The preceeding blog discussed what is probably the “granddaddy” of all cleaning challenges – burned-on carbon. But, as those of us who have been in the cleaning business since Noah launched the ark know, there are a bunch of other cleaning challenges that defy modern technology. In general, these challenges fall into a small group of categories.
The first category is insoluble contaminants bonded to a part surface. These are materials that don’t qualify as “particles” because they are bonded to the part surface by adhesive forces. This category of contaminants includes paints, rubber, plastic, epoxy and silicone compounds like RTV as well as many others. In most cases, these materials are purposely formulated to have excellent adhesion properties and be resistant to chemical and/or mechanical attack. In short, they were never meant to be removed. In addition, many of these materials have resilient physical properties which prevents fracturing them as a viable means of removal. Again, as in removing burned on carbon, these contaminants are often adhered to substrates that are, themselves, susceptible to chemical attack by most of the likely candidate cleaning chemistries.
The second category is removing a “contaminant” that is of the same composition as the substrate it is being removed from. Here, the culprit is usually the cohesive forces that bond the contaminant to the substrate. Good examples of this are removing glass particles from ground glass surfaces and removing carbon particles from machined or polished carbon substrates. Mechanical intervention in the form of brushing, high pressure sprays and ultrasonics are beneficial in most of these applications. A word of caution though – – Any cleaning technology that is able to break the cohesive bond between the contaminant and the substrate is probably just short of being able to break down the substrate itself so control is a major issue. “Overkill” is not an option in these applications.
Magnetism, although not the same as cohesion presents nearly the same problem. Magnetic particles, even if initially removed, tend to re-attach as quickly as they are removed. The problem of magnetized parts or particles was discussed in a previous blog.
The third category of challenging parts cleaning applications involves parts that have a geometry that limits or excludes access to the surface to be cleaned. One typical example of this is tube and plate heat exchangers and reactors with internal contaminants that can not be removed chemically. Their physical properties preclude access for introducing mechanical means to assist in cleaning. Long tubes, parts with deep blind holes or internal passageways and parts with “hems” also fit into this category. A further challenge in cleaning parts in this category is that inspection to assure cleanliness is usually just as difficult as the cleaning itself. In the worst case, cleaning parts in this category may require costly and time consuming dis-assembly which often tilts the scale toward replacement.
As if the above components of challenge aren’t enough, most challenging cleaning applications exhibit more than one component of challenge. Those heat exchangers mentioned above – – Not only is there a lack of access, but the contaminant, usually a buildup of calcium or other material which has been baked on, is only minimally soluble making chemical cleaning alone a non-viable option.
The above list comprises a “red flag” guide to those with cleaning requirements and those attempting to solve them. It, of course, is not all inclusive but applies in a high percentage of cleaning requirements still in search of a solution.