Tray deck design

FIG. 14-79 Cost of trays in plate towers. Price includes tray deck, bubble caps, risers, downcomers, and structural-steel parts. The stainless steel designated is type 410 Peters and Timmerhaus, Plant Design and Economics for Cbemical Engineers, 4th ed., McGraw-Hill, New York, 1.9.91). 

Lieberman gives two rules of thumb for troubleshooting fractionators that could also be used as checks on a design. First, the pressure drops across a section of trays must not exceed 22% of the space between the tray decks, to avoid incipient flood. Mathematical , hold... 

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. 

This concept is the basis for tray design for perforated tray decks. While various valve tray vendors maintain that this rule does not hold for their equipment, it is the author s industrial experience that valve trays leak just as badly as do sieve trays, at low vapor hole velocities. To summarize ... 

As the weir height of the trays is 3 in, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 3.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

From the designer s point of view, the top tray of the stripper must have a several times greater number of sieve holes or valve caps on its tray deck than the bottom tray. If, however, all the trays in the stripper are identical, then either the bottom tray will leak (see Chap. 2), or the top tray will flood. Either way, stripping efficiency will suffer. 

Tray deck nonconventional active area design... 

In the last decade, the nonconventional tray added downcomer active area design (Fig. 3.4) has claimed good success. Please note that this design offers a larger tray deck active area than the conventional one in... 

Fig. 3.1. Also note that this nonconventional design has the downcomer outlet area as additional active tray area. This additional active area is the tray deck area under the downcomer having valves, bubble caps, or sieve holes that allow the gas to pass through under the liquid downcomer area of the next tray up. ICPD tray programs dealing with the design and rating of sieve, bubble cap, and valve-type trays allow this active area input. This is an option shown in Table 3.1, which is offered in the three tray design/rating computer programs given in this book.

Valve sticking can be alleviated by proper selection of valve type. Several designs have the valves seated on tumed-down nibs which hold the disk about 0.1 in (138) above the tray deck to prevent sticking (Figs. 6.5a, 6.6a, b, d). This clearance, however, may enhance the weeping tendency and somewhat reduce turndown. One experience has been reported (85) where weeping from this type of valve at low liquid rates did not permit adequate liquid circulation in a pumparound loop. 

Trays can flood even below design loads because of fouling of the tray decks. Flooded trays lose fractionating efficiency (see Chapter 19). Check the pressure drop across the suspect trays. If the pressure drop per... 

The shear clip must be designed to break away from the vessel wall at some predetermined stress. Otherwise, a powerful pressure surge could deform the wall of the vessel itself. However, if the shear clips can be designed to be strong enough to withstand a AP across the tray deck of about 0.6 psi (i.e., about 2 ft of hydrocarbon liquid) the downcomers would blow clear before the shear clips would fail. Hence, the downcomers would act as a partial safety bypass and protect the tray decks from damage. 

Because this problem occurred in December, it was possible to operate the isostripper well below the design pressure. To increase the volumetric vapor flow (but not the mass flow) through the tray decks, we suggested that the tower pressure be lowered from 120 psig to 65 psig. The objective was to increase the dry tray pressure drop by about 1 in. of liquid per tray. 

Purchase new tray decks with valves especially designed for a high turndown ratio. This suggestion is only applicable to nonfouling services because trays of this type have flat valves that easily stick to the tray deck. Note that the existing downcomers can be retained. 

Grid tray Similar to a sieve tray. A perforated distillation tray deck, but with no movable valve caps. Represents modern distillation tray design. 

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. 

Contract maintenance workers often will not replace the tray manways unless the tray manway is adjacent to a tower external manway. They reason that once the tray manways that are visible from the tower manway are closed, there is no way for someone to inspect the other trays. This problem is not just common— it is imiversal. The maintenance force at the Good Hope Refinery pulled this nasty trick on me at the coker fractionator. Equipped with my crescent wrench, I opened the tray internal manway below the side tower manway. 1 discovered that the 12 trays below this point had their manways stacked in their downcomers. In 1990,1 worked on a project to improve fractionation at the Chevron Refinery crude distillation unit in El Segundo, California. When the tower was opened to implement my design, the tray manways were found lying on the tray decks below the diesel draw tray. The lesson is, inspect each tray and then witness the closure of each tray manway, separately. 

Tray deck panels aligned correctly (as per design). 

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