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Interface level

Interface level is the height of the boundary between settled water and the oil above it in a separator, dehydrator or desalter. Controlled by the water outlet valve, it must sit below the electrodes and above the water outlet.

Interface level explained

If the interface rises towards the electrodes, conductive water shortens the electrical path and the power unit draws high current or trips. If it falls too low, oil leaves with the effluent water. Emulsion and solids at the interface make it hard to measure, which is why instrument type matters.

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Rag layer
A rag layer is a band of emulsion and solids that collects at the oil-water interface in a separator or desalter. As it grows it reduces settling volume, confuses level instruments and may reach the electrodes or the water outlet.
Emulsion (water-in-oil)
A water-in-oil emulsion is a dispersion of small water droplets in crude oil, held apart by natural surface-active films of asphaltenes, resins, waxes and fine solids. The droplets will not settle until those films are broken and the droplets merge.
Electrostatic coalescer
An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.
Crude oil desalter
A crude oil desalter is an electrostatic coalescer operated with wash water mixed into the crude upstream. The water dilutes the salty droplets in the crude, and the coalescer then removes the water and most of the salt with it.
Crude oil dehydrator
A crude oil dehydrator is an electrostatic coalescer that removes produced water from crude oil to meet a water specification, usually stated as basic sediment and water. No wash water is added. It is the same equipment as a desalter.
Wash water
Wash water is low-salinity water injected into crude oil upstream of a desalter to dilute the salty water droplets it carries. It is typically added at 3 to 10 per cent of the crude volume, then removed in the desalter.

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