Considerations regarding the choice of materials in applications that use oxygen

by Mitch Spronck

Usually, we consider oxygen to be pretty harmless. It’s an abundant element as it accounts for about 21 % (volume percent) of the atmosphere that surrounds us. As mammals, we breathe the air that contains oxygen, plants produce it and even most bacteria rely on it for energy production.

Oxygen and the combustion triangle

However, oxygen is not all fun and games. It can be toxic at increased partial pressures, it is the reason that metals can become rusty (oxidation) and oxygen is one of the three components of the combustion triangle – besides heat and fuel. Oxygen (just like other oxidizers) lowers the ignition temperature of  fuel, making it easier to light. This article will focus on that last point and will explain why oxygen can be extremely dangerous in environments where stainless steel is used.

Don’t play with fire

Say you want to light a candle. Whether you use a match or a lighter, you will need fuel, heat, and oxygen. In order to light the match, you rub the match against the side of the matchbox and whoosh…fire! Using a lighter is even simpler: Just press the button and there you go, fire.

Considering how simple it is to make a fire with 21 % (volume percent) oxygen, we can extend this exercise to making a campfire. This time, we will not use a match or lighter, but we will start the fire ourselves. What do we need?

• Two branches
• Some dry tinder (not the app …) or some other easily flammable material

By rubbing the two branches together, enough friction (heat) is created to ignite the tinder. This can in turn be used to ignite the bulk material (in this case wood), producing a fire. In order to keep the tinder ignited, blowing on it is a commonly used technique since the air we breathe out also contains about 17% oxygen and will lower the ignition temperature by bringing fresh oxygen to the source.

For the next example, you will need a bit of imagination. Imagine a large hermetically sealed room with pure oxygen in it; so instead of air, which contains nitrogen, argon, carbon dioxide, etc., it is just oxygen. Now imagine that the inside of this room is built completely from stainless steel and there is a bullet shot into the room. The bullet will ricochet off the wall somewhere and can create a spark in the process. The energy in the form of a spark (heat) together with the stainless steel (fuel) and the oxygen, could be sufficient to raise the ambient temperature to the auto-ignition temperature of the bullet or steel, thereby completing our combustion triangle and creating a huge explosion.

Risks in stainless-steel applications making use of oxygen

Moving our attention back to the real world, we can extend this last thought experiment to processes in which stainless-steel and pure oxygen are used. You can imagine that it may have the same effect when a small piece of debris is flung into a stainless-steel tank, piece of piping, or –   the reason we like to consider this topic in so much detail – a back pressure regulator. This small piece of debris, just like the bullet in the chamber, could create a spark.
Although one of the most likely causes, this is not the only cause of increased temperatures and ignition. Several other factors can contribute to the auto-ignition of steel (or related materials):

• Flow velocity: The velocity of a fluid through a valve cannot be over 61 m/s (200 ft/s) when steel is used. This is due to the fact that velocity increases the so-called kinetic energy of the fluid in question. Kinetic energy is defined as the energy that an object possesses due to motion and can be converted into thermal energy (heat). When this happens, the threshold for auto-ignition could be overcome and a fire or explosion could occur.
• Vibration: Internal friction through vibration can cause sufficient heat for the auto-ignition of certain materials. Any contact between fast-flowing fluids and solid objects (piping, walls, regulators, valve, etc.) will cause friction. Friction can – as well as increased flow velocity – be seen as a form of kinetic energy and can therefore also be converted to thermal energy.
• Adiabatic or rapid compression of gas: Rapid compression of gases can cause abnormally high gas temperatures, which can, in turn, ignite materials in the valve or piping. In the early 19^th century, Joseph Louis Gay-Lussac, a French chemist-physicist discovered that the temperature and pressure of a gas (of fixed mass and volume) are directly proportional to one another. In other words, when the temperature of a gas rises, the pressure does too, and vice versa. When gas is compressed very quickly, all atoms or molecules will collide more quickly with each other, increasing the – you guessed it – kinetic energy.

In summary, as harmless as oxygen may seem, using stainless-steel regulators for pressure control in applications that use oxygen is not without risk. Fortunately, other types of materials can be used for applications that operate with pure oxygen to overcome this risk.

Material selection for pressure control applications using oxygen

One of the best and most widely used materials is Monel, which is a nickel-copper alloy. Other advantages of Monel over stainless steel are its high resistance to corrosion and ability to remain ductile and not become brittle, even at extremely low temperatures.

Similarly, seals that usually are not made from metals have certain prerequisites. It is recommended that all polymeric materials are made from fluoroelastomers. This means that commonly used materials like FKM (Viton ©), FFKM (Kalrez ©), PTFE, etc. can be used in this kind of application.

Lastly, all parts that come in contact with a medium like pure oxygen have to be degreased to minimize ignition risks. This procedure is applied to remove any greases or similar compounds, eliminating any substances that can easily ignite.

In conclusion, choosing the appropriate materials and applying adequate degreasing procedures is the best way to mitigate risks involved with the use of oxygen in your pressure control application. Contact Pressure Control Solutions if you would like to discuss material selection for your application or read more about material selection for pressure regulators on our Spare Parts and Maintenance page.