Vapor cleaning

Recently several patents have been issued (16—18) describing the use of 1,2-dichloroethylene for use in blends of chlorofluorocarbons for solvent vapor cleaning. This art is primarily driven by the need to replace part of the chlorofluorocarbons because of the restriction on their production under the Montreal Protocol of 1987. Test data from the manufacturer show that the cleaning abiUty of these blends exceeds that of the pure chlorofluorocarbons or their azeotropic blends (19). 

These units cannot handle liquid slugs or dirt particles hence, they require gas/vapor cleaning before entering the unit. For units with cooling jackets and automatic temperature regulation of the process gas, the temperature can range as high as 302°F [20]. 

Ultrasonic systems using less harmful and corrosive materials have been successfully used as an alternative to vapor cleaning. For example, stainless steel, which used to be cleaned with distillate-spray wash and vapor rinse using 1,1,1 -trichlo-roethane, has been replaced by immersion in ultrasonic bath containing trichloro-trifluoroethane and methanol. Hence the beneficial substitution of one organic solvent by a more environmentally friendly cleaner is possible. 

The vapor is air (or an inert gas such as nitrogen), enriched with vaporized cleaning solvent. 

The commonly used method of reclassification by repeated draining and flushing yields three or more times the volume of the transformer of PCB contaminated fluids for disposal. At 2000 ppm PCB and higher concentrations, chemical decontamination processes which can be used with the transformer on.line do not compete cost-effectively with incineration. The use of a vapor cleaning process to minimize the quantity of fluid for disposal then becomes cost-effective. The process is a physical requirement when the PCB concentration of the residual mineral oil is about 2000 ppm PCB. 

PCBs, a simple flushing or vapor cleaning process leaves about 2.5% of the original askarel contained in the core/coil assembly. For a typical 250 gal. askarel transformer the impregnated fluid amounts to about 6 gal. A PCB concentration of 500 ppm PCB in a 250 gal. transformer is... 

If the concentration level of PCBs can be reduced to such an extent that subsequent leaching does not yield a concentration which is above the 500 ppm PCB limit imposed by the EPA 90 days after the process has been completed then, according to the letter of the law, the unit can be reclassified as PCB contaminated. However, experimental data on units which have been retrofilled with a simple vapor cleaning process have shown that the PCB leaching rate under normal load conditions is in the range of 1.5 2.5 ppm PCB per day. If the trapped PCBs are able to leach out into the bulk retrofill fluid at a rate of as little as 1 ppm PCB... 

The following discussion concerns the quantitation of the effect of repeated retrofill operations using a vapor cleaning system and describes how an alternative procedure can mitigate the leaching problem in a single operation. 

The experimental conditions shown in Figure (2) were found convenient because a series of complete leaching curves could be compared over a period of about 30 days rather than the 300 days found for a 750 KVA transformer. It is clear that sufficient time has elapsed to be able to compare a conventional vapor cleaning retrofill system with the processing necessary to avoid the leaching problem. 

It was found that processing beyond simple vapor cleaning could be continued to the point where leaching was completely stopped. In actual practice, however, such a rigorous level need not be achieved since, if the leaching rate is reduced by, say, an order of magnitude from the measured value of 1.3 ppm PCB/day then, instead of about one year of reclassified use, the unit would retain its classification for 10 years. In this example, if the transformer s life-expectancy was only for a further 5 years then clearly it would remain reclassified for the remainder of its useful life. The extent of processing can be tailored to the level of reclassification required and the life-expectancy of the transformer,... 

It has been mentioned above that a vapor cleaning system has been proven successful in removing surface PCB contamination from areas previously considered inaccessable. Tbe method has tbe advantage that a minimum quantity of PCB contaminated fluid is derived for disposal in contrast to tbe drain and flush method wbicb produces several times tbe volume of tbe transformer tank. Also, as tbe level of PCB contamination in tbe mineral oil increases tbe vapor cleaning method becomes increasingly necessary and eventually becomes tbe only viable cost-effective procedure. 

Diffusion coefficients have been measured and used to calculate the rate of PCB leaching from impregnated insulation into oil used to retrofill askarel transformers. It has been shown that it is theoretically unlikely that a simple vapor cleaning system alone can be used to reclassify a transformer to non-PCB status. 

The vapor cleaning method becomes a practical requirement when the concentration of the PCB contamination is greater than about 1000 ppm PCB. The method has a significant cost advantage over the conventional drain and flush process since vapor cleaning yields a minimum volume of contaminated fluid for disposal. 

Degreaser, low emission vapor (cleaning) A degreaser where the cleaning solvent is contained in an enclosed cleaning chamber, then pumped away before the cleaning chamber is opened. The vapors are condensed and returned to the cleaning liquid sump. 

Vapor cleaning Cleaning by the condensation of a solvent vapor on a cold surface above a hot liquid sump. The condensed solvent and contamination flow off into the sump. Cleaning continues until the part reaches the temperature of the vapor and condensation ceases. Also called Vapor degreasing. 

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