Trays deck levelness

For smaller diameter towers a visual check of tray deck levelness is sufficient. For two-pass trays, a small-diameter tower is less than 8 ft. For single-pass trays, a diameter of less than 6 ft is small. 

For towers of 10 ft or more in diameter, check for out-of-levelness of a tray check using a carpenter s laser level, available in hardware stores for about 40. Purchase a level that has short tripod legs. Use the bubble to level up the legs. Set the level on one end of the tower, and check the height of the red beam at the other end and at the center of the tray for out-of-levelness. As it is often dim and dusty in the tower, the trace of the red laser may be clearly visible. Low points and areas of the tray deck that are out of level can now be easily identified. 

The more level the tray, the better the mixing efficiency between vapor and the liquid. Certainly, if the tray out-of-levelness is greater than the height of the weir, tray efficiency will be badly degraded. 

Checking for weir out-of-levelness is easy. Set the laser level on the edge of the weir. Using the bubble glass level indicator, adjust the laser level to a true horizontal position. The line of red light compared to the top of the weir will indicate how much of the weir is out of level. A weir that is more than 0.5 inch out of level should be re-adjusted. If it is not, stagnant liquid pools behind the higher section of the weir, as described in the prior chapter, will result and ruin the tray s efficiency. 

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To summarize. I ll select a hole area for the tray, so that the velocity of vapor flowing through the holes will be big enough to keep the tray from leaking. Of course, if the tray decks are badly out of level, the above calculations are meaningless. So don t forget to inspect your tray installation for tray deck levelness (see my book. Process Equipment Malfunctions, McGraw-Hill, 2011). 

The sum of the crest height plus the weir height equals the depth of liquid on the tray deck. One might now ask, Is not the liquid level on the inlet side of the tray higher than the liquid level near the outlet weir While the answer is Yes, water does flow downhill, we design the tray to make this factor small enough to neglect. 

We have yet to discuss the most important factor in determining the height of liquid in the downcomer. This is the pressure drop of the vapor flowing through the tray deck. Typically, 50 percent of the level in the downcomer is due to the flow of vapor through the trays. 

Figure 1.8 is a realistic picture as to what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is the flow of vapor through the tower. The spray height of this entrained liquid is a function of two factors ... 

One of the most frequent causes of flooding is the use of carbon steel trays. Especially when the valve caps are also carbon steel, the valves have a tendency to stick in a partially closed position. This raises the pressure drop of the vapor flowing through the valves, which, in turn, pushes up the liquid level in the downcomer draining the tray. The liquid can then back up onto the tray deck, and promote jet flood, due to entrainment. 

On the other hand, bubble caps (or even the more ancient tunnel cap trays) are different, in that they do not depend on the vapor flow to retain the liquid level on the tray deck. More on this later. For now, just recall that we are dealing only with perforated tray decks. 

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck. 

The common reason for out-of-levelness of trays is sagging of the tray decks. Sags are caused by pressure surges and sloppy installation. Sometimes, the tray support rings might not be installed level or the tower itself might be out-of-plumb (meaning the tower itself may not be truly vertical). 

Let s further assume that the orifice plate distributor is 1 in out-of-level. This could easily happen in a 14-ft 0-in-ID tower. Figure 7.2 shows the results. The flow of internal reflux or liquid through the higher portion of the tray deck falls to zero. Worse yet, vapor starts to... 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

The most common cause of tray deck flooding in any fractionator is high liquid level. When the liquid level rises above the steam inlet, the stripper will flood. I asked Zip if they were maintaining a normal liquid level in the bottom of the diesel oil stripper. In response, he indicated a circular chart. A pen inscribed a perfect red circle on the chart at the 65% level. The fact that the indicator level did not vary made me suspicious. 

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. 

Make sure trays are installed level. Tray decks with low points will leak and lose tray efficiency at relatively high vapor rates. The more level the tray, the greater the tower s turndown ratio. 

Most often, tower packing supports are dislodged, or distillation trays upset, when a tower is operated with an excessive bottom liquid level. Forcing heat into a tower when the liquid level is several feet above the bottom tray deck often results in dislodging the bottom few trays. Occasionally, trays are mis-assembled during a turnaround. The results of either of these misadventures is diminished fractionation efficiency. 

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. 

Foam-induced flooding has been considered a problem occurring on tray decks or inside packed beds. Certainly this is correct. For many columns, however, it is a high foam level formed in the bottom of the tower which causes premature flooding. When this foam level rises to cover the reboiler vapor return nozzle, flooding results. 

I assume this has happened because 1 have accidentally resealed the downcomer above the out-of-level tray deck. 

However, the largest operating cost for many process units is the energy supplied to the reboilers. We should therefore avoid high reflux rates, and try to achieve the best efficiency point for distillation tower trays at a minimum vapor flow. This is best done by designing and installing the tray decks and outlet weirs as level as possible. Damaged tray decks should not be reused unless they can be restored to their proper state of levelness, which is difficult, if not impossible. 

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