Desalting operation can be very challenging due to changing process variables and crude assay. The desalter will be the predominant source of wastewater in a refinery and upset in desalter will not only increase sewers loading but also increase valuable crude losses. In other way, after the crude has been washed and mixed as an emulsion of oil and water, demulsifying chemicals are injected and electrostatic fields are used to break the emulsion and concentrate suspended water globules in the bottom of the setting tank . A dedicated balance must be maintained while controlling chemical desemulsifier feed and other parameters for the specified quality of the crude at outlet and the minimum under carry at bottom discharge. Depending on crude assay (from high potential yield to opportunity crudes) and process variables , the Desalter Under Carry Barrier guarantees less than 50 to 300 PPM crude contamination at the waste water discharge valve .