General information only. This paper describes a diagnostic method. It is not an instruction or authorisation to modify, bypass or operate equipment. Dehydrators and desalters involve high voltage, pressure and flammable fluids. Follow your site procedures, permit-to-work system and management of change process.
What the vessel is trying to do
An electrostatic coalescer, whether it is called a dehydrator or a desalter, removes water from crude oil by making small water droplets merge into larger ones that settle under gravity. Basic sediment and water (BS&W) at the outlet is high when droplets leave with the oil before they have settled. That happens for one of three reasons: the droplets are too small, they settle too slowly, or the flow carries them out.
Why droplet size and temperature dominate
Stokes’ law gives the settling speed of a small droplet: speed equals gravitational acceleration times the square of droplet diameter times the density difference between water and oil, divided by eighteen times the oil viscosity. Three illustrative cases show the sensitivity. They use a density difference of 150 kg/m³ and are arithmetic examples, not data from any unit.
| Case | Droplet diameter | Oil viscosity | Settling speed |
|---|---|---|---|
| A: reference | 100 µm | 5 mPa·s | about 9.8 mm/min |
| B: cooler crude | 100 µm | 10 mPa·s | about 4.9 mm/min |
| C: smaller droplets | 50 µm | 5 mPa·s | about 2.5 mm/min |
Halving droplet diameter costs a factor of four. A fall in temperature that doubles viscosity costs a factor of two. This is why the method below checks temperature and the electrostatic field before anything inside the vessel.
Step 1: confirm the measurement
Establish that outlet BS&W is in fact high. Check that the sample point is on the oil outlet and drawn from a flowing line, that the same method is being used as before (centrifuge to ASTM D4007, distillation to ASTM D4006 or Karl Fischer to ASTM D4928 give different numbers), and that the sample time matches the excursion. Compare with any online water-in-oil analyser.
Step 2: look for a feed change
Ask what changed upstream at the time the excursion began. Common causes are a new well or a well returned after workover, a change in crude blend, slop or tank bottoms being reprocessed, a change in water cut, and production chemicals such as corrosion inhibitor or acid returns. Incompatible blends can precipitate asphaltenes that stabilise emulsion.
Step 3: check treating temperature
Compare the inlet temperature with the design value and with the period of good operation. A fouled exchanger, a heater running below duty or a higher throughput through the same heater all lower temperature and raise viscosity. Temperature is usually the cheapest variable to restore.
Step 4: read the power unit
Record secondary voltage and current on each power unit, and compare with commissioning values.
- Normal voltage with low current indicates the field is established and the emulsion between the electrodes is not highly conductive.
- Low voltage with high current indicates a conductive path: a high interface, a wet emulsion band reaching the electrodes, or a failing bushing or insulator.
- No voltage indicates a tripped or failed power unit. A vessel operating without its field is a settling tank, and outlet water will rise sharply.
See coalescer power unit tripping for causes of electrical faults.
Step 5: check the interface
Confirm the interface level reading with the try-cocks or sample taps at several heights. An emulsion band, often called a rag layer, that has grown between the water and the oil reduces settling volume and may reach the electrodes. Solids in the vessel bottom reduce water volume and residence time; check when the mud-wash system was last used.
Step 6: check chemistry and mixing
Confirm demulsifier type, dose rate and injection point, and that the injection pump is delivering. On a desalter, record the mixing valve pressure drop and the wash water rate. Too high a pressure drop shears the water into droplets too fine to settle; reduce it in small steps and hold each setting for several residence times while recording outlet BS&W and salt.
Step 7: check hydraulics
Compare throughput with design. Above design rate, upward oil velocity can exceed the settling speed of the droplets the field produces. If rate is within design and the earlier steps are clear, suspect internals: a damaged or blocked inlet distributor causes channelling, and a displaced electrode grid leaves part of the vessel untreated. These need an internal inspection at the next opportunity.
Recording the results
Record each check with the date, the value found and the value expected. If the excursion is not resolved, the record is the data set an engineer needs for a diagnosis. The troubleshooting index lists the data we would request.
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.