Q & A > Question Details
Can anyone suggest/advise the procedure to reduce upsets during a water shot problem in the CDU during tank changeover or slop processing? Either design changes in the desalter or any equipment internals, modification etc.
18/03/2021 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
I am not aware of any practical ways to eliminate this problem by design changes in the CDU. Prevention by good tank management is the key. Your procedures should include:
• Adequate settling time after import before putting a tank on feed. How long is adequate is something most sites learn by experience, and can be different for different crudes due to density, viscosity and water content variation. 24-48 hours is typical.
• Frequent water drawing during the settling period.
• Pre-feed checks for water at the water-draw nozzle and oil nozzle elevation.
• Introduce the tank to the crude unit at low rates, then increase up to the target percentage after a couple of hours if everything is stable. If the crude is a problem crude or there has been pressure to feed the crude before the usual settling time, the tank should be introduced at an even lower percentage initially.
• Management of the build-up of solids at the bottom of the crude tanks. If wax-banks or sediment are allowed to accumulate, they can hold water back and prevent it reaching the water-draw. If the tanks have a long inspection interval, it may be necessary to take intermediate tank shut-downs to clean the tank. Specialist companies can scan a tank to draw a profile of wax-banks with the tank still in service.
• Slop segregation to minimise slop water in the slop oil tank and vice versa.
• Continuous pressure on slop production and slop processing to minimise the requirement to transfer slop to a crude tank.
• If water does reach the CDU, the operator would usually reduce the furnace temperature rapidly but it may be too late to prevent damage. Signs of wet crude include:
o Sudden spike in atmospheric tower pressure.
o Erratic readings of furnace pass-flow meters.
o Changes in crude or product temperatures in the pre-heat train.

I hope that is helpful.