7.6 Compressed Gas Safety
The compression of a gas results in a large amount of potential energy. Therefore, compressed gas cylinders are high energy sources and are potential explosives. Compressed gas cylinders can act as a rocket or fragmentation bomb. If the gas is flammable or reactive there is also the potential for a fire or explosion to occur. In addition to physical hazards, gases also often have health hazards (e.g. toxic gases), though even inert gases such as nitrogen can be hazardous in high concentrations which can cause asphyxiation.
The Laboratory Standard defines a compressed gas as: "Any material or mixture having in the container either an absolute pressure greater than 276 kPa (40 lbf/in2 ) at 21 C, or an absolute pressure greater than 717 kPa (104 lbf/in2 ) at 54 C, or both , or any liquid flammable material having a Reid vapor pressure greater than 276 kPa (40lbf/in2 ) at 38 C."
Before working with compressed gases, one should become familiar with the general guidelines for handling and use, in addition to any other physical or health hazards associated with the particular gas.
- Always secure gas cylinders in an upright position, even when they are not in active use or transport. Straps need to be substantial, placed between the middle and the top curve of the cylinder, and should attach the cylinder to a stable structure, such as a wall or lab bench. Cylinders should not be secured to movable furniture or anything unstable. Lecture bottles and round-bottom cylinders are not required to be stored upright, but they must be stored in a secure location (i.e., where they won't fall off a shelf or roll off a table).
- The cylinder valve should be accessible at all times.
- Keep gas cylinders away from heating sources, including sunlight, heating components, boilers, steam pipes, etc. Keep gas cylinders away from temperatures below -20°F, unless designed for that use.
- Gas cylinder storage should be segregated from the storage of other chemicals and incompatible classes of gases should be further segregated. Do not store gas cylinders in the same cabinet as corrosive materials, even if the gas is corrosive itself.
- Empty cylinders should be segregated from full cylinders. Hazard compatibility segregation still applies to empty cylinders (e.g. keep empty flammable and empty oxidizing gases separate).
- The quantity and size of flammable and hazardous gas cylinders in laboratory work areas shall comply with the Minnesota Building Code. Contact your UHS Research Safety Professional for a consultation if you are storing more than two standard sized cylinders of toxic, flammable, or oxidizing gas, or you are storing more than 25 lecture bottles in a research lab or more than 10 in an instructional lab.
Gas Storage Ventilation Requirements
In addition when storing flammable, toxic or corrosive gases the following guidelines must be adhered to:
- A "highly ventilated enclosure" is required for storing gases which are highly toxic or pyrophoric. A highly ventilated enclosure is a Fume hood or Gas Cabinet. Highly toxic gases will have a GHS Hazard statement H330 "Fatal if inhaled". Pyrophoric gases will have the GHS hazard statements H220 - extremely flammable gas AND H250 - catches fire spontaneously if exposed to air
- A "well-ventilated area" is required for storing all other compressed gases. Examples of well-ventilated areas are a standard lab room or outdoor location or loading dock area. Gases which are toxic, flammable, corrosive, oxidizing or inert should be stored here. Note gases are only pyrophoric if they have the H250 hazard statement. E.g. Flammable gases can have the following GHS codes H220 - extremely flammable gas or H221 flammable gas or H222 extremely flammable aerosol or H223 flammable aerosol. Without the H250 code they are not pyrophoric.
- A "low ventilation" or "recirculated air" area is FORBIDDEN from storing compressed gases. Examples include offices or closets.
- Only move gas cylinders when they are securely strapped down to an approved cylinder cart.
- Wear gloves, safety glasses, and hard-toed shoes when moving cylinders. Use protective valve caps on cylinders when they are in transit or not in use. Lecture bottles do not need valve caps.
- Handle gas cylinders carefully – do not drop them, strike them against things, or roll them along the ground.
- Cylinders must always be transported using the elevator and never the stairs. Prudent Practices 6.C.4 Transport of Chemicals recommends that passengers should not be in the elevator. UMN interprets passengers as extra riders the person handling can ride with the cylinder and should discourage other passengers.
- Cylinders are often picked up and returned to central cylinder storage areas. Pick up cylinders promptly from these locations, return shared cylinder carts and report any problems to the area manager.
- If you need cylinders moved between campuses or plan to ship cylinders, contact DEHS at (612) 626-6002 before doing so.
Set-Up and Active Work
- When setting up a gas supply system, the entire system should be checked for leaks and degradation initially and at regular intervals after.
- When setting up gas supply systems, joints should not be greased.
- All pressure regulators should be equipped with pressure relief valves.
- Cylinder valves should be opened slowly.
- When equipment is not operating or left unattended, the cylinder valve must be closed.
- Build a purge step into all experimental procedures to ensure that no gas is left in the tubing. This is especially important if you are disconnecting the tubing, removing it from a fume hood, or using it elsewhere.
- When the pressure approaches 172 kPa (25 psi), the cylinder is considered empty and the valve must be tightly closed to prevent contaminants from entering. Remove the regulator, replace the valve cap and prepare the cylinder for return. Empty cylinders still contain gas and need the same precautions as full cylinders.
- Transfilling of gas cylinders (filling your own gas cylinders from another source) has specific requirements that must be followed. If this is required, contact UHS at (612) 626-6002 for further guidance.
When working with compressed gases that are flammable, corrosive, irritating or toxic, the following additional guidelines should be observed:
- The relief valve on cylinders of hazardous gases should vented to a hood or other safe location.
- For highly hazardous gases, perform leak checking with an inert gas, such as helium or nitrogen first, before using the highly hazardous gas in the system.
- When opening valves on irritating or toxic gases, it should be done in a fume hood or specially designed cylinder cabinet.
Cryogens and Liquid Gases
Cryogenic liquids have boiling points of less than -73 °C ( -100 °F). Commonly used cryogenic liquids include liquid nitrogen, helium, argon, and slush mixtures of dry ice with isopropanol. All of these substances are commonly used in cold traps. Other materials such as oxygen and hydrogen are, also, often used in the liquid state. The extreme cold of cryogenic liquids necessitates special care in their use. The main hazards associated with using cryogenic liquids are fire or explosion, pressure buildup, embrittlement of structural materials, frostbite, and asphyxiation.