Manifolds allow the connection of multiple cylinders or containers of the same gas to a common supply line that is then delivered to the pressure control device or process. This can be as simple as two cylinders connected to a pressure regulator using flexible hose assemblies as in a protocol switchover station, (see figure 1), or as complex as a fully automatic switchover that can interface with remote alarms, building management systems, or even send email alerts when cylinders need replenishment (see figure 2).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 and accommodate the addition of more cylinders without the need to shut down the process. A flexible, expandable manifold design might feature diaphragm isolation valves for leak integrity and positive closure and modular metal-to-metal seals that allow for system expansion. Additionally, system designers should select high-quality materials, (preferably 316L stainless steel) for diaphragms and for appropriately rated flexible hose assemblies. Hose assemblies should also be specified with integral check valves to prevent cylinder backfilling and reduce system exposure to ambient air.
Designing and specifying gas-handling systems that are as expandable as a child’s LEGO set is today a reality, thanks to innovative systems designed with flexibility in mind. Whether one is expanding research and development facilities, ramping up production for late-stage clinical trials or moving to full-scale production, the more modular, reconfigurable and flexible gas-handling systems there are, the better.
By using switchovers, manifolds and similar technologies, systems can start small and expand as need and frequency progress in moving from cylinders to dewars and microbulk, without having to replace original equipment.
In so doing, processes are continuous and efficient, and savings are indeed measurable.Figure 1. Incorporating a protocol switchover station into a gas delivery system is a simple way to connect multiple cylinders to a common supply line.
Sizing Does Matter
As for the available gas connected to a particular system, it should be sized so one side of the system has enough gas to last for a minimum of one- to two-weeks’ use. Deploying a modularly designed cylinder header system assures process system planners that, as instruments are added and more cylinders are required to meet required demand, the system will remain easily expandable by simply adding additional pigtails to auxiliary ports or by attaching extensions that add more stations, as shown in figure 3.
When it comes to the purity requirements of today’s process analytical technologies, such as gas chromatographs (GCs), or inductively coupled plasma mass spectrometers (ICP MS), the gases must be at least 99.999% pure or better. Typically for GC carrier gases like helium, the only supply option source is from high-pressure gas cylinders or pallets of high-pressure cylinders. The manifold best suited for this type of application traditionally is one capable of a differential pressure switchover in which the switching pressure and resulting residual gas left in the cylinders could be as high as 200 psig over the required line pressure.
While it is impossible (and not desirable) to deplete cylinders to below approximately 150 psig — because of pressure drop in long pipelines — there is also the risk of impurities, particularly moisture, that may increase at lower residual pressures in the cylinders. With the increasing cost-per-cylinder of high-purity helium, the ability to easily change switching pressure can be cost-effective. Achieving this goal can only be accomplished with a system in which the switching pressure is determined by an electronic or computer-controlled input value, one that can be programmed to switch at as low a pressure as realistically possible. For example, if the setpoint allows the switching pressure to be reduced by 100 psig, the helium cost savings can add up to as much as 5% per year.
For gases including nitrogen, argon, oxygen or carbon dioxide in which the initial source is from high-pressure cylinders, there are alternative supply sources that become more attractive as the volume of gas needed increases.
These gases, for example, can be supplied in a cryogenic form delivered in insulated portable dewars that hold up to the equivalent of 18 high-pressure cylinders. Such gases can also be delivered to small stationary cryogenic micro-bulk tanks that are filled on-site, able to contain three times the volume. The benefit, particularly for nitrogen and argon delivered in cryogenic form, is higher purity; in most cases, equal to that of high-purity cylinder grades at a fraction of the cost.
Luckily, there are systems available now that not only can be used with high-pressure cylinders when a particular need demands it — say argon to feed an ICP MS is for only one instrument — but also can be used with cryogenic sources by simply pushing a button that configures the system for the lower pressures found in cryogenic delivery forms.
However, there are two pitfalls that can reduce the cost savings potential of gases supplied in cryogenic form. First, any container that is not in use will build pressure to a level in which the dewar or micro-bulk pressure relief device actuates. Under these circumstances, the container may vent between 2 - 3% of its contents per day. That can mean that 10% to as much as 15% of the product will be wasted, also known as evaporation loss. Fortunately, there are systems that manage either cryogenic or high-pressure sources that incorporate an economizer feature (see figure 3) that senses when the container not in use is about to start to vent and automatically switches to supply the end-use points from that container, reducing pressure and avoiding venting.
The second pitfall concerns what is commonly referred to as “residual return,” a condition often caused by false alarms when the pressure in the primary container drops below the switchover point — even when there is significant liquid left in the vessel. It is caused by overdrawing the capacity of the dewars to maintain pressure as the containers get closer to empty. This can add up to as much as 15 - 25% of idle container’s contents, and the amount that is typically left in the container when it’s thought to be empty.
There are solutions available to address the discrepancy. For example, on its switchover system, CONCOA incorporates a look-back program feature that ensures that the first time the primary side drops below the programmed switching pressure it will switch over but not alarm, the system actuating 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 such that it is above the switching pressure, it 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% or less.
As an example, the typical ICP MS likely uses approximately 175 cu. ft. of argon per day if running 24/7. If the initial application called for the instrument to be operational (at most) for four hours every other day, when it is in purge mode it is only using a quarter of maximum consumption. If that is the case, having a system that had four high-pressure cylinders each containing 336 cu. ft. would be sufficient to supply that single-use point. However, once it was joined by three other ICP MS each running 24/7, the consumption would exceed 525 cu. ft. per day, well within the volume of one cryogenic container for primary supply. Therefore, a primary and one in reserve would be the best, most efficient option resource wise.
But if the reserve were to be venting for five days, you would waste 10% of the contents or nearly 52 days’ worth per year, which translates into many thousands of dollars. In addition, if the residual return is not minimized, it is equal to an additional 12% of wasted gases and avoidable expense and an example of the true dollar costs that can be saved by eliminating these two pitfalls as detailed in table 1.
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 while being flexible enough to switch from one supply source to another if usage drops or increases.
Published in the May 2013 edition of Pharmaceutical Manufacturing magazine
About the Author
Larry Gallagher is Specialty Gas Products Manager for CONCOA, Virginia Beach, Va., manufacturers of gas pressure and flow control equipment for industrial, medical and specialty gas applications, as well as distribution systems for laser materials processing, 800-225-0473, www.concoa.com, email@example.com.