Materials become easier to ignite because their flammable ranges start to expand and their autoignition temperatures begin to drop in the presence of an oxygen-rich environment. This is a concern, since the residue could spontaneously auto-ignite and cause an explosion. So, it is important to ensure all oxygen system components like pipes, hoses, tubes, regulators and meters are all clean and dry. Oxygen system components are often dirtied during the manufacturing process and must be properly cleaned before inspection, assembly and packaging.
A contaminant is basically anything that could potentially cause combustion, autoignition or affect the purity of the oxygen system. Possible contaminants typically fall into three categories. Organic hydrocarbon-based greases and oils. Inorganic pollutants such as nitrates, phosphates, water-based cutting oils and other acids and solvents. And lastly, particulate including things like lint, dust or welding slag.
Multiple cleaning methods to choose
Most gaseous oxygen system components are typically cleaned and inspected to meet rigorous industry quality standards, including ASTM G93 – 03(2011), the Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments.
Depending on the soil encountered, there are different ways to effectively clean oxygen system components.
- Mechanical cleaning uses wire brushing, sandblasting or grinding to remove scale, paint, coatings or welding dross
- Aqueous cleaning, with or without detergents and spray agitation, typically uses very hot water or steam to remove water-soluble contaminants. However, the detergent must be rinsed off and the parts dried before further processing
- Alkaline or caustic cleaning using caustic salt dissolved in water removes grease, wax and hydrocarbon oils. The caustic solution is applied using swabbing, spraying or immersion, but the cleaning solution must be rinsed well to prevent parts damage
- Semi-aqueous cleaning uses a water emulsion to clean heavy waxes and greases and other hard-to-clean soils. This method typically requires agitation to retain the fluid mixture, and again the emulsion must be rinsed from the parts using polished water
- Acid cleaning with hydrochloric phosphoric acid removes rust, scale and other oxides or strips plating and coatings from parts. However, in all instances the acid solution needs rinsing and may also require neutralizsation.
Vapour degreasing: A better, higher purity cleaning alternative
Vapour degreasing is a lesser-known but highly effective method for cleaning oxygen system parts. Vapour degreasing cleans, rinses and dries parts in just one step inside a single machine.
Parts are placed inside a vapour degreasing machine filled with a solvent-based cleaning fluid. Most solvent-based cleaning fluids are a mixture of compounds that can include hydrocarbons like mineral spirits, isopropanol and ethanol. Depending on how the compounds are combined determines the cleaning fluid’s effectiveness and its material compatibility. The vapor degreaser can use just one type of cleaning fluid or can be mixed, blended or custom formulated to remove a specific soil from a specific substrate, maximising cleaning effectiveness.
Parts are immersed in the continuously filtered and distilled cleaning fluid to dissolve the soils from the parts surface. In some instances, ultrasonic agitation is added for additional cleaning muscle. No additional detergents or additives are needed to dissolve the soils. As the parts are removed from the cleaning fluid, they undergo a brief vapour rinse and drying process. The cleaning fluid condenses and drips back into the vapour degreaser to be reused.
The vapour degreaser reuses and recycles the cleaning fluid for hundreds of times before it needs to be refreshed or replaced. This helps reduce the cost hazardous waste removal. The parts come out clean, rinsed, dried and ready for inspection, assembly and packaging.
Since the cleaning, rinsing and drying are done in one machine, the vapour degreaser cleans a high volume of parts quickly (about 6-20 minutes). Some of the more popular cleaning fluids used in production facilities include n-propyl Bromide (nPB), Trichloroethylene (TCE) and Perchloroethylene (Perc). However, many companies are moving away from these older solvents to improved, updated cleaning fluids. The new fluids clean just as effectively, but without the worker safety or air quality concerns of the older solvents. There are a number of good sustainable, low GWP (global warming potential) and zero ODP (ozone depleting potential) cleaning fluids on the market today.
Some real cleaning advantages..
- Lower surface tension for thorough cleaning: Vapour degreasing fluids have a lower surface tension and are less viscous than water. This means the liquid solvent and the solvent vapours more readily get into, and even more importantly, evaporating out of blind holes, end holes and other tight spaces on the parts
- Lower temperature cleaning: Modern vapour degreasing fluids are ideal for use on a variety of materials including stainless, steel, copper and low carbon steel. They have a low boiling point and heat of vaporization. This translates to greater safety for sensitive system parts, especially delicate gaskets and seals. It also means less energy usage and overall production cost savings
- No reside, no rinsing: Unlike aqueous, alkaline or acid cleaning, solvent-based cleaning fluids do not require an extra rinsing step. The vapuor degreasing fluid cleans, rinses and dries oxygen system components in one operation. Components dry spot-free without stains or potentially flammable leftover residue. And since the cleaning fluids are water-free, they remove the potential for rust.
Additional benefits of vapour degreasing…
- Improved workplace safety: Many modern vapour degreasing fluids are non-flammable for improved safety in the workplace. Their azeotropic properties ensure they are thermally stable and sound to use. This could also translate into company insurance savings
- Easily adaptable to a clean room: Unlike aqueous cleaning that requires engineers to ensure the chemistry of water, detergents, and other additives are consistent, modern solvent-based cleaning fluids in a vapour degreaser remain consistently pure. Plus, there are no stabilisers or acid acceptance testing required as with older chlorinated solvents. Since there is no need for complicated process controls at every stage in the cleaning process, production managers get peace of mind that there is no variation in the cleaning fluid chemistry. This makes initial product validation, and ongoing process controls for operation in a cleanroom much simpler and less costly.
Vapour degreasing also helps maintain required cleanroom environmental conditions including temperature and humidity. They do not generate dust, fumes, heat, or moisture, so they do not require special blowers and fans or any special climate controls to maintain cleanroom air quality or ambient temperature and humidity.
- A smaller workplace footprint: In addition to the simplicity of operation, a major advantage of using a vapour degreaser is the small footprint relative to the amount of production output. A small footprint means the vapour degreaser takes up little space while sustaining high output, allowing for lower overall operating costs in the expensive cleanroom environment
- Cleans both big and small: Vapour degreasers provide excellent cleaning results on parts of any size and almost any geometry. One vapour degreasing machine is very adaptable and can clean a variety of valves and control devices as well as finished assemblies from very small to big with excellent consistency. In general, if the part (or finished assembly) fits into the vapour degreaser, the part can be effectively cleaned. For most parts there are no special fixtures (other than a basket to hold the parts) to clean a wide variety of part geometries and part sizes. This also means it is easy to clean just one part, or parts in multiples with minimal set-up.
Ask for help
Cleaning systems designed to transport and store pure liquid or gaseous oxygen requires the greatest care and enormous attention to safety because almost any residue or contamination could cause a catastrophic explosion.
The design of these systems makes cleaning even more difficult. Tubing, gages, pumps, filters and valves can all entrap contaminants. The contamination varies widely: from simple particulate to challenging grease, wax and oil residue. Vapour degreasing using modern, non-flammable vapour degreasing fluids is an excellent method to effectively clean these highly flammable soils and mitigate the potential for fire or explosion when exposed to oxygen-rich environments.
For those looking for help in selecting the best non-flammable, fast-drying and residue-free cleaning fluids, seek the help of a fluid manufacturer with expertise in both vapour degreasing and oxygen system cleaning. They can help determine which fluids will work best on your particular system substrates and contaminants.
About the author
Elizabeth Norwood is a Senior Chemist at MicroCare, LLC, which offers precision cleaning solutions. She has been in the industry more than 25 years and holds a BS in Chemistry from the University of St. Joseph
Norwood researches, develops and tests cleaning-related products. She currently has one patent issued and two pending for her work.