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Reply To: Main Tower Corrosion

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Mike Kimbrell

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.

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