Risks Associated with Oxygen-Enriched Environments
Cleaning for oxygen service, or oxygen cleaning, is the process of contaminant removal from components to be used in oxygen service. An enriched oxygen environment (defined by OSHA as greater than 23.5% oxygen by volume) significantly increases the risk of ignition and fire. Contaminants such as metal shavings, lint, oil, and other particulates must therefore be removed from any surface of equipment in contact with an oxygen enriched environment. A loose contaminant in a piping system will travel throughout the system once pressure is applied. In an oxygen enriched environment, the particulate may strike the inside surface of the pipe and cause a spark. The end consequence could be a fire or an explosion.
Alternatively, and actually more commonly, particulates within the piping system can be ignited by adiabatic heat or friction. Adiabatic heat is the heat picked up by the oxygen from rapid pressurization of the piping system. We all are familiar with the heat that can be generated by friction. That’s why you rub your hands together to get them warm! Gas velocity, friction, adiabatic heat and contaminants within the system can all contribute to the generation of heat or become a source of ignition and subsequent fire.
Oxygen Cleaning Processes
Piping systems fabricated at FIBA are utilized in the medical, food processing, manufacturing, and other industries where a sterile product is critical. Cleaning for oxygen service not only eliminates a potential fire hazard, but also ensures the product is conveyed in its intended pure form. Any foreign matter in a hospital breathing machine or equipment used to process food is obviously unacceptable. Cleaning for oxygen service is not limited to oxygen service; it can be applied to other gases such as nitrogen, argon, neon, or krypton.
The most important aspects of oxygen cleaning are properly followed procedures and documentation. Only employees trained in these procedures may participate in the process. Established solution ratios, temperatures, pH readings and cleaning times must be strictly adhered to. To minimize contamination, oxygen cleaning is performed in a designated clean room. The enclosed room has systems to eliminate dust and other particulates. Precautions must be taken to avoid oil, lubricants and other manufacturing contaminants from entering the clean room on employees’ hands, gloves or tools.
Components that are to be cleaned for oxygen service first go through a series of pre-cleaning steps. This includes abrasive blasting, brushing, scraping, swabbing, or otherwise physically removing contaminants from the piece. Pre-cleaning can typically remove up to 75% of the contaminants. Based on the size and geometry, the next step is to clean the parts in an aqueous solution. Smaller components are submerged in an ultrasonic washer. As the name suggests, an ultrasonic washer radiates sound waves through a tank full of solution with alternating phases of high and low pressure. The low pressure produces millions of microscopic bubbles. This is referred to as cavitation. The high pressure phase collapses the bubbles releasing enormous amounts of energy. This release, repeated many times per second, dislodges contaminants from the component and suspends it in the solution. The microscopic bubbles, acting as scrub brushes, are able to reach all recesses and crevices. Depending on the soil level, parts may be submerged in the tank from 20 to 90 minutes.
Larger assemblies are cleaned in an agitating lift washer. Like the ultrasonic washer, the parts are submerged in an aqueous solution. An automatic lift then repeatedly raises and lowers the assembly. The resulting agitation is comparable to a household washing machine. The contaminants are released and suspended in the solution. A rinse cycle completes the cleaning process.
Inspection is a critical step in cleaning for oxygen service. Components are inspected under bright light to detect any presence of visible oil or particulate matter. Because many oils used in the manufacturing process exhibit fluorescence, black lights are utilized to examine surfaces. A bluish-white glow will indicate the presence of these hydrocarbon films. If any contamination is observed, the part must be re-cleaned.
NVR and Particle Analysis
Other inspection processes include, non-volatile residue (NVR) analysis and particle counts. NVR is a qualitative test using solvent extraction to supplement the visual inspections. NVR analysis is most effectively used when surfaces to be inspected are not accessible such as cylinders, tubes, piping or internal chambers. A solvent or water is typically used to extract and quantify contaminant levels within the parts being inspected. The components are typically cleaned and rinsed using a controlled process and then a secondary rinse is introduced to extract a controlled sample for analysis. This known sample size is filtered using a special membrane filter of known porosity and that liquid is heated until evaporated. After evaporation, the remaining residue is weighed using precision scales to detect and quantify the total contamination. Additionally, the membrane filter is analyzed under a high-powered microscope wherein the particles are inspected and quantified by size and concentration. The final results of both tests are then compared against the ASTM G93 specification and/or customer specification limits for compliance.
Maintain Cleanliness After Inspection
After an item has been cleaned and has passed inspections, precautions are taken to ensure that it remains clean and corrosion free until delivered to the customer. Pipe end caps and sealed plastic bags are often used. When practical, an assembly is pressurized with an inert gas such as dry, oil-free nitrogen.
As mentioned previously, proper procedures and documentation are the keys to a successful cleaning for oxygen service program. Procedures will help ensure a satisfactory finished product and the documentation provides a trackable written record to satisfy quality and environmental audits.