V. Standards for Handling Compressed Gas Cylinders
Users of compressed gases should be familiar with the pertinent equipment and the characteristics of the gases. Gases are supplied in cylinders under great pressures, some as much as several thousand pounds per square inch. If the valve is broken off the cylinder neck, the cylinder becomes a potentially deadly rocket, propelled with great momentum and high speed. Gas cylinders have been documented to cause extensive property damage, injury, and death. For this reason, all gas cylinders, full or empty, must always be strapped or chained to a sturdy support to prevent the cylinder from falling and breaking off the valve. All cylinders of compressed gas should be treated as high-energy sources and therefore regarded as potential explosives. The contents of a cylinder may also present such hazards as flammability, toxicity, corrosivity, excessive reactivity, and potential asphyxiation (if the volume of air displaced by the contents of the cylinder is sufficient).
- Any material or mixture having in the container an absolute pressure greater than 40 psi at 70F or
- Regardless of pressure, one having an absolute pressure greater than 104 psi at 130F, or
- Any liquid material having a vapor pressure greater than 40 psi absolute at 100F
The hazards of compressed gases can be categorized into the following basic hazards:
- Inerts which displace oxygen causing simple asphyxiation (e.g., nitrogen, argon, and helium)
- Toxics which cause adverse health effects depending on the type of gas, route entry, and dose. (e.g., phosgene and CO)
- Flammables which cause fire or explosion when ignited. (e.g., CO, CH4, and H2)
- Reactives which can be subdivided into:
a. Corrosives that erode and deteriorate human flesh, or equipment
b. Oxidizers that are not flammable by themselves, but which react violently with flammable or combustible materials
*Many gases fall into more than one category
Hazards can result from improper handling of gas cylinders and high pressure equipment, which exist in many University facilities. For example, a leaking cylinder could produce an atmosphere that is toxic, anesthetic, asphyxiating, or explosive; and in the event of a rapid escape, the cylinder becomes a randomly directed missile. The main purpose of properly handling compressed gases is, therefore, to prevent uncontrolled escape of the gas. All handling, storage and utilization of compressed gases must comply with the Compressed Gas Association Standards.
The following information is offered in order to familiarize personnel with cylinder parts and terminology:
- Valve handwheel: used to open and close the cylinder valve. Valves are occasionally not equipped with handwheels and require special wrenches to effect operation.
- Valve pack nut: contains packing gland and packing around stem. Adjusted only occasionally; usually tightened if leakage is observed around valve stem. Should not be tampered with for diaphragm-type valves.
- Valve outlet connection: for connection to pressure-and/or flow-regulated equipment. Various types of connections are provided to prevent interchange of equipment for incompatible gases. Usually identified by a CGA (Compressed Gas Association) number, for example, No. 350 for hydrogen service.
- Safety device: to permit gas to escape if the temperature gets high enough to endanger the cylinder by increased unsafe pressures.
- Cylinder collar: holds cylinder cap (6) at all times, except when regulating equipment is attached to cylinder valve.
- Cylinder cap: to protect cylinder valve.
- DOT number: This number signifies that the cylinder conforms to Department of Transportation specification DOT-3A governing materials of construction, capacities, and test procedures; and that the service pressure for which the cylinder is designed is 2,000 pounds per square inch at 70F.
- Hydrostatic Test Date: This number indicates the date month and year of initial hydrostatic testing. Thereafter, hydrostatic pressure tests are performed on cylinders. For most gases this is done every five years to determine their fitness for further use. At this time, new test dates are stamped into the shoulder of the cylinder. Present regulations permit visual tests in lieu of hydrostatic tests for low-pressure cylinders for certain gases free of corrosive agents. Special permits allow for hydrostatic pressure tests at ten-year intervals for cylinders in high-pressure service for certain gases.
- Original inspector's insignia: for conducting hydrostatic and other required tests approve the cylinder under DOT specifications.
- Valve outlet cap: protects valve threads from damage and keeps outlet clean; not used universally.
B. GENERAL STANDARDS
- All cylinders must be marked as to content. Do not accept cylinders with unidentifiable contents.
- Unless the pressure regulator is attached, keep the valve protection cap on securely.
- Cylinders must be secured with heavy-duty chains, at about 1/3 and 2/3 the height of the cylinder (if only one chain is used, secure at 2/3 cylinder height, to prevent them from falling. Chains rather than straps should be used to ensure that cylinders remain upright in the event of a fire. Cylinder clamps anchored to a support wall or fixed lab bench should be used for securing cylinders. Never use unsecured tables, benches or chemical hoods to anchor cylinders.
- Segregate empty and full cylinders.
- Group cylinders by type of gas. For example, store oxidizing gases at least 20 feet away from flammable gases.
- Cylinders must not be stored near sources of heat, ignition, oil, grease, or where they might become part of an electric circuit.
- Protect from direct sun.
- Cylinders can be stored in the open but should be protected from the ground beneath to prevent rusting.
- Bond and ground all cylinders, lines, and equipment used with flammable compressed gases.
- Limit the storage of corrosive gases to about three months. The cylinder valve stem should be worked frequently to prevent freezing. The valve should be closed when not in use.
- Cylinders should not be subjected to low temperatures because many steels undergo decreased ductility at low temperatures and could crack.
- Avoid subsurface storage locations.
- Before connecting a regulator, the cylinder valve should be opened slightly and close immediately unless the gas is toxic.
- Do not try to force a regulator to fit the cylinder. A poor fit probably indicates that the regulator is not intended for use with that particular compressed gas.
- Open the cylinder valve slowly. Rapid release of a compressed gas will cause an unsecured gas line to whip dangerously and also may build up a static charge which could ignite a combustible gas. Never direct high pressure gases at a person, or use compressed gas or compressed air to blow away dust, etc.
- Never attempt to repair or alter cylinders, valves, or safety relief devices.
- Do not wipe or touch the valve outlet of an oxygen cylinder valve in such a way that organic residues which might be subsequently ignited by exposure to high oxygen pressure are deposited.
- DO NOT EMPTY A CYLINDER COMPLETELY. This will prevent a "suck back" and a possible explosive mixture.
- When discharging gas into liquid, a trap or suitable check valve must be used to prevent liquid from getting into the cylinder or regulator.
- Do not use copper tubing with acetylene.
- Never drag or roll cylinders, even for short distances. Move cylinders only on a hand truck. Use chains to secure them to the hand truck. Do not lift cylinders by the cap.
- Cylinders must not be charged except by the vendor.
- Use soapy water or a commercial leak detector to detect gas leaks when setting up and before each use.
- Employees must not attempt to repair cylinders or cylinder valves or to force stuck or frozen cylinder valves.
- Always wear safety goggles when handling or using compressed gases.
- Only those tools approved by the cylinder vendor should be used on cylinder connections. Do no modify or alter cylinders or their attachments. Use cylinders and manifold systems only with their appropriate pressure regulators.
- Always use a trap to prevent back siphonage of liquid chemicals, and a check valve to prevent back flow of Gases into the cylinder. When gas is passed from a cylinder into a vessel containing a liquid, contamination of the cylinder gas with other chemicals is a real possibility. Such contamination makes the gas unsuitable for future use and may result in explosion with resultant injury, damage, or even death. Use of a safety trap to contain liquid and a check valve to prevent back flow of gas will eliminate this possibility. These are installed immediately after the pressure regulator, and before the vessel containing the liquid. The safety trap should have a volume of about one and one half times the total liquid volume in the system.
- Use cylinders only in well–ventilated areas. Corrosive gases should be used only in locations with access to safety showers and eyewash stations. Corrosive, toxic, and flammable gases should be used in chemical hoods designed for use with that particular gas or group of gases.
- Do not expose cylinders to temperatures higher than about 50C (122F). Some rupture devices on cylinders will Release about 65C (149F). Some small cylinders, including those not fitted with rupture devices, may explode if exposed to high temperatures.
- Transport cylinders in freight-only elevators, if possible. If transport in a passenger elevator is necessary, perform a leak test prior to placing the cylinder into the elevator. Schedule with someone to meet the cylinder at the destination floor elevator (provide them with information on the contents of the cylinder). Post the elevator “Do not enter” and place the cylinder (securely fastened to a cylinder cart) along with the SDS into the elevator. The person removing the cylinder from the elevator should then remove the sign.
C. RESTRICTED PRODUCTS (TOXIC GASES)
1. Notification of intent to work with highly toxic gases should be made prior to their proposed purchase to allow time for assuring that physical facilities and work practice controls are sufficient for the expected hazard. Large cylinders of toxic gases should not be purchased if it is possible to use small cylinders.
2. Some of these gases are extremely toxic and may require isolated laboratory space and specially ventilated cabinets. For this reason, clearance should be requested well in advance of the proposed use. Examples of gases considered to be extremely toxic are chlorine, arsine, boron trifluoride, bromine, fluoride, phosgene, and phosphine. There are many others which are not listed here.
3. The total quantity of highly toxic gases stored should be limited to immediately foreseeable requirements. They should be stored in ventilated cabinets. Allowable quantities are listed in the table at the end of this section.
4. All personnel working in the immediate must be instructed as to the toxicity of the gas or gases being used or stored, the appropriate methods of protection against harmful exposure, and first aid treatment in case of exposure.
5. Because of the hazardous nature of highly toxic and poisonous gases, persons handling such gases are advised to contact the supplier for more complete information than will be found on the Safety Data Sheet with regard to usage and first aid.
6. Cylinders of all gases having health hazard ratings of 3 or 4 and cylinders of gases having a health hazard rating of 2 with no physiological warning properties shall be kept in a continuously mechanically ventilated hood or other continuously mechanically ventilated enclosure. There shall be no more than three cylinders of gases with health hazard ratings or 3 or 4 per hood or other enclosure.
7. Do not transport cylinders containing toxic gases in passenger elevators.
D. FLAMMABLE GASES (HYDROGEN, ACETYLENE, AND OTHERS OF THIS CLASSIFICATION)
Because of the fire and explosive hazards that can result when these products are used in confined spaces, special care must be used.
- When cylinders are kept inside the building, two or more cylinders should not be manifolded together. However, several instruments may be operated from one cylinder.
- Considerations for the number of highly flammable gas cylinders to be placed in a laboratory room will include size of the room, airflow, other equipment in use, ease of access to cylinders, etc. The quantity and size of cylinders in laboratory work areas shall comply with NFPA regulations.
- Standby cylinders of flammable gases (full reserve cylinders) or empty cylinders must not be stored in the laboratory. Empty cylinders will be removed from the laboratory when the full cylinders are received.
- When practical, valves on flammable gas cylinders should be closed before all employees leave the laboratory at night.
- Piping must be compatible with the gas, (e.g., no copper for acetylene; no plastic tubing in any high pressure portion of a system, etc.)
E. ACCEPTANCE OF CYLINDERS FROM VENDORS
1. The contents of cylinders must be identified with decals, stencils, glued or wired-on tags, or other markings on the cylinders. Color codes alone or tags hung around the necks of the cylinders must not be used. Cylinders lacking proper identification must not be accepted from the vendor.
2. Cylinders must not be accepted from the vendors unless the valve safety covers are in place and properly tightened.
3. Vendors moving cylinders into University buildings must use hand trucks or cylinder carts; cylinders must not be dragged or rolled.
4. Cylinder valves must conform to standards of the National Compressed Gas Association.
F. HANDLING AND STORAGE OF CYLINDERS
a. Cylinders should never be dropped or permitted to strike each other violently.
b. The valve safety covers must be left on the cylinders until they are secured to walls, benches, or stable pieces of equipment, or until non-tip bases are attached.
c. Cylinders must be transferred only by carts, hand trucks, or dollies. They must not be rolled or dragged. The valve safety covers must be in place and the cylinders secured to the carts during transport.
d. Cryogenic containers of twenty gallon capacity or more should be transported only on 4-wheeled carts designed for that purpose.
e. Empty cylinders must be marked "EMPTY" or "MT" with grease pencils. Generally, this marking should be on a large piece of adhesive or masking tape stuck on the cylinder rather than on the tank itself. However, some cylinders have tags wired to the valve that identify their contents; in this case, the bottom half of' this tag may be torn off to indicate an empty cylinder. In all cases, empty cylinders must be easily identified so as not to be confused or stored with full cylinders.
Storage of gas cylinders is an important factor in gas safety. Always assign a definite area for cylinder storage.
a. Store them upright in racks.
b. Keep the area cool, dry, and well-ventilated. Dryness avoids rust and corrosion, and makes maneuvering around storage areas safer. Avoid heat or direct sunlight. Prevent sparks and temperatures greater than 130F, particularly around flammables.
c. Cylinders that are necessary for current laboratory requirements shall be stored in a proper cylinder storage area.
d. The quantity and size of cylinders in laboratory work areas shall comply with NFPA standards.
e. Keep cylinders away from live electrical equipment. Any electrical spark to the cylinder can present heat and weaken the steel at that spot. Ventilation is necessary in case of leaks.
f. Separate full and empty cylinders. Empty cylinders should be marked to indicate that they are empty.
g. Separate non-flammables and oxidizers from flammables by at least 20 feet or by a noncombustible barrier having a fire rating of at least one hour.
h. Smoking is not allowed near flammables or oxidizers.
i. Cylinder storage areas should always be secured. Cylinders should always be strapped or chained.
j. NFPA recommends that corrosive gases not be stored for longer than six months. Examples of corrosive gases are: boron trichloride, boron trifluoride, bromine trifluoride, carbonyl fluorine, carbonyl sulfide, chlorine, chlorine trifluoride, fluorine, hydrogen bromide, hydrogen chloride, hydrogen fluoride, hydrogen iodide, hydrogen selenide, hydrogen sulfide, iodine pentafluoride, nitrogen dioxide, nitrogen tetroxide, nitrogen trioxide, nitrosyl chloride, phosgene, phosphorous pentafluoride, and sulfur tetrafluoride.
k. 1,3-butadiene, ethylene oxide, vinyl bromide, vinyl chloride, and vinyl fluoride should not be stored for longer than six months because problems may arise due to polymerization. Note: ethylene oxide should not be stored for more than three months unless provisions are made for refrigerated storage.
G. PRESSURE REGULATORS AND NEEDLE VALVES
1. The valve fittings of cylinders used to store different families of gases are different and will only allow regulators or needle valves to be attached that are safe for use with those gases. Cylinders must not be purchased or accepted whose fittings do not conform to standards of the National Compressed Gas Association. Only pressure regulators and needle valves approved for the gases may be used.
2. Threads and points of unions must be clean; these surfaces must be inspected before they are connected. Personnel must not attempt to lubricate threads or fittings.
3. When attaching regulators or needle valves, personnel must tighten the connections firmly. Non-adjustable wrenches of the proper size should be used. Pliers or adjustable wrenches should not be used, as they damage the nuts, most of which are brass and rather soft. Need for excessive force often indicates that the regulators or needle valves do not fit the cylinders. Leaks at the unions between the regulators and the cylinder valves are usually due to damage to the faces of the connections. Attempts to force a tight fit may damage the previously undamaged half of the connection. If the cylinder valve faces are damaged, the cylinders must be returned to the vendor. Employees must not attempt to repair them. Damaged regulators must not be used until repaired.
4. After attaching the pressure regulator to the cylinder, personnel should turn out the delivery pressure adjusting screws of the regulators until they turn freely. The cylinder valves should be opened slowly. Laboratory personnel should avoid standing directly in front of the regulators at this time as the pressure of the cylinders may blow the glass from the front of a faulty gauge (always wear safety goggles when handling compressed gases). The cylinder valve handles should be left attached to the valves while the cylinders are in use. Cylinder valves that "stick" and do not open when the usual amount of force is applied may be damaged. Personnel must not attempt to force them open, but should return these cylinders to the vendors, stating on the cylinders that the valves are stuck.
5. Pressure in full cylinders should be as indicated on the cylinders or labels. Lack of full pressure may indicate leaks at the connections between the cylinders and valve regulators, damaged regulators, or incompletely filled cylinders.
6. Employees should connect delivery lines to the low pressure outlets of the regulator valves or to the needle valves. Where low pressure lines are used, their valves should be closed and line pressure adjusted by turning the regulator delivery pressure adjusting screws until the desired pressures are shown on the delivery pressure gauges.
7. If the gases are not to be used over a considerable length of time (i.e., 24 hours), the cylinder valves should be closed, the lines bled, and the pressure adjusting screws turned back until they turn freely. Damage to the gauges may result if pressure is left on the gauges during extended periods of nonuse.
H. LEAK TESTING
1. Leak testing using "snoop" or a soap solution should be done twice. The first test should be made before the regulator or needle valve is attached to determine if there are leaks at the union of the cylinder and the cylinder valve and to determine if the valve is leaking. The second test should be made after the regulator is attached and the cylinder valve is opened to detect leaks around the valve stem packing, the connecting fittings, the regulator or needle valve, and the transfer lines to the instrument.
2. Compressed gas cylinders are tested for leaks when they are filled; however, leaks have been detected when new cylinders were received in Clemson laboratories. Personnel should not attempt to repair cylinder leaks or leaks caused by loose valve stem packing. Leaking cylinders of nontoxic, nonflammable gas may be taken to a loading dock or other place having suitable air flow for the vendors to pick up. Leaks from cylinders of toxic or flammable gases require immediate attention. Decisions of how to handle the problem will depend on the kind of gas, the size of the leak, the area where the cylinder is located, and other factors. Personnel must wear gas masks or SCBA and appropriate protective clothing when attempting to move leaking cylinders of toxic gases. Immediately evacuate the area and call the fire department.
Report leaking or damaged cylinders to the University Fire Department at 911. As always, when an accident or emergency situation:
- Remain calm.
- Give your name
- Report your location
- Describe the nature of the emergency (e.g., leaking high-pressure cylinder of toxic gas)
NOTE: Do not attempt to move a leaking or damaged high pressure cylinder even if it is not toxic gas unless you have been specifically trained to do so and have the necessary protective equipment and personnel back up present.
I. EMPTY CYLINDERS
1. A small amount of gas must be left in the cylinders and the cylinder valves must be closed to prevent contamination of the inside of the cylinders.
2. Empty cylinders should be marked "EMPTY" or "MT" and stored apart from full cylinders.
3. Valve safety covers and the labels showing contents must be in place.
J. HYDROSTATIC TESTING
Hydrostatic pressure tests are performed on most cylinders every five years to determine their fitness for further use. At this time, new test dates are stamped into the shoulder of the cylinder. Present regulations permit visual tests in lieu of hydrostatic tests for low-pressure cylinders in certain gases free of corrosive agents. Special permits allow for hydrostatic pressure tests at ten-year intervals for cylinders in high-pressure service for certain gases (cylinders that require hydrostatic testing every 10 years have a five-pointed star stamped next to the hydrostatic test date). Original inspector’s insignia for conducting hydrostatic and other required tests to approve the cylinder under Department of Transportation specifications must be on the cylinder.
K. LECTURE BOTTLES
In addition to standard precautions, the following shall apply to work with lecture bottles in the laboratory:
- Lecture bottles shall be stored where the temperature does not exceed 50C. Unlike larger cylinders, lecture bottles do not have pressure-relief devices to prevent rupturing.
- If labels and valve tags do not agree, or if there is any question as to the contents of a lecture bottle, return it to the supplier.
- Lecture bottles should be purchased from suppliers who will accept the return of empty or partially empty bottles or leased from a supplier who offers a leasing option for lecture bottles. Contact RS for names of suppliers who offer leasing options for lecture bottles.
The following special rules apply to work with cylinders of acetylene:
- Acetylene cylinders should be stored in an upright position if possible. If a cylinder of acetylene has been stored in a non-upright position, do not use it until it has sat upright for at least 30 minutes.
- Acetylene cylinders should be tightly secured with straps or chains and stored where they are unlikely to be struck.
- When connecting an acetylene cylinder, be sure to use a flash arrester at the outlet of the cylinder and the correct kind of tubing to transfer the gas. Some tubing materials, such as copper and lead solder, form explosive acetylides.
- Never exceed the pressure limit indicated by the warning red line of an acetylene pressure gauge.
- Never under any circumstances attempt to transfer acetylene from one cylinder to another, to refill acetylene cylinders, or to mix any other gas with acetylene in a cylinder.
- Acetylene cylinders should be stored away from oxygen cylinders when not in immediate use. (keep at least 20ft. apart or separate by a 5ft. wall with at least a 1 hour fire rating).
Special Precautions for Working with Cryogens: Some of the hazards associated with cryogens (fluids used to maintain extremely low temperatures) are fire, pressure, embrittlement of materials, and skin or eye burns upon contact with the liquid. Cryogens can condense nearly pure liquid oxygen from the air, creating a severe fire risk. A pressure hazard exists because of the large expansion ratio from liquid to gas, causing pressure build up in containers. Many materials become brittle at extreme low temperatures. Brief contact with materials at extreme low temperatures can cause burns similar to thermal burns. Carefully observe all special precautions.
1. Equipment should be kept clean, especially when working with liquid or gaseous oxygen.
2. Mixtures of gases or fluids should be strictly controlled to prevent formation of flammable or explosive mixtures.
3. For flammable cryogens the precautions provided in the "Flammable/Combustible Materials" section of this booklet should be used.
4. Always wear goggles when handling cryogens. If there is a splash or spray hazard, a face shield over the goggles, an impervious apron or coat, cuffless trousers, and fully-covering, non-lacing shoes should be worn. Watches, rings, and other jewelry should not be worn. Gloves should be impervious and sufficiently large to be readily thrown off should a cryogen be spilled. Cryo-gloves or pot holders should also be used. Respirators may be required if the cryogen is toxic and sufficient local exhaust ventilation is not available. Contact RS for exposure monitoring.
5. Containers and systems containing cryogens should have pressure relief mechanisms.
6. Containers and systems should be capable of withstanding extreme cold without becoming brittle. Glass containers should be taped solidly around the outside or encased in plastic mesh.
7. Funnels should not be used for pouring liquid nitrogen or any other cryogen.
8. Large mobile Dewars or LN2 refrigerators (or the trolleys carrying these) used for transporting cryogens within a building or between buildings should be equipped with a braking mechanism.
9. Large mobile Dewars at risk for tipping should be transported on appropriate carts. Wheeled trolleys may not be used if the vessel must pass over elevator thresholds or other slots/crevasses wider than 25% of the wheel width.
10. Dispensing stations designed to allow research staff to fill smaller vessels from a larger self-pressurizing Dewar must be located in non-public areas, and should be posted with standard operating procedures.
11. Smaller vessels of liquid nitrogen or other cryogens transported by hand within or between buildings must have a handle or bail, and must be covered.
N. COMPRESSED AIR
Compressed air is used for many purposes at the University and the following safety guidelines should be observed:
1.Air lines must be reduced in pressure so as to not exceed 30 psi at the nozzle.
2.Inspect lines regularly.
3.Portable air lines must be stored so they do not hang or extend into machine work areas or traffic patterns. Suitable spring hangers, hose reels, sway bracing, vibration dampers, etc. must be provided.
4.A pressure gauge or a valved connection for a pressure gauge should be located at theoutlet of each pressure- reducing valve.
5.Air receivers must be installed so that all drains, hand holes, and manholes therein are easily accessible. Air receivers should be supported with sufficient clearance to permit a complete external inspection and to avoid corrosion of external surfaces. Under no circumstances shall an air receiver be buried underground or located in an inaccessible place. The receiver should be located as close to the compressor or aftercooler as possible in order to keep the discharge pipe short.
6. A drain pipe and valve should be installed at the lowest point of every air receiver to provide for the removal of accumulated oil and water. Adequate automatic traps may be installed in addition to drain valves. The receiver should be completely drained frequently and at such intervals as to prevent the accumulation of excessive amounts of liquid in the receiver.
7 .Every air receiver should be equipped with a visible pressure gauge with one or more spring-loaded safety valves. The total relieving capacity of the safety valve should be such as to prevent pressure in the receiver from exceeding the maximum allowable working pressure of the receiver by more than 10%.
8. No valve of any type is to be placed between the air receiver and its safety valve or valves.
9. Safety appliances, such as safety valves, indicating devices and controlling devices, are to be constructed, located, and installed so that they cannot be rendered inoperative by any means.
10.All safety valves are to be tested frequently and at regular intervals to determine whether they are in good operating condition.
11.Extreme care should be taken in cleaning the worksite with compressed air. Small particles can be given sufficient velocity to cause severe eye damage. Adequate eye protection such as safety goggles must be worn by all persons engaged in cleaning with compressed air.
12. Use of compressed air to "blow down" or clean oneself off at anytime is prohibited.