16.4 Compressed Gases16.4 Compressed Gases
Compressed gases are commonly used in laboratories.
- Gas cylinders may contain gases that are flammable, highly toxic, toxic, corrosive, asphyxiant, or oxidizing.
- A risk assessment, such as a POSHER (docx) review, may determine that a gas cabinet, leak detection, and an emergency shut off button in the hallway may be required.
- Unsecured cylinders can be tipped over, causing serious injury and damage. Impact can shear the valve from an uncapped cylinder, causing a catastrophic release of pressure leading to personal injury and extensive damage.
- Mechanical failure of the cylinder, cylinder valve, or regulator can result in rapid diffusion of the pressurized contents of the cylinder into the atmosphere; leading to explosion, fire, runaway reactions, or burst reaction vessels.
- A minimum 1 cubic foot/minute/square foot (cfm/ft2) of room ventilation is required for the storage and use of compressed gases due to the hazards listed in this chapter.
- In accordance with NFPA 704, gases or liquids having a hazard ranking of Health Class 3 or 4, Flammability Class 4, or Instability Class 3 or 4 that are carried in pressurized piping above 15 pounds per square inch gauge (psig) (103 kPa), an approved means of leak detection and emergency shutoff or excess flow control shall be provided.
Backflow prevention or check valves shall be provided where the backflow of the hazardous gas could create a hazardous condition or cause the unauthorized discharge of hazardous materials.
Piping, tubing, valves, fittings and related components shall be designed and fabricated from materials that are compatible with the material to be contained and shall be of adequate strength and durability to withstand the pressure, structural and seismic stress and exposure to which they are subject.
Use a restrictive flow orifice (RFO) or needle valve to restrict flow to only that needed for the experiment. Consider using a dilution of the gas that is suitable for the research, but falls outside of the code requirements.
- Manual valves or automatic remotely activated fail-safe emergency shutoff valves shall be installed on supply piping and tubing and provided with ready access at the following locations:
- The point of use.
- The tank, cylinder or bulk source.
Manual emergency shutoff valves and controls for remotely activated emergency shutoff valves shall be identified and the location shall have access clearly visible and indicated by means of a sign.
- Template (docx) for writing a Standard Operating Procedure- Use this template for developing protocols a specific hazardous gas system.
16.4.1 Safe Storage and Handling of Compressed Gas Cylinders16.4.1 Safe Storage and Handling of Compressed Gas Cylinders
- Restrain cylinders of all sizes by straps, chains, or a suitable stand to prevent them from falling;
- Keeps cylinders in storage upright, secure, and interlocked into a compact group;
- Do not use compressed gases in occupiable environmental chambers, cold rooms, or other similar stand-alone units. These typically do not provide outside air ventilation;
- Keep cylinders capped and the regulator off when not in use;
- Keep cylinders labeled with a full/in-use/empty tag.
- The contents of any compressed gas cylinder must be clearly identified. Labels on cylinders may be stenciled, stamped, or a label or tag attached to the cylinder. Do not rely on the color of the cylinder for identification because color-coding is not standardized and may vary with the manufacturer or supplier.
- For short term experiments using hazardous gases, always select the smallest returnable cylinder available. In cases where the gas will be used over an extended period of time (several months to more than one year), you should order a gas quantity that will last for three to six months;
- Some small cylinders, such as lecture bottles and cylinders of highly toxic gases, are not fitted with rupture devices and may explode if exposed to high temperatures;
- Do not store or use incompatible gases next to each other;
- Cylinders of oxygen must be stored at least 20 feet away from cylinders of any flammable gas.
- Isolate them by storing in a gas cabinets.
- Segregate full cylinders of low hazard gases from "empty" cylinders awaiting return to the vendor;
- Do not expose cylinders to temperatures higher than 50 ˚C (125 ˚F);
- Never place cylinders where they may become part of an electric circuit;
- Corrosive gases should be returned to the gas supplier within one year to avoid regulator and cylinder valve problems due to corrosion;
- Avoid areas that are damp or subject to other corrosive materials;
- When transporting cylinders:
- Always use a hand truck equipped with a chain or belt for securing the cylinder. Full size cylinders weigh up to 300 pounds.
- Make sure the protective cap covers the cylinder valve.
- Never transport a cylinder while a regulator is attached.
- Avoid riding in elevators with compressed gas cylinders. If this is necessary, consider using a buddy system to have one person send the properly secured cylinders on the elevator, while the other person waits at the floor by the elevator doors where the cylinders will arrive.
- Do not move compressed gas cylinders by carrying, rolling, sliding, or dragging them across the floor.
- Do not transport oxygen and combustible gases at the same time.
- Do not drop cylinders or permit them to strike anything violently.
16.4.2 Fume Hood Applications16.4.2 Fume Hood Applications
Fume hood applications: This does not apply to highly toxic and toxic compressed gases, in cylinders large than 20ft3 at STP due to the face velocity requirements.
- The smallest possible cylinder should be used for the experiment (a six-month bottle supply for routine use gases is appropriate, while smaller cylinder supplies are suggested for short term use);
- Cylinders of toxic and highly toxic gases that are 20 ft3 or smallerat NTP are allowed within fume hoods as long as there are no incompatible materials also used in the hood or storage of any items inside.
- Make an effort to obtain gas cylinders in returnable bottles;
- Order bottles with lowest cylinder pressure possible;
- Use a flow restricting orifice or needle valve to restrict flow to only that needed for the experiment;
- Consider using a dilution of the gas that is suitable for the research, but falls outside of the New York State code requirements;
- Place the cylinder in rear of the hood. High pressure leaks can readily escape the hood and capture is best in the rear of the hood;
- Assure all components in experiment can withstand full bottle pressure or incorporate pressure relief (run relief line into a hood slot);
- All gas lines connected to the hazardous gas source, including purge lines and gas supply lines, must be completely contained inside of the hood. If this is not possible, the cylinder must go into a gas cabinet.
16.4.3 Operation of Compressed Gas Cylinders16.4.3 Operation of Compressed Gas Cylinders
The cylinder valve hand wheel opens and closes the cylinder valve. The pressure relief valve is designed to keep a cylinder from exploding in case of fire or extreme temperature. Cylinders of very toxic gases do not have a pressure relief valve, but they are constructed with special safety features. The valve outlet connection is the joint used to attach the regulator. The pressure regulator is attached to the valve outlet connector in order to reduce the gas flow to a working level. The Compressed Gas Association has intentionally made certain types of regulators incompatible with certain valve outlet connections to avoid accidental mixing of gases that react with each other. Gases should always be used with the appropriate regulator. Do not use adaptors with regulators. The cylinder connection is a metal-to-metal pressure seal. Make sure the curved mating surfaces are clean before attaching a regulator to a cylinder. Do not use Teflon tape on the threaded parts, because this may actually cause the metal seal not to form properly. Always leak test the connection.
Here is a link to the Airgas material compatibility chart: http://airgassgcatalog.com/catalog/ap020.pdf as well as the available excess flow valves: http://airgassgcatalog.com/catalog/E113_TAG_096.pdf
Basic Operating guidelines include:
- Make sure that the cylinder is secured.
- Attach the proper regulator to the cylinder. If the regulator does not fit, it may not be suitable for the gas you are using.
- Attach the appropriate hose connections to the flow control valve. Secure any tubing with clamps so that it will not whip around when pressure is turned on. Use suitable materials for connections; toxic and corrosive gases require connections made of special materials.
- Install a trap between the regulator and the reaction mixture to avoid backflow into the cylinder.
- To prevent a surge of pressure, turn the delivery pressure adjusting screw counterclockwise until it turns freely and then close the flow control valve.
- Slowly open the cylinder valve hand wheel until the cylinder pressure gauge reads the cylinder pressure.
- With the flow control valve closed, turn the delivery pressure screw clockwise until the delivery pressure gauge reads the desired pressure.
- Adjust the gas flow to the system by using the flow control valve or another flow control device between the regulator and the experiment.
- After an experiment is completed, turn the cylinder valve off first, and then allow gas to bleed from the regulator. When both gauges read “zero”, remove the regulator and replace the protective cap on the cylinder head.
- When the cylinder is empty, mark it as “Empty”, and store empty cylinders separate from full cylinders.
- Attach a “Full/In Use/Empty” tag to all of your cylinders, these tags are perforated and can be obtained from the gas cylinder vendor.
Precautions to follow:
- Use a regulator only with gas for which it is intended. The use of adaptors or homemade connectors has caused serious and even fatal accidents.
- Toxic gases should be purchased with a flow-limiting orifice.
- When using more than one gas, be sure to install one-way flow valves from each cylinder to prevent mixing. Otherwise accidental mixing can cause contamination of a cylinder.
- Do not attempt to put any gas into a commercial gas cylinder.
- Do not allow a cylinder to become completely empty. Leave at least 25 psi of residual gas to avoid contamination of the cylinder by reverse flow.
- Do not tamper with or use force on a cylinder valve.
16.4.4 Return of Cylinders16.4.4 Return of Cylinders
Disposal of cylinders and lecture bottles is expensive, especially if the contents are unknown.
- Make sure that all cylinders and lecture bottles are labeled and included in your chemical inventory. Before you place an order for a cylinder or lecture bottle, determine if the manufacturer will take back the cylinder or lecture bottle when it becomes empty.
- If at all possible, only order from manufacturers who will accept cylinders or lecture bottles for return.
16.4.5 Hazards of Specific Gases16.4.5 Hazards of Specific Gases
- Inert Gases - These can cause asphyxiation by displacing the air necessary for the support of life.
- Examples: Helium, Argon, Nitrogen
- Cryogens are capable of causing freezing burns, frostbite, and destruction of tissue.
- Cryogenic Liquids - Cryogenic liquids are extremely cold and their vapors can rapidly freeze human tissue.
- Boiling and splashing will occur when the cryogen contacts warm objects.
- Can cause common materials such as plastic and rubber to become brittle and fracture under stress.
- Liquid to gas expansion ratio: one volume of liquid nitrogen will vaporize and expand to about 700 times that volume, as a gas, and thus can build up tremendous pressures in a closed system. Therefore, dispensing areas need to be well ventilated. Avoid storing cryogenics in cold rooms, environmental chambers, and other areas with poor ventilation. If necessary, install an oxygen monitor/oxygen deficiency alarm and/or toxic gas monitor before working with these materials in confined areas.
- Oxidizers- Oxidizers vigorously accelerate combustion; therefore keep away from all flammable and organic materials. Greasy and oily materials should never be stored around oxygen. Oil or grease should never be applied to fittings or connectors.
- Examples: Oxygen, Chlorine
- Flammable Gases - Flammable gases are easily ignited by heat, sparks, or flames, and may form explosive mixtures with air. Vapors from liquefied gas often are heavier than air, and may spread along the ground and travel to a source of ignition and result in a flashback fire.
- Examples: Methane, Propane, Hydrogen, Acetylene, flammable gas mixtures.
- Flammable gases present serious fire and explosion hazards.
- Do not store near open flames or other sources of ignition.
- Cylinders containing Acetylene should never be stored on their side.
- Corrosive Gases- Corrosive gases readily attack the skin, mucous membranes, and eyes. Some corrosive gases are also toxic.
- Examples: Chlorine, Hydrogen Chloride, Ammonia
- There can be an accelerated corrosion of materials in the presence of moisture.
- Due to the corrosive nature of the gases, corrosive cylinders should only be kept on hand for 6 months (up to one year maximum). Only order the smallest size needed for your experiments
- Poison Gases- Poison gases are extremely toxic and present a serious hazard to laboratory staff.
- Examples: Arsine, Phosphine, Phosgene
- Poisonous gases require special ventilation systems and equipment and must only be used by properly trained experts. There are also special building code regulations that must be followed with regard to quantities kept on hand and storage.
- The purchase and use of poisonous gases require prior approval from EHS. Contact the Chemical Hygiene Officer at 607-255-8200.
16.4.6 Gas Generators16.4.6 Gas Generators
Gas Generators for Nitrogen or Hydrogen Gas
Gas generators are an alternative to having compressed gas cylinders in the laboratory. These provide high purity nitrogen or hydrogen and can be connected to lab equipment often without the New York State code implications due to the small quantities that are being stored.
Hydrogen Gas Generators:
- Produce gas on demand via electrolysis of water:
- Store up to 100 ml of gas:
- Require 15 megohm quality deionized water;
- Outlet pressures (psig) and flow rate requirements must be determined for installation.
Nitrogen Gas Generator:
- Produce gas via filtration from the air:
- This is a continuous supply;
- Have varying delivery flow rates. Typically maximum flows are 35 to 70 Liters/minute;
- Operate at maximum pressures of 116 psi (8 bar).
At flow rate requirements above 15 psi (1.05 bar), the Authority Having Jurisdiction may require gas detection. At lower flow rates they may waive this requirement. An engineering review is necessary. These generators do add to the heat load of the room in which they are located.