Some gases are chemically incompatible with other gases and they can explode without a spark or heat source present, by just mixing them together. Because of this, a check valve or other back flow protection apparatus must always be used in any system where the possibility of a reverse flow of incompatible gases could occur.
Compressed Gas Cylinder Safety
Common hazards associated with compressed gases and cryogenic liquids may include:
Cryogenic liquids (liquid argon, nitrogen and oxygen) and certain other liquefied gases are at extremely low temperatures. Coming in contact with them can rapidly freeze and destroy skin tissues. Also, many materials are incompatible with the low temperatures of these liquids and liquefied gases. Certain piping materials, for example, that are perfectly rugged at ambient temperatures lose ductility and impact strength at cryogenic temperatures.
Many gases are stored under high pressure. When released suddenly or under uncontrolled conditions, high pressure gases striking someone can erode or remove human tissue. Also, a sudden release of high pressure gas from a ruptured cylinder or from one with its valve accidentally broken off, can propel the cylinder in an uncontrolled manner.
Except for oxygen, all gases are asphyxiants. Even though a gas is completely nontoxic, it can easily cause suffocation unless it is mixed with sufficient oxygen to support life. Nitrogen, for example, is a harmless, inert gas which makes up about 78% of the air we normally breath. However, only a few breaths of pure nitrogen could render a person unconscious because nitrogen alone will not support life.
In the presence of an oxidizer, some gases will burn if ignited by static electricity or by a heat source such as a flame or hot object. Oxygen and other oxidizing gases do not burn but will support combustion of flammable materials. Increasing the concentration of an oxidizer accelerates the rate of combustion. Materials that are nonflammable under normal conditions may burn in an oxygen enriched atmosphere.
Many flammable gases will explode. Even gases of relatively low flammability, in proper concentrations, can explode if trapped in a confined space.
Some gases are toxic and can cause injury or death if inhaled, absorbed through the skin or swallowed. The degree of toxicity varies from gas to gas. For example, carbon monoxide is a toxic gas emitted by automobile exhaust systems. It can be harmful to individuals exposed to this gas in concentrations of more than 50 parts per million (ppm) over an 8 hour period. Arsine, on the other hand, is a highly toxic gas. It can be harmful to individuals exposed to this gas in concentrations of more than 0.05 ppm over an 8 hour period. Concentrations of 50 ppm for carbon monoxide and 0.05 ppm for arsine are measures of toxicity called Threshold Limit Values (TLVs) or Permissible Exposure Limits (PELs) for these gases. [NOTE: TLVs are measures of toxicity established by the ACGIH while PELs are measures of toxicity established by OSHA. Because the OSHA PEL for a given substance may or may not be the same as the TLV established by ACGIH].
Some gases are corrosive. They chemically attack and irreversibly damage human tissues such as eyes, skin or mucous membranes. They also chemically attack and eat away metal, rubber and many other substances. Some gases are not corrosive in their pure form, but can become extremely destructive in the presence of substances such as moisture or other gases. A tiny leak of hydrogen sulfide, for example, can become a large leak as H2S reacts with oxygen in the air to eat away the container in which it is stored.
Some gases are oxidizers and may create a fire hazard without actually being flammable. Oxygen, for example, is nonflammable but it vigorously supports combustion. In other words, anything that burns will burn faster and hotter in the presence of oxygen.
Some gases are pyrophoric and do not need a spark or a heat source to ignite them. These gases can burst into flame whenever they come in contact with air. Silane is an example of such a gas.
Before ordering or moving compressed gas cylinders to a laboratory or laboratory support space it is important to ensure the following facilities requirements, engineering controls, and management practices are in place:
Ventilation - All compressed gases, except oxygen, may present an asphyxiation hazard during an unexpected or uncontrolled release. Therefore, proper ventilation and, if needed, monitoring must be in place before compressed gas cylinders are used.
- General Ventilation Requirements - all cylinders must be stored in a well ventilated area.
- Specific Ventilation Requirements - all cylinders of the following gases shall be kept in a continuously mechanically ventilated hood or other continuously mechanically ventilated enclosure:
- All gases that have Health Hazard Ratings of 3 or 4.
- All gases that have a Health Hazard rating 2 without physiological warning properties.
- Pyrophoric gases
Storage Limits - Total quantities on site should be limited to the foreseeable requirements.
Grouping - Where gases of different classes are stored at the same location, containers should be grouped, by types of gas, and compatibility. Full and empty containers should be stored separately with the storage layout so planned that containers comprising old stock can be removed first with a minimum handling of other containers. The best way to ensure that you understand all of the hazards associated with the materials you are using, including compatibility, is to read the Safety Data Sheet.
External Corrosion - Containers should not be exposed to continuous dampness and should not be stored near salt or other corrosive chemicals or fumes. Corrosion may damage the containers and may cause the valve protection caps to rust or stick.
Mechanical Damage - Containers shall be protected from any object that will produce a harmful cut or other abrasion in the surface of the metal. Containers shall not be stored near elevators, gangways, and unprotected platform edges or in locations where heavy moving objects may strike or fall on them.
Once a compressed gas cylinder is brought into a research laboratory the user shall follow all of the safe storage requirements listed below:
- All cylinders larger than lecture bottle size must be secured to a wall or other sturdy device using a strap or chain designed for this purpose.
- The user shall keep the container valve closed at all times (charged or empty) except when the container is in use.
- Protective caps must be kept on at all times unless containers are connected to equipment.
- Containers shall not be placed where they might become part of an electrical circuit.
- A flame may never be permitted to come into contact with any part of the compressed-gas cylinder.
- Containers shall not be subject to artificially created low temperatures without approval from the vendor and Brown EHS. Steel can undergo significantly decreased impact resistance and ductility at low temperatures.
- Compressed Gas Cylinders must be stored and used to avoid incompatible gases from mixing. The best way to ensure that you understand all of the hazards associated with the materials you are using, including compatibility, is to read the Safety Data Sheet.
Regulator Leak Testing, Maintenance, and Use
- When not in use, store the regulator in a clean, dry, and safe place.
- Inspect and test at least every 6 months after first use.
- Have only qualified repairmen service, test, and clean the regulator. Brown university faculty, students, or staff are not authorized to make regulator repairs or to conduct regulator maintenance.
- The gauge lenses are made from Polycarbonate - Use only soapy water to clean, then wipe dry using soft cloths. DO NOT USE SOLVENTS.
- Use thread sealants that are compatible with the gas being used.
Procedures for attaching a pressure reducing regulator to a compressed gas cylinder
- Turn the regulator's adjustment screw out (counter-clockwise) until it feels loose.
- Stand behind the cylinder with the valve outlet facing away from you.
- Observe the high-pressure gauge on the regulator from an angle, do not pressurize a gauge while looking directly at the glass or plastic faceplate.
- Open the valve handle on the compressed gas cylinder S-L-O-W-L-Y, until you hear the space between the cylinder valve gently fill the gas. (You can also watch the pressure rise on the high-pressure gauge. If you turned the regulator's adjustment screw back properly, there should be no gas flow out of the regulator or pressure rise on the low-pressure gauge.)
- If you are using a nontoxic, nonflammable gas, you can ensure purity by shutting off the cylinder valve and gently cracking the CGA connection at the cylinder valve. (Generally, three pressurizations with venting will ensure the interior of the connection has a clean, representative sample of the gas in the compressed gas cylinder. For toxic or flammable gases, you can purchase special venting regulators that can be safely vented to a fume hood or vented gas cabinet.)
- When you are ready to use the compressed gas cylinder, fully open the cylinder valve until you feel it stop. Then, close it one-quarter turn.
How to perform a functional test of a regulator and check for internal leaks before use
- Close the regulator by turning the adjusting key counterclockwise.
- Close cylinder valve.
- Drain downstream line.
- The low pressure gauge will indicate zero. The cylinder (high pressure) gauge will read full pressure. Any pressure drop will indicate leakage. Repair before use, or replace with a properly functioning unit.
- A gauge should read zero when all the pressure is removed. If it does not, it may be damaged. Locate and correct the cause of the damage and replace the gauge.
Shutdown and removal of regulator from cylinder
- Close downstream valves.
- Close supply valve on the cylinder or line.
- Bleed off gases - oxygen first - then close downstream valves.
- Turn pressure adjusting key counterclockwise until free.
- Remove regulator from cylinder.