By David Durkin

Specialty gases fall into several categories that include high-purity, oxidizers, flammables, corrosives, and toxic gases. They are packaged in cylinders that have been carefully prepared, filled, and analyzed to perform specific tasks:

( Span and calibration gases to calibrate gas chromatographs.
( Carrier gases to exclude impurities and sweep a sample through the column.
( EPA protocol gases to monitor atmospheric discharge from power plants, chemical plants, and refineries.
( Process gases to promote specific reactions.

The first component in most gas use systems is a regulator or an automatic switchover system. The sole function of any pressure regulator is to reduce and control gas pressure from a high level in the cylinder to a lower-use pressure.

In the case of specialty gases, the regulator must perform this pressure reduction and assure that the gas enters the system in the same condition as it is in the cylinder, meaning that the regulator cannot add anything to the gas stream (impurities) or subtract anything (absorption). This is accomplished by the careful selection of materials contacting the gas stream.

The first material to consider is the regulator body. A brass regulator is compatible with many of the specialty gases and is relatively economical because the metal is easy to machine. There are two ways to make a brass regulator - through forging and using barstock (figure 1). Though a forged brass body regulator is economical, internals can be relatively rough and have high-surface area, rendering cleaning more difficult and increasing the opportunity for contamination. Forged body regulators are recommended for medium-purity specialty gases up to 99.995% purity.

A barstock body regulator's surfaces are machined, including all the internal surfaces, giving the manufacturer more control of internal surface finishes and wetted surface area. Barstock body regulators also will have a lower surface area, making cleaning easier and the chance for contamination lower. Barstock body regulators are suitable for all purity levels of specialty gases.

Many specialty gases are corrosive and incompatible with brass body regulators. A few examples of these gases are chlorine, hydrogen chloride, ammonia, and hydrogen sulfide, for which stainless steel or other materials may be necessary. The most widely used material is 316L stainless steel, highly compatible with corrosive gases in their anhydrous (dry) state. However, if moisture is present, the gas readily combines with it to form the acid phase of the gas that is highly corrosive and can rapidly lead to regulator failure.

Special care for corrosive gases comes in two forms - either by using more corrosive-resistant materials such as Hastelloy, Monel, or electroless nickel plating, or through system purging when changing cylinders. Because the atmosphere is full of moisture, when the regulator is removed from the cylinder during cylinder change, a small amount of moisture enters the regulator. It is enough to cause damage to the regulator and other downstream equipment if not removed with an effective purging procedure.

The most effective purge is the Deep Purge (figure2) with a snorkel tube design. The snorkel tube allows purge gas to enter the system directly at the cylinder valve and performs a positive displacement purge, physically pushing moisture and other contaminants out of the system. The typical purge gases used are nitrogen, argon or helium because they are inert and very dry. Purging with the deep purge greatly improves system safety and regulator longevity.

Another form of special care is to provide a continuous gas supply using an automatic switchover system (figure 3), which virtually eliminates downtime caused by contamination entering the piping to an analyzer. The AutoSwitch system offers several benefits, from an increase in the connected gas supply to the ability to switch from a primary to a reserve supply.

Sizing the system properly eliminates downtime created when changing cylinders, avoids running out gas on long production runs, and decreases the overall gas residual returned to the supplier. Many facilities automatically change cylinders on a Friday afternoon so they don't have to worry about the weekend.

With an AutoSwitch system, the reserve supply is already connected so the primary supply can be used until depletion. Upon depletion of the primary supply, the reserve will automatically supply the system. The system is built to avoid gas contamination during cylinder change by including check valves in the cylinder pigtails or with built-in purge capability for consistent gas purity to add an extra level of special care.

Additionally, many switchover systems offer alarm capability, with optional features such as telemetry and/or computer interface to further enhance the security of the system. All of the above enhance laboratory or process application by reducing downtime and improving system purity.

Often overlooked is the gas piping system, which should be considered an extension of the gas cylinder. While it is convenient to run plastic, rubber, Teflon or other flexible tubing, it can be a big mistake. Specialty gases are highly purified and analyzed to assure that the gas in the cylinder is a specified purity. Once the gas leaves the cylinder, there are two things that can happen. The gas can be degraded by the piping system or remain at the same purity level as in the cylinder. Plastic or other styles of flexible tubing will degrade the purity by allowing atmospheric contamination to enter the piping system by permeating through the tubing.

Special care in this case is to use either stainless steel or copper tubing. If copper tubing is chosen, the connections should not be soldered or silver-brazed. The flux required for silver brazing is hydrocarbon-based and will off-gas and add contamination to the system. The recommended connections for copper tubing are instrumentation-quality compression tube fittings. For stainless steel tubing, either orbital-TIG-welding or compression-tube fittings are recommended.

By taking special care with the gas use system, specialty gases can easily be delivered to point of use without contamination to perform designated tasks.

CONCOA
1501 Harpers Rd.
Virginia Beach, VA, 23454