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Reply To: Tray Collapse

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Mike Kimbrell

There are a few common mechanisms that can lead to damaged trays in the upper portion of the fractionator.

The first is corrosion of the trays due to ammonium chloride salt formation. Keeping the tower top temperature above the salt point is the mitigation strategy. Cokers make ammonia, so limiting the chloride salts in the feed is how to lower the salt point by good salt removal in the desalters in the upstream crude unit. Monitoring the fractionator overhead accumulator water for chlorides, ammonia, iron and any other metals that the overhead condensers are constructed to monitor trends and to predict the ammonium chloride salt point temperature is something that should be done routinely, once per week as a minimum. The water dew point should also be calculated routinely and the tower top temperature should be comfortably higher than the water dew point at all times. Ammonium chloride salts will adsorb water vapor if the gas stream is too close to the water dew point and make wet salts that are extremely corrosive if they have been deposited inside the tower.

Excessive vapor rates that increase the tray pressure drop higher than their design uplift capacity is another way trays are damaged. Excessive steam rates during on-line spalling or during the little steam step could cause high vapor rates that could overcome the tray’s ability to resist. As the other two have indicated, liquid water that rapidly vaporizes will generate excessive vapor rates that can damage the trays from excessive uplift.

Flooding the trays enough to back up liquid to the tray above can result in excessive force being put on the tray and exceeding the tray uplift limit. This requires a two phase liquid at high velocity to be pushed through the holes on the tray and that increased vapor density increases the uplift. The flooding can be caused by salt deposition on the tray or corrosion products restricting the tray openings.

There are a few other less common mechanisms. What are the symptoms of your tower problem? Did it just show up or was there an event on the unit that initiated it?

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