Compressed Gases

Compressed Gases

Compressed gases refer to gases and some liquids contained within a vessel at pressures significantly higher than the surrounding atmosphere. Most laboratory compressed gases have internal cylinder pressures on the order of thousands of pounds per square inch (PSI), making them a potentially catastrophic physical hazard in the event of cylinder rupture, valve failure, or another event that results in rapid loss of contents. The large quantities of material that can be stored compressed in a cylinder and their ability to rapidly diffuse in air can also make them significant health hazards. Compressed gases can present any of the health or physical hazards associated with chemicals. The table below outlines some basic properties of compressed gases commonly used in the laboratory.

• Density refers to the relative density of the gas once released into room air. Those greater than 1 are heavier than air and will tend to accumulate at low levels while those below 1 are lighter than air and will rise.
• LEL is the lower explosive limit, the lowest concentration at which the gas can form a flammable/ explosive mixture in air.
• Primary hazards are briefly listed – consult Safety Data Sheets for more specific hazard information.
• Regulator refers to the specific threading and construction type of the regulator that must be used for each given gas; ultra-high purity gases often require a different regulator. Regulators are specified by Compressed Gas Association (CGA).
• Additional information about compressed gases is covered in the sections Safe Management of Chemicals and Good Work Practices, UVA’s chemical safety and waste training, and from the EHS on-line module, Compressed Gas Cylinder and Regulator Safety.

Safe Handling of Compressed Gases

• Know the contents of a cylinder and be familiar with the properties of that gas. Never use a cylinder which cannot be positively identified; cylinder color coding varies among gas vendors and is an unreliable identifier of cylinder contents.
• All cylinders in operation or not, must be firmly always secured by an adequate bench- or wall-mounted cylinder clamp or chain. Keep in mind that breakage of the valve body on a compressed gas cylinder can easily transform the cylinder into a “torpedo” capable of going through concrete walls!
• Cylinders of compressed gases must be handled as high energy sources and therefore as potential explosives. Cylinder valves should be opened slowly. Never tamper with any part of a valve such as the safety relief or packing nuts.
• Upon receipt of a new cylinder from the vendor immediately check the cylinder valve for leaks with a soap solution. Leaks in cylinders should promptly be reported to the shop personnel and corrected.
• When installing a new cylinder, write your name on the cylinder information tag and attach it to the valve stem. Ensure that cylinders are prominently posted / labeled as to their contents, and tagged as “Full”, “In Use”, or "Empty” as appropriate.
• Use cylinders only with matched connectors and proper Compressed Gas Association (CGA) regulator. Never install cylinder adapters on a regulator. Teflon tape must never be used on any CGA cylinder valve fitting.
• Oxygen regulators should be used only on oxygen tanks. Contamination of oxygen regulators with the oil present in other gases can result in a serious explosion hazard when the regulator is again used for oxygen. Oxygen, or oxygen-rich gas mixes, can explode in high pressure or shock situations where any organic materials (e.g., plastic gaskets) are present. Open valves slowly and wear appropriate PPE.
• Leak- test all connections to a cylinder with a soap solution. CAUTION! Any gas, regardless of its health hazard may cause asphyxiation by displacing oxygen.
• Lubricants and greases should never be used on regulators or gas cylinder fittings. Refer to supplier technical bulletins for when to use Teflon tape, washers, or special consumable seals.

Pressure-relief devices protecting equipment attached to cylinders of flammable, toxic, or otherwise hazardous gases should be vented to an exhaust duct or fume hood.

• When not in use, the regulators on cylinders should be depressurized. If the cylinder is not to be used for a long time, the regulator must be removed. Never leave partly assembled apparatus attached to gas cylinders. Never attempt to refill a cylinder.
• When storing or moving a cylinder, always attach the safety cap securely to protect the valve stem, and transport gas cylinders of size 2 or larger on a specifically designed wheeled cart.
• Cylinders should be in the lab so that the cylinder valve is accessible at all times. The main cylinder valve should be closed as soon as it is no longer necessary that it be open (i.e., it should never be left open when the equipment is unattended or not operating.) When storing or moving a cylinder, have the cap in place to protect the valve stem and never expose cylinders to temperatures higher than 50 oC.
• A cylinder should never be emptied to a pressure lower than 172kPa (25 psig): leave a slight pressure to keep contaminants out and notify the vendor with a note if draw- down occurs. Empty cylinders should not be refilled by anyone except the gas supplier. Remove the empty cylinder regulator and replace the valve cap. Keep the empty cylinder chained until pickup by the gas vendor. Be sure that a cylinder tag is attached and indicates the proper status of the cylinder (full, partially full, empty).
• Cylinder discharge lines should be equipped with approved check valves to prevent inadvertent contamination of cylinders that are connected to a closed system where the possibility of flow reversal exists. Sucking back is particularly troublesome in the case of gases used as reactants in a closed system. If there is a possibility that a cylinder has been contaminated, it should be so labeled and returned to the supplier.
• When ordering toxic or flammable gases, whenever possible request a Flow Restrictor cylinder Valve. The FRV orifice considerably reduces the full-open leak rate in event of a major leak (e.g., regulator diaphragm failure).
• When not in use, store cylinders by gas type, separating oxidizing gases from flammable ones. Hydrogen and oxygen cylinders should be kept 20 ft apart when not in use.
• Hydrogen is lighter than air and highly flammable, with a very wide range of explosive concentrations. The Virginia fire code greatly limits the quantity of flammable compressed gases that can be stored on elevated floors or the basement. If hydrogen is needed regularly consider a hydrogen generator instead of compressed gas cylinders. The same kind of technology can be used to produce high purity oxygen. Contact EHS for information.
• Most inert gases are permitted in UVA laboratories without special ventilation, but compressed toxic, reactive, and most flammable gases require storage inside a chemical fume hood or a dedicated exhausted gas cabinet. Certain gas storage and delivery systems also require active leak detection and alarms.

Cryogenic Liquids

Cryogenic liquids are extremely cold liquids that pose a risk of serious tissue damage from immersion or splash contact. Their very high liquid-to-gas expansion ratios can also pose an asphyxiation hazard from oxygen displacement, cause explosion-like events when sealed containers are rapidly warmed up or result in fires and explosions from the uncontrolled release of a flammable cryogenic liquid. The table below summarizes the properties for some of the more common cryogenic liquids used in laboratories.

Safe Handling of Cryogenic Liquids and Dry Ice

• Cryogenic liquids are supplied in special insulated (“Dewar”) vessels with pressure relief fittings. To the extent possible, avoid damp areas since moisture can result in excess ice formation around the relief valve. Periodically inspect cryogenic liquid vessels.
• Store cryogens and dry ice in well-ventilated areas since they are continuously “boiling off” or sublimating to a gaseous state. Do not store cryogenic liquid or dry ice in cold rooms, as they are not ventilated spaces.
• Cryogenic liquids and dry ice can easily damage laboratory countertops and sinks, and floor tiles; avoid prolonged contact with any material not designed for extreme low temperatures. Never dispose of dry ice in lab sinks; instead, allow dry ice to sublimate in a loose- fitting cooler in a well-ventilated space.
• Avoid transporting cryogen tanks and liquid nitrogen tanks in passenger elevators. Use service elevators when available, and do not ride with the tank in the elevator. Use a “buddy system” to intercept passengers if you must transport up or down several floors.
• Upgrade personal protective equipment with a face-shield (over safety glasses) and heavy insulated forearm-length gloves when dispensing cryogenic liquids.
• Oxygen deficiency detection, alarms, and control systems may be needed in enclosed rooms where appreciable quantities of cryogenic liquid are stored and, in such places, where filling or dispensing operations occur. Contact EHS for additional information.
• In case of liquid nitrogen tank icing: Attempt to tighten the liquid dispensing valve. Be sure to wear insulated gloves. If icing increases or stays steady, move the tank outside, and call Praxair to have them pick it up. When in doubt, call the vendor, or contact EHS.
• For laboratories that have Praxair tanks, the Praxair call number should be posted near the EHS number for quick reference. (contact numbers – Praxair Technician 434-531-7858, and the general service number 804-231-1191).