Strictly, the Jeronimo and Sawistowski correlation predicts clear liquid heights at the froth to spray regime transition. However, it has been shown (27,92) that clear liquid height in the spray regime is much the same as clear liquid height at that transition.

Valve trays. Development of a clear liquid height correlation for valve trays has been inhibited by difficulties in measurement of clear liquid heights on these trays. A number of correlations have been proposed (86,93-95), but questions have been raised about their applicability (12,86,87). None has been tested against a sufficiently large data base to be recommended for general use. Lockett (12) prefers Dhulesia's »86) and Brambilla's (95) correlations to the others.

&3.6 Turndown

The turndown ratio is the ratio of the normal operating (or design) vapor throughput to the minimum allowable vapor throughput. The minimum allowable throughput is usually at the excessive weeping limit, while the normal operating throughput is a safe margin away from the relevant flooding limit.

Sieve trays have a poor turndown (about 2:1). The minimum operating throughput in sieve trays is almost always restricted by excessive weeping. Turndown of sieve trays can be improved by

■ Blanking some tray holes: Blanking reduces the fractional hole area on the tray, thus decreasing weeping, However, reducing fractional hole area also decreases maximum capacity. A shutdown in which the blanking strips are removed is necessary before full capacity is reestablished. Blanking practices are discussed elsewhere (1).

■ Using low fractional hole areas: A fractional hole area reduction to about 5 percent of the bubbling area typically boosts sieve tray turndown to about 3 to 4:1 at the expense of a lower maximum capacity, i.e., of a larger column diameter. This technique is not recommended because traying the column with valve trays is normally a cheaper alternative,

Valve tray turndown is normally about 4 to 5:1. The minimum operating rate in valve trays is usually restricted by excessive weeping, but it may also be restricted by the onset of vapor channeling (Sec. 6.2.13).

Special valve designs achieve higher turndown. One design (7,74,96; Fig. 6.3b) has a light orifice coverplate contained within a heavier cap. The cap can move vertically within a cage. At low vapor rates, the light orifice coverplate is lifted. At its maximum travel, it is pushed against the heavier cap. At high flow rates, both the plate and the heavier cap travel upward until reaching the cage. FRI data (96) demonstrate a turndown better than 6:1 to 10:1 with this design.

Another high-turndown valve design (Figs. 6.3c and 6.4d) has rectangular valves oriented with their long edge parallel to the liquid flow so that the slot opening facing the approaching liquid flow is minimized. Data by FRI and by Van Winkle et al. (9,97) demonstrate a turndown better than 7:1 to 9:1 with this design. A third design (98) has a caged vertically moving plate with an or ifice drilled in the plate. At low rates, vapor issues only from the orifice; at higher rates, the plate itself moves. Turndown of up to 12:1 (98) was claimed with this design.

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