January 17, 2020 at 6:22 am #31035
What are the analyze in the streams of the main tower to know the corrosion level in this equipment ? At the sour water of top vessel we analyze Chloride, pH, NH3, H2S, Fe, Cu, Ni, Zn and in the hydrocarbon streams we are analyzing chloride. Is there any technique that is possible and reliable to measure the thickness of main tower in operation ?
January 20, 2020 at 4:24 pm #31038
I think you are measuring the necessary components to have notification of a change in the corrosion in the overhead system. If you have a cooling water condenser in the overhead circuit you should measure hardness in terms of calcium and magnesium ions in the sour water. A tube leak can put cooling water into the overhead accumulator and that hardness will foul the sour water stripper.
Corrosion in the overhead system requires liquid water, so calculating the water dew point and keeping the tower top temperature at least 25 F higher than that calculated temperature is important to keep water from forming inside the tower and corroding the tower trays or the vessel itself. Sometimes that 25 F margin is not enough, typically due to ammonium chloride salts. Using the information from the analysis of the sour water and the operating conditions of the tower the ammonium chloride salt point can be calculated. As the Delayed Coker generates ammonia as one of the products of the thermal decomposition of the feed, keeping the chlorides in the feed to a minimum by optimizing the desalter in the crude unit is the primary control strategy for minimizing chlorides in the overhead of the Coker fractionator.
Preventing water from condensing until the overhead condenser is the typical corrosion control strategy for Coker overhead systems. At the overhead condenser, wash water is added to force a liquid water phase. Enough water is added to dilute the acidic materials so they do not cause any metal loss. An adequate water wash is also used to prevent damage from cyanides in the overhead system.
It is common to have inspectors measure the overhead piping wall thickness on some regular schedule using UT while the unit is in service. This gives you information on the day of the inspection but does not provide any ongoing information. I have been involved in gluing an UT transducer to the piping and routing that signal into the control room. There is real time wall thickness information from that one point. Having multiple transducers provides a more general metal loss picture.
I am aware of a technique called pulsed eddy current testing that can provide the average wall thickness of the piping between the pulse generator and the detector. I do not have any experience with that technique.
January 26, 2020 at 3:03 pm #31050
We usually have a huge corrosion in the wall on the top section of main tower, above LGOk. Is there any technique that is possible to measure the wall thickness of the main tower running ? In our case we stopped the tower, cooled down and measured the wall thickness by ultrasom. We also we are using the technique Pulsed eddy current testing with the main tower in operation. Are this technique reliable to measure the wall thickness of the main tower in operation ? It seems that the corrosion on the tower wall is from HCl formed on the top . How do we detect this before the tower damage occurred ?
January 26, 2020 at 4:06 pm #31051
The typical strategy to prevent corrosion of the fractionator is to have the tower top temperature higher than the water dew point and the ammonium chloride salt point. This type of corrosion is usually caused by wet salts. Ammonium chloride salts that are dry are not corrosive; however, they can adsorb moisture from the vapors and become wet. These wet salts can be very corrosive. Additionally, these salts can cause the pressure drop through that section of the tower to increase such that the tower will flood at lower than design rates. These salts can also plug the tower and prevent liquids from traveling down the trays. Monitoring the tower pressure drop is a way to know if these salts are forming. If they have formed there will be associated corrosion.
The concentration of chlorides and ammonia in the overhead accumulator water can be used to calculate the salt point at the top of the tower. Chloride concentrations of as low as 5 ppm can correlate to a salt temperature as high as 125 C. There are a number of assumptions in that comment. Still, if the Coker fractionator tower top temperature is less than 125 C and the chlorides in the overhead water are over 5 ppm, there is a very good likelihood that ammonium chloride salts are depositing inside the tower.
I usually recommend the tower top temperature be a minimum of 15 C higher than the calculated water dew point. I then calculate the ammonium chloride salt point. If that temperature is higher than the water dew point, I would recommend increasing the tower top temperature to prevent ammonium chloride salt deposition while attempting to reduce the chloride content in the Coker feed.
Improving desalter performance to reduce the chlorides in the feed to the Coker is the first line of defense. Raising the Coker fractionator tower top temperature is the second option. There are ammonium chloride salt dispersants that have been supplied by the various chemical vendors, but those dispersants do not remove the salts from the tower.
I am not aware of any other technique to measure the tower wall thickness while the unit is in service.
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