Tray deck leaking

K = 0.18 to 0.25 tray operation close to its best efficiency point K = 0.35 to 0.40 tray suffering from entrainment—increase in reflux rate, noticeably reduces tray efficiency K = >0.5 tray is in fully developed flood—opening a vent on the overhead vapor line will blow out liquid, with the vapor K = 0.10 to 0.12 tray deck is suffering from low tray efficiency, due to tray deck leaking... 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

K = 0.10 to 0.12 Tray deck is suffering from low tray efficiency, due to tray deck leaking. 

Possibly 90 percent of the trays seen in the plant are of these types. Perforated tray decks all have one feature in common they depend on the flow of vapor through the tray deck perforations, to prevent liquid from leaking through the tray deck. As we will see later, if liquid bypasses the outlet weir, and leaks through the tray deck onto the tray below, tray separation efficiency will suffer. 

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 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. 

Tray 2 now has most of its vapor feed flowing up through its outlet downcomer, rather than the tray deck and most of its liquid flow, leaking through its tray deck. 

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. 

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. 

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. 

Figure 18—13 illustrates how sagging tray decks can aggravate the problem of low vapor flows. Also, the preceding discussion is almost as applicable to valve trays as to sieve trays. Valve trays most certainly leak at low vapor rales regardless of any vendor claims to the contrary. 

For perforated tray decks (sieve or grid trays), I calculate the pressure drop of the vapor flowing through the holes (inches). The pressure drop I want is the weight of liquid on the trays that I calculated above in step c. The idea is to keep the tray from leaking through the tray deck perforations. 

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). 

Trays are leaking because the deck hole area is excessive for a given vapor rate. 

---