Original Date: 07/25/2005
Revision Date: 09/12/2006
Best Practice : Critical Cleaning of High-Voltage Assemblies
Northrop Grumman Electronic Systems’ Electronics Repair Center representatives worked closely with several chemical companies to identify chemistry that would meet or exceed specified requirements to clean critical high- voltage assemblies that have electrically arced. A hydrofluoether-based material proved to be superior with low global-warming potential. The new process continues to save more than $30,000 annually in solvent expense.
Northrop Grumman Electronic Systems’ (NGES’) Electronics Repair Center (ERC) cleans every high voltage assembly after a breakdown with a special cleaning system designed to satisfy all of the cleaning requirements. Elements from the decomposition of the dielectric gas sulfur hexafluoride are contained in fielded units that have electrically arced. This high-voltage arc leaves sulfur powder and various corrosive fluorine compounds over many portions of the equipment. The cleaning requirements for critical high- voltage assemblies ensure the removal of conductive ionic contamination, particulate contamination, and the elimination of chemical degradation of dielectric materials. The cleaning must be capable of removing all forms of contamination without causing product degradation.
Because several of the cleaning options available to accomplish the required task were either not completely adequate or presented harmful effects to the environment, NGES’ ERC representatives worked closely with several chemical companies to identify chemistry that would meet or exceed specified requirements. Many chemistries were identified over a 15-year period of trials, with Freon being the chemical of choice for most high voltage applications until it was banned as an ozone-depleting compound. Environmental concern became an important consideration in the selection of the cleaning agent. To meet changing needs, criteria were established requiring the new process to use high pressure wash and to recycle all used solvent. The solvent could not use an ozone-depleting compound nor be a greenhouse-warming gas, and it had to remain stable under exposure to various contaminants.
A hydrofluoether- (HFE-)based material proved to be superior, with low global warming potential. Any release of oxygenated materials to the environment would break down within 4.3 days, with no harmful effects to the earth or atmosphere. Although the engineered fluid HFE-72 DA exhibited a boiling point of 95°F, experimentation proved the chemistry worked best when sprayed at -2°F. A self-contained refrigeration unit was built with an attached still for reclamation of used solvent.
NGES’ Critical Cleaning of High-Voltage Assembly process continues to save more than $30,000 annually in solvent expense, saves many hours over hand cleaning, and does a thorough job. There are also additional unknown savings of parts for units that pass all testing after going through only the cleaning process.
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