Desalters are specially configured electrostatic dehydrators, usually with 2-stages of desalting; with fresh water injection ahead of the second stage of dehydration and separated water from the second stage recycled to the first stage. In general, the fresh water, at 3-5% of the feed, is injected into a mixing valve and/or a static mixing element to ensure intimate mixing of the dilution water with the crude oil without creating emulsions. Often, there is a degassing vessel just upstream of the desalting vessel and often mounted above it to mitigate gas break-out. Electrostatic grid systems do not tolerate free gas evolution as this creates arcing and damage to the electronic components, entrance bushing, and grid hangers. Similarly, electrostatic grids do not tolerate high levels of water entering the grids. For effective desalting (dehydration), the first stage desalter often utilizes an AC configured electrostatic grid system while the second stage is most often AC/DC configured. Two types of desalters are configured: those that are over-pressured by pumping a “dead” or degassed crude and those that are processed with interstage degassing via natural pressured flow.

Reasons for Crude Oil Desalting
Most refineries require relatively low salt content in the crude oil, anywhere from 5-10 ptb (pounds of salt per thousand barrels) prior to shipment to the refinery. The principal reason for removing the salt from the crude is corrosion of the trays in the crude oil fractionators. Highly salty crudes without desalting can generate 200 ptb with dehydration alone. Most refineries also have desalting which removes the salt content to <1 ptb. Residual chloride ion in the crude causes severe deterioration of the stainless steel trays in the crude unit distillation tower.

The Process
Desalters are normally processed through electrostatic dehydrators without heating internals—these vessels normally are referred to as Electrostatic Coalescers. The vessel is fitted with electrostatic internals, inlet and outlet distributors, but without firetubes. The necessary heat required for desalting is then accomplished in a combination of heat exchangers and separate high capacity heaters (cabin type). Degassed crude flows to electrostatic coalescer and is dehydrated to 0.5-0.1% BSW. Oil flows to the second stage desalter, where fresh water is injected into the crude just ahead of the mixing valve and often followed by a static mixer to intimately mix the fresh water with the crude oil. The fluids are directed to the second stage desalter for deep dehydration 0.2-0.05% BSW. The desalter feed must be free of emulsion (extremely small droplets of water) as emulsion is unaffected in processing through the electrostatic coalescer.  A desalting chemical is added upstream of the desalter so that all emulsion is resolved. The principal components of the electrostatic grid system are a mineral oil filled 480-690 VAC single phase, high-voltage transformer with a 100% reactance core, a Teflon entrance bushing, Teflon hangers, and a network of structural elements. Special controllers can be added to improve the operation of the transformer and improve dehydration effectiveness and overall performance.


Options and Features

  • Degassing Separators
  • Voltage to 50,000 Volts AC or AC/DC
  • FPSO Configured Internals
  • Transformer Monitoring Panel (IP66) Hazardous Area Classification Zone 2, IIB, T4
  • Coalescer Load Controller (CLC), PLC Based with HMI
  • Internal Coating
  • Alloy Materials
  • Factory Piped and Assembled
  • Skid Mounted
  • Ladders and Platforms
  • Heat Exchangers, Plate and Frame or Shell and Tube
  • Sand Jet and Drain Systems
  • Mixing Valves
  • Static Mixing Elements