Jet flooding

Capacity cause Steam reboiler flow set above column jet flood limit... 

Jet Flood. Flooding generally occurs by jet flood or downcomer backup. Reference 15 gives Equations 1, 2, and 3 for Jet flood, using Ballast trays. 

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

Because the tray is not near jet flooding, referring to Figure 8-149,... 

Note that jet flooding capacity is fairly insensitive to system physical properties, but that the system limit capacity is strongly dependent on physical properties. 

System limit flooding is similar to jet flooding, due to low surface tension and low density difference between liquid and vapor. Terminal velocity of some entrainment droplets is less than the upward vapor velocity, and hence they are carried up into the tray above, thus reducing tray efficiency and capacity. 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

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. 

Of course, any factor (dirt, polymers, gums, salts) that causes a reduction in the open area of the tray deck will also promote jet flooding. Indeed, most trays flood below their calculated flood point, because of these sorts of problems. Trays, like people, rarely perform quite up to expectations. 

Have about 15 percent less capacity because, when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding. 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

The Vload factor is used to determine what is known as jet flood. Jet flood is simply the liquid jetting, causing liquid to recycle from one tray back to the tray above, from which the liquid passed. In some cases jetting can be so severe that it blocks the gas passage with pressure buildup. The following equations calculate Vload ... 

Referring to the explanation of liquid jet flooding (see Eq. (3.29), tray Vload factor). Jet flooding is accountable in a factor named the capacity factor for valve-type trays CAFO. The following equations derive CAFO. For vapor density Dv (lb/ft3) equal to or less than 0.170, then ... 

Equation (3.91) is the jet flood equation. The chief difference between this equation and the entrainment flood equations, (3.88) through (3.90), is the area references. Equation (3.91) is based on the total sieve tray hole area for gas passage, and Eq. (3.88) through (3.90) are based on the tray active area. Again, the tray active area is simply the tower cross-sectional area less the total downcomer area. 

The jet flood equation is also based on the work of Souders and Brown [12]. This equation computes the ratio of the square power of the vapor load (noted in this chapter as Vload) to a constant, 8.75, to derive the tray flood. It has been used for over three decades by tray vendors (Koch and F.W. Glitsch [3]) to design and rate sieve-type trays. In many cases, especially for sieve-type tray design, jet flood governs tray flood and thus is the primary sieve tray design and rating equation. In contrast, Eqs. (3.88) through (3.90) are the principal equations for flood determination for both valve- and bubble cap-type trays. 

Please note that HOLHA is the total hole area in ft2 on a single tray deck. It is used in Eq. (3.91) to calculate sieve tray jet flood and will be used to calculate sieve tray pressure drop as well. 

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

Flooding- mechanism Entrainment (jet) flood only Tray typee Sieve or valve trays only Pressure 1,5-500 psia (Note 1)... 

Jet flooding occurs due to liquid entrainment induced by vapor jets passing through the liquid flowing on the tray. The entrained droplet may carry into the tray area above and reduce tray efficiency and capacity. 

Jet flooding includes situations where the rising vapor velocity is sufficient to prevent liquid droplets from disengaging from the vapor above the tray deck. Jet flooding can be reduced by increasing tray spacing. 

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