Design and Analysis of Impregnation Chamber Used In Vacuum Pressure Impregnation Process

Abstract

The simple explanation is that in nature and in manufacturing, things leak. Vacuum impregnation stops leak. The ultimate goal of vacuum impregnation is to seal leak/migration paths without impacting the functional, assembly or appearance characteristics of a part. The impregnation chamber which is used in VPI process operates maximum up to 80 to 150 psi.it is important to analyse and design the pressure vessel that will provide safety, durability and serviceability to the company. Accomplishing this task will require a very good understanding of behaviour and a good knowledge of parameters that affecting the pressure vessel due to varying loads, pressure and thickness of shell element. The most important one is that the given geometry of pressure vessel must be analysed to assure it should meet the design standards. Vacuum impregnation as an industrial process has been in commercial use for more than 60 years. For the world's largest manufacturers, it continues to be the preferred process through which to guarantee the pressure-proof, leak-proof, and corrosion-proof requirements of parts and components in critical operations. The ultimate goal of vacuum impregnation is to seal leak / migration paths without impacting the functional, assembly or appearance characteristics of a part. Functional characteristics include the ability for fluids or gasses to flow only where needed in order to enhance in-service performance of the components' design. Assembly characteristics, which must be maintained, include performance of tapped holes; the integrity of mating and sealing surfaces, the elimination of residual internal contamination in water jackets, sockets, surfaces and dimensional areas. Appearance characteristics include oxidation and discoloration. Vacuum impregnation is governed by military standard MIL-STD-276A as well as numerous proprietary and customer specifications. The vacuum component of the process removes the air that occupies the migration path commonly known as porosity in cast or pressed metals. The impregnation component of the process fills the void with a durable and stable material suitable for field of use. These materials can be silicate or epoxies. (7) During normal operation, transformers are subjected to the failure of the insulation system. How long an insulation system will be serviceable depends on the materials chosen and the service environment. Thermal, mechanical, voltage and environmental stresses all combine to reduce the service life of the transformer. Solid- state Controls, Inc. (SCI) manufactures all of its transformers using the Vacuum Pressure Impregnation (VPI) process. This system strengthens the insulation system and extends the service life of the transformer. The VPI process is the most advanced system in use today. The VPI process is the most effective way known to eliminate the dead air spaces that cause hot spots within the transformer coils. These hot spots can be 20 0 higher than the average coil temperature. The VPI process, along with a good resin, provides a low thermal resistance path that lowers the average operating temperature of the transformer.The resin seals the transformer against environmental conditions and bonds all components of the insulation system together for good mechanical strength. This is very effective in reducing mechanical vibrations. This greatly reduces the audible noise level of the transformer. The VPI process and resin also enhances the dielectric capability between windings and between the windings and ground. This allows the transformer to survive higher voltage stress levels without failure. The general VPI process contain following parts In the impregnation chamber (also known as an autoclave, pressure vessel or vacuum vessel) air is evacuated from the leak path in the part by using a deep vacuum. The evacuated leak path is filled with sealant by covering the part with the sealant and applying pressure. More energy is required to penetrate the porosity with sealant than to evacuate the air. In the recovery station (also known as drain station, centrifuge) excess sealant is recovered for reuse. In the wash/rinse station (also known as surge station, rolling rinse or pump over station) residual sealant is washed from the part's internal passages, taps, pockets and features where sealant is undesirable. In the cure station (also known as standing, rolling cure or pump over station) the sealant, impregnated into the walls of the part, is polymerized in the leak path. In general, vacuum impregnation includes four steps.