The Best Sources, Systems, and Delivery Methods
While there is great diversity in testing methods available and industries represented, these laboratories have one thing in common: a desire for precise and accurate results. As such, these labs often rely on high-purity carrier, combustion, and calibration gases.
When considering laboratory gas sources, there is growing pressure among lab managers to stay abreast of the requirements for the latest analytical equipment while also attempting to find solutions that optimize efficiency gains and minimize costs. When it comes to laboratory gas solutions, managing capital costs, safety, convenience, quality, and long-term returns are all considerations. If you are thinking of changing, updating, or modifying your current gas supply, consider that the overall success of your analytical workflow may depend on making the correct decision.
Traditionally, most specialty gases have been supplied in compressed gas cylinders. These cylinders are typically 4 feet tall and weigh between 75 and 80 pounds with their contents pressurized to around 2,000 pounds per square inch (psi). While compressed cylinders are still by far the most commonly used method for supplying analytical gases, they do present a number of limitations. Specifically, they present risks to worker safety, require specific storage and handling equipment, and may have cylinderto- cylinder quality variations that make sensitive analyses difficult. Although cylinders present some challenges for the analytical lab, they remain widely used and, coupled with engineered solutions for safety and regulation, still present an attractive choice for many labs.
Alternately, the adoption of point-of-use gas generators to produce a continuous supply of compressed gasses such as zero air, nitrogen and hydrogen has become popular for a broad range of instrumentation. As examples, zero air is used for liquid chromatography (LC) and gas chromatography with flame ionization detection (GC-FID); nitrogen is used with GC-FID, thermal analysis (TD), inductively coupled plasma spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), and liquid chromatography with mass spectrometry (LC-MS); and hydrogen is used both as a combustion gas for multiple purposes and as a carrier gas for gas chromatography (GC), where it offers increased speed, resolution, and sensitivity over helium (especially when used with FID). On-demand laboratory gas generators are available in a variety of configurations and output capacities suitable for supplying single or multiple instruments. However, while gas generators offer a number of safety, reliability, and convenience benefits to the user, these benefits come at the expense of higher initial capital cost, require ongoing maintenance, and may pose delivery pressure limitations in some situations.
A matter of choice
For many labs, previous commitments and constraints on capital investment may prohibit conversion to on-site gas generation. However, there are other reasons that a lab may choose a cylinder setup, and in many cases the benefits of cylinders may outweigh those of on-site gas generation. Regardless of what type of gas you are using, there are essentially two methods for setting up your gas cylinders: single, point-of-use cylinders and a multi-cylinder configuration. Choosing a suitable gas solution will depend largely on the size and demands of your particular lab.
Point-of-use cylinders are generally best suited for smaller laboratories with single systems or a limited number of gas applications. In this setup, the cylinder is typically mounted to the wall in close proximity to the instrument it is serving. Close proximity to the device operator offers the advantage of easy monitoring and flow control if necessary. Additionally, there is less tendency for variations in pressure when the gas supply is close to the device in use. However, the main disadvantage of this setup is that your process must stop whenever a gas canister must be switched out, which may result in considerable instrument downtime.
For large labs, and those with high gas demands or several systems requiring a specific gas, a multi-cylinder configuration is often the best choice. In such systems, multiple cylinders are racked together and connected to manifolds with a switchover system. Gas supply is initiated from one end of the switchover system and is automatically switched as the primary source is depleted. This setup avoids many of the problems and much of the downtime associated with cylinder change-out, because the primary cylinder can be replaced while gas delivery continues from the secondary cylinder. Although multi-cylinder configurations with switchovers are less disruptive to laboratory operations, consider that they are more expensive to set up and may require a trained technician to handle switchovers and cylinder changes.
Another critical choice when setting up your gas delivery system is the selection of a suitable pressure regulator. Gas pressure regulators are used to reduce the pressure of gas supplied from a high-pressure canister to a level that can be safely used for instrument and equipment operation. There are two types of regulators to choose from: single-stage and two-stage. As their names suggest, single-stage regulators reduce cylinder pressure to the outlet pressure in one step, while two-stage regulators perform the same function in a two-step process. Selecting a regulator will depend largely on the type of application for which it is required.
If your system involves the supply of several instruments by a centralized gas supply, you may experience supply pressure variations, especially if automatic switchover manifolds are used. In this situation, a two-stage regulator with a narrow supply pressure effect should be used. Alternately, if you are using your gas for a shortduration calibration, a single-stage regulator with a wider supply pressure effect but a constant flow should be used.
Materiality is also a concern when selecting a gas regulator. Gas regulators must be constructed from materials that are suitable for the application in question. For general use, brass regulators with elastomeric diaphragms will be adequate for many applications. However, the use of stainless steel diaphragms will prevent adsorption of gases on the diaphragm and eliminate air diffusion. Such diaphragms are necessary for trace analyses where components may be adsorbed on an elastomeric diaphragm, or for applications such as GC, which can be affected by diffusion of atmospheric oxygen or outgassing of monomers and dimers from the elastomeric components.
Kicking the canister
On-site generation of laboratory gas is becoming increasingly popular in many analytical labs by offering an increased level of safety and convenience as well as considerable long-term cost savings when compared to using high-pressure canisters. On-site gas generators typically operate at low pressures and store only small volumes of gas (< 50 cm3), greatly reducing risks associated with gas leaks such as asphyxiation or explosion. Further, once installed, a gas generator will provide continuous supply to a single or multiple instruments without the need to remove, replace, and reorder new canisters on a regular basis.
It is important to note that in some cases, gas generators may present delivery pressure limitations, resulting in problems for labs requiring high pressure gas or for labs that wish to serve multiple instruments from a single generator. In such situations, it is best to position the generator as close to the instrument as possible to avoid supply line pressure drops, and keep in mind that you may require multiple generators to meet the needs of your lab.
While on-site gas generators present a very attractive option in terms of convenience, be advised that they do require some maintenance to maintain optimum performance and reliability. Depending on the type of gas generator used, it may require replacement of filters or desiccant cartridges on a semiannual basis as well as replacement of valves, sensors, heaters, or thermocouplers every couple of years. Complete knowledge of the maintenance requirements and the associated costs is important when purchasing a gas generator.
Gas generators present a very attractive argument in favor of safety. Their low storage volumes and low pressure minimize much of the risk associated with working with either flammable gases such as hydrogen, which can easily reach explosive levels from a small leak in a multi-cylinder line, or inert gases such as nitrogen, which can quickly displace breathable atmosphere, resulting in asphyxiation. Canisters may also present physical hazards when they are being moved or if they are inadvertently dropped. Given these risks, it is important to note that laboratories have one of the best safety records regarding compressed gas usage, and through proper use of engineered equipment and safety protocols, much of the risk can be mitigated.
Dollars and sense
If there is a major limitation to on-site gas generation, it is that it requires a relatively high initial capital investment. This is a consideration for many laboratories, and obtaining management buy-in is often key in making a switch from canister to on-site generation. However, most manufacturers estimate return on investment of less than one year, depending on your usage level. If your usage is high, accrued savings from canister purchase, delivery and handling may outweigh the initial investment in a matter of months. While cylinder gas is still an attractive alternative for many—if not most—labs, considering all the available options and alternatives as well as fully understanding the demands of your particular application will undoubtedly be critical for developing a gas delivery program that makes sense for your lab.
For more information on specialty gases and gas generators visit www.labmanager.com/gas-generators