Q1 2009 / Specialty Gas Valves – Part 1: The spec valve difference: high performance for custom applications

All compressed gas packages share fundamental similarities – they consist of a compressed medium, they store gas in a cylinder, and they contain it and flow it through a valve. And, on the surface at least, a valve is a valve.

The typical industrial valves, also known as packed or o-ring valves, are the long-standing stalwarts of the gas industry. Most industrial valves use elastomeric o-rings or polymeric packing to prevent gas from escaping out of the top of the valve when the valve is open. These valves are excellent for many applications.

When contamination is a concern

However, certain elastomers and polymers can react with and therefore contaminate gases. True, in some cases the component contamination is negligible. For example, the helium that goes into a balloon does not need the same purity as run through lab equipment. Therefore in the specialty gas industry, purity is much more critical, and requires a much lower margin for eror. 

Analytical chemistry and electronic applications, for example, demand that the gas delivered, even if it is inert, be as pure as possible. The other concern with using the typical industrial valve for a specialty gas application is corrosion resistance. Highly corrosive gases can react with certain elastomers and polymers leading to high particle generation as well as valve malfunction.. 

Specialty gas packed valves

The specialty gas packed valve is a more advanced version of an industrial valve (Figure 1). These valves use specially formulated polymeric packings to seal the valve against the outside atmosphere. Since the simpler industrial valves have internal components that are not as resilient to corrosive gases the better packings in specialty gas packed valves can stand up to highly corrosive atmospheres and products that advancing technology increasingly employs. 

pecialty gas packed valves also offer improved leak performance. For example, industrial valves have a leak rate in the order of 1x10^-3 mbarL/sec He. This is roughly verifiable using a bubble test in water. Specialty gas packed valves, by contrast, offer a leak rate in the vicinity of 10^-5 or 10^-6. Since differences in leak rate measurements are exponential, any improvement is significant out in the field.

Besides greater resistance to corrosion, specialty gas packed valves also deliver higher purity levels. Still improvements continue to be sought since today’s industry requirements continue to demand ever greater purity levels from cylinder valves.

Diaphragm Packless valves

The next evolutionary step in achieving higher purity levels was the development of the diaphragm packless valve (Figure 2). These are also sometimes referred to as “spring diaphragm” or simply “diaphragm” valves. 

O-rings and packing can be a source of off-gassing, where chemical components of these construction materials can release and blend into the gas stream. In addition, some valve construction materials can absorb critical elements of the gas mixture itself which might alter calibration analysis. Again, with higher purity levels becoming ever more critical, even the smallest levels of chemical variability can be catastrophic to the integrity of a specialty gas system. 

 

There is always a tradeoff in applications

Of course all designs have their advantages and disadvantages. The diaphragm packless valve with its metal seal offers better atmospheric protection, greater purity, and improved tightness with a performance in the 10^-7 range or better.

The tradeoff, however, is that, this type of valve is not always the best choice for highly corrosive applications. Although with certain mixes at lower concentrations valve corrosion may not be a problem. However, when highly corrosive products are involved they can affect the function of the diaphragm packless valve. Highly corrosive products can attack the spring, for example. This results in particle generation and creates the possibility that the valve may not function properly during operation. If the spring does not force the seat up, the valve can essentially stick.

Some recommendations

Here’s the bottom line: Specialty gas packed valves are the right choice for highly corrosive applications. Choose diaphragm packless valves for applications involving poisonous gases, or those demanding higher purity levels where corrosion is not an issue. Their improved tightness will also help reduce the chance of fire and other industrial hazards whenever they can be used.

High purity with corrosive gas

There are some applications in which high purity is required with a corrosive gas. For these situations the tied diaphragm valve was developed (Figure 3). The tied diaphragm valve is not only ideally suited for the ultra high purity applications found in the electronics industry, but its superior leak integrity combined with low gas wetted volume makes it highly effective in high purity corrosive applications. 

It may sound trite, but it is important to have the right tool for the job. This is equally true for valve selection in the specialty gas industry. By selecting the valve that will perform best in your application, you can have confidence that the package you provide will deliver the utmost quality. SGR

All pictures provided courtesy of Ceodeux

William Hald is technical sales/key account manager at Ceodeux where he has worked with cylinder valves and equipment for specialty and UHP gases for ten years. Hald earned a BA from St. Vincent College in Latrobe, PA and attended Duquesne University’s graduate school of business. He can be reached at: hald.william@CEODEUX.com

Read Part 2: Specialty Gas Valves for specialized applications, by Sherwood Valve’s Michael Devitt