Startup stability diagram

Below the dump point (100 percent weep), tray efficiency is a small fraction of its normal value, and mass transfer is extremely poor. Since no liquid enters the downcomers, they lose the liquid seal that prevents vapor from rising through them. Operation below the dump point can be accompanied by severe hydraulic instability due to unsealing of downcomers, as was demonstrated by field experience (76). The startup stability diagram (1), which defines the range of vapor and liquid rates needed for satisfactory startup, has the dump point as the lower limit- The tendency of dumping increases when (77-79)... 

Figure 6.17 Typical startup stability diagram. (Henry Z. Kister, Hydrocarbon Processing, February 1979. Reprinted courtesy of Hydrocarbon Processing.)...

Both curves in Fig. 6.17 (especially the upper curve) are very sensitive to some of the downcomer layout design parameters, particularly to the clearance under the downcomer, and the design of inlet weirs and seal pans. Figure 6.18 illustrates the effect of reducing the clearance imder the downcomer, from 1.5 to 1.0 in on the startup-stability diagram shown in Fig. 6.17. 

Figure 6.18 shows the effect of the clearance under the downcomer on the startup-stability diagram. The smaller the clearance, the higher the upper curve in the diagraun, and the easier the startup. Smaller clearances restrict vapor flow up the initially empty downcomer during startup. Consequently, the impedance to downflow is reduced. 

Equations for constructing startup stability diagrams were presented in Kister, H.Z., Hydrocarbon Proc. Feb., 1979, p. 89. Below are revisions and updates to the set of equations in the above reference. The equation number and the symbols match those in the original reference. 

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