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Gas and Cryogenic Safety

Gas Safety

If pressure equipment fails in use, it can seriously injure or kill people nearby and cause serious damage to property. In addition, sometimes the gases are chemically active. The addition of chemical energy makes the tank ticking time bombs that must be safely handled. Please treat gas with due respect.

Examples of pressure systems and equipment

  1. Compressed air systems (fixed and portable)
  2. Pipework and hoses
  3. Gas cylinders

General Rules

  1. During storage and use, all cylinders need to be clearly labelled. Flammable, toxic, pyrophoric gasses must be distinguished by the color of the tags and stored in segregated areas. Please see the appendix for examples.
  2. All gas cylinders need to be chained to the wall. The chain must either be at half height or two chains at 1/3 and 2/3 height. This ensures that the cylinder stays vertical in the event of cylinder valve failure, preventing it from flying through the walls like a missile.
  3. Please ensure the fitting/regulators being used are rated for the pressure they are being subjected to.
  4. Cylinder must be located away from other hazards. Maintain an exclusion zone around a cylinder as per standard given below.
  5. The cylinders come with a valve guard. This protects the cylinder valve in case the cylinder falls.  Please keep it on if the cylinder is not being used.
  6. Always install the cylinder with an output pressure gauge and shut-off valve. The former makes sure an unsafe pressure is not maintained in the line. The latter is useful in an emergency where the gas supply needs to be turned off.
  7. Cylinders must always be moved on carts. Do not roll them horizontally. If handling a gas cylinder manually, you must two people holding the cylinder
Fig: Proper method of cylinder transport. Don’t roll the cylinders.
Fig: Rules to follow during use. Maintain an exclusion zone around the cylinder. Don’t remove valve guards. Use gas regulators. Always restrain cylinders at 1/2 or 1/3+2/3 height.
  1. The spindle valves of tanks come in different designs. On the left is an older design, where there is danger of spindle loosening while the valve is being turned on. There are two threads in series. Opening one can open the other. The probability of this is significantly lower in newer spindle valves (see figure on the right). Request all lab in-charges to do a self audit on the cylinders in their labs. If they have cylinders with old designs, please get those changed with the more modern ones. I will add his information in the safety manual as well. The squiggly bracket in the picture can be a guide to tell the difference.
Fig: Older vs. newer spindle design. Strongly advise all to only accept cylinders with newer spindle design.

Hazardous gases

Any gas that is categorized as toxic or pyrophoric is a hazardous gas. These are considered significantly more dangerous than compressed gasses and need additional handling.

  1. Hazardous gasses need a gas cabinet to be operated safely. Gas cabinet shut off the gas flow in case of alarms or gas leaks. The gas cabinets may be completely automatic or semiautomatic. The need of gas cabinet must be decided on a case to case basis. In general, highly toxic and/or pyrophoric gasses require a gas cabinet.
  2. Hazardous gases must only be used with welded, face-seal fittings (VCR® or equivalent). No Swagelok® or other compression fittings. No plastic tubing. In extreme cases coaxial lines are needed. IISc prefers purged coax lines instead of evacuated ones.
  3. The design of all gas manifold of hazardous gases must be certified by two faculty members, at least one of whom is the PI.
  4. All welded fitting carrying hazardous gases must be He-leak checked.
  5. Lab facilities with hazardous gasses are supposed to install automatic sensors for detection and warning of harmful gas leaks. Hand held detectors are strongly recommended.
  6. Proper respirators (self-contained breathing apparatus) must be used whenever hazardous gas cylinders are being installed/changed.
Fig: Additional requirement for hazardous gases

Cryogenic Safety

Cryogenic liquids are liquefied gases that are maintained at very low temperatures. These liquids are gases at normal temperatures and pressures. Since these liquids are extremely cold and can expand to very large volumes of gases they are a safety hazard. The following guidelines should be followed while working with cryogenic liquids.

Types of Cryogenic Liquids

  1. Inert Gases: Inert gases do not react chemically to any great extent. They do not burn or support combustion. Examples of this group are nitrogen, helium, neon, argon and krypton.
  2. Flammable Gases: Some cryogenic liquids produce a gas that can burn in air. The most common examples are hydrogen, methane and liquefied natural gas.
  3. Oxygen: Many materials considered as non-combustible can burn in the presence of liquid oxygen. Organic materials can react explosively with liquid oxygen. The hazards and handling precautions of liquid oxygen must therefore be considered separately from other cryogenic liquids.

Types of Cryogenic liquids Container

Cryogenic liquids are shipped and used in thermally insulated containers. These cryogenic liquid containers are specifically designed to withstand rapid temperature changes and extreme differences in temperature.

  1. Liquid Dewar Flasks: Liquid Dewar flasks are non-pressurized, vacuum-jacketed vessels, somewhat like a “Thermos bottle”. They should have a loose fitting cap or plug that prevents air and moisture from entering, yet allows excess pressure to vent. Flasks containing helium, hydrogen and other low- boiling liquids have an outer vessel of liquid nitrogen for insulation.
  2. Laboratory Liquid Dewar Flasks: Laboratory liquid Dewars have wide-mouthed openings and do not have lids or covers. These small containers are primarily used in laboratories for temporary storage.
Fig: (left) Liquid dewar flasks of different sizes. (right) laboratory liquid dewar flask
  1. Liquid Cylinders: Liquid cylinders are pressurized containers specifically designed for cryogenic liquids. This type of container has valves for filling and dispensing the cryogenic liquid, a pressure-relief valve with a frangible (bursting) disk as backup protection. Never remove/disable the safety  valves, otherwise the tank can explode with disastrous consequences. Please read: http://blogs.sciencemag.org/pipeline/archives/2006/03/08/how_not_to_do_it_liquid_nitrogen_tanks
Fig: left (cross-section liquid cylinders. (right) top view of the liquid cylinders showing the safety relief valve and rupture disks.

Health Hazards of cryogenic liquids

There are three groups of health hazards associated with cryogenic liquids: extreme cold, asphyxiation, and toxicity.

  1. Extreme Cold Hazard: Cryogenic liquids and their associated cold vapors and gases can produce effects on the skin similar to a thermal burn. Brief exposures that would not affect skin on the face or hands can damage delicate tissues such as the eyes. Prolonged exposure of the skin or contact with cold surfaces can cause frostbite. The skin appears waxy yellow. There is no initial pain, but there is intense pain when frozen tissue thaws. Unprotected skin can stick to metal that is cooled by cryogenic liquids. The skin can then tear when pulled away. Even non-metallic materials are dangerous to touch at low temperatures. Prolonged breathing of extremely cold air may damage the lungs. Always handle the cryo liquids with personal protective equipment: face-shield, gloves and apron.
  2. Asphyxiation Hazard: When cryogenic liquids form a gas, the gas is very cold and usually heavier than air. This cold, heavy gas does not disperse very well and can accumulate near the floor. Even if the gas is non-toxic, it displaces air. When there is not enough air or oxygen, asphyxiation and death can occur. Oxygen deficiency is a serious hazard in enclosed or confined spaces. Small amounts of liquid can evaporate into very large volumes of gas. For example, one litre of liquid nitrogen vapourizes to 695 litres of nitrogen gas when warmed to room temperature (21°C).
  3. Toxic Hazards: Each gas can cause specific health effects. For example, liquid carbon monoxide can release large quantities of carbon monoxide gas, which can cause death almost immediately. Refer to the material safety data sheet for information about the toxic hazards of a particular cryogen.

Flammability Hazard of Cryogenic Liquids

Fig: Required personal protective equipment for handling cryogenic liquids

Several types of situations exist that may result in a flammability hazard including: fire, oxygenenriched air, liquid oxygen, and explosion due to rapid expansion.

  1. Fire Hazard: Flammable gases such as hydrogen, methane, liquefied natural gas and carbon monoxide can burn or explode. Hydrogen is particularly hazardous. It forms flammable mixtures with air over a wide range of concentration (4 percent to 75 percent by volume). It is also very easily ignited.
  2. Oxygen-Enriched Air: Liquid hydrogen and liquid helium are both so cold that they can liquefy the air they contact. For example, liquid air can condense on a surface cooled by liquid hydrogen or helium. Nitrogen evaporates more rapidly than oxygen from the liquid air. This action leaves behind a liquid air mixture which, when evaporated, gives a high concentration of oxygen. This oxygen-enriched air now presents all of the same hazards as oxygen.
  3. Liquid Oxygen Hazard: Liquid oxygen contains 4,000 times more oxygen by volume than normal air. Materials that are usually considered non-combustible, (such as carbon and stainless steels, cast iron, aluminum, zinc and Teflon (PTFE),) may burn in the presence of liquid oxygen. Many organic materials can react explosively, especially if a flammable mixture is produced. Clothing splashed or soaked with liquid oxygen can remain highly flammable for hours.
  4. Explosion Due to Rapid Expansion: Without adequate venting or pressure-relief devices on the containers, enormous pressures can build up. The pressure can cause an explosion called a “boiling liquid expanding vapour explosion” (BLEVE). Unusual or accidental conditions such as an external fire, or a break in the vacuum which provides thermal insulation, may cause a very rapid pressure rise. The pressure relief valve may not be able to handle this increased pressure. Therefore, the containers must also have another backup device such as a frangible (bursting) disc.

General Rules

The correct usage with proper protective equipment + use of both hands to hold the tank and tube.
  1. Always use cryo rated gloves, face-shield, and apron when handling cryo liquids. Cryo liquids should not be handled without open-toed shoes.
  2. Handle the cryo liquids safely. This means use of both hands to hold dewar and duct.
  3. Never store, transport cryogenic liquids in non-rated containers.
  4. Never store cryo liquids above eye level, i.e. above 5 ft. If a safety valve blows up. Please investigate. There is a reason it blew up
  5. Never disable, replace or weld shut a safety valve. As irritating as they might be, they keep us all safe
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