Where an electrostatic coalescer sits in the process
An electrostatic coalescer sits downstream of gas-liquid separation and heating, where the crude carries only emulsified water. In an oilfield train it is normally the last stage before storage or export. In a refinery it sits in the crude preheat train, upstream of the crude unit heater, at the temperature that gives the best balance of viscosity, water solubility and vapour pressure. The crude oil treatment overview shows the full train.
Inside an electrostatic coalescer vessel
The internals of an electrostatic coalescer are few, and each has one job. Select a number on the drawing, or read the descriptions under it.
- Transformer and power unitMounted on or beside the vessel, the power unit steps the supply up to the high voltage applied to the electrodes. Repeated tripping of the power unit usually points to a conductive path inside the vessel, such as a high interface, a wet emulsion layer or a failed insulator or bushing.
- High-pressure entrance bushingThe entrance bushing carries the high-voltage conductor through the pressure boundary into the vessel while insulating it from the shell. It is both a pressure part and an electrical insulator, and it is one of the most commonly replaced spare parts.
- Electrode insulators and hangersInsulators suspend the energised electrode grids from the vessel while keeping them electrically isolated from it. Surface contamination or cracking lets current track to earth and drags down the field voltage.
- Electrode gridsThe grid assembly creates the electric field between energised and earthed electrodes. Water droplets passing through the field are polarised, collide and coalesce. Grid spacing and voltage are matched to the crude and the water cut.
- Inlet distributorThe distributor spreads the incoming emulsion evenly along the vessel at low velocity so that the whole grid area is used. Its position relative to the interface depends on the vessel design.
- Oil-water interface and level measurementCoalesced water settles into the lower water phase. The interface level is measured and controlled: too high and conductive water approaches the grids; too low and oil leaves with the effluent water.
- Outlet collectorThe collector header gathers treated crude from the top of the vessel along its length, keeping upward velocity uniform so that droplets are not carried over.
- Effluent water outletWater and the salt dissolved in it leave from the bottom of the vessel under interface level control and go to water treatment.
Design and selection considerations for electrostatic coalescers
The design of an electrostatic coalescer is set by the crude, the water and the duty. The main choices are:
- Field type. Alternating current (AC) fields tolerate high water cuts and are the most common. Designs that combine AC and direct current (DC) fields add electrophoretic movement of droplets and can treat to lower water contents.
- Grid arrangement. The number of grid levels and their spacing set the field strength and the area available for treatment.
- Vessel size. Diameter and length follow from grid loading, settling time at treating viscosity, and the water volume needed for stable interface control.
- Inlet distribution. Poor distribution wastes grid area and disturbs the interface. It is a common limit in older vessels.
- Interface measurement. The rag layer confuses simple level instruments. Technology is chosen for the emulsion and solids expected.
- Solids handling. Where sediment is expected, a mud wash system keeps the vessel bottom clear without a shutdown.
Materials of construction for electrostatic coalescers
Electrostatic coalescer shells are normally carbon steel with a corrosion allowance, with internal coating or cladding in the water phase where brine chemistry requires it. Insulators, hangers and the entrance bushing use PTFE or comparable insulating materials rated for the treating temperature and pressure. Gaskets and seals are selected for aromatics content and temperature.
Common operating problems with electrostatic coalescers
The common operating problems with electrostatic coalescers are high water or salt at the outlet, low grid voltage or power unit trips, growth of the emulsion band at the interface, and oil carried under into the effluent water. Most trace to four causes: feed that has moved away from the design basis, low temperature, chemical dosing, or a failing electrical component. The troubleshooting symptom index lists probable causes and the data that confirms each.
Capacity enhancement and revamp options for electrostatic coalescers
An electrostatic coalescer can often treat more crude, or wetter crude, within its existing shell. Revamp options include replacing damaged or fouled grids, fitting new entrance bushings and insulators, upgrading power units, improving the inlet distributor and upgrading interface measurement. Our design review service identifies the binding limit first. Parts for existing units, including units built by others, are listed under electrostatic coalescer spare parts.
