COMPRESSED GAS SAFETY
Compressed gas cylinders can present a variety of hazards due to their pressure and /or contents. All compressed gases used at NC State University must be ordered through University Central stores. Section 100-15 of this manual covers requirements which must be followed for the use of all compressed gases. In addition to the standard required work practices for inert gases, hazardous gases may require additional controls and work practices including, but not limited to, the use of gas cabinets, gas monitors, emergency shutoffs, proper equipment design, leak testing procedures, and the use of air supplying respirators for certain highly toxic gases. The Industrial Hygiene section of the Environmental Health and Safety Center (515-6860) can be contacted for assistance with these requirements and to provide assistance with the safe design of equipment which involves the use of hazardous gases.
Assure that employees handling compressed gases are adequately trained in the inherent hazards of the cylinders and their contents, as well as proper handling, storage, and use according to OSHA and NC State requirements.
General handling and storage requirements apply to all use of compressed gases at NC State University. Cylinder procurement and shipping procedures described herein apply to "main campus" use of compressed gas cylinders.
Work supervisors and lab Principal Investigators are responsible for assuring that the requirements of this section are followed by all persons under their supervision who uses or handle compressed gas cylinders.
Please review the following compressed gas use applications.
Class 1 Application - Use of Inert Gases - Gases which are non-flammable and non-toxic, but which may cause asphyxiation due to displacement of oxygen in poorly ventilated spaces
Class 2 Application - Use of Flammable, Low Toxicity - Gases which are flammable but act as non-toxic, simple asphyxiants (e.g.hydrogen, methane)
Class 3 Application - Use of Pyrophoric Gases and Liquids - Gases or liquids which spontaneously ignite on contact with air at a temperature of 130 F or below.
Class 4 Application - Use of Corrosive, Toxic, and Highly Toxic Gases - Gases which may cause acute or chronic health effects at relatively low concentrations in air
(See Appendix A of Gas Monitoring Program for additional details on this definition and examples)
Class 5 Application - Use of Compressed Gases in Fume Hoods
Use of Cryogenic Liquids - Use of liquids with a boiling point below -238 F (-150 C). Refer to Application #1 and Cryogenic Liquids section for requirements.
Use of Fuel Gases for Welding, Cutting, Brazing - Use of oxygen and fuel gases (e.g propane, acetylene) for gas welding and cutting applications. Refer to Application #2 and Use of Fuel Gases for Welding and Cutting.
Gas Use Requirements - Engineering Applicability (Legend)
Gas Use Requirements - Administrative & Procedural Applicability (Legend)
X1 Not required if flow restricting orifice is installed in a cylinder valve. May be required for semiconductor applications
X2 Based on the outcome of hazard review
X3 Required in lab and inside gas cabinet for new installations
X4 For new installations
X5 Typically not required, may be required for semiconductor applications
X6 For corrosive gases
X7 See Fume Hood Use
X8 See Gas Monitoring for details
X9 See Hazard Review
X10 See Cryogenic Liquid
This includes a listing of typical engineering controls, referenced in the matrix above. In some cases, NC Building Code may require additional controls. Additional controls or deviations from the controls listed below may be appropriate for your experiment. The controls appropriate for your operation will be identified through the hazard review process.
1. Gas Cabinets - With the exception of cylinders containing a non-toxic, flammable gas, and cylinders used in fume hood applications, hazardous gas cylinders must be housed in gas cylinder cabinets. These cabinets must be equipped with sprinkler protection, and must be constructed and ventilated according to NC code requirements. These requirements include, but are not limited to, the need to provide 200 fpm air flow at the cabinet window.
2. Interlocks - In addition to automatic shutoff of gas flow due to loss of power or ventilation (described below), it will often be appropriate for an automatic shutdown of gas flow due to conditions such as high system pressure, high gas delivery pressure, loss of vacuum, loss of cooling, or other conditions identified through the hazard review process.
3. Emergency Off - Where gases are used in gas cabinets, the emergency off buttons should be located at the lab doorway. Activation of this button will cause pneumatic valves to shut, stopping gas flow. Typically, this button should kill electrical power to hazardous lab equipment as well.
4. Equipment Enclosures and Ventilation - Experimental apparatus using hazardous gases should be contained in an enclosed and exhausted tool enclosure. These enclosures must be connected to the exhaust ventilation system. Ventilation rates must be sized to allow for 100 fpm of air flow through the largest open enclosure door. Mass flow controllers carrying hazardous gases must be housed in a separate ventilated enclosure (or in an enclosed compartment of a larger tool enclosure) so that 100 fpm exhaust flow is available at the largest open door to the enclosure. All components should be readily accessible for maintenance.
5. Smoke Detection - All labs using hazardous gases will have a smoke detector which is connected to the building alarm system. In certain cases, it may be necessary to interlock smoke detector activation with the shutdown of hazardous gas flow.
6. Sprinkler Protection - Where hazardous gases are contained in gas cabinets, sprinkler protection should be provided to the interior of the gas cabinet. In some cases, this protection is required by code. Sprinkler protection is recommended in all labs using hazardous materials.
7. Emergency Power - Emergency power is recommended to power exhaust fans connected to hazardous gas enclosures. In certain cases, this protection is required.
8. Pneumatic Shutoff Valves - All corrosive, toxic, flammable, and pyrophoric gases will contain a normally closed pneumatic shutoff valve, rated for at least full cylinder pressure, and located immediately downstream of the cylinder valve. This valve will shut in the event of power failure, remote actuation of an emergency off button (see this topic), or other appropriate conditions such as hazardous gas alarm activation.
9. Scrubbers - When hazardous waste gases are generated, it is often advisable to treat/react these gases prior to exhaust from the building. This may involve the use of bubblers in a fume hood or sophisticated units for larger scale hazardous gas processes. Note that in some cases (e.g minimal volumes of hazardous gases produced) scrubbers may be not necessary or even unadvisable. Where scrubbers are used, they need to be carefully reviewed as part of the hazard review. Maintenance requirements and procedures need to be clearly understood and followed. See NCSU Scrubber information.
10. Vacuum Pumps - Vacuum pumps used for hazardous gases need to be carefully selected. Depending on the gases being pumped, special precautions may be necessary. For processes where pyrophoric gases are used, pumps need to be continuously purged with nitrogen, with loss of nitrogen flow causing the pyrophoric gas supply valves to close. Pumps used for oxygen service will need to be prepared for this services which includes the elimination of hydrocarbon oils for use due to flammability concerns. In some cases, such as the use of highly toxic gases, vacuum pumps will need to be housed in a ventilated enclosure.
11. Flow Restrictors - A means to limit hazardous gas flow rates to just over maximum flow needed must be installed immediately downstream of each hazardous gas cylinder. For small scale experiments, such as fume hood use, a needle valve is sufficient. For large cylinders a flow restricting orifice, installed by the gas supplier in the cylinder valve or installed in the gas purge panel is required.
12. Ventilation Alarms - All ducts connected to enclosures used to exhaust hazardous compressed gas cylinders or gas carrying components must be connected to a ventilation alarm. Typically, activation of this alarm will cause pneumatic gas supply shutoff valves to close.
13. Eyewash and Showers - A safety shower or eyewash with a wand is required to be present in areas where corrosive gases are used or stored.
14. Purge Panels - Where corrosive, pyrophoric, or toxic gases are in use, the gas installation must include means to adequately purge the area between the cylinder valve and the regulator with an inert gas prior to breaking these connections for maintenance or cylinder change. Inert gases used for this purpose must be used solely for this purpose and not connected to other apparatus. Failure to adequately purge cylinders can result in lack of ability to close the cylinder valve or "regulator creep" which allows full cylinder pressure to be transferred to the low pressure side of the regulator. Review the requirements listed in Purge Panels for your existing or planned installation. 15. Gas Monitors - See Gas Monitoring Program for a complete description of University gas monitoring requirements.
16. Piping and Fittings - All gas piping must be compatible with the gases used and capable of withstanding full cylinder pressure. For example, tygon tubing should never be used with hazardous gases or low hazard gases unless one end is open to atmosphere. Fittings should be selected based on the service needs. Face seal or welding fittings should be used for hazardous gas service wherever possible. All gauges and components subject to leakages which carry hazardous gases must be contained in an exhausted enclosure.
17. Hardware - Never lubricate, modify, force, or tamper with a cylinder valve. Use the appropriate regulator on each gas cylinder. Adaptors or homemade modifications can be dangerous. Assure all components of the experimental apparatus that can handle full cylinder pressure or are otherwise protected. Oil or grease on the high-pressure side of an oxygen, chlorine, or other cylinder of an oxidizing agent can lead to an explosion. Whenever back siphoning ofchemicals into the cylinder might be a problem, use multiple traps or check valves.
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