If you have worked around ultrasonic cleaning systems, you may have noticed that most of them are plastered with stickers warning against the use of flammable materials as cleaning agents. To those who are not familiar with various ultrasonic phenomena, these warnings may seem a bit extreme. In fact, however, flammable liquids may become even more flammable when subjected to ultrasonic excitation. Some of the rules like keeping the temperature below the flash point and avoiding mixtures between the LEL and UEL cease to assure safety when dealing with ultrasonically activated liquids.
Ultrasonic energy may produce suspended droplets or a “fog” over the top of ultrasonically activated liquids. This effect is the basis of operation for ultrasonic nebulizers often used in respiratory therapy. Ultrasonic devices are also often used in home humidifiers to enhance the vaporization of water. In these cases, the suspended particles or “fog” act to maximize the surface area of the liquid that is exposed to the surrounding air to enhance evaporation of the liquid. It is not difficult to imagine that suspended droplets vs. an undisturbed liquid surface vastly enhances the evaporation process.
Ultrasonic atomization is not unlike the effect of a carburetor in an internal combustion engine mixing gasoline and air to form the proper mix for flammability. Because of enhanced evaporation, ultrasonically suspended particles above a flammable surface may significantly affect the local flammability conditions. A “combustible” liquid, may produce a “flammable” mixture once vaporized. A practical example of this is baking flour. One normally doesn’t consider flour as a flammable material yet, when suspended in air, common baking flour becomes quite flammable and, in fact, explosive! Another example is coal. Lumps of coal, although flammable, do not produce the same rate of combustion as the same coal if it is powdered and mixed with air.
With the above considered, all of the rules about flash point as well as upper and lower flammability limits may not strictly apply to ultrasonically excited liquids. In ultrasonic cleaners, this is a concern because of the fact that nearly all ultrasonic cleaners produce ultrasonic energy using some form of electrical device. Electrical devices are potential sources of sparks and heat!
The only way to safely use ultrasonics with flammable solvents involves breaking the combustion triangle. In enclosed areas where the potential for an electrical spark exists, this is usually accomplished by using nitrogen or some other non-oxidizing gas to replace air in a process called “purging.” Outside of the enclosure, where contact between air and solvent are unavoidable, the combustion triangle is broken by eliminating sources of ignition. Basically, all nearby sources of ignition are either eliminated or contained. It is worthy of note that just because an ultrasonic cleaning (or any other) system is approved for use with flammable solvents does not mean that it is safe to use it in an area that is otherwise uncontrolled with regard to sources of additional vaporized solvent or ignition.
When considering the use of flammable solvents in an ultrasonic cleaner, users should consult not only the equipment supplier but with local officials as well to assure that the installation meets applicable codes.
In conclusion, unless you have totally covered all bases, please heed those warnings on your ultrasonic cleaning device. They are there for your safety and do not absolve the user from the responsibility to apply safe practices in the area of use.
– FJF –