What is specific to dehydrator duty
Dehydrator duty differs from desalter duty in three ways. No wash water or mixing valve is used, so the emulsion arriving at the vessel is the natural one, not one created by mixing. The inlet water cut is higher and more variable, because it follows the wells. And the target is a water content, not a salt content, although salt falls in step because it is dissolved in the water removed.
The vessel, grids, entrance bushing and power unit are the same as in desalter duty. They are described, with an annotated drawing, on the electrostatic coalescers page.
Where a crude oil dehydrator sits in the process
A crude oil dehydrator sits after the low-pressure separator and the heater, and before storage, export pumps or a desalter stage. Gas must be released upstream: gas breaking out inside an electrostatic vessel disturbs settling and can create a hazard around the energised grids, which is why operating pressure is kept above the crude’s vapour pressure at treating temperature.
Design considerations for dehydrator duty
The design of a dehydrator duty turns on the water cut profile over field life. A unit sized for early-life water cut will be overloaded later, so the design basis should state the maximum water cut the grids must tolerate and the point at which upstream free-water removal is added. The other inputs are viscosity at treating temperature, emulsion stability from bottle tests, solids loading and the BS&W specification.
Heating is the main operating cost of dehydration. The treating temperature is chosen as the lowest that meets specification with a sensible demulsifier dose, because every extra degree costs fuel and drives light ends into the gas phase. See crude oil heaters.
Common operating problems in dehydrator duty
The common operating problems in dehydrator duty are high outlet BS&W, grid voltage collapsing when a wet slug arrives, and rag layer growth fed by solids and production chemicals. Slugging from upstream separators is a frequent root cause, because it changes water cut and flow faster than interface control can follow. Symptoms, probable causes and the data needed to confirm them are in the troubleshooting symptom index.
Revamp options when water cut rises
When water cut rises beyond the design basis of a dehydrator, the options are to remove more free water upstream, raise treating temperature, upgrade the grids and power unit within the existing shell, or add a second vessel. A design review compares these against the forecast water cut, not just today’s.
