Why there are three names for one vessel
Salt is not dissolved in crude oil. It is dissolved in the small droplets of produced water that the crude carries. Removing water therefore removes salt, and both jobs come down to getting small water droplets to settle out of oil. The electrostatic coalescer does that. Oilfield engineers tend to call it a dehydrator or electrostatic treater, because their specification is water content. Refinery engineers call it a desalter, because their specification is salt content.
What changes between the two duties
| Aspect | Dehydrator duty | Desalter duty |
|---|---|---|
| Purpose | Reduce water in crude to the export or downstream specification | Reduce salt in crude to protect downstream equipment or meet a sales specification |
| Specification measured | Basic sediment and water (BS&W), vol% | Salt, in pounds per thousand barrels (PTB) or mg/L, and BS&W |
| Wash water | None | Typically 3 to 10 vol% of crude, mixed in upstream |
| Mixing valve | Not used | Used; its pressure drop sets how finely wash water is dispersed |
| Usual location | Oilfield production facility | Refinery crude unit preheat train; also oilfields with a salt specification |
| Vessel, electrodes, power unit | The same in principle; sized for the crude, the temperature and the water load | |
How dilution removes salt
A coalescer never removes every droplet. Whatever water stays in the crude keeps its dissolved salt. If that remaining water is concentrated produced brine, a small volume of it still carries a lot of salt. Desalting works by mixing in wash water of low salinity first, so that the droplets left behind are diluted brine. Outlet salt depends on two things: how much water remains, and how salty that water is. Wash water rate and mixing efficiency set the second; the coalescer sets the first.
When a field needs both
Where produced water is very saline, dehydration alone may meet the water specification and still miss the salt specification. The usual arrangement is two coalescers in series: the first in dehydrator duty, the second in desalter duty with wash water added between them. See single-stage vs two-stage desalting for how staging affects salt removal and water use.
What this means when you specify or buy
- Ask for the duty, not the name. State inlet water cut, produced water salinity, and the outlet BS&W and salt you need.
- A dehydrator can often be converted to desalter duty by adding wash water injection and a mixing valve, provided the vessel has capacity for the extra water.
- Spare parts are common to both duties. Entrance bushings, electrode insulators and grid assemblies are listed under electrostatic coalescer spare parts.
Talk to an engineer about your unit
Tell us the equipment, the duty and what you are seeing. Partial data is normal: send what you have and we will tell you what else matters. We acknowledge every enquiry within 1 working day.