Leakage from tray

LEAKAGE AND WEEP HOLES. To ensure a minimum of liquid leakage from one tray to another, bubble-cap risers should be fitted firmly or sealed into the trays, and an effective seal is necessary around the tray supports. In large columns, the trays are often made in sections for ease of installation. Each of these sections must be installed carefully to minimize leakage. 

To evaluate evaporative loss of fluid from the capillary gap, the instrument was programmed to dispense 100 )xL of colored tap water into the gaps created between coverplates and clean, blank slides. The waste tray was filled with 50 mL of water, and the instrument lid was closed. Slides were observed for a 2-h period, and fluid loss was assessed. For the first 60 min, there was no appreciable fluid loss. At 90 min, an average of 7 mm (fluid height in the gap) was lost. At 120 min, an average of 12 mm of fluid was lost. These measurements correspond to a total loss of fluid from the gap of 24% over the 2-h period. Whether the fluid loss was the result of evaporation or slow, gravity-induced leakage from the capillary gap was not assessed. 

The effect of liquid leakage from the tray is more important for stripping trays than for rectifying trays. For stripping trays, liquid leakage carries more volatile components toward the draw point for the less volatile product, thus reducing the effectiveness of the stripper. Fortunately, for most bubble cap trays, there is no tray leakage of consequence. 

Only specific incidents were included. For instance, a statement such as "Leakage from chimney trays in refinery, vacuum columns can be reduced by seal-welding tray sections does not constitute a case history. On the other hand, a statement such as "One vacuum column experienced severe chimney tray leakage at low-rate operation. Seal-welding tray sections reduced leakage to acceptable levels does. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Cable tray fires have resulted from the accumulation of process leakage, residues or combustible dusts or debris on top of cable trays with densely packed wires and cables. Covering shields above cable trays have been used to minimize such accumulations however, such shields should be used with caution as they can limit air circulation and increase cable and wire temperatures in densely loaded trays particularly those carrying power cables. Covering shields may also block fixed fire protection water spray or manually applied firefighting water spray from reaching the cables. 

A technique which can positively eliminate leakage is using a welded internal coliunn head instead of a chimney tray for liquid collection (Fig. 4.11c, d). The vapor can travel from the column section below the head to the section above either through risers or via an external pipe. This technique is expensive, and its use is restricted to situations where no leakage below the collection point can be tolerated. 

In some services (e.g., refinery fractionators), vapor approaching the chimney tray is hotter than the chimney tray liquid. Heat will be transferred from the vapor to the liquid. If the vapor is condensable, some will condense on the bottom face of the chimney tray. The net result is analogous to leakage. The author is familiar with situations where refractory was installed on the bottom face of the chimney tray. In all these cases, steps were also taken to minimize leakage, making it difficult to independently assess the effectiveness of the refractory. For multicomponent, partially condensable vapor condensing on an uninsulated bottom face of a chimney tray (e.g., in a refinery fractionator), a typical heat transfer coefficient is 15 Btu/(h ft °F) (237). 

Weep holes usually range in size from Vi to % in (48, 73, 86, 237, 248, 257, 371, 409). Small holes are preferred in clean, noncorrosive services but should be avoided in fouling or corrosive applications. The recommended hole area is 4 in per 100 ft of tray area (48, 257, 371, 409). This hole area will drain a column containing 50 bubble-cap (or other leak-tight) trays with 4-in weirs in about 8 hours (48, 371). Alternatively, the required weep-hole area can be derived from the required drainage time (usually a few hours) by using Bolles (48, 371) or Lockhart s (248) procedure. Excessive hole area will cause leakage, and should be avoided, especially in draw pans one case in which this was troublesome was reported (231). 

A valve tray used in trapout service excessive leaked. The leakage was eliminated seal-welding trqy sections, and by welding a strip onto the periphery of the tray, a few inches from the siqpport ring. 

Lube oil was drawn as a side cut from a trapout tn (total drawoff). When all tower bafQe trays were r laced by valve tntys, lube oil rate declined 1 12% due to leakage at the trapout tray. To restore the original draw rate, the traqxnit tray was replaced first by a bubble-cap tray, then by a valve tray with venturi openings finally the outlet nozzles were 63q>anded. Each of these steps progressively further lowered the lube oil rate. Installation of a seal-welded chimn trapout tray solved the problem and achieved a lube oil rate 19% above original. 

Premature flooding occurred on the third tray from the bottom because of variations on this trty that caused vapor leakage into the downcomer. Gamma scans identified troublespot... 

Small-diameter columns. When column diameter is less than 3 ft, it is difficult to access the column from inside in order to install and maintain the trays. Cartridge" trays are often installed, or an oversized diameter is used. Either option is expensive. Cartridge trays also run into leakage and hold-down problems (40). Packing is normally a cheaper and more desirable alternative. 

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. 

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