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Cylinder Valves For Highly Reactive and Oxidizing UHP Gases

By Paul Kremer, William Hald, Robert Newton
Reprinted with permission from Gases & Instrumentation™ Volume 1 / Issue 3
November/December 2007

Proper valve design can result in safe operations for fluorine gases and their derivative compounds.

For the compressed gas industry, the many varied applications employed by customers necessitate supplying a wide variety of gases. We deal with gases that are as innocuous as the air we breathe (albeit better in some areas than others) and other gases that are extremely toxic and reactive. As many of us in the industry are aware, fluorine and its derivatives (NF₃, ClF₃, etc.) are highly reactive oxidizing gases, but have essential roles in many applications as fluorinating agents. Substances such as these are both highly dangerous and highly useful. In order to make the best use of fluorine and other fluorinated. products, design maximizing product safety and efficiency is key.

The high reactivity and oxidizing properties of fluorine make it a valuable part of etching and cleaning applications. Active fluorine is released only when a sufficient level of energy is applied to a system. Once the reaction is initiated, the reaction is self-propagating, and presents a hazard for any material that is not compatible with fluorine. Fluorine, in fact, “… is the most reactive member of the periodic table,” and “…is the most powerful oxidizing agent known.” [1] There have been several instances of flash fires involving equipment containing fluorine mixtures at elevated pressure. In all of these cases, a plastic material (e.g. valve seat, sealing washer) burnt. In some of these fires, the metallic parts that were in contact with or surrounding the plastomers also ignited. The pressure of the system even projected burning materials into the surrounding area—a major safety hazard.

In order to best explain both the risks inherent with fluorine and its derivatives, and the best way to handle those risks, it helps to begin with the fundamentals of what is involved with a fire. As many of us know, even as far back as grade-school fire presentations, there is a “triangle of fire”. The triangle of fire explains what is required for combustion to occur. More recently, a fourth element, an uninhibited chemical reaction, has been added. This creates a “fire tetrahedron” or “ fire pyramid”. (See Figure 1)

The four elements are thus:

The presence of oxygen
A combustible material in contact with the oxidizer to act as fuel (e.g. stainless steel or PCTFE)
A source of ignition energy or heat
Uninhibited chemical reaction

This model is especially relevant to the gas and chemical industries when dealing with reactive/oxidizing gases, such as fluorine and its derivatives. The chemical reaction component involves the return of heat to the fuel to maintain the fire. Such chemical reactions between materials can create the heat energy needed for the fire. These same materials are also available as a fuel to be combusted.

If any one of the parts of the tetra-hedron is eliminated, the fire will not be able to sustain itself. From a safety standpoint, if we in the gas industry can limit the strength of any of these three elements we are reducing both the risk of ignition or, at least, the potential severity of a fire. How can this be done? Let us briefly consider the model at hand.

While removal of oxygen is an excellent firefighting method, it is probably not practical to encase all fluorine systems in a vacuum environment; so it will be best to focus elsewhere. What remains are heat energy and fuel, which as the tetrahedron model explains, interact with each other to be both heat source and fuel. Thus, it is worth further examination of how the components of a fluorine system interact and cause combustion.

First, consider the gas that is being used. The gas can be a factor depend-ing on pressure, decomposition temperature (for compounds), and the velocity in the pipelines. The greater the pressure of the gas, the higher the potential energy of the system will be. For compounds, such as fluorine derivatives, the temperature at which the molecule breaks down into its constituent elements is also important. Namely, is the gas more or less volatile once the compound separates into its individual elements? The velocity in the pipelines is a factor on several fronts as it affects how the gas will react with surrounding materials. High gas velocity will create heat by particle impact or flow friction on the material, especially in tortuous passages and/or small crevices. This generation of heat may initiate a local combustion if the auto-ignition temperature of the material in contact with highly oxidizing gases (NF₃,ClF₃,F₂) is reached.

Furthermore, adiabatic compression may occur for example, when a valve is opened quickly and the system is rapidly pressurized with gas. This sudden increase in the pressure of a highly oxidizing gas will result in a rapid temperature increase. At very high rates of temperature increase, there may be insufficient time for heat exchange to take place with the materials in contact with hot gas, thus initiating the auto-ignition of a non-metallic material.

Next, the materials used in con-structing the handling systems should be reviewed. This is especially pertinent to our discussion as the gas in the system is a constant, but we may have the freedom at times to choose different materials. One of the factors to consider in construction materials is the specific heat of the material. That is, how much energy is required to raise the material’s temperature? Metals and polymers that have higher specific heats will be much more resistant to combustion as they will require more external energy to raise their temperature. Likewise, the heat of combustion for the mate-rial is important. This is the amount of heat that a substance gives off when combusted/oxidized. Heat of combustion is critical since the more heat given off, the more heat energy that is added to surrounding materials, put-ting them at risk for ignition. We must also consider how prone the material is to self-propagation of ignition (self-combustion).

Beyond the chemical properties of the materials used in construction, there are physical properties to con-sider. What is the size and shape of the material being used? How much is being used in the system? What is the cleanliness of the material? Dirt, oils, and other impurities may present combustion risks as particulates released into a system can be a source of fric-tion or be chemically reactive. Also, how mechanically stable is the system as a whole? Mechanical shocks (e.g. rapid operation of flow control equipment) may also result in the generation of heat, potentially igniting sensitive materials such as plastomerics.

After considering all of this information, it is clear that we must be cogniscent of the large number of risk factors when dealing with gases such as fluorine and derivatives thereof. In the past, a “one size fits all” mentality dominated gas handling design. Valves, regulators, and other equipment were designed to perform well in most applications. Over time, experience has shown that there are gases that have a personality all their own, and require special design considerations. Take, for example, something we see in the world of sports quite often. A football team will often prepare special defensive schemes to counter a dangerous player on an opponent’s team in order to minimize the player’s effect. We see two defenders assigned to great receivers, extra linebackers to stop great runners, etc. One may have a good team, but there are some special players who deserve extra attention. Likewise in the gas industry, we must consider our best possible defense for certain dangerous gases like fluorine and its derivatives. When addressing these highly reactive and oxidizing gases, some of the design considerations we have found beneficial are:

Correct choice of materials for construction. Metals and polymers that are resistant to the effects of these gases and possess strong thermochemical properties are the best choice. For example, nickel is especially beneficial when compared with stainless steel as it is less reactive and has a higher ignition temperature.
Minimization of the effects of adiabatic compression. This can be accomplished by various internal mechanical design features.
Gas velocity. Again, integrating features into the design that keep velocity as low as possible.
Cleanliness and passivation. It is critical to have good internal processes (clean assembly environment, quality control, etc.) and cleaning to eliminate foreign substances that may react with the gas or damage internal parts.
Sealing technology. Use sealing technology such as tied diaphragm seals to reduce particle generation. By contrast, spring diaphragms have more moving parts, and a greater risk of particle generation.
High tightness. Use seals that provide low leak rates to the atmosphere, thus reducing the risk of reactions with ambient air.
Easy operation and safe handling. Design products with user safety in mind. Complicated operating procedures can introduce user error.
Low maintenance. Design products with high cycle life to reduce the possibility of failure in service.
High flow performance. This will allow for ease of filling and withdrawal as well as reducing gas velocity and adiabatic compression.
Minimum gas wetted volume. Use seals such as the tied diaphragm design that feature low dead space. Gas trapped in dead space can react over time with atmospheric gases and moisture. Spring diaphragm valves have a higher wetted volume.
Resistant against impact. Sturdy construction that will not fail easily during transport or use will protect against impact damage that could compromise safety.
Secondary sealing mechanism. This is necessary for increased For increased safety in the event of an emergency or failure.
Manual and pneumatic actuation.

Today’s marketplace requires both options be available, and must be accommodated in design.

Taking into account all of these considerations, a valve can be designed that maximizes its use for gases such as fluorine, NF₃, and silane. (See Figure 2). Such a design should incorporate internal gas wetted parts constructed of nickel, with its high auto-ignition temperature. Also, a low gas velocity should be utilized. For example, the valve shown in Figure 2 has a high flow rate, with a Cv of 0.55 to reduce gas velocity. The spindle features an integrated central pin that aids in the transfer of heat energy, also reducing auto-ignition. Users should look for a secondary metal seal in the valve for safety in case an ignition was to take place and a seat designed with a minimal amount of the soft seat disc exposed to the gas.

Safety in the workplace is an ever-growing concern, as accidents not only have huge ethical implications, but financial fallout as well. By working with gas suppliers who have the needs of end-users in mind, gas handling equipment suppliers are able to develop solutions that maximize safety, efficiency and quality.

Yaws, Carl L. Matheson Gas Data Book. Seventh Edition. New York: McGraw-Hill, (2001) p.386.
What are the Fire Triangle and Fire Tetrahedron? National Fire Prevention Agency. January 5, 2007. http://www.nfpa.org/itemDetail.asp ?categoryID=266&itemID=18935&URL=Research%20&%20Reports /Charles%20S.%20Morgan%20Library/FAQ/facts%20and%20lore#3

Paul Kremer is the senior R&D Manager for Ceodeux s.a. (rotarexgroup), 24 route De Diekirch, L-7505 Lintgen, Luxembourg. educated as a Mechanical engineer, Paul has 11 years experience in R&D for development of valves and pumps for the chemical industry. in addition, he has spent 25 years involved in the development of valves and regulators for industrial and semiconductor gases. he can be reached at +11 352 32 78 369, or by e-mail at kremer.paul@rotarex.com

William Hald is technical sales/key account Manager at Ceodeux, 221 Westec Drive, Westmoreland Technology Park I, Mt. Pleasant, PA 15666. he has been with Ceodeux for nine years. he has spent his tenure there handling Cylinder valves and equipment for specialty and UHP gases. Bill earned a B.A. from St. Vincent College in Latrobe, PA in 1998, and attended Duquesne University’s graduate school of business. he can be reached at 724-696-4340 or hald.william@ceodeux.com

Robert Newton is product manager, specialty gas valves for Ceodeux, 221 Westec Drive, Westmoreland Technology Park I, Mt. Pleasant, PA 15666 he can be reached at 724-696-4340 or newton.robert@ceodeux.com.