Typical published ranges. The removal figures on this page are typical values from open literature for well-operated units. Actual performance depends on the crude, temperature, wash water quality, mixing and chemical treatment, and is confirmed per project.
How a single stage works
Wash water is injected into the crude and dispersed through a mixing valve. The small wash water droplets meet the brine droplets already in the crude and dilute them. The desalter then coalesces and settles most of the water. Salt leaves with the settled water. The salt left in the crude equals the volume of water carried over multiplied by the salinity of that water.
How two stages work
In a two-stage arrangement the crude passes through two desalters in series. Fresh wash water goes to the second stage only. The water drawn from the second stage, which has picked up little salt, is pumped back and used as the wash water for the first stage. The crude therefore meets the cleanest water last, and the same water does two jobs. Two stages remove more salt than one stage using the same quantity of fresh water.
Single-stage and two-stage desalting compared
| Criterion | Single stage | Two stages in series |
|---|---|---|
| Typical salt removal | About 90 to 95 % | About 99 % |
| Fresh wash water | Typically 3 to 10 vol% of crude | Similar total, used twice through inter-stage recycle |
| Equipment | One vessel, one power unit, one mixing valve | Two of each, plus an inter-stage water pump |
| Plot space and cost | Lower | Higher |
| Tolerance of upsets | An upset passes directly downstream | The second stage catches carry-over from the first |
| Best fit | Moderate inlet salt; outlet target within reach of one stage | High inlet salt, tight outlet target, or limited fresh water |
A worked example
Take a crude arriving with 100 PTB of salt, where PTB is pounds of salt per thousand barrels. At 93 % removal, one stage leaves 7 PTB. That meets a 10 PTB sales specification but not the 1 PTB that many refineries set ahead of the crude distillation unit. A second stage removing 90 % of what remains leaves 0.7 PTB. These figures are illustrative arithmetic, not a performance statement for any unit.
How to decide
- Establish inlet salt across the range of crudes and water cuts you expect, not the design case alone.
- Fix the outlet salt target and who sets it: a sales contract, or a downstream corrosion and fouling limit.
- Work out the removal needed. If it is above about 95 %, one stage is unlikely to hold it reliably.
- Check wash water. Availability, quality and effluent disposal often decide the matter before equipment cost does.
- Consider future crudes. Heavier or more saline crudes later in field or refinery life are a common reason for adding a second stage.
Adding a second stage to an existing unit
A second stage is a common revamp. Before committing to it, it is worth checking whether the existing stage is performing as it should. Poor mixing valve settings, low treating temperature, a worn electrode system or an uncontrolled interface can each cost several per cent of removal. A design review establishes what the existing vessel can deliver and what a second stage would add.
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