Flexible Gas Handling
Computerized and expandable systems allow for continuous processing and cost efficient lab expansion.
Specifying gas handling systems that are as expandable as a child’s LEGO® set is a reality today, thanks to innovative systems designed for laboratory use with flexibility in mind.
By using switchovers, manifolds, and the like, you can start with a small system and expand as need and frequency of use increase, moving from cylinders to dewars and microbulk storage without having to replace original equipment. This means processes remain continuous and efficient, and savings are measurable.
Manifolds enable multiple cylinders or containers of the same gas to be connected to a common supply line that delivers gas to the pressure-control device or process. This setup can be as simple as two cylinders connected to a pressure regulator using flexible hose assemblies, such as in a protocol switchover station (Photo 1), or as complex as a fully automatic switchover that can interface with remote alarms and building management systems or even send email alerts when cylinders need replenishment (Diagram 1).
Though it may seem simple enough to increase the available gas on hand for almost any instrument or process, the design and functions of needed equipment are dictated by the specific process and gas involved. Where possible, the system should be able to expand to accommodate more cylinders without requiring that the process be shut down. Essential features in the manifold design include diaphragm isolation valves to ensure leak integrity and positive closure, and metal-to-metal seals with a modular design that allow for system expansion. Additionally, high-quality construction is important, preferably employing 316L stainless steel diaphragms and high-quality, appropriately rated flexible hose assemblies with integral check valves that prevent the backfilling of cylinders and reduce system exposure to ambient air.
The gas supply connected to any particular system should be sized so that one side of the system has enough gas for a minimum of one to two weeks’ use. As instruments are added and more cylinders are needed to keep up, a cylinder header system with a modular design allows for expansion by simply adding additional pigtails to auxiliary ports or by attaching extensions that add more stations, as shown in Photo 2.
Today’s laboratory instruments or systems, such as gas chromatographs (GCs) and inductively coupled plasma mass spectrometers (ICP-MSs), require gases of 99.999 percent purity or better. Typically, for gases such as helium used as a carrier gas for GCs, the only supply option is high-pressure gas cylinders or pallets of high-pressure cylinders. The conventional manifold for this type of application has been a differential pressure switchover in which the switching pressure and pressure of the residual gas in the cylinders could be as high as 200 psig over the required line pressure.
Depleting the cylinders to below approximately 150 psig is not only impossible but undesirable because the resultant pressure drop in long pipelines impedes performance, and a lower residual pressure in the cylinders increases the risk of impurities, particularly moisture. With the increase in cost per cylinder of high-purity helium, the ability to easily change switching pressure can be cost-effective, but this can be accomplished only with a system in which the switching pressure is programmed by an electronic or computer-controlled input value to switch at as low a point as realistically possible. If such a system can reduce the switching pressure by 100 psig, the savings could be as much as 5 percent of the helium costs per year.
With respect to gases such as nitrogen, argon, oxygen, and carbon dioxide, which may be supplied initially in high-pressure cylinders, alternative supply sources may become attractive as the needed volume of gas increases. These gases can also be supplied in a cryogenic form in insulated, portable cryogenic vessels, commonly called dewars, which have a storage volume equivalent to that of 18 high-pressure cylinders; or the gas can be supplied in small, stationary, cryogenic microbulk tanks that are filled on-site and contain three times that volume. The benefit is that the purity levels, particularly for nitrogen and argon delivered in cryogenic form, are in most cases equal to that of high-purity cylinder grades at a fraction of the cost. Fortunately, today’s systems can be used not only with high-pressure cylinders to meet a particular demand for, say, argon to feed only one instrument, but also with cryogenic sources, as shown in Photo 3, by simply pushing a button that configures the system for the lower pressures found in cryogenic delivery forms.
However, when gas is supplied in cryogenic form, two pitfalls can reduce the financial savings potential. First, any container that is not in use will build pressure to a level in which the dewar or microbulk pressure-relief device actuates and causes the container to vent between 2 and 3 percent of its contents per day. Consequently, as much as 10 to 15 percent of the product disappears in what is termed evaporation loss. Fortunately, some of the systems that can now be used for either cryogenic or high-pressure sources have what is called an economizer— the system recognizes that the container not in use is about to start venting and switches to supply the end-use points from that container, reducing its pressure so it does not vent.
The second pitfall is termed residual return, which is caused by false alarms when the pressure in the primary container drops below the switchover point, even when a significant volume of liquid remains. It is caused by overdrawing the capacity of the dewars to maintain pressure as the containers get closer to being empty. This can mean that as much as 15 to 25 percent of the container’s contents are not used and typically remain in the container when the container is thought to be empty.
To address residual return, units such as the CONCOA IntelliSwitch incorporate what is called a look-back feature. This ensures that the first time the primary side drops below the programmed switching pressure, the device will switch over but not alarm, and the system goes through a residual contents test that challenges the primary unit to prove it is truly empty. If the primary side builds pressure within a specified period of time to a point above the programmed switching pressure, the device will switch back to the primary side and continue to use what it is capable of supplying. On average, the residual return is reduced to as little as 3 percent or less.
As an example, the typical ICP-MS would use approximately 175 cu. ft. of argon per day if running 24/7. If initially it were to be used at most for four hours every other day so that in purge mode the system is using a fourth of that volume, a system with four high-pressure cylinders each containing 336 cu. ft. would be sufficient to supply that single-use point. However, if the system were joined by three other ICP-MSs each running 24/7, consumption would exceed 525 cu. ft. per day—a demand that could easily be met by using one cryogenic container as the primary and one as the reserve.
But if the reserve container vented for five days, 10 percent of the contents, or nearly 52 days’ worth per year, would be wasted, which translates into many thousands of dollars. In addition, a failure to minimize the residual return would result in an additional 12 percent of wasted expense.
Gas costs in the laboratory can be considerable if not dealt with by using intelligent, computerized, and expandable systems that incorporate an algorithm to monitor and limit these risks and are flexible enough to switch from one supply source to another if usage drops or increases.