Crude unit is usually seeing water shots due to insufficient tank settling time, However, as soon as DCS operator notices increase in Desalter amps he first suspects high water carryover from Crude. Therefore, he reduces wash water make up to Desalter to reduce net incoming water to Desalter. As per our refinery Desalter control philosophy, Desalter water level is controlled by outgoing brine and wash water flow set point has been given as per requirement. So whenever there is water carryover with crude is occurring, manual intervention is required to control wash water make up. The query is, has any refinery tried to change the control philosophy of desalter to control desalter interface level with wash water addition rate and fixing the outgoing brine flow based on wash water flow requirement% basis. This is to reduce operator intervention during water shot upset.May-2022
Jake Gotham, InSite Technical Services, email@example.com
The objective of the wash water is to remove the salts arriving with the water in the crude. If a crude has higher water content it will have a higher salt content and it is useful to increase wash water to remove the additional salts. What you are proposing would reduce wash water and reduce the effectiveness of the desalter. As you describe, there may be occasional upsets where large amounts of water are carried into the crude unit, and temporarily cutting wash water may be necessary to prevent water carryover into the distillation tower. Rather than design a control system for these events that could be counterproductive in normal operation, effort should be concentrated on preventing water slugs entering the crude unit in the first place:
- Proper tank settling and water-draw procedures.
- Removal of wax banks in crude tanks that might prevent water-drawing being fully effective (consider sonar surveys).
- Introduce tanks to crude feed gradually if they have recently received an import.
- Consider adding a water analyser upstream of the preheat train. This provides warning and allows the operator to prepare by reducing throughput, reducing desalter interface level and adjusting the ratio of crude tanks.