compressedgases
compressedgases

Compressed Gases and Cryogenic Liquids

» Compressed Gases;

» Transporting Compressed Gas Cylinders;

» Flammable Compressed Gases;

» Storage of Flammable Compressed Gases;

» Using Flammable Compressed Gases;

» Cryogenic Liquids;

» Transporting Cryogenic Liquids;

 

Handling Compressed Gas Cylinders

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Figure 1.  A selection of compressed gas cylinders

Q. What is a compressed gas?

A.  Compressed gases are materials (or mixtures of materials) in gas state stored under pressure in steel cylinders.  Thousands of products are available in this form.  There are three major groups of compressed gases stored in cylinders:  Liquefied, Non-liquified and Dissolved gases.

Liquified Gases:  Liquiefied gases are gases which can become liquids at normal temperature when they are inside cylinders under pressure.   They exist inside the cylinder in a liquid-vapour balance.  As gas is removed from the cylinder , liquid evaporates to replace the gas keeping the pressure in the cylinder constant.  Eg  Propane, Nitrous Oxide and carbon Dioxide are examples.

Non-Liquified Gases:  also known as compressed, pressurized or permanent gases.   These gases do not become liquid when they are compressed at normal temperatures even at high pressures.  Eg.. Oxygen, Nitorgen, helium and argon.

Dissolved Gases:   Acetylene is the only commonly dissolved gas.     Acetylene is chemically ver unstable.   Even at amospsheric pressure, acetylene gas can explode.  Acetylene cylinders are fully packed with an inert, porous filler.   The filler is saturated with acetone or other suitable solvent and the acetylene, when added to the tank, dissolves into the acetone.

Q.  Why are compressed gas cylinders hazardous.

A.   Aside from the chemical and toxicological properties of the materials in the cylinders, the compressed gases are often at very high pressures.  Gas can be released by opening the valve, or from a broken or leaking valve or safety device.   Even at relatively low pressure, gas can flow rapidly from an open or leaking cylinder.   Severe damage to a cylinder may result in the cylinder becoming an uncontrolled “torpedo or pinwheel”.

Safety Procedures

  1. Read the material safety data sheets (MSDS) and labels for all of the materials you work with. MSDS are available online from the Praxair web site (http://www.praxair.ca/en/resource-library/sds)

  2. Know all of the hazards (fire/explosion, health, chemical reactivity, corrosiveness, and pressure) of the materials you work with.

  3. Verify that the chemical in the tank is the correct one for its intended use.  Check the label, not the cylinder colour, to identify the gas.

  4. Store compressed gas cylinders in cool, dry, well-ventilated areas, away from incompatible materials and ignition sources. Ensure that the storage temperature does not exceed 52°C (125°F).

  5. Store, handle and use compressed gas cylinders securely fastened in place in the upright position. Never roll, drag, or drop cylinders or permit them to strike each other.

  6. Move cylinders in handcarts or other devices designed for moving cylinders. (see below)

  7. Leave the cylinder valve protection cap in place until the cylinder is secured and ready for use.

  8. Discharge compressed gases safely using devices, such as pressure regulators, approved for the particular gas.

  9. Use the correct gas regulator for the type of gas you are using. Some materials used in regulators may not be compatible (i.e., will react badly) with some gases.  Ensure that the regulator is the correct one for the gas and the application.  

  10. Our compressed gas supplier offers a service to check used gas regulators and ensure they are safe to use.  Contact them for details and pricing. 

  11. Never force connections or use homemade adaptors. 

  12. Carefully check all cylinder-to-equipment connections before use and periodically during use, to be sure they are tight, clean, in good condition and not leaking.

  13. Carefully open all valves, slowly, pointed away from you and others, using the proper tools.

  14. Close all valves when cylinders are not in use.

  15. Never tamper with safety devices on cylinders, valves or equipment.

  16. Do not allow flames to contact cylinders and do not strike an electric arc on cylinders.

  17. Always use cylinders in cool well-ventilated areas.

  18. Handle "empty" cylinders safely: leave a slight positive pressure in them, close cylinder valves, disassemble equipment properly, replace cylinder valve protection caps, mark cylinders "empty" or "MT," and store them separately from full cylinders.

  19. Wear the proper personal protective equipment for each of the jobs you do.

  20. Know how to handle emergencies such as fires, leaks or personal injury.

  21. Follow the health and safety rules that apply to your job.

(adapted from www.CCOHS.ca  Working Safely with Compressed Gases)

 

Procedures for Transporting Compressed Gas Cylinders

 

  • Always transport cylinders with valve caps or other valve protection in place.

  • Do not pull cylinders by their valve caps,

  • Do not roll cylinders on their sides.

  • Do not drag or slide cylinders.

  • Rolling cylinders on their bottom edge ("milk churning") is only acceptable for short distances provided you have the physical strength required.

  • Never lift cylinders with magnets or chain or wire rope slings.

  • Only Transport cylinders on specially built hand carts or trolleys (available in the cylinder storage areas) or other devices designed for this purpose. All transport devices must have some way of securing cylinders to prevent them from falling.

 

Gas Cylinder TruckCylinder Truck

 Figure 2.  authorized cylinder transport carts

 

  • If use of the elevator is required.  Cylinders should always be accompanied in an elevator by the person responsible for the transportation.  No one else should enter the elevator car.

 

Note: Transporting Liquid Nitrogen

Liquid Nitrogen is not a compressed gas although the tank it may be stored in may be under pressure.  There are different procedures for transporting cryogenic liquids which must be followed.   Please follow the procedures in “Safe Procedures for Handling and Transporting Cryogenic Liquids”

 

Flammable Compressed Gas Cylinder Safety Guidelines.

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 Figure 3.  TDG Flammable Gas placard

Flammable Compressed Gases are common in many labs.   They are used in a variety of laboratory work including as fuel for Atomic Absorption Spectrometers, fuel for flame ionization detectors in Gas Chromatographs and as the Carrier gas in GC’s.    Flammable compressed gases include  Propane, Acetylene, Hydrogen, Butane as well as many other pure gases and mixtures.

  Natural Gas (which is primarily methane)  is supplied to many of the Science Buildings through a pipe line system.  Unlike Cylinders, the pressure of this gas tends to be quite low, however, as it is a centralized system the volume would be unlimited.  In addition Natural Gas has had mercaptan, a pungent smelling but otherwise harmless chemical, added to assist in the detection of Natural Gas leaks.

Flammable Compressed Gases in cylindesr are a unique risk in that they may contain relatively large amounts of gas, at relatively high pressures and in some cases may be in a liquefied state (Acetylene).

Storage of Flammable Compressed Gas Cylinders.

Flammable Compressed Gas Cylinders should not be stored indoors until they are ready to be used.

Praxair will deliver all Flammable Compressed Gas Cylinders to either the outside storage compound at the SC loading dock area, or the outside flammable gas storage cage at DNA Loading Dock B.   Keys for the storage cage can be obtained from Science Facilities.

Stored Cylinders must always have their protective caps properly attached and secured.  

Do not “purge” empty tanks or leave the main valve open as there is always residual gas or liquid in the tank.  Label the tank as empty, close main valve properly and replace the protective cap.  Return the cylinder to the outside  storage compound or cage.

Using Flammable Compressed Gases in Laboratories.

  •  Read the MSDS for the gas you will be using  BEFORE you start to use it.  Ensure you understand the hazards involved and the safety procedures and equipment you should put in place prior to using the material.

  • Never “purge” a new tank of flammable compressed gases, by opening the main valve and “blowing off”.  This may, spontaneously or in the presence of static electricity, cause the material to ignite.

  • Oxidizer and Flammable gas cylinders shall not be stored side by side.  There should be a minimum of 6 m between each for indoor storage or use or they should be separated by a wall that meets the requirement of 2 hour fire rating (see the Ontario Fire Code for more information).

  • Acetylene tanks shall be used and stored in an upright position at all times.

  • All connections and tubing shall be inspected and leak tested every time a new tank is connected to a device or piece of equipment.  Connections inside instruments should also be inspected and leak tested on a regular basis and at a minimum of every 6 months.   As well, when tubing is changed, or new connections added, the system should be leak tested.

  • If using a flammable gas as a carrier in a device (such as a GC) where the gas runs through the device continuously and is not combusted, ensure the residual gas is vented to an appropriate exhaust (fumehood or exhaust snorkel).  Investigate whether the carrier gas can be turned off when the device is not in use.

  • Ensure that flammable gases are only used in rooms with adequate ventilation (to avoid accumulation of gases).

  • If using gases which are significantly denser than air ensure there is low level exhaust and conversely if using gases which are significantly lighter than air ensure there is adequate high level exhaust.

  • Supervisors should develop Standard Operating Procedures by which common tasks like replacing fuel cylinders on devices are described in detail and to which lab personnel can refer to performing these tasks.

  • When performing Leak Tests,  ensure the methods and materials used are safe and effective for the gas and tubing materials you are testing.

  • Flammable compressed gas cylinders must be properly secured using the appropriate gas cylinder holder racks or chains.  The straps or chains shall be a secured around a tank at a minimum height of at least half the height of cylinder,

  • Flammable compressed gas cylinders shall not be stored or used near electrical panels or directly adjacent to lab exits, eyewash or deluge showers.

*Special Considerations*

Hydrogen Gas

Hydrogen is a highly flammable gas and can form explosive mixtures with air if it is in concentrations of 4% - 74 %.  The temperature of spontaneous ignition in air is 500 oC.  Hydrogen fires is extremely hot and is almost invisible in air making hydrogen flames difficult to detect.  If not handled properly hydrogen gas can pose a serious threat to the health and safety of laboratory personnel and emergency responders as well as property.

Acetylene

Acetylene is dissolved in acetone within a porous material inside the cylinder. When the cylinder valve is opened, acetylene in gaseous form flows out, just like carbon dioxide when a bottle of carbonated water is opened. To prevent flammable acetone from emerging from the cylinder, acetylene cylinders must be vertical while gas is flowing, or must be placed with the cylinder valve at least 40 cm higher than the cylinder base.

Cryogenic Liquids

Cryogenic liquids are gases that are kept in their liquid states at very low temperatures e.g. liquid Nitrogen (LN2), liquid Helium (LHe).

Following these basic general safe practices will help protect you from the hazards of cryogenic liquids:

  1. Read the material safety data sheets (MSDS) and labels for all of the materials you work with.

  2. Know all of the hazards (fire/explosion, health, chemical reactivity, pressure) of the materials you work with.

  3. Use only containers specifically designed for liquid cryogenic storage.

  4. Store cryogen containers in cool, dry, well-ventilated areas, away from incompatible materials and ignition sources.

  5. Store, handle and use cryogen containers securely fastened in place in the upright position.

  6. Ensure that pressure relief valves are working properly.

  7. Never tamper with safety devices on vessels, valves or equipment. Never use a vessel where the safety devices (relief valves or burst discs) are not working properly.

  8. Never roll, drag, or drop vessels or permit them to strike each other.

  9. Move containers on handcarts or other devices designed for moving cryogenic liquid vessels.

  10. Ensure proper ventilation in areas where cryogens are stored or used to reduce the risk of fire, explosion or asphyxiation.

  11. Fill containers only with the liquids for which they were designed. Label each container. Fill vessels to the indicated level only. Do not overfill.

  12. Proceed slowly when filling a container or inserting objects into a cryogen to minimize boiling and splashing.

  13. Prevent frostbite by never allowing cryogenic liquids to touch your skin.

  14. Never wear watches, rings, bracelets, or other jewellery that could freeze to your skin.

  15. Always wear loose fitting insulated gloves when handling anything that may have been in contact with a cryogen.

  16. Wear safety glasses whenever you are near a cryogen, and a face shield when pouring a cryogen.

  17. Wear the proper personal protective equipment for each of the jobs you do.

  18. Know the location of eyewash stations and safety showers.

  19. When finished working with LN2, let it evaporate in a fume hood.  Never pour down any plumbing system. 

  20. Never store any cryogenic liquid (LN2. LO2, LHe) or Dry ice (CO2 in a walk-in fridge, freezer or environmental chamber

  21. Never transport LN2 inside a vehicle e.g. car, truck, van without the appropriate ventilation.

  22. Obtain proper training on how to use all of the materials and equipment you are using.

  23. Know how to deal with emergencies (fires, leaks, personal injury).

  24. Follow the health and safety rules that apply to your job.

(adapted from www.ccohs.ca)

Transporting Cryogenic Liquids

  • Move cryogenic liquid containers carefully.

  • Do not move a container by rolling it on its lower rim.

  • Always use a hand truck, cart, or other proper handling device.

  • Use a strap to secure the container to the handcart. Keep the cryogenic liquid containers upright at all times except for the minor tilting on the cart during transport. Always push the container (don't pull) as pushing reduces the chance of the container falling on you or a co-worker.

  • If cryogens must be transported by elevator, take adequate precautions to prevent possible injury.  If a power failure occurred, a passenger could be trapped in the confined space of an elevator with the cryogen. Excessive amounts of the cryogen could vaporize and displace the oxygen. Transport cryogenic liquid containers by elevators only at times when the elevator is not heavily used (early morning is preferable). Send cryogenic liquids in elevators without any passengers and ensure that no passengers get on the elevator while the cryogen is being transported.  Station personnel at elevator openings on all floors between the first floor and the destination floor to ensure no one enters the car with the material.

  • Never transport cryogenic liquids inside a vehicle e.g. car, truck, van unless using a “dry-shipper.  Insulated packages containing refrigerated liquid nitrogen fully absorbed in a porous material and intended for transport, at low temperature, of non-dangerous products are not subject to Dangerous Goods Regulations provided the design of the insulated packaging would not allow the build-up of pressure within the container and would not permit the release of any refrigerated liquid nitrogen irrespective of the orientation of the insulated packaging.  Contact us for more information.

 

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Figure 4.  approved cryogenic containers and transport containers.