David Escobar, Director of Engineering at Metso Automation
Industrial oxygen is used for many purposes: in a basic oxygen furnace for making steel, water pollution countermeasures, including sewage treatment, habitability and superfund site rehabilitation, and chemical processes such as production of vinyl chloride, nitric acid, epoxyethane and hydrogen peroxide. It is also used for medical treatment, life support in harsh environments and industrial gasses for welding and other processes.
The production of oxygen has risen from approximately 470 billion cubic feet in 1991 to over 1.5 trillion cubic feet in the U.S. and more than 4 trillion cubic feet globally in 2014.
Oxygen is the most common oxidizing gas and is, of course, highly reactive. When dealing with an oxygen-enriched environment, it is important to control the sources of ignition. Ignition can be caused by many things, among them:
- Electrical arcs, which can come from electrical equipment or even static discharge
- Friction, which can be generated by the sliding contact of materials within the oxygen-enriched environment
- Impact of particles or projectiles internal or external to the enriched environment can generate heat
- Resonance, which is vibration-induced heating
- Heat of compression (HoC) is the most common cause of explosion due to contamination. Heating is caused by the adiabatic compression of a fluid; this is often called auto-ignition.
Auto-ignition is the phenomenon of spontaneous ignition of a fuel source due to the heat generated by the sudden compression of a gas or HoC. When a valve in a high-pressure or high-velocity oxygen flow is opened or closed quickly, the kinetic energy is converted to increased temperature and potential energy in the form of increased pressure. If the temperature generated by the compression exceeds the temperature needed to ignite non-metallic seals or even the pipe itself, the result is a spontaneously explosion or auto-ignition. When this happens in oxygen systems, the effect can be devastating.
Because the HoC is substantial and can generate thousands of degrees of temperature even at moderate pressure ratios, oxygen systems are designed to limit the pressure drops to control HoC and limit temperature within the autoignition temperatures of the system components.
Thus, it is absolutely essential that contaminants, which can introduce lower auto-ignition temperatures than even the non-metallic seats and seals, be removed from any oxygen system. Any method that achieves the desired cleanliness level is acceptable. CGA 4.4 and the recently issued MSS-SP-138 provide excellent recommendations for cleaning processes.
CONTAMINANTS TO BE REMOVED
Basically, anything that promotes combustion or impact product purity is considered a contaminant. ASTM G93 categorizes contaminants into three types:
- Volatile Organic Compounds (VOC)
- Hydrocarbon-based greases and oils
- Water-based detergents and cutting oils
- Particles, lint and fibers
- Dust – Weld slag, etc.
Specifications vary on cleanliness level, methods and validation, and include how much residue is acceptable, what method of cleaning can be used and what kind of inspection must be conducted.
Mechanical cleaning is used to remove scale, coatings, paint, weld slag and other solid contaminants and can include grit or ice blasting, wire brushing and grinding.
Aqueous cleaning can be with hot water and steam cleaning or alkaline cleaning. Hot water and steam cleaning is effective against water-soluble contaminants, and is normally used with detergent. Alkaline cleaning uses caustic salt in water to create a highly alkaline solution. It is effective against hydrocarbon oils, grease and waxes, and generally is enhanced by agitation and/or jet spraying. Typically this is used for industrial parts washers. This process is greatly enhanced by ultrasonic agitation, but the solvent residue must be removed as well.
Semi-aqueous cleaning uses hydrocarbon solvent and water emulsion, which is effective for removing heavy contaminants from parts like heavy grease wax or hard to remove soils. Emulsion may require agitation to maintain the mixture, and parts must be rinsed before the emulsion can dry. Otherwise, contaminants may re-deposit on the part that was cleaned.
Acid cleaning varies substantially based on the acid used.
- Hydrochloric acid is used to remove scale, rust and oxides. and to strip platings (chrome, zinc, cadmium, etc.) and other coatings
- Chromic and nitric acid are used to for passivating, deoxidizing, brightening and removing alkaline residues in addition to cutting oils
- Phosphoric removes oxides, light rust and fluxes
Acids must be removed completely from the part prior to drying and, depending on the acid strength, may need a neutralizing process.
Solvents can be used without water dilution or emulsion. Alcohol is a common solvent often used to revisit areas of concern identified by black (UV) light inspection. Solvents like alcohol evaporate completely, leaving no residue.
Vapor degreasing is a process in which a solvent is heated until it vaporizes, while the part is maintained at a lower temperature. The solvent then condenses and dissolves contaminants. The part must be oriented so that the condensed solvent can drain from the part by gravity. This method is very effective for inaccessible areas on parts but requires a contained environment for the part during the process.
Any combination of cleaning methods that achieve the desired cleanliness level is acceptable.
Visual inspection can be direct, including white light, which is effective in detecting contamination down to 500 mg per square meter. UV (black) light visual inspection identifies contaminants that fluoresce and is effective in detecting contamination down to 40 mg per square meter.
Indirect visual inspection is done in two ways: wipe test and solvent filtering. A wipe test can identify contaminants in locations that have no direct line of sight. Typically, both white light and UV light are used on the wipe cloth, and are effective in detecting contamination down to 30 mg per square meter. Solvent filtering rinses the inaccessible area in solvent, which is then filtered to capture contaminants. The filter is then visually inspected and can detect 100 ml per square foot of low residue solvent and it also uses white and UV light.
Quantitative inspection is done by evaporating the solvent used for cleaning and obtaining the weight of the remaining effluent. Acceptable levels of residue vary according to user requirements.
Clean room: This provides a designated location where the environment limits dust airborne particles, where clean tools and clean assembly and test equipment can be stored. It can also provide controlled lighting for visual inspections.
Clean test equipment: Pressure test equipment contains contaminants in hoses and pumps. If a test machine cannot be dedicated for clean testing, give special consideration to cleaning of test equipment or alternate testing with clean gas.
Packaging: After cleaning, give specific instructions on how to package the product to preserve cleanliness in shipping and subsequent storage. Consider the role of desiccant as a possible contaminant. Use compatible products or control desiccant to prevent contamination. Consider the addition of actuation and accessories to the valve. Can the actuator be installed and set up without violating the protection? If the protection is compromised, are there procedural steps to identify and remediate any contamination?
Oxygen cleaning is used to remove contaminants that can significantly reduce the temperature of auto-ignition. There are many methods for doing the actual cleaning. CGA 4.4 and the recently issued MSS-SP-138 provide excellent recommendations, but any method that achieves the desired cleanliness level is acceptable. It is important to know what level of cleanliness your standard process produces. Process validation using a quantitative measurement allows the supplier to have confidence in process quality when using qualitative inspections for production work.
Editor’s Note: The following article was posted on April 20, 2015 in Valve Magazine.com. It is reprinted here for the Reaction Research Society (RRS) with permission from the author and Valve Magazine. The information here is very useful in amateur rocketry and is intended to make our readers aware of the importance of a proper oxygen cleaning process for lines and valves. High purity oxidizers must be handled with care and cleanliness is of paramount importance. The RRS would like to thank David Escobar of Metso Automation and Judy Tibbs, Director of Education at the Valve Manufacturers Association and Editor in Chief of VALVE Magazine.
David Escobar is director of engineering at Metso Automation. Reach him at email@example.com.
CGA refers to the Compressed Gas Association. Founded in 1913, the CGA is an organization dedicated to the development and promotion of safety standards in the industrial, medical and food industry. The CGA is comprised of over 110 member companies worldwide working together through the committee system to create technical specifications, safety standards and educational materials; to cooperate with governmental agencies in formulating responsible regulations and standards; and to promote compliance with these regulations and standards in the workplace.
For more information, go to the CGA website:
MSS refers to the Manufacturers Standardization Society of the Valve and Fittings Industry. Standard practices (SP) documents are available related to many applications including the standardized practice of oxygen cleaning (ANSI/MSS SP-138). ANSI or the American National Standards Institute has adopted the standard for oxygen cleaning of valves and fittings.
ASTM stands for the American Society for Testing and Materials. It is now an international organization known simply as “ASTM International” with its headquarters in West Conshohocken, Pennsylvania. ASTM publishes voluntary consensus technical standards including ASTM G-93 for the Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments.
For more information, go to the ASTM International website: