Equipment used for the storage or transfer of oxygen must be scrupulously clean. Any contaminants on the surface that are combustible such as hydrocarbon oils and greases, or anything that could ignite or spark, like bits of metal swarf, dust and lint, etc., must be removed.
An oxygen-enriched /pressurized environment will result in much more vigorous combustion (potentially explosive) and can cause ignition in materials that would not normally burn in atmospheric air.
There are various recognized specifications that list the different ways in which the desired cleanliness can be achieved – and how it can be tested/certified afterwards. CGA G-4.1 and ASTM G-93 are known internationally.
More locally, BOC, Air Products and EIGA have their own specifications, but for all intents and purposes, they’re all very similar.
Firstly, bits of dust, paper and swarf, are flushed / blown out before the cleaning process begins. If there is loosely-adhered debris such as rust, or in some cases tightly-adhered material such as mill-scale or excessive weld oxide, a chemical cleaning process using strong, corrosive acids or abrasive shot-blasting may be required.
For the cleaning itself, there are various products recommended in each of the aforementioned specifications, and they are broadly categorised.
Each of these have their advantages and disadvantages which will vary dependent upon the task in hand
Detergents are perhaps the easiest product to use (from a handling perspective), but can sometimes foam excessively – and are not always as effective as something like a hot caustic product on very dirty surfaces.
Alkaline products are also very effective at removing oil and grease, but they are often most effective at high temperatures, which may bring logistical challenges to certain sites and situations.
Acid cleaners can work at ambient temperatures, but aside from being not as effective at removing oils as the other options, they are also more likely to have compatibility issues with the system materials. On the plus side, they’ll often remove superficial rust.
Solvents are great at removing oils, but bring many additional hazards into the mix (vapour spaces, static charge build-up, etc., dangerous fumes – not to mention the specialized equipment required to handle the chemical.)
The choice of cleaning agent will be determined by the type of job:
After cleaning, the surfaces should be thoroughly dried to ensure that there is no moisture left within the system. With pipework, this is often done using filtered clean air or nitrogen. The air/gas is blown through the lines until they are dry. This can be determined by the use of a dewpoint meter.
Once dried, the system is inspected in order to validate the process. There are four methods, and are either qualitative or quantitative.
Easy to access areas (vessel walls, large bore pipework, valve bodies, etc.) that can be visually inspected can be tested in one of three ways. In practice, the inspection is often a combination of all three.
The simplest inspection is of course to just look at the cleaned surfaces. This should be done under a bright white light (ideally, natural light), above 500 lux. Any visible debris or staining will result in the components being re-processed / cleaned.
A white lint-free cloth wetted with demineralised water or perchloroethylene is lightly wiped over the surface and visually inspected for evidence of discolouration. Any dirty marks on the cloth would indicate that the cleaning has not been sufficient.
Many hydrocarbons fluoresce beneath an ultra-violet light. Inspecting the components/surfaces/swabs in a dark environment with a UV light will immediately identify any hydrocarbon contamination.
Harder to reach areas – such as the internal surfaces of pipework – will need inspecting in a different manner. With short runs of pipe, it may be possible to pass a small, wetted plug (or soft pig) through the pipe and inspect it, much like the swab test described above.
In most cases however, this can be tricky. For starters, it won’t work if the line is made up of different diameter pipe. Even when it isn’t, it can be difficult to ensure a good ‘fit’ or a consistent light swabbing effect as it passes through the line and, not to mention, there’s always the risk of losing your swab inside.
Solvent test. A section of pipe is selected and a quantity of perchloroethylene is swilled through and collected. The solvent will drag out any hydrocarbon debris. The collected sample is then evaporated, leaving the hydrocarbon debris behind which is weighed to give an accurate measurement in mg/m2.
Each of the recognised specifications provide guidance as to what an acceptable mg/m2 reading will be. Again, this will be determined by the type of equipment and nature of the service.
If you have equipment that is used for handling oxygen, INOX Passivation Ltd are able to carry out all of the requirements detailed in the recognised specifications.
We will audit your system, select the most appropriate/effective methodology, produce the necessary RAMS and testing protocol, then carry out the required inspections before certifying the system as fit-for-use.