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.