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This instalment discusses the storage and use of carrier and detector gases, gas filtration, and the means by which pressures are controlled on the way from the gas supplies to a GC instrument.
This month's column discusses the safe storage and use of calibration, carrier, and detector gas cylinders for small or large organizations.
Carrier and detector gases, when not generated directly on-site, are transported to laboratory facilities in gas cylinders and stored there until used. A recent "GC Connections" instalment (1) explored the sources, both natural and synthetic, of carrier and detector gases for gas chromatography (GC). When put into service, secured gas cylinders or laboratory generators are connected to pressure- and flow-regulating devices that establish appropriate flow and pressure conditions for delivery to one or more gas chromatographs. The delivery system may be a simple length of tubing or it may be a more complex manifolded arrangement with intermediate regulation and valving. Ideally, the gas stream passes through a final purification stage positioned close to each chromatograph.
Much of the equipment associated with gas supplies in the laboratory ensures the purity and control of carrier and detector gases; the rest provides necessary safety measures for day-to-day encounters with flammable or asphyxiant gases that come in heavy high-pressure containers. Both the correct equipment and the procedures associated with gas deployment are critical for achieving the safest work environment and the best possible results.
In the United States, both the federal and state government authorities promulgate regulations and guidelines for the safe handling and use of gases according to their delivery system and chemical nature. In 29 CFR 1910.101, the United States Occupational Health and Safety Administration (OSHA) incorporates standards issued by the Compressed Gas Association (CGA) (2). Several organizations have made their policies and procedures for compressed gas handling available on-line; among these, the State University of New York (SUNY) campus at Stony Brook Compressed Gas Safety Guide (3) is one that is easily accessible and useful for typical chromatography laboratory scenarios.
Industrial gas consumers — including both companies and their employees — are responsible for the safe storage and use of gas cylinders and their contents from the moment the tanks come off a delivery truck until the gas supplier collects the empties for return. During this period of responsibility, certain procedures and equipment are used to mitigate the hazards and get the best use out of the cylinders' contents. In addition, companies should enact specific training programmes to qualify employees to handle compressed gases safely and effectively. The contents of this article are intended as general professional advice and are not meant to reduce or replace any requirements, procedures, or regulations imposed by companies or their local authorities.
Gas cylinders are ubiquitous in chromatography laboratories. Their presence in nearly all chromatography work environments, combined with the rarity of accidental injury or property damage, engenders a sometimes too-relaxed attitude towards them on the part of laboratory workers and managers alike. Other hazards, such as burns from hot GC inlets or punctures suffered while cutting fused-silica columns, are much more common and can be mitigated by following easily understood and simple procedures. It is safe to say that the gas cylinders in a chromatography laboratory present an array of hazards that, short of the presence of toxic chemicals, comprise the greatest personal and property safety challenges chromatographers are likely to encounter.
Gas cylinder users do not always follow safety procedures. I often see cylinders that are not properly restrained or a lack of valve-protection caps that should be installed when the cylinders are not in use. Sometimes I observe incorrect transfer of cylinders from one location to another. For example, years ago at a university a gas delivery worker tilted two cylinders 15° from vertical, one with each gloved hand, and proceeded to roll them on their bottom edges down a hallway and into a laboratory. The cylinders did not fall, but I wonder how many times they had fallen during his tenure in that job and what kind of damage had been caused.
And how about moving tanks with a regulator attached and the main valve open, or without a protective valve cap? Not a good idea, but I've seen it done more than once. So, why not handle and use compressed gas cylinders without following the prescribed safe procedures? There are lots of reasons not to do so, and they can be grouped into three categories: Physical hazards, energy hazards, and chemical hazards.
Physical Hazards: Gas cylinders are massive. A 49-L capacity steel cylinder, such as what is commonly used for helium or nitrogen carrier gas, weighs about 63 kg (138 lb) and is physically unstable in its normal vertical position. Such a mass of steel can do considerable damage to limbs and toes that happen to be in the way as it falls.
Energy Hazards: In addition to being bulky, massive, and unstable, compressed gas cylinders constrain a considerable amount of potential energy as pressurized gas. Should that pressure be released suddenly, a cylinder can become a spinning projectile that can penetrate walls and potentially inflict deadly harm. In an earlier "GC Connections" instalment (4), I calculated that a full helium cylinder could attain a velocity of more than 108 km/h (67 mph) in a few seconds if the gas were released along the cylinder's main axis. The result depends on how quickly the gas is released, but by an estimate that is enough energy to do a lot of damage.
Chemical Hazards: In the case of hydrogen, a gas cylinder represents considerable combustible chemical energy as well. Hydrogen concentrations can reach the lower explosive limit (LEL) of only 4% in air in any enclosed space into which sufficient quantities are released.
Cylinders that contain gases other than air also pose an asphyxiation hazard that results from the potential displacement of oxygen should the gas be released more rapidly than ambient air can be replaced by ventilation.
Some laboratories also use cryogenic liquefied gases such as liquid nitrogen or carbon dioxide. The proper use and storage of cryogenic liquids is a separate topic about which the interested reader can learn more from the references at the end of this article. Specific personnel protective equipment such as face masks, gloves, and boots should always be used when handling cryogenic liquids.
Gas cylinders for the laboratory include some built-in safety measures. They are protected against over-pressurization caused by heating or inadvertent over-filling by a burst or rupture disk in the cylinder valve stem. Safety burst disks will open at pressures greater than the normal operating pressure for the cylinder, but less than the cylinder's test pressure. For a laboratory carrier-gas tank, the disk will open when the tank pressure exceeds about 225 MPa (3200 psi). This pressure is greater than what would be attained should a completely full cylinder be stored at up to 60 °C; the maximum recommended storage temperature for nonreactive gases is around 50 °C. A ruptured disk will release the contents of the cylinder safely (and loudly). A so-affected cylinder must be returned to the supplier for inspection and refurbishing.
Now let's examine how to manage and handle gas cylinders safely. A cylinder passes through three stages at a laboratory site: Receipt and storage, in-use, and ready to return. These stages usually represent three separate locations within a facility; the first and last have nearly identical requirements, while the in-use stage is different.
Regardless of the stage of a cylinder's lifetime, store inert laboratory gases separately from reactive gases, including hydrogen fuel gas or any other gases used for other purposes. Hydrogen has some special venting and storage requirements that are detailed in the standards; keeping different types of gases separate helps avoid mix-ups.
Receipt and Storage: Receiving personnel should check the labelled contents of all cylinders upon delivery, as well as the overall appearance and condition of the cylinders. Any exceptions should be brought to the attention of the supplier as soon as possible. Suppliers don't deliver damaged or incorrect cylinders intentionally, but a quick check is in order. Ideally the date of the most recent hydrostatic pressure test should be verified; however, this information can be difficult to ascertain from the markings on a cylinder. Cylinder pressure can be verified if the valve assembly includes an indicating regulator; otherwise it's better to wait until a cylinder is put into service before checking the fill pressure.
Gas cylinder contents generally are colour-coded on the cylinder, but the colour scheme is not consistent from supplier to supplier. Thus, it is not possible to rely on the colour of a cylinder for identification. An attached label or tag will give the necessary information. If there is no tag or label, then the cylinder must be returned immediately to the supplier — or another supplier if the original supplier is unknown — for proper disposition.
Gas cylinders must be moved and stored properly from receipt until they are returned to the supplier. Outdoor and indoor storage are both possible. Indoor storage is more convenient if space is available, while outdoor storage should provide protection from weather and dirt. Cylinders must be kept dry in all cases to avoid corrosion.
Suitable cylinder storage locations include any area with limited access where they can be properly restrained. For small organizations the laboratory itself is often the best spot. Storage in a public hallway is not desirable, of course, while too-limited access might cause a problem should quick access be required in an emergency. Each location where gas cylinders are used is unique and, if at all possible, the question of proper cylinder locations should be considered carefully.
Often, and especially in situations where a larger number of cylinders are used, there may be more than one location for cylinder storage. For example, some companies store full cylinders in a location near a loading dock while keeping in-use cylinders in the laboratory or in a shared area that feeds multiple locations.
In-Use: Cylinders are classified as in-use if a regulator is attached or the protective valve cap is removed, whether or not gas is flowing at any particular time. With the protective cap removed or with a regulator attached, the high-pressure cylinder valve is exposed and represents an increased hazard. A cylinder that falls on its side with the cap or protective collar in place is unlikely to decompress explosively, whereas one with an exposed valve is much more likely to do so. Therefore, the in-use category requires very careful attention to cylinder restraints.
Ready to Return: Used cylinders ready for return should be kept in a dedicated empty-cylinder area that is convenient for their removal from the building, or at least the empties should be marked clearly as such. Once empty, or otherwise no longer needed, cylinders fall back into a category with the same requirements as full cylinders waiting to be put in use — the protective valve cap must be installed; otherwise the valve could be damaged. Even non-obvious damage to the valve can be a problem for the supplier when the cylinder is refilled.
If possible, gas cylinders should not be emptied completely before they are returned, since doing so may expose the inside of the cylinder to ambient air and moisture. The worst case is for an empty cylinder to be stored outside with an open valve. It will be difficult for the supplier to completely purge and clean the cylinder before the next filling. A residual pressure of 175–700 kPa (about 25–100 psi) is suitable.
The image of two large cylinders being rolled by hand on their edges down a hallway remains with me more than 30 years after I witnessed it. The right way to move cylinders is of course with a cylinder cart or hand-truck. These are available in single- and dual-cylinder models with a variety of configurations. The better designs include static-dissipating wheels, which are much appreciated when moving hydrogen cylinders. Never move a cylinder with the regulator attached or the valve cover missing. And never use a gas cart as a permanent cylinder stand. Some tilting and rolling is required to manoeuvre a cylinder onto the cart, so caution is appropriate. Wear gloves and protective goggles and footwear.
If a cylinder starts to fall from upright toward prone, do not attempt to catch it. Jump back quickly out of the way. Return a fallen or otherwise physically stressed cylinder to the supplier immediately. A fall can introduce weaknesses in the cylinder that may make it potentially unsafe for continued use.
Proper restraint of gas cylinders is extremely important for their safe use. When set into position, cylinders must be prevented from tipping over, especially when in use with regulators attached. Gas cylinder restraints provide protection from accidental tippage that could compromise attached gas lines or, in the extreme case, cause a regulator or the tank valve to break or detach.
Several types of restraining systems are available, including single- or dual-tank straps that attach to a fixed object or a wall; individual free-standing cylinder floor stands; small multicylinder corrals with a restraining chain; and completely enclosed and lockable cylinder cages.
Do not place cylinders on their sides: Moving them down to the floor or back to the upright position is a lifting hazard at the least, and the movement greatly increases the chances for dropping the cylinder.
Tank Straps: These are available in several forms. They all feature an attachable strap or brace that encircles one or two cylinders approximately 1 m above the floor. The chain, woven belt, or metal strap is attached with a snap, buckle, or bolt so that the cylinder is prevented from falling over or moving out of position. Tank straps should be affixed permanently to an immovable object such as a wall or a laboratory bench that is bolted to the wall or floor. Some straps feature a clamp that attaches temporarily to a benchtop and allows the strap to be relocated easily. Although this type does not provide strict immobility it may be sufficient in some locations, but only if the bench is secured.
Floor Stands: Floor stands are single-cylinder stands that clamp or strap onto a cylinder and provide a larger footprint than the cylinder itself. They provide improved stability but are not immune to tipping or movement, so they are recommended only for temporary cylinder placement.
Cylinder Corrals: A cylinder corral is affixed permanently to the floor or wall. The corral admits two or three cylinders across and two or three deep, and the cylinders are restrained by a chain that attaches onto several hooks welded to the corral. This arrangement is suitable for stored or in-use cylinders where single tank straps are not enough — for example, in a gas analysis laboratory where multiple calibration gas cylinders plus carrier gas, hydrogen, and air are required. A cylinder corral is the most flexible arrangement and requires the least amount of linear wall space for multiple cylinders.
Cage Enclosures: A cylinder cage or locker is a large box with heavy mesh walls and usually a floor as well. The door is firmly secured with a lockable bolt. Cylinder cabinets are similar, but they have solid walls. A cylinder cage holds up to 16 or more laboratory gas tanks, but only for storage purposes as it is not a good practice to snake gas lines through the cage mesh. It is the most secure method of cylinder storage. Cages or cabinets are also the most expensive storage method, but they are well worth the investment in safety and security for organizations that have to manage a large number of cylinders.
After verifying gas cylinders and their content upon receipt, safely moving and securing them in place, and organizing them by their current use, the next step is to attach the appropriate gas regulators, tubing, filters, and fittings to bring the high-purity gas supplies up to one or more gas chromatographs. The next "GC Connections" instalment will discuss these topics and more, as we continue to follow the multiple gas paths through a GC system.
John V. Hinshaw is a senior scientist at Serveron Corporation in Beaverton, Oregon, USA, and is a member of the LCGC Europe editorial advisory board. Direct correspondence about this column should be addressed to "GC Connections", LCGC Europe, 4A Bridgegate Pavilion, Chester Business Park, Chester, CH4 9QH, UK, or email the editor-in-chief, Alasdair Matheson, at email@example.com
(1) J.V. Hinshaw, LCGC Europe 27(1), 33–36 (2014).
(2) Compressed Gases, in 29 CFR 1910.101 (U.S. Government Printing Office, Washington, D.C.). Available at https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9747.
(3) "Compressed Gas Safety Guide," at http://www.stonybrook.edu/facilities/ehs/occupational/cg.shtml (SUNY Stony Brook University, January 2014).
(4) J.V. Hinshaw, LCGC North Am. 20(6), 532–536 (2002).