Compressed Gas Cylinder Safety

Compressed gases expose users to both chemical and physical hazards. Gases contained within cylinders can be toxic, flammable, oxidizing, corrosive, inert, or a combination of these hazards. Because the pressurized chemical is released in gaseous form, a leak from the cylinder, regulator, or any part of the system used to deliver the gas can quickly contaminate a large area. Therefore, it is necessary to be familiar with the chemical hazards of the gas and possibility of asphyxiation. In addition to the chemical hazards, there are hazards from the gas pressure and the physical weight of the cylinder. A gas cylinder falling over can break chemical containers and crush feet. The cylinder can itself become a dangerous propelled object if its valve is broken off. Appropriate care in the handling and storage of compressed gas cylinders is essential and information is provided below.

Airgas is currently the gas cylinder vendor on campus. If you have any questions or concerns regarding gas cylinders, please contact the Airgas University Specialist, Mike Chenoweth, at 309-698-9700 or mike.chenoweth@airgas.com or the Division of Research Safety.

Hazard Categories for Compressed Gases

Corrosive

Gases that corrode material or tissue with which they come in contact, or do so in the presence of water, are classified as corrosive. 

Flammable

A gas at normal atmospheric temperature and pressure that can be ignited and burned when mixed with the proper proportions of air, oxygen, or other oxidizers is considered flammable. Changes in temperature, pressure, or oxidant concentration may cause the flammability range to vary considerably.

Inert

Gases that do not react with other materials at standard temperature and pressure are classified as inert. They are colorless, odorless, nonflammable, and nontoxic. The primary hazards of these gases are the high pressure and potential for asphyxiation. These gases are often stored at pressures exceeding 2,000 psi. They can displace the amount of oxygen necessary to support life when released in a confined place. Use of adequate ventilation and monitoring the oxygen content in confined places will minimize the danger of asphyxiation. 

Oxidizer

Gases that do not burn but will support combustion are classified as oxidants. 

Toxic

Gases that may produce lethal or other harmful effects on humans are classified as toxic. The degree of toxicity and the effects will vary depending on the gas. The Safety Data Sheet should be consulted to determine the toxicity. 

SDS (safety data sheets) for compressed gases can be found on the Airgas website http://www.airgas.com/sds-search.  If you are having trouble locating a SDS, contact Airgas. 

Types of Cylinders and Valves

There are two types of gas cylinders used on campus.

  1. Refillable Cylinders – These are provided as a rental from a gas vendor and must be returned to them when they are empty or the gas is no longer in use. This is the recommended practice for managing gas cylinders.
  2. Non-refillable Cylinders – Including lecture bottles, these are purchased from the gas vendor; the university is responsible for disposing of them properly. These cylinders are not rented; disposal costs can range from $0 (non-hazardous gases only) to over $1000 even if the container is empty. The primary reason for purchasing non-refillable gas cylinders is the size. These cylinders can be small (ca. 12-15 inches long and 2 inches in diameter) and conveniently portable. However, most gas companies now have small cylinders for rent.


Disposable Lecture Bottle

The Division of Research Safety encourages all researchers to rent refillable gas cylinders.


Small Refillable Gas Cylinder

Most gas vendors are able to provide a complete line of small quantity gases for rent in refillable, returnable cylinders which are generally about 20 inches long and 4 inches in diameter. These refillable, returnable cylinders are as portable as lecture bottles and almost always cost less than the same product delivered in a lecture bottle. The cylinders contain more product than lecture bottles and may be returned to the gas vendor for free even if they are not empty.

Standard vs. RPV valves

For many gases, there are two types of valves used. A Standard valve can be seen throughout campus on many gas cylinders. For Ultra High Purity gases (UHP), Airgas has the option of a RPV valve (Residual Pressure Valve). The RPV valve does not allow the cylinder to be emptied completely by the user. This keeps ultra-high purity gas inside and allows the gas company to refill the container and maintain the high purity of the contents. Standard valves can empty the cylinder completely, resulting in the need to evacuate and backfill the cylinder to guarantee high purity gas.

Below are pictures of the two valves. The small difference should be understood. The RPV valve (right) has the valve outlet connection offset from the stem. Both types of valves can be connected to the same regulator for the same gas.

At UIUC, adapters must not be used to make new connections between valves and regulators. The use of adapters to alter the CGA connection can lead to elevated risk for accidents involving gas cylinders and regulators. For more information: https://www.drs.illinois.edu/News/Airgas-Valve-Alert.

Standard Valve
RPV Valve

Storing Gas Cylinders

The amount of hazardous compressed gases permitted within a laboratory work area is regulated by NFPA 45 as outlined in the table below.

Flammable Gas

Oxidizing Gas

Health Hazard 3 or 4 (NFPA)

Amount allowed per 500 ft2 

6.0 scf

6.0 scf

0.3 scf

Number of cylinders allowed per 500 ft2

3 cylinders (9"x51" cylinder)

3 cylinders (9"x51" cylinder)

1 lecture bottle

Amount of gas allowed per ft2 of lab space

0.012 ft3

0.012 ft3

0.0006 ft3

In addition to the maximum quantities listed in the table above, the number of lecture bottle cylinders is limited to 25.

  1. Always secure gas cylinders and lecture bottles upright (with valve end up) to a wall, bench top, cylinder rack, or post unless the cylinder is specifically designed to be stored otherwise. Specially designed cylinder clamps can be purchased for securing a cylinder against a bench top. Gas cylinders must not be stored on gas carts. Do not strap cylinders to other cylinders.
  2. Cylinders must be strapped or chained at, or slightly above, the midpoint. The strap must be tight around the cylinder to prevent it from tipping or moving.
  3. Cylinders should be stored in a well-ventilated area away from sparks, flames, or any source of heat or ignition.
  4. Cylinders should not be exposed to conditions that may cause corrosion.
  5. Never store gas cylinders in a location that could block exit routes.
  6. Cylinders may be stored outside on a slab if they are protected from the direct rays of the sun. Do not expose cylinders to temperatures above 125 ºF.
  7. Mark empty cylinders with an “empty” label.

Transporting Gas Cylinders

  1. Always use a suitable cylinder cart for transporting cylinders, with the cylinder securely chained or strapped to the cart. Do not roll or drag a cylinder to move it or allow cylinders to strike each other or any other surface violently. 
    - To secure the cylinder to a cart, first move the cart within two feet of the cylinder and then slowly walk the cylinder to the cart. A cart should be used to transport the cylinder short as well as long distances. 
  2. Protective valve caps must be secured when moving cylinders. Do not lift or move the cylinder by the cap.

General Use 

  1. The gas label provided by the vendor must be kept in its original condition.
  2. Caps used for valve protection should be kept on the cylinder except when the cylinder is in use. A cylinder's cap should be screwed on tightly. 
  3. If a cylinder or valve is noticeably corroded, the gas vendor should be contacted for instructions. Any other damage that might impair the integrity of the cylinder should be called to the attention of the gas vendor before the cylinder is returned.
  4. Transferring compressed gases from one commercial cylinder to another is not permitted.
  5. Do not use compressed gas cylinders for any purpose other than for transporting and supplying gas.
  6. Never tamper with, repair, or alter cylinders, regulators, or any pressure-relief devices. Return defective cylinders to the gas vendor immediately. 
  7. Do not attempt to remove a stuck cylinder cap by using a lever in the cap ports. The lever may accidentally open the valve when the cap turns. 
  8. Do not place cylinders where they might become part of an electric circuit or allow them to come into contact with an electrically energized system. 
  9. Use pressure regulators that are equipped with pressure relief devices.
  10. Users must be properly trained in using gas cylinders by someone with experience.

Regulators

              

A regulator allows the high pressure in a gas cylinder to be reduced to a manageable and usable pressure. A regulator controls delivery pressure and should not be used for flow control. Chose a regulator with an appropriate delivery-pressure range for the application. The maximum should be at least twice as high as the desired pressure.

Regulators can be single-stage or two-stage. You cannot tell the difference between a single and two-stage regulator by the number of gauges it has. A single-stage pressure regulator is used when inlet pressure does not vary greatly or when readjusting the inlet pressures does not cause a problem with the experiment/application. As the pressure inside the cylinder drops, the outlet pressure increases due to the lack of pressure to raise the poppet. The pressure adjustment handle will need to be regularly adjusted to maintain a constant outlet pressure.A two-stage pressure regulator is used with gas cylinders to deliver a constant pressure of gas without constant minor adjustments of the pressure adjustment handle when the pressure inside the cylinder changes. The first stage of this regulator decreases the inlet pressure to a preset pressure. The second stage then reduces this pressure to the desired delivery pressure.

A two-stage pressure regulator is used with gas cylinders to deliver a constant pressure of gas without constant minor adjustments of the pressure adjustment handle when the pressure inside the cylinder changes. The first stage of this regulator decreases the inlet pressure to a preset pressure. The second stage then reduces this pressure to the desired delivery pressure.


Single-Stage Regulator1


Two-Stage Regulator2

Different gases and cylinder sizes have different requirements for regulators. The cylinder connection can be male or female and be composed of different materials. A CGA number printed on the gas cylinder and the regulator is used to find the correct match. Below are a few examples of different connectors and their CGA numbers.

For flammable gases, a left-handed thread is used. This is often indicated by a notch or line on the adaptor.

The diaphram within the regulator can be made from different materials. Depending on the gas used, some materials are preferred or even required. Two common materials include elastomers (e.g., neoprene) and stainless steel. The diaphrams made of elastomers can introduce contaminates to the system as they can absorb moisture or contaminates from the air. To maintain purity it is recommended to use a stainless steel diaphram whenever possible.

Regulators do not last forever. Depending on the type of gas and frequency of use, regulators can become compromised. Contact Airgas to determine if the regulator you are using is safe.

Never use a connection adapater to mount the wrong regulator on a cylinder. The CGA connections must match between the cylinder outlet valve and the regulator. The CGA connection depends on the type of gas and also the size of the cylinder. Modifying connections can lead to a serious risk using the cylinder.

Teflon tape should not be used at on the CGA connection. This can cause leaks, connection to fail, or contamination of the regulator. The CGA connection is designed so no additional materials are needed to improve the seal. Do not lubricate fittings. Lubricants can react with certain gases or introduce impurities to the system that can damage connections and the regulator. Carefully inspect the CGA connection for damage before using the gas. If there is any damage, contact Airgas.


Consult with the gas supply company for questions about the correct type of regulator and materials for the gas being used.

Guidelines for Specific Hazards

Flammable Gas Safety

In addition to the above guidelines, the following measures should be taken when handling flammable gases:

  1. Cylinders containing flammable gases (empty or full) should be separated from cylinders containing oxidizing gases by a minimum distance of 20 feet or by a barrier at least 5 feet high that has a fire-resistance rating of at least one-half hour; e.g., a concrete block wall. 
  2. Storage of flammable gases in a ventilated, fire resistant enclosure is recommended (e.g., an approved gas cabinet or chemical fume hood). If this is not possible, flammable gas cylinders should be stored in a well-ventilated space. 
  3. The quantity of flammable gases in a laboratory should be kept to a minimum. A maximum of three full-size cylinders of flammable gas are permitted in one laboratory (500 ft2). 
  4. The use of flow restrictors or surge protectors on flammable gas cylinders is recommended to prevent a sudden large flow of gas in case of a rupture or other unexpected release. 
  5. There may be circumstances where using a pure flammable gas may pose unacceptable risks. Under these circumstances, it may be necessary to use the flammable gas in a mixture with an inert gas to lower the flammability of the gas (e.g., 1% hydrogen in 99% argon). If in doubt, contact the Division of Research Safety.

Toxic Gas Safety

In addition to the general guidelines, the following measures should be taken when handling toxic gases:

  1. Gases that have an NFPA Health Hazard Rating of 3 or 4 (e.g., hydrogen sulfide) must be stored in a continuously mechanically ventilated gas cabinet.
  2. Gases that have an NFPA Health Hazard Rating of 2 without warning properties (e.g., carbon monoxide) must be stored in a continuously mechanically ventilated gas cabinet. 
  3. The quantity of toxic gas in a laboratory should be kept to a minimum. 
  4. Flow restrictors are required on most toxic gas cylinders. 
  5. Ensure that pressure-relief devices vent directly to a laboratory exhaust system.
  6. Gas detection systems are required in laboratories using:
    - 20 scf/ 4 lb or more of “highly toxic” gas or
    - 810 scf/ 150 lb or more of “toxic” gas according to NFPA 55.
    Gas detection is not required if the gas has physiological warning properties below the permissible exposure limit (PEL). The table at the end of the document marks highly toxic and toxic gases with “HT” or “T”, respectively.


Gas Cabinet for Toxic Gas

Oxidizing Gas Safety

In addition to the general guidelines, the following measures should be taken when handling oxidizing gases: 

  1. Do not permit oil or grease to come in contact with compressed oxidizing gases; explosions could occur. Regulators and tubing used with oxidizing gases must be specially cleaned to remove oil and other reducing agents. 
  2. Cylinders containing oxygen or other oxidizing gases (empty or full) should be separated from cylinders containing flammable gases by a minimum distance of 20 feet or by a barrier at least 5 feet high and having a fire-resistance rating of at least one-half hour (e.g., a concrete block wall). 
  3. Do not store oxidizing gases near flammable solvents, combustible materials, or near unprotected electrical connections, heat sources, or sources of ignition. 

Corrosive Gas Safety

In addition to the general guidelines, the following measures should be taken when handling corrosive gases:

  1. Cylinders containing corrosive chemicals should be checked periodically to ensure that the valve has not corroded. If a cylinder or valve is noticeably corroded, contact the gas vendor for instructions.
  2. Be cautious if flow does not immediately start when a valve is opened slightly. There could be a plug in the valve, and if the valve is opened more, the plug could clear suddenly with unexpected excessive flow.
  3. Remove regulators after each use and flush with air or nitrogen to reduce regulator corrosion.

Pyrophoric Gas Safety

In addition to the general guidelines, the following measures should be taken when handling pyrophoric gases:

  1. Pyrophoric gases must be used only in spaces that have sprinkler systems.
  2. Cylinders must be stored in continuously ventilated and fire suppressive gas cabinets.
  3. Gas plumbing must have a leak detector.
  4. Plumbing of the inboard/outboard style is recommended.
  5. Flame resistant lab coats must be worn when using pyrophoric gases.


Gas Cabinets for Pyrophoric Gases

Disposal Guidelines

All gas cylinders that are empty or partly full that will no longer be used must be disposed of properly.

Refillable Cylinders

Rented cylinders must be returned to the vendor. Contact the vendor or campus unit that supplied the cylinder and arrange a pickup according to the rental plan. 

Non-refillable Cylinders

Lecture bottles and disposable gas cylinders must be managed as potentially hazardous waste, as they are purchased outright. Examples of disposable gas cylinders are cylinders of propane, butane, helium, or calibration gases. Indicate on the non-refillable gas cylinder whether it is empty or still contains product above one atmosphere of pressure, and dispose of the cylinder through DRS.

Hazard Reference Table

Gas

Flammable Limits in Air (Vol. %) (1)

Oxidizer

Inert

Corrosive

Toxic

(NFPA rank)

Acetylene

2.5 - 82

Ammonia

 15-28

X

3

Argon

X

Arsine

5.1 - 78

4

Boron Trichloride

X

3

Boron Trifluoride

X

3

1,3-Butadiene

2.0 - 12

2

n-Butane

1.6 - 8.4

Butenes

1.6 - 10

Carbon Dioxide

X

Carbon Monoxide

12.5 - 74

3

Chlorine

X

X

4 (T)

Diborane

0.8 – 98

(spontaneously ignites)

4 (HT)

Dichlorosilane

4.1 - 98.8

X

4 (T)

Dimethylamine

2.8 - 14.4

X

3

Ethane*

3.0 - 12.5

Ethylene

2.7 - 36

Ethylene Oxide

3 - 100

3

Fluorine

X

4 (HT)

Halocarbon-13 (Chlorotrifluoromethane)

X

Helium

X

Hydrogen

4.0 - 75

Hydrogen Bromide

X

3

Hydrogen Chloride

X

3

Hydrogen Cyanide

5.6-40

X

4 (HT)

Hydrogen Fluoride

X

4 (T)

Hydrogen Sulfide

4-44

  4 (T)

Isobutane

1.8 – 8.4

Iso-Butylene

1.8 - 9.6

Krypton

X

Methane

5.0 - 15.0

Methyl Chloride

8.1 - 17.4

2

Monomethylamine

4.9 - 20.7

X

3

Neon

X

Nitric Oxide

X

X

4 (HT)

Nitrogen

X

Nitrogen Dioxide

X

X

3 (HT)

Nitrogen Trifluoride

X

Nitrous Oxide

X

Oxygen

X

Ozone

X

4

Phosgene

4 (HT)

Phosphine

1.6 – 99

(spontaneously ignites)

X

4 (HT)

Propane

2.1 - 9.5

Propylene

2.0 – 11.1

Silane

1.5 – 98

(spontaneously ignites)

Sulfur Dioxide

X

3

Sulfur Hexafluoride

X

Sulfur Tetrafluoride

X

4 (HT)

Trimethylamine

2.0 – 11.6

X

3

Vinyl Chloride

3.6 - 33

2

Xenon

X

[1]Single Stage Pressure Regulator” by Mintrick at en.wikipedia is licensed by CC-BY-SA-3.0
[2]Two Stage Pressure Regulator” by Mintrick at en.wikipedia is licensed by CC-BY-SA-3.0

Other information about compressed gas safety can be obtained from the Division of Research Safety at 217-333-2755 or e-mail to labsafety@illinois.edu.

Last Update: 7/31/2017