Tray deck leakage

Many towers equipped with valve or sieve trays do not operate efficiently at low feed rates. This is due to tray-deck leakage. As the pressure drop of the vapor flowing through the sieve holes falls below the weight of the liquid on the tray deck (as determined by the height of the weir), the tray will start to leak. 

K for sieve trays is 0.3 K for grid trays is 0.6. Only an idiot would use movable valve trays. The valve caps stick to the deck and do not greatly retard tray deck leakage. 

Don t use movable caps (i.e., valves). They do not greatly decrease tray deck leakage at low vapor rates. But they do stick to the tray deck and cause premature flooding. 

The mass-transfer devices may be sieves (holes), fixed valves, moveable valves, or bubble caps. Fig. 2 shows a selection of mass-transfer devices. The purpose of the device is intimate mixing of the vapor and liquid on the tray deck. An ideal device has high capacity, high flexibility, low leakage, low pressure drop, and low cost. 

Valve trays cannot be tested for leaks. One cannot simply look at a tray installation and conclude it is tight. To measure leakage, a water level must be established on the tray deck with the rate of leakage actually measured, and the rate of leakage cannot be determined with a valve tray. 

Common distillation tower valve trays are assembled from a number of sections bolted together inside the tower. The seal between the tray sections (unless gasketing strips are used) is far from leakproof. The valves themselves (i.e., the little bubbling devices on the tray decks) are also subject to leakage. 

Replacing tray decks and downcomers with Monel steel (high nickel content) will help prevent this corrosion. The area underneath the downcomers are especially prone to this type of leakage due to hydrochloric acid attack. 

Trays suffer from lost tray efficiency as a result of both flooding and dumping. Trays have some entrained droplets of liquid lifted by the flowing vapors to the trays above. This tends to blow butane up into the lighter propane product. Perforated trays always have some leakage of liquid through the tray deck to the trays below. This tends to drip propane down into the heavier butane product. 

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