Positive downcomer seal

FIG. 14-25 A common design practice of ensuring a positive downcomer seal. (From Henry Z. Kister, Chem. Eng., December 29, 1980 reprinted courtesy of Chemical Engineering.)... 

Used in low liquid rate systems to maintain positive downcomer seal... 

Inlet weirs. These are used to insure a positive downcomer seal at high vapor rates and low liquid rates as shown in Fig. 2.23a. 

Inlet weir height should equal the cleju ance under the downcomer but be less than that of the overflow weir. Excessive inlet weir height will lead to excessive downcomer backup and excessive weep through the inlet row of perforations and should therefore be avoided. If a positive downcomer seal is required, the inlet weir needs to be higher than the clearance under the downcomer, but this may cause a reduction in downcomer capacity. 

Figure 5.5 Inlet weir provides a positive downcomer seal.

The bottom edge of the downcomer from the tray above should be 0.25 to 0.5 inch below the top edge of the weir of the tray below. This is called a positive downcomer seal. Without a positive downcomer seal, vapor will flow up the downcomer and displace the downflowing liquid. This will cause flooding due to excessive downcomer backup. 

Inlet weirs (Fig. 6.21a) and recessed seal pans (Fig. 6.216) are primarily used for achieving a downcomer seal in cases where a potential sealing problem exists and clearance under the downcomer is limited by one of the design criteria previously cited (Sec. 6.19). These devices provide a positive seal on the tray imder all conditions and ensure that the second and third sealing criteria (Sec. 6.18) are always satisfied. Sometimes it is argued that these devices improve liquid distribution to the tray, but this function is usually performed satisfactorily by the downcomer outlet (48, 172, 257, 404) and can rarely justify using either device. One exception is when the downcomer is circular... 

The need for positively sealing the downcomer is determined in this phase. This could be achieved by installing an inlet weir, which is a weir installed at the tray inlet to keep the downcomer outlet immersed in liquid. A similar device, which extends below the tray floor, is a seal pan (Fig. 17). Both devices provide positive assurance against vapor rising up the downcomer, but they may also trap solids and dirt and cause blockage. A seal pan must always be used in the downcomer from the bottom tray otherwise there is nothing to prevent vapor from rising up the bottom downcomer. 

Figure 5 shows typical test facilities used by different investigators. In configuration, they all consist of a fast column or riser, a gas-solids separator, a downcomer for solids recycle, and a loop seal valve and/or an additional controlling device for adjusting solids circulation rate, mounted in positions appropriate with the inlet and outlet geometry. These facilities can be grouped into three types. 

The segmental, or chord downcomer, shown in Figure 11.26a is the simplest and cheapest form of construction and is satisfactory for most purposes. The downcomer channel is formed by a flat plate, called an apron, which extends down from the outlet weir. The apron is usually vertical but may be sloped (see Figure 11.26b) to increase the plate area available for perforation. If a more positive seal is required at the downcomer at the outlet, an inlet weir can be fitted (see Figure 11.26c) or a recessed seal pan used (see Figure 11.26d). Circular downcomers (pipes) are sometimes used for small liquid flow rates. 

For sloped downcomers adjust W (weir length) to the correct value for the downcomer bottom edge. Downcomer clearance, //dcc, is typically a minimum of lin. (25.4 mm) and is usually 0.5 in. (12.7 mm) less than the outlet weir height to maintain a positive seal on the downcomer. Calculate the velocity under the downcomer ... 

The discharge end of each downcomer pipe must be positioned close to the surface of the plate below, so that the free end will be immersed in the liquid level on that plate. This forms a liquid seal over the open end to keep vapor from entering the pipe. By positioning successive downcomer pipes on opposite sides of each sieve tray, the liquid flows across each plate, minimizing any stagnate flow sections and helping move any solids that might accumulate from the distillation column. 

Vapor may flow up the downcomer. A positive seal may be needed to prevent this, as discussed below. 

Figure 4.12 Downcomer trapout pan arrangements, (a) No positive seal b, c) with positive seal (c ) with pumparound, no positive seal (e) with pumparound and positive seal.

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