Cleaning for Oxygen Service
Do we clean our components for oxygen service?
It’s a question we hear all the time. Often it is because our customer will use the regulator or valve in oxygen service, but not always. Many times, our customers will request O2 cleaning even though their media is anything but oxygen. This level of cleaning has become a benchmark in the specialty gas market for the specific level of cleanliness of wetted flow path parts. Primarily, if a regulator or valve is cleaned for oxygen service, it means that all components in contact with the process media are free of hydrocarbons (oil, grease, and other contaminants), as well as particles large enough to causes pars that can lead to fires.
ASTM G – 93 and CGA G-4.1 are standards defining what methods and equipment are suggested to achieve and verify a specific level of cleanliness for oxygen service. These standards do not attempt to identify a specific construction that is suitable for oxygen service (acceptable materials of construction for oxygen service are covered by other industry standards).
Instead, they recommend the methods for cleaning, and specify the methodology for verifying the resulting level of cleanliness of piece parts and assemblies. ASTM G-93 defines several levels of cleanliness for various applications for equipment used in oxygen-enriched environments, along with the methods used to verify these levels of cleanliness. Tescom uses ASTM G-93 as its standard for cleaning. In fact, any component that we sell through the industrial division is packaged with a label that includes the following:
“This product is cleaned in accordance with ASTM G – 93 and CGA G-4.1 (This assures removal of visible particles and combustible residues).”
Kindling Chain Ignition
So what does this all mean to the end user? Essentially, through a series of cleaning and evaluation steps, the component has been cleaned, assembled, and packaged in a controlled manner that assures the end user that no particles or combustible residues are present on the wetted surfaces. These particles and combustible residue can then act as ignition mechanisms for an oxygen fire under the right conditions.
Every day, someone learns about the need for leaning components used in oxygen service the hard way Most common among the metal working industry, oxygen fires often occur in the regulator set of an oxy/acetylene welding rig. The welder may have carelessly tossed the regulator set on the ground when changing cylinders, or perhaps didn’t cap the end of the cylinder after a long day of welding, allowing dirt to enter the outlet of the cylinder valve. When the regulator is connected to a cylinder of oxygen, and the cylinder valve is opened, high pressure oxygen flows into the regulator at a high velocity.
The oxygen reaches the main valve of the regulator, which is usually in the closed position. In this situation two opportunities for an oxygen fire exist. The first is from the impingement of dirt and other particles moving at high velocity within the component. The second is due to heat of compression. Heat is generated from the conversion of mechanical energy when a gas is compressed from a low to a high pressure. High gas temperatures can result if this compression occurs quickly to simulate near – adiabatic conditions.
This temperature rise may be high enough to cause the plastic seat or o-ring material in the regulator to ignite. Plastics and elastomers have what is called an auto- ignition temperature that refers to the temperature at which the material will ignite in the presence of 100% oxygen. A seat fire or o-ring fire will promote consequential burn of the surrounding metal. This condition is often referred to as promoted ignition or kindling chain ignition.
Avoiding a fire
Impingement occurs when a particle or particles entrained in the gas are moving at a high velocity through the component, and begin hitting a wall or other fixed object in the body of the component. When the particle impacts the body of the component at high velocity, there is a chance that a spark is created at the moment of impact. This spark can trigger a fire, selected fueled by the body material, and supported by the oxygen – rich atmosphere. For metals, there is a property called Absolute Threshold Pressure that refers to the pressure at which the metal will support self-sustained combustion in oxygen atmospheres. All of the 300 series stainless steels have threshold pressures near 500 psi while aluminum supports combustion at pressures as low as 35 psi. + Once the oxygen fire begins, it continues until the system pressure is reduced to a point where combustion is no longer possible. This usually occurs when the fire has burned through the wall of the regulator resulting in a dangerous explosion of fire and material.
When looking for a component to be used in an oxygen environment, choosing plastic parts with high auto-ignition temperatures and metals with higher threshold pressures will substantially reduce the potential for oxygen fires. Brass and Monel are commonly used metals while PCTFE and Viton® are commonly used soft goods. For more information about oxygen-compatible constructions, please contact Tescom engineers for assistance.
We’ve discussed the need for special cleaning for oxygen service, but why would someone specify this level of cleaning for applications of other gases? There are no other industry standards that quantify a level of cleanliness as specific as cleaning for oxygen service. It is the benchmark of clean, you might say. So, rather than spelling it out in a long, windy text specification how the device should be cleaned, verified, and packaged, the customer simply requests cleaning for oxygen service. Of course, this by no means implies that any component cleaned for oxygen service is suitable for oxygen service, but we now know that the device is hydrocarbon and particle free, making it suitable for all manner of analytical, pharmaceutical, and industrial applications. Tescom’s Industrial Controls Division cleans and packages all pressure regulators and valves for oxygen service as a standard feature of our product offering. There is no need to specify it separately.
[Courtesy: Shavo Technologies Pvt. Ltd.]