Tray damage flooding

Accounting for a loss in fractionation is a common troubleshooting assignment. For example, crude unit operators find that they can no longer meet furnace oil end-point specs unless they sacrifice furnace oil yield. On one unit, furnace oil production had dropped from 7,000 B/SD to 4,000 B/SD. Possible explanations for this type of problem are tray flooding, improper heat balance, and tray damage. 

A distillation column can flood due to tray damage, undersized liquid downcomers, high liquid level in the bottom of the tower, fouling, or excessive vapor velocity. Only the latter two difficulties are commonly encountered in natural gas conditioning. The troubleshooter should first check for flooding due to excessive vapor velocities. The following correlation may be used for trayed columns 2 ft or more in diameter with a standard 2 ft tray spacing ... 

If a tower does become flooded in the bottom section, a common operator error is to try to pump the level out too quickly. This can easily damage trays by imposing a downward acting differential pressure produced by a large weight of liquid on top of the tray and a vapor space immediately below the tray. To eliminate the flooding, it is better to lower feed rate and heat to the reboiler. It is important to be patient and avoid sudden changes. 

Here, we refer to small amounts of water rather than large slugs that could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood. 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

If the liquid layer is deep (i.e., several of the lower trays may be flooded), vapor may travel through the liquid in the form of slugs. These can loosen bolting and other fasteners, damage trays, or lift trays off their supports (61,150a, 192, 207, 295), collapse packing supports (237), and cause column vibration and structural damage. 

LPG Lean oil stripper Column was pressured up throu a connection in the overhead system while liquid circulated throu its valve tr. The gas could not travel downward, causing mechanical damage to top 12 trays. This later resulted in premature flooding. Always pressure columns fn>m the bottom up, espedaUy when column contains valve trays. 

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