficiency is a serious hazard in enclosed or confined spaces.

Small amounts of liquid can evaporate into very large volumes of gas. For example, one litre of liquid nitrogen vapourizes to 695 litres of nitrogen gas when warmed to room temperature (21°C).

Biological hazards associated with cryogens

Some Cryogens eg oxygen  , also continuous production and leakage of some cryogenic fluids e.g. oxygen can support growth of aerobic microorganisms or accelerate growth of infectious aerobic microorganisms.  

Toxic Hazards

Each gas can cause specific health effects. For example, liquid carbon monoxide can release large quantities of carbon monoxide gas, which can cause death almost immediately. Refer to the material safety data sheet for information about the toxic hazards of a particular cryogen.

Physical hazards associated with cryogenic fluids

Liquid nitrogen can cause explosion due to blockage of tubes or pipes due to formation of ice where moisture from the air is frozen.

Cryogen’s are transported in glass vessels; these is normally a very delicate endeavor because the glass may fall or be subjected to thermal shock which could lead to spillage

Chemical hazards associated with cryogenics

Some cryogenic fluids e.g. oxygen are fire risk because they support combustion and in case of any leakage they can ignite and support fire.

Safe use of cryogen’s

1. Avoid all body contact with cryogens by wearing protective clothing.
2. Ensure proper ventilation to discourage pressure build up.
3. Use the right containers, which can sustain and withstand the rapid changes in temperature.
4. Never mix different cryogens together.
5. Never use oil or grease on the equipment containing liquid oxygen.
6. Keep all organic compounds away from liquid oxygen.
7. Ban smoking in the immediate area.
8. Always use goggles, face shield and cold resistant gloves when handling cryogenic liquids.
9. Make sure that any container used to hold cryogenic liquids are suitable for use with extreme cold

Hazards of compressed gases

1. Inert nontoxic gases like Helium, Argon and even Nitrogen reduces the concentration of oxygen in the air below the safe level especially in confined areas. These may slowly lead to drowsiness to the laboratory staff leading to doing mistakes and accidents.
2. Toxic gases e.g. Cl, NH₃, and SO₂ can lead to toxic anoxia especially when released in large quantities also even small leakage of these gases can lead to irritation.
3. Compressed gases can asphyxiation due to a slow leak that goes undetected.
4. An otherwise harmless gas can displace enough oxygen in a room to kill.
5. Flammable gases e.g. methane, acetone etc. may cause fires and explosions especially when they are in contact with   some metallic elements.
6. Decomposing gases e.g.  Ethane releases carbon and hydrogen   accompanied by release of large quantities of heat Cylinders carrying compressed gases may explode due to exposure to heat and if dropped d carelessly

Safe use of compressed gases

1. Gas cylinders are color coded for easy identification,

The color codes are as follows;

Hydrogen cylinder              – Red

Ethylene acetate cylinder – Maroon

Normal air cylinder              – Gray

Oxygen   cylinder                 – Black

2. The actual names of the gases should also be given on the cylinder together with other information  should be recorded on the neck of the cylinder e.g. date of filling , pressure  and capacity of the cylinder  etc.
3. When cylinder are received in the laboratory, they should be checked immediately for any damage and for correct identification. It is important to ensure that no cylinders and cylinder fittings are damaged.
4. Cylinders should be fixed in an upright position.
5. Never temper with cylinder heads, valves or pressure control gauges or any cylinder safety device or attempt to use a regulator that is not designed for use with the gas you are using.
6. Never use an adapter to make a regulator fit your cylinder. Use the regulator that fits.
7. Always use a cylinder cart to move compressed gas cylinders from one location to another, and always secure the cylinder to the cart with a chain or strap and buckle.
8. Do not drop cylinders. Do not allow cylinders to strike each other. Keep all compressed gas cylinders capped until ready to use, and then place the correct regulator on the valve, recap when work is finished.
9. Always keep the cap on the cylinder while moving it and keep it in place until the cylinder is secured. Always cap empty cylinders.
10. Never use oil, grease, or other combustible substances with oxygen cylinders, valves or regulators
11. Never deface or remove the product identification label on a cylinder.
12. Do not use unlabeled cylinders
13. When you empty a cylinder, leave some positive pressure in the cylinder, close the valve, replace the cap, and mark the cylinder empty.
14. Cylinder Status Tags are very convenient for marking cylinders as full, in use or empty. Use a piece of label tape, write “empty” on it and place it on the cylinder.
15. When using compressed gas cylinders, you should know the properties and safety precautions of the gas before using it. Some gases can be particularly harmful.