Compressed gas is a generic term used for describing: compressed gases, liquefied compressed gases, refrigerated liquefied gases (cryogenic gases), and dissolved gases. Compressed gases may also be referred to as gas cylinders.
The class of gases that are termed compressed gases are non-liquefied gases. This means that they do not become liquid at normal temperatures, even at high pressure. They are either:
- A gas or mixture of gases in a container having an absolute pressure exceeding 40 pounds per square inch (psi) at 70° F (21° C);
- A gas or mixture of gases in a container having an absolute pressure exceeding 104 psi at 130° F regardless of the pressure at 70° F(21° C); or
- A liquid having a vapor pressure exceeding 40 psi at 100° F (38° C); as determined by ASTM D-323-72.
Examples include: helium, nitrogen, oxygen, and argon.
Liquefied Compressed Gases
The class of gases that are referred to as liquefied compressed gases become liquid at normal temperatures when they are pressurized inside a gas cylinder.
Examples of liquefied gases include: ammonia, chlorine, methane, natural gas and liquefied petroleum gas or liquid petroleum gas (lpg) which are propane and butane.
Refrigerated Liquefied Gases (Cryogens)
The class of gases that are referred to as refrigerated liquefied gases are also known as cryogens. They are:
- Kept at very low temperatures.
- Extremely cold with boiling points below minus 150° C.
- Heavier than air under cold temperature conditions and can accumulate near the floor.
- Able to expand into very large volumes of gas from small amounts of refrigerated liquefied gas liquid.
Examples of refrigerated liquefied gases (cryogens) include: helium, liquid nitrogen, and liquid argon. Carbon dioxide and nitrous oxide, which have slightly higher boiling points, are also included in this category.
Bulk storage and container transport of cryogenic gases is in liquid, double-walled, vacuum-sealed containers that can be either pressurized or non-pressurized.
Pressurized containers have vents, dispensing hoses, and pressure gauges. Pressure relief valves are spring loaded devices set at a specific pressure that relieve excessive pressure, reclose and reseal to prevent further release of product.
Non-pressurized containers are typically open-topped vessels or Dewars with loose fitting covers to minimize evaporation or off gassing and range from one to 50 liters in size.
This class of gases that are referred to as dissolved gases are those that are dissolved in another substance.
Acetylene is the only commonly used dissolved gas and it is dissolved in acetone. It is chemically unstable and flammable.
Gases are classified in different ways according to definitions by different regulations including Department of Transportation (DOT) and Fire Code regulations as well as by classification systems such as the Globally Harmonized System of Classification and Labelling of Chemicals (GHS). These classifications are different and overlap in some ways. Users should be aware of all the hazards associated with the gases they are using. The links below are to the Fire Code definitions for compressed gases Web page and GHS and DOT definitions with pictograms to help identify how chemicals are classified.
Fire Code Definitions for Compressed Gases
GHS-DOT Classification Comparison Chart (PDF)
Cylinders stored and used on campus must be clearly labeled. The labeling must list contents, concentrations, hazard classifications, safety precautions and the manufacturer or supplier according to United States Department of Transportation (DOT) and Occupational Safety and Health Administration (OSHA) regulations. Do not remove manufacturer applied labels.
Cylinder manufacturers include engraved marking on cylinders in order to allow for proper identification of substances in container by manufacturers. Do not attempt to identify the compressed gas in the cylinder on your own. Be sure to contact the manufacturer if you are unsure of cylinder contents.
Cylinder Status Tags
All cylinders should bear a cylinder status tag stating one of the three conditions: full, in-service, or empty.
Both physical and health hazards are present with use of compressed gases.
- The high pressure in cylinders (4.4 to 6,000 psig) makes the gas cylinder a potential physical explosive rocket that could punch through walls.
- Some gases may be corrosive which could result in change in tissue and/or equipment at the point of contact.
- Cryogenic gases have dangers of low temperature, potential frostbite, and they may expand into large volumes of gas that could displace oxygen and result in suffocation.
- Inert gases and oxidizing reactions may create oxygen deficiency hazards (ODH) by displacing oxygen and may lead to suffocation. The early symptoms may be dizziness and weakness, which may lead to unconsciousness and death. This is also termed asphyxiation.
- Flammability is a concern especially with the gases acetylene, hydrogen, and propane.
- The permissible exposure limits for toxic materials is very low and so even small exposure is considered to be poisonous.
- Oxygen leaks may create oxygen enriched atmospheres which increase the risk of fire and explosions. See a video link demonstrating the hazard at http://youtu.be/ZNiZaVT1mBY.
- See additional hazards as listed on Safety Data Sheets (SDSs).
Compressed gas cylinders must be handled with caution at all times. It is particularly important to protect cylinder valves from breakage and store gas cylinders upright. The following are safety and handling instructions:
- Make sure cylinders have proper labels including: contents, concentrations, hazard classifications, safety precautions, manufacturer or supplier’s name, and a tag that indicates whether the cylinder is full, in-service, or empty. Accept only properly identified cylinders. If the cylinder is not properly labeled or the label cannot be read, return the cylinder to the supplier.
- Make sure gas cylinders are not damaged and do not show signs of corrosion. If you notice that they have damaged labels, dents, gouges, burn/heat marks, or show signs of corrosion, then do not accept them and return them to the supplier.
- Assume all cylinders contain gas under pressure and treat all gases as hazardous chemicals.
- Label all gas lines leading from gas cylinders clearly.
- Use, store, and transport cylinders in an upright position unless they qualify to be stored horizontally. (See Storage section below for more information.)
- Use cylinders in a well-ventilated area. If you need to use a gas cylinder in spaces with inadequate ventilation confer with EH&S (206)543-7388 to conduct a hazard assessment. Spaces with inadequate ventilation may need oxygen alarms or ventilation failure alarms.
- Move cylinders with a cart or hand truck designed for strapping on cylinders.
- Secure cylinders properly during storage, transport, and use so that they cannot be knocked over.
- Make sure caps are in place when the cylinder is not in use and during transport.
- Make sure access to the cylinder valve is unobstructed at all times.
- Make sure pressure regulators are equipped with pressure release valves.
- When turning off the cylinder, turn the gas supply off at the cylinder valve first, de-pressurize the system, and turn off the regulator.
- Keep incompatible gases stored separately. If the cylinder is not in use, separate oxidizing gases from flammable gases by 20 feet or a one-hour firewall. Note: one backup cylinder stored in the area with the one in use may also be considered to be “in use” and not subject to incompatibility storage requirements. (See Compatibility section below for more information.)
- Store highly toxic gases in exhausted enclosures (gas cabinet or fume hood).
- Avoid sources of ignition and open flame.
- Do NOT purchase more or larger cylinders than necessary.
- Do NOT store flammable gases next to an exit or near oxygen cylinders.
- Do NOT use or permit contact of oil or grease on cylinders or their valves.
- Do NOT empty gas cylinders to a pressure lower than 25 psi (172 kPa). At lower pressures, suction and backflow can cause contamination of residual contents with air if the valve is open.
- Do NOT use Teflon tape on cylinder or tube fitting connections, which have metal-to-metal face seals or gasket seals.
Personal Protective Equipment (PPE)
When using gas cylinders, make sure to wear closed-toe shoes, and safety glasses.
Refrigerated Liquefied Gas/Cryogen PPE
- Wear insulated leather gloves when handling cryogens
- Wear protective eyewear and face shields
- Wear smock or lab coat
Tailor PPE requirements to each hazard.
Compressed Gas Piping and Tubing
Permanent Gas Piping Systems
- Installed by qualified personnel
- Durable and heat resistant materials
- Steel, copper, stainless steel tubing/pipe
- Fuel gas Grade T flexible gas tubing with appropriate hose clamps must be used for all petroleum-based products
- Piping material must be compatible with gas
- Components: Shut off valves, point of use valves, regulators and pressure relief valves
- Appropriate labeling
- May serve an entire building
Temporary Gas Piping Systems
- Piping/tubing runs should be as short as possible
- Piping may not serve other rooms
- Appropriate plastic and soft copper tubing acceptable
- Piping material must be compatible with gas
- Regulator must step down pressure significantly for piping system
- Tubing/piping may not be charged (pressurized) when unattended
- It is only used for short term, experimental process development
- Appropriate labeling
- Only experienced lab staff should operate the system
♦ Pipelines can suffer pressure changes depending on the ambient temperature resulting in an over-pressurization and must include a pressure relief device.
♦ Pipelines must include appropriate labeling, zone shut-off valves, and methods for venting lines between regulators and shut-off valves.
♦ Do NOT use copper fittings and tubing on acetylene tanks.
Pressure regulators lower the gas pressure to a useable level.
There are two kinds of pressure regulator designs which appear similar: single and two stage.
- Single stage regulators are used when precise control of delivery pressure is not required.
- Two-stage regulators give precise control.
♦ Keep regulators clean, free of surface oil and grease. (Especially oxidant gases).
♦ Always use the proper regulator for the gas in the cylinder. Connection fittings, stamped CGA numbers, plaques, and/or decals on the regulator indicate which gas the regulator is designed for.
♦ Do not use TeflonŽ tape, putty, or other such materials on the threads unless specifically required (or applied) by the manufacturer/vendor.
♦ A volume restriction orifice installed downstream of the regulator is required for all toxic and highly toxic gases. Specify pressure and flow requirements when ordering compressed gas so that the vendor provides the proper restriction orifice.
- Secure the container with a bracket, chain, strap, or other approved restraint to a fixed object, such as a wall or bench, with one or more restraints.
- One strap or chain meets the minimum requirement.
- Two straps are recommended or chains located at 1/3 and 2/3 of the cylinder height above the floor because cylinders secured by a single strap have been found to escape the strap during an earthquake.
- Secure the container to a cart, hand truck, or other mobile device designed for the movement of compressed gas containers, cylinders, or tanks.
- The cylinder must be in good condition with an operable valve. Cylinders not in use should be capped.
- Compressed gas cylinders should be stored in an organized, ventilated, well-lit place away from combustible materials.
- All gas cylinders must be stored upright and firmly secured to a wall. (Refer to: Securing Cylinders above)
- Do not remove or damage manufacturer applied labels, decals, or cylinder content information. If the label is no longer legible, contact the vendor for pickup, or contact EH&S at (206) 616-5835 for more information
- Any storage area must be protected from excessive heat, open flame, or ignition sources.
- Storage outside should be above grade, dry, and protected from weather conditions.
- Store cylinders so oldest products get used first.
- Compressed gas containers, cylinders and tanks must be stored in the upright position. There are two exceptions:
An example of a cylinder that may be stored horizontally is a lecture gas cylinder.
- Containers designed for use in the horizontal position
- Compressed gas containers with a water volume less than 5 liters are allowed to be stored in the horizontal position
Gas types should be separated from incompatibles and the areas marked. Guidelines are as follows*:
- Keep incompatibles a distance away of not less than 20 feet or use noncombustible partitions extending not less than 18 inches above and to the sides of the containers, cylinders, or tanks
- Use approved storage cabinets or exhausted enclosures
* Exception includes: cylinders in use including one spare backup cylinder stored in the same location as the cylinder in use.
More information from EH&S about compatibility is available at: http://www.ehs.washington.edu/fsosurveys/chemcompat.shtm
Consideration should also be given to using compatible materials for containers, tubing, and reaction vessels. Several compatibility guides are available on the Web, such as
Flammable and Poisonous (Toxic, Highly Toxic) Gas Specifics
- Store and use flammable gas cylinders in adequately ventilated areas segregated from other gases.
- Store and use poisonous (toxic, highly toxic) gas cylinders in a gas cabinet or exhausted enclosure. A chemical fume hood may be acceptable for lecture and other small bottles. Confer with EH&S.
- Toxic and highly toxic cylinders must be provided with a restriction orifice by the vendor.
- Respirators may be necessary when working with poisonous (toxic, highly toxic) gas. Contact EH&S at (206) 543-7388 for evaluation.
When receiving compressed gases, they may be labeled according to DOT regulations and include the weight of the gas in pounds. However, when entering the quantity of gas into MyChem, we prefer you enter the amount in cubic feet. The Praxair and Airgas catalogs are useful in showing the conversion from pounds of gas to cubic feet of gas.
If you purchased your gas from the UW preferred vendor, Praxair, and have additional questions on calculating the amount of gas in your container you may contact them with the contact information provided below and they will provide you with further assistance.
UW-specific Email: firstname.lastname@example.org
General Email: email@example.com
Gases entered into MyChem are hazard coded by EH&S for fire code compliance. You can see the hazards when viewing the chemical inventory and on the caution sign. If you find that your gas has not yet been hazard coded by EH&S, send an email to firstname.lastname@example.org for assistance.
After an open competitive solicitation process and in consultation with several University departments, Praxair, Inc. was selected for a University contract to provide compressed and liquid (cryogenic) gases.
Special Fire Department permits and engineering controls such as a gas storage cabinet may be required to use toxic or corrosive gases. Prior to ordering these gases, you are encouraged to contact EH&S at (206) 543-7262 for an assessment and to ensure proper safeguards are met. Extra precautions must also be documented in your SOP's for these chemicals.
Toxic and Highly Toxic Gases
ALL toxic and highly toxic gases are to be purchased through Praxair, unless unavailable.
Ordering information is provided on the UW eProcurement Web page.
The user must coordinate with the vendor for delivery so that the gas is not left unattended until it reaches its end-use location. Compressed toxic gases may not be stored on loading docks or other temporary locations unless specifically approved by EH&S.
Always cap a cylinder before you move it. Cylinders must always be transported using a hand truck or cart designed for that purpose. Safety glasses and closed toe shoes should be worn when handling cylinders.
When moving compressed gas cylinders:
- Have the metal outlet cap/plug installed,
- Have the valve cap installed if the cylinder has one, and
- Secure cylinders in a cart or container designed to prevent the cylinder from falling over while being moved.
- Use service or freight elevators.
Additional requirements for cryogens:
- Transport cryogens (for example, liquid nitrogen) using one of the following methods:
- Exclude people from the elevator by posting a sign on the cart carrying the cylinder. The sign should read Do Not Enter. Compressed Gas Transport. Ideally, someone sends the elevator up and another person waits on the receiving floor to take the cryogens out of the elevator. If this is not possible then another plan should be devised to ensure that the cryogen is taken out of the elevator once it reaches the needed floor.
If you are going to offer a cylinder for shipment commercially, you will need to be trained. See Shipping Hazardous Materials for more information about training. If you are going to transport a cylinder, please contact EH&S for guidance at email@example.com.
Compressed Gas Shipments
- Whenever possible, researchers should purchase compressed gas through the preferred supplier, Praxair. See Procurement section.
- Inspect the cylinder when it arrives to make sure it is the gas you ordered.
- Never accept a cylinder with damaged labels, dents, gouges, or burn/heat marks.
♦ Cylinders of oxygen, nitrogen, helium, argon or other normal constituents of air may be vented. To dispose of empty cylinders, do the following:
- Remove or deface all labels
- Punch a hole in the cylinder (if cylinder contained flammable gas, leave open in well-ventilated area for 24 hours prior)
- Draw a circle around hole and write the word “empty” next to it
- Dispose of as scrap metal. (At UW Seattle, contact UW Recycling & Solid Waste at firstname.lastname@example.org to arrange pickup.)
♦ For cylinders containing other trace constituents in air, such as calibration gases, please contact EH&S by submitting a Waste Evaluation Form to ensure that the contents are safe to vent.
- Leave the area immediately and get to fresh air.
- Call 911.
- Complete a report in the Online Accident Reporting System (OARS).
EH&S is responsible to help departments identify risk and provide consultation and support to improve their safety.
Online and a hands-on classroom compressed gas training courses are available. Details are available at www.ehs.washington.edu/psotrain/corsdesc.shtm.
For additional information on the OSHA Globally Harmonized System of Classification take the GHS-Globally Harmonized System (HazCom) – Online course provided by EH&S.
For assistance, contact EH&S at (206) 616-6261.
Seattle Fire Code
NFPA 55 - Compressed gas and Cryogenic Fluids Code
WAC: Permissible Exposure Limits for Airborne Contaminants.
OSHA: Compressed Gases
Department of Transportation
Additional hazard communication information provided by EH&S
A Guide to the Globally Harmonized System of Classification and Labeling of Chemicals (OSHA)
List of Chemicals Immediately Dangerous to Life and Health (IDLH)
Liquid Nitrogen and Alarms in University Research Space