In depth
Why the gas has to come out before the field goes on
An electrostatic dehydrator works on liquid. Its electrode grids hang in a vessel that is meant to run liquid-full, with the field applied across crude that carries emulsified water and nothing else. Crude that arrives from a low-pressure separator, or that has just been heated, is rarely in that state: lowering the pressure and raising the temperature both release dissolved gas, and that gas comes out of solution inside the dehydrator if it is given no other chance. Bubbles rising through the grid zone disturb the field, carry water droplets back up with them, and collect as a gas pocket under the top of the shell that the vessel was never designed to hold. The usual symptoms are an unsteady interface, power units that hunt or trip, and a basic sediment and water (BS&W) figure that will not settle.
The degasser dehydrator removes that gas first. The upper vessel is a degasser: the crude enters it, flashes at the package operating pressure, and the released gas leaves from the top to the vent or low-pressure gas system. The de-gassed liquid then passes down into the dehydrator below it, enters through the distributor beneath the electrode grids and is treated as in any electrostatic coalescer: the field merges the water droplets, the water settles to the bottom and leaves under interface control, and dry crude leaves from the top of the liquid-full vessel.
Why the two vessels are stacked
Stacking the degasser above the dehydrator is not only a matter of plot area. The height of the upper vessel gives a hydrostatic head that drives the liquid into the lower vessel through a down-comer without a pump, and keeps the dehydrator liquid-full at a pressure a little above the flash pressure in the degasser, so that no further gas can break out around the grids. One skid carries both vessels, the power units on their platform, the down-comer and interconnecting piping, the inlet distributor, the level and interface instruments, and the local control panel. The package arrives as one lift, is piped up at four nozzles, and takes the plot of a single horizontal vessel.
The dehydrator itself is the same electrostatic coalescer that Axsia Howmar supplies as a stand-alone vessel, with the same grids, insulators, entrance bushings and power units, so the spare parts and the service support are the same. Where the crude also needs desalting, wash water injection and a mixing valve are added ahead of the degasser and the package runs in desalter duty.
Performance surveillance: catch the fault while the crude is still on specification
The signals that tell you how a dehydrator is really running are already in the control room: interface level, the secondary voltage and current of each power unit, inlet temperature, the pressure drop across the mixing valve, wash water and demulsifier rates, treated crude flow, and the daily laboratory BS&W and salt. What is usually missing is anyone reading them together, every day, against what the vessel was designed to do. Operations staff watch the alarms; the slow drifts that come before a failure never reach an alarm limit, and the treated crude is the last thing to show them.
Axsia Howmar offers performance surveillance for this package and for dehydrators and desalters of any make: a daily export from the plant historian, read by the engineers who design the equipment against the design model of the vessel, and returned as a green, amber or red note with the cause and the action. In a published case from a heavy-oil desalter, that kind of review picked a voltage imbalance out of the power unit data within a week and traced it to an over-tightened internal connector, replaced on a planned visit for the price of a small part while the crude had been on specification throughout.
