An improved separator for desalting petroleum crude oils which may be operated in a continuous manner under automatic control; the improved desalter is therefore well suited to modern refinery operation with minimal downtime. A portion of the emulsion layer is withdrawn from the desalter through external withdrawal ports according to the thickness and position of the emulsion layer with the selected withdrawal header(s) being controlled by sensors monitoring the position and thickness of the emulsion layer. The withdrawn emulsion layer can be routed as such or with the desalter water effluent to a settling tank or directly to another unit for separation and reprocessing.