How Crude Oil Desalters Work: Interface, Emulsion and Control

A crude oil desalter mixes crude with wash water so dissolved salts and fine solids can transfer into the water phase, then uses electrostatic coalescence and residence time to separate the phases. Stable interface and emulsion control are central to reliable operation.

The desalting process

  1. Wash water is added to contact the crude and dissolve inorganic salts.
  2. Mixing disperses the water through the crude so contaminants can transfer.
  3. An electrostatic field promotes coalescence, helping small water droplets combine and settle.
  4. Separated brine leaves the vessel while treated crude continues to the next refinery stage.

Why the oil-water interface matters

If the interface is too high, water or emulsion can approach the crude outlet and upset downstream equipment. If it is too low, hydrocarbon can leave with the brine. A thick or unstable emulsion band can also reduce effective separation volume and make a single assumed interface unreliable.

Variables that affect desalter performance

  • Crude density, viscosity and conductivity
  • Temperature and residence time
  • Wash-water rate and distribution
  • Mixing intensity and pressure drop
  • Demulsifier selection and dosage
  • Electrical field conditions
  • Interface and emulsion-band behaviour

Interface measurement requirements

The measurement technology must be selected for the expected phases, emulsion range, vessel geometry, process conditions and required control response. The Agar ID-200H Series provides a two-wire 4-20 mA signal with HART and is described for 0-100% hydrocarbon-water concentration by volume at the probe.

Measurement is one part of the control strategy. Installation location, representative process contact, commissioning thresholds, alarm logic and maintenance access all need application-specific review.

Related technical guidance

Continue with the ID-200 desalter applications page, the desalter sensor guide, or ask PMA about a specific vessel.