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Stokes’ law

Stokes' law gives the settling speed of a small droplet in a liquid. Speed rises with the square of droplet diameter and with the density difference between the two liquids, and falls as the viscosity of the surrounding liquid rises.

Stokes’ law explained

The law explains the main levers in crude dehydration. Heating lowers oil viscosity. Coalescence increases droplet diameter, and doubling diameter gives four times the settling speed. Heavy crude is hard to treat because its density is close to that of water and its viscosity is high.

  • Electrostatic coalescers

    An electrostatic coalescer is a pressure vessel that uses a high-voltage electric field to merge small water droplets in crude oil into drops large enough to…

  • Crude oil heaters

    A crude oil heater raises crude to the temperature at which water can be separated from it. In oilfield treatment this is usually an indirect fired…

  • Crude oil dehydrators

    A crude oil dehydrator is an electrostatic coalescer used to remove produced water from crude so that it meets its basic sediment and water (BS&W) specification.…

  • Crude oil treatment

    A crude oil treatment system removes water, salt and sediment so that crude meets its sales or refinery specification. It works as a train: separation, heating,…

  • Troubleshooting and performance diagnostics

    Troubleshooting a desalter, dehydrator or hydrogen pressure swing adsorption (PSA) unit starts with the symptom, because each symptom has a short list of probable causes and…

Emulsion (water-in-oil)
A water-in-oil emulsion is a dispersion of small water droplets in crude oil, held apart by natural surface-active films of asphaltenes, resins, waxes and fine solids. The droplets will not settle until those films are broken and the droplets merge.
Electrostatic coalescer
An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.
Crude oil dehydrator
A crude oil dehydrator is an electrostatic coalescer that removes produced water from crude oil to meet a water specification, usually stated as basic sediment and water. No wash water is added. It is the same equipment as a desalter.
Demulsifier
A demulsifier is a surface-active chemical injected into crude oil at parts-per-million levels to displace the natural films that stabilise water droplets. With the films weakened, droplets can merge and settle. It works together with heat, time and electrostatic fields.
Interface level
Interface level is the height of the boundary between settled water and the oil above it in a separator, dehydrator or desalter. Controlled by the water outlet valve, it must sit below the electrodes and above the water outlet.
Rag layer
A rag layer is a band of emulsion and solids that collects at the oil-water interface in a separator or desalter. As it grows it reduces settling volume, confuses level instruments and may reach the electrodes or the water outlet.

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