Twelve Rules – specialty gas systems
Larry Gallagher
Specialty Gas Products Manager
CONCOA
Process and analytical instrumentation used in today’s chemical processing units and quality control laboratories requires a continuous supply of high-purity carrier, support, process, or calibration gas supplied in a safe and cost-efficient manner that does not compromise purity and sample integrity. These goals can be met by a gas sampling and control system whose components are compatible with reactive or condensable samples and the conditions imposed by a specific application.
The most important factors to consider when developing the specifications for a gas delivery system include the materials of construction and the pressures to which they will be subjected. It is extremely important that all surfaces with which the gas comes in contact be compatible with the gas. Should a reaction occur, the component may deteriorate and ultimately lead to leakage.
Improved accuracy, service life, and safety are achieved when materials are properly specified. Poor specifications can result in invalid results, increased costs, or even create a dangerous condition.
When considering materials compatibility for calibration or process gas applications, it is necessary to look at the type of metal seal components, surface plating or treatment, and polymer or elastomer components. The results of selecting the appropriate materials are increased accuracy in calibration, increased service life of components, and increased safety. Selecting the wrong components can result in invalid results, increased costs, and unsafe operating conditions.
A review of the general compatibility principles should serve as a guide in selecting the appropriate pressure or flow control systems. For specific gases or conditions, however, you should always consult with the manufacturers and review their various tables of materials compatibility.
Following are the twelve rules that should act as a guiding principle in determining what type of gas control system is right for your customer.
Metal Components
Rule 1: Metals are not all created equal. Be careful in their selection.
This rule applies when considering the difference between forged metal parts or machined bar stock components. For high-purity and high-integrity systems, metal components that come in contact with the gas stream should be made from machined bar stock, not forgings. During the forging process, the metal becomes more porous – both the metal itself and the resulting surface. The greater porosity permits impurities to be trapped within the surface, and could result in particulate generation. Conversely, bar stock has a much smoother surface finish, which greatly reduces the possibility of contaminants becoming embedded.
Rule 2: Stainless is usually the best. Use it where greater corrosion resistance is called for.
The 455 Series Regulator (Figure 1) features internal Hastelloy components and Monel gauges, which ensure greater corrosion resistance as compared to 316L stainless steel.
In some applications, materials that are equal to or superior to 316L can be used. These include Hastelloy for internal components and Monel for gauges. (See Figure 1.) Both of these materials, which are high nickel alloys, have improved corrosion resistance and are superior to 316L stainless steel. However, their high cost usually limits their use to special parts or applications that demand these properties.
In the high-pressure end of pure oxygen systems – specifically cylinder hoses or pigtails – Monel is required. Stainless steel in a high pressure oxygen system is considered a safety hazard and Monel is required not just preferred.
Rule 3: Use brass for most inert applications.
While this is true, the material should be a low copper alloy bar stock. It is worth noting here that acetylene forms reactive acetilide by-products if exposed to copper. When piping acetylene, stainless steel, or carbon, the material used should be steel.
Rule 4: Consider using aluminum, which is suitable for certain mildly corrosive applications.
Aluminum is suitable, particularly for anhydrous ammonia applications; however, the aluminum must be anodized. Its low cost and ease of machining make it an attractive material. However, it is not a replacement for 316L stainless.
Weigh the Advantages of Surface Plating versus Treatment.
Rule 5: Use common sense when you consider chrome-plating as a means to improve corrosion resistance.
Rule 6: Consider nickel plating to improve corrosion resistance in acid gas applications.
Electroless nickel-plated brass is preferred when handling highly corrosive acid gases.
Rule 7: Always passivate fluorine systems.
Fluorine systems must be passivated prior to use. This process creates an impervious metal fluoride layer that controls exposure of the wetted surface to low levels of fluorine. This procedure should be performed only by the manufacturer. Untreated surfaces will ignite when exposed to fluorine.
Rule 8: Ensure the integrity of low level reactive mixtures by glass-coating or surface-inerting.
The blue-to-rainbow finish on the 316L regulator body shown in Figure 2 is the result of a chemically deposited silicon glass layer.
To ensure the integrity of low part-per-million mixtures of reactive gases (hydrogen sulfide, reduced sulfurs, ammonia, or nitric oxide), a regulator that has a chemically deposited layer of silicone compounds, such as the Silicosteel treatment is essential.
Polymer versus Elastomer: Know their Limits
Rule 9: Avoid the use of elastomers, which are poor substitutes.
Butyl rubber, neoprene, and Buna-n are examples of elastomers that are often used in industrial regulators or pressure control devices. The application of regulators with these coatings in high-purity or reactive gas systems must be avoided. They are prone to off-gassing impurities, and they also diffuse moisture and air into the gas stream.
Rule 10: Since fluoropolymers are generally suitable for high-purity systems, use them with discretion.
The seat material in regulators used in high-purity systems should be PTFE (Teflon), PCTFE (KelF), Kelrez, or similar inert polymers. They are resistant to nearly all reactive gases and provide an inert seal. Although they are suitable for internal components, caution should be applied when considering them for tubing or high-pressure pigtail applications. In both cases, they may diffuse materials across these barriers.
Rule 11: Consider the operating temperature of the system.
This vaporizing 452 Series regulator shown in Figure 3 operates at temperatures as high as 380F (193C), and can be applied with confidence because its seat material is made of Arlon (PEEK).
Rule 12: Consider the operating pressure of the system.
In addition to ensuring that the manufacturer certifies that the system or component is suitable for the operating pressure, examine the seat material to ensure that either PCTFE or PEEK is used for higher inlet pressures – 4,500 psig and above.
Larry Gallagher is the specialty gas products manager for CONCOA, Virginia Beach, Va., manufacturer of gas pressure and flow control equipment for industrial, medical, and specialty gas applications, as well as distribution systems for laser materials processing. For additional information call +1 800-225-0473 visit: www.concoa.com.
1501 Harpers Rd., Virginia Beach, Virginia 23454, USA
