Possible Causes Corrective Action
Desalter capacity exceeded by
handling heavier oil than design basis.
Decrease throughput.
Increase operating temperature.
Blend heavy oil with lighter oil.
Insufficient wash water rate. Increase wash water rate to between 4% and 8% of oil flow rate.
Inadequate mixing. Increase mix valve P in 1-2 psi increments to establish optimum.
Low operating temperature. Increase temperature of untreated oil, close all unnecessary heat exchanger bypasses.
Low electrode voltage. Check electrical system for operating problems.
Insufficient demulsifier dosage or ineffective demulsifier.
Increase demulsifier chemical injection rate and/or change type.
Work Aid 4B: Troubleshooting Desalter Problem of High Water Carryover in Desalted Oil
Possible Causes Corrective Action
High oil/water interface level. Check water level by using interface sampling lines; decrease level to lowest possible with good effluent water quality and clear water at 30 in. level.
Excessive mixing valve P. Open mixing valve completely, allow
amperage to stabilize, and increase mixing valve pressure drop slowly (allow about one-half hour per adjustment) to establish optimum setting.
Excessive water injection. Reduce wash water injection rate to between 4% and 6% of oil flow rate.
Very high BS&W content in oil feed. Sample crude for BS&W; decrease wash water injection rate to compensate for excess water in feed.
Electrical failure. Check voltage and amperage readings; if transformer or entrance bushing failure identified, or power cannot be restored
immediately, discontinue wash water injection.
Work Aid 4C: Troubleshooting Desalter Problem of Oily Effluent Water (Black Water)
Possible Causes Corrective Action
Low oil/water interface level. Check water level by using interface sampling lines;
raise level until clear water is obtained at the 30 in.
level and effluent water quality is acceptable without excessive water carryover into desalted oil.
Excessive mixing valve P. Open mixing valve completely until operation
stabilizes, then increase P in small increments until optimized. If wash water rate too high, decrease to between 4% and 6% of oil flow rate.
High effluent water pH. Check effluent water pH. If greater than 7.5,
reevaluate wash water components, acidify wash water with H2SO4 until effluent water pH is between 5.5 and 7.0.
Sludge in desalter. Clean desalter. If not possible, try operating with higher interface levels as long as salt removal efficiency is not impaired.
High solids concentration in effluent brine. (Excessive oil content in solids.)
Check wash water for particulates and minimize where possible. Investigate incorporating improved solids wetting agent in chemical additives package.
Excessive asphaltenes in crude.
Increase water residence time in desalter by raising interface level, providing this does not interfere with desalting efficiency. Avoid blending light naphtha with heavy oils.
Low operating temperature. Close any unnecessary bypasses to maximize preheat, if operating temperature is below normal.
Insufficient or ineffective Increase chemical demulsifier dosage and/or change
Work Aid 4D: Troubleshooting Desalter Problem of a Wide Emulsion Band
Possible Causes Corrective Action
Oil feed properties -- high BS&W, low gravity, waxy constituents, high particulate loading, emulsifiers from oil field recovery.
Slug feed chemical (e.g., 2 to 4 x normal rate) for a maximum of 2 to 3 hours -- then lower injection rate to less than 10 ppm to stabilize operation. Investigate offsite crude handling procedures. Check for
alternative chemical additive package with more effective solids wetting agent.
Excessive mixing valve P Open mixing valve completely, allow amperage to stabilize and slowly increase P to optimum value.
Work Aid 4E: Troubleshooting Desalter Problem of Voltmeter and/or Ammeter Readings Varying Widely and Continuously
Possible Causes Corrective Action
Water level in desalter too high.
Check water level using interface samples; decrease to lowest level that gives good quality effluent and clear water at 30 in. level. Check interface level controller and valve for proper operation; check sensor calibration if necessary.
Stable emulsion formed in
desalter. Increase injection rate and/or change type of demulsifier chemical.
Excessive water injection. Check that wash water rate is between 4% and 6% of oil flow rate; stop wash water injection if controller or water flow meter operation is questionable.
Gas forming in desalter
vessel. Operating temperature too high or back pressure insufficient. Check backpressure valve operation.
Work Aid 4F: Troubleshooting Desalter Problem of Continuous Low Voltage and/or High Ammeter Readings
Possible Causes Corrective Action Stable emulsion has entered
desalter.
Stop wash water injection and operate without water for about 30 minutes. If unsuccessful, decrease interface level and stop desalter operation for about 2 hours and then resume. When voltage returns to normal, resume wash water injection with mixing valve wide open;
slowly increase mixing valve P to optimum. Increase injection rate and/or change type of demulsifier
chemical.
Water/oil interface too high. Check level versus set point using interface-sampling system. Lower water level and confirm proper
operation of interface level control system.
Temperature too high. Check desalter-operating temperature. Check oil conductivity-temperature relationship with desalter vendor. Operate desalter at temperatures where oil is less conductive.
Failed entrance bushing. Check bushing and replace if necessary. Ascertain that transformer connected to bushing is not source of problem before checking bushing.
Failed insulator inside desalter.
Take desalter out of service. Empty and purge the vessel. When entry is permitted, enter vessel, determine which insulator has failed by visual
inspection and/or electrical resistance test, and replace it.
Work Aid 4G: Troubleshooting Desalter Problem of Sharp Increase in Current Draw (Amperage)
Possible Causes Corrective Action Water slug entering with
crude.
Reduce wash water injection rate and check offsite crude handling procedures.
High water level in desalter. Check level controller setting by using interface sampling system. Lower level while retaining good effluent water quality and clear water at 30 in. level.
Work Aid 4H: Petreco Troubleshooting Guide
GLOSSARY
Atm. Col. Atmospheric column in Crude Unit.
APS Atmospheric pipe still.
BS&W Basic (or bottoms) sediment and water content in crude oil expressed as volume percent and determined by a centrifuge procedure.
Demulsification/
Demulsifying
breaking an emulsion
Desalting Efficiency
The percentage of the original salt removed by desalting.
dewatering efficiency
The percentage of wash water plus water contained in the incoming crude that is removed in the desalter.
Emulsification/
Emulsifying
creating an emulsion
kVA Kilovolt-ampere.
kW Kilowatt.
Mixing Efficiency The percentage of feed water used for perfect mixing.
Mixing Index The ratio of the optimum salt content to the actual salt content in the treated oil.
Oil-In-Water Emulsion
Oil as the dispersed phase in a continuous water phase. The effluent brine from the desalter may be an oil-in-water emulsion.
Optimum Salt
Content The best possible desalting obtained when all of the brine droplets are coalesced with all of the wash water droplets dispersed into the crude during the mixing process, and the dispersed water is reduced to the practical minimum in the electrical dehydration step.
Process Efficiency The ratio of the actual to the optimum salt removal efficiencies.
ptb Salt content in oil is expressed as ptb. One ptb is one pound of salt (as NaCl) per thousand barrels of oil, and, depending on the specific gravity of the oil, corresponds to approximately 2.85 wppm.
Stable Emulsion Either a water-in-oil or oil-in-water emulsion wherein the dispersed phase does not coalesce or separate from the continuous phase. Stable emulsion layers can grow in a desalter and result in excessive water and salt carryover into the treated oil, as well as a very oily effluent brine sometimes referred to a "black water."
Water-In-Oil Emulsion
Product of the dispersion of water (dispersed phase) into oil (continuous phase) with the water droplets larger than colloidal size. The feed to the desalter is a water-in-oil emulsion.
REFERENCES
(1) Bartley, D., "Heavy Crudes, Stocks Pose Desalting Problems, Oil & Gas Journal, February 1, 1982.
(2) Non-proprietary information from the ER&E Desalter Handbook and Operating Guide, August 1986.
(3) Manual of Petroleum Measurement Standards.
(4) Non-proprietary information from EPRCo Production Operations Division Surface Facilities School, "Crude Oil Desalting," Volume I, March 1986.
(5) Vendor Brochures.
ADDENDUMS
Section ... Page
Addendum A: Desalter Shutdown and Start Up Instructions...A-85 Addendum B: Desalting Equipment Vendors...B-89 Addendum C: Typical Chemical Analysis of Sea & Aquifer Water... C-91 Addendum D: Relative Desaltability of Various Crudes... D-92 Addendum E: Water Solubility in Crude Oil...E-94