Upset Tray Decks

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Although trays can corrode through, a more common cause of damage is unit upsets. A high liquid level, above the flash zone, will cause the trays to be bumped by the up-flowing vapors. Slugs of water can dislodge tray decks when the water suddenly flashes.

It seems that many tray upsets occur during short unit outages. At such times steam may back into the fractionator and form pockets of water. Also, any operating malfunction that causes a pressure surge in the overhead or flash zone (check the pressure recorder) can disrupt tray decks. If the operating engineer sees a loss in fractionation following such incidents, he should proceed as follows:

1. Quantify the reduced fractionation. This is done by measuring the 5%-95% gap. For example, the gap between heavy naphtha and furnace oil is calculated by subtracting the naphtha, 95 vol% American Society for Testing Materials (ASTM) temperature, from the furnace oil, 5 vol% ASTM temperature. Compare this gap against historical data. A reduction of 20°F to 30°F in the gap is significant.

2. Check the pressure drop across the trays. Low pressure drop is symptomatic of broken tray decks. If the pressure drop per tray (in feet of liquid) is less than 10% of the tray spacing, tray damage and/or dry Irays are quite likely.

3. X-ray the tower. On one 11-ft diameter fractionator, up-ended tray decks could distinctly be seen on the X-ray film.

Recommending to management that a crude unit be shut down to repair trays can be a milestone in an operating engineer’s career. If the trays are found intact when the tower is opened, the recommendation can be an albatross around one’s neck. X-raying is a good way to avoid this risk.

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