Cleaning for oxygen service is best defined as the removal of combustible contaminants from the surface of any equipment or system in oxygen service, including all parts thereof. Essentially, any component that may come into contact with an oxygen rich environment.
The combustible contaminants include organic and inorganic substances such as hydrocarbon material for example oils and greases, paper, fibre, coal dust, solvents, weld slag, rust, sand and dirt. If these contaminants are not removed properly, in a worst case scenario, this can cause combustion in an oxygen atmosphere or at the least rejection of the product due to unacceptable product purity.
Oxygen in its own right is not flammable but it supports combustion. Oxygen can react with most materials. The higher the oxygen content and/or pressure in a system the more vigorous the combustion and the lower the ignition temperature required. Materials that do not normally ignite in atmospheric air will burn and may explode in an oxygen rich environment. In addition the oxygen rich environment will give rise to a higher flame temperature and combustion velocity and the devastating consequences thereof.
The recognition of oxygen's reactivity has led to stringent requirements regarding the cleanliness of equipment in oxygen service. Strict guidelines exist to ensure that care must be taken in the selection of equipment including all materials and components, all of which need to be oxygen compatible. They must also be free from combustible contaminants as described above.
With this in mind special consideration must be given to any cleaning processes employed in the manufacture and maintenance of all oxygen service systems.
Specific consideration must be given to the following:-
- - cleaning standard to be achieved (how clean is clean?)
- - cleaning procedure specified (or not)
- - cleaning agent to be used
- - surface properties of the parts to be cleaned
- - shape and geometry of the material
- - types and amounts of contaminants
- - the degree of automation required
The size and capacity of the equipment is determined from
- - the size of the material or components to be cleaned
- - the required throughput
Your starting point should be the cleaning standard and procedure. For example *G93 indicates that solvent cleaning is preferable. Solvent cleaning and solvent vapour phase cleaning of components consists of the removal of contaminants by immersion in the solvent, possibly with the addition of ultrasonic agitation and the action of continued condensation of solvent vapour on the component surfaces. The procedure requires that the oxygen equipment, system or component is colder than the solvent boiling point. This allows the vapour to condense on the components and perform a final rinse.
The major significant advantage of solvent cleaning is that re-vaporised solvent is always clean and the contaminants remain in the evaporator liquid section which requires only periodic cleaning out, thus causing a reduction in the frequency of system downtime.
It is also important to note **G127–95 (Reapproved 2000). The effectiveness of a particular cleaning agent depends upon the method by which it is used, the nature and type of the contaminants and the characteristics of the article being cleaned, such as size, shape, and material. Final evaluation of the cleaning agent should include testing of actual products and production processes.
All equipment must, together with the cleaning chemistry, fulfil as a minimum the legislation for health, safety and environment.
The choice of equipment has to be based on the efficiency of cleaning versus cost bearing in mind what is the cost of the problem? If there is no cost there is no problem.
The efficiency is controlled by utilising typical samples, written procedures and requested criteria for cleanliness.
If you need to clean to ASTM G93 - 03(2011) Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments then all of the above needs to be given due consideration.
*G93 - Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments
**Designation: G127 –95(Reapproved 2000) Standard Guide for the Selection of Cleaning Agents for Oxygen Systems.
Phil Dale
Co-contributor Handbook for Critical Cleaning
Liquid Displacement Drying Techniques
Handbook for Critical Cleaning, Second Edition - 2 Volume Set Hardcover – April 4, 2011by Barbara Kanegsberg (Editor), Edward Kanegsberg (Editor)
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