Turndown valve trays

A wide range of loadabihty (turndown ratio) is required. Valve trays are particiilarly desirable in this case. 

At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor. 

Figure 8-71 A. Nutter MVO high performance fixed valve tray with 4 1 turndown ratio. Used in new installations and to replace sieve trays. Used by permission, Nutter Engineering, Harsco Corp., Bui. CN-4.

Turndown can be applied to all types/styles of tray columns however, it is more relevant to sieve and valve trays. The generally accepted explanation of turndown is as follows [199] (also see Figure 8-101) ... 

Bubblecap trays are used only when a liquid ievei must be maintained at iow turndown ratio they can be designed for lower pressure drop than either sieve or valve trays. 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Fixed valve and sieve trays prevail when fouling or corrosion is expected, or if turndown is unimportant. Valve trays prevail when high turndown is required. The energy saved, even during short turndown periods, usually justifies the small additional cost of the moving valve trays. 

Typical open-slot areas for moving valve trays are 14 to 15 percent of tfie bubbling area. Here the higher hole areas can be afforded due to the high turndown of the valves. 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

The turndown of valve trays is much better than sieve trays, but not as good as bubble-cap trays. Bubble-cap trays are the moBt suitable to handle extremely low liquid rate applications (less than 2 gpm per foot of average flow width (10)]. 

Turndown About 2 1. Not generally suitable for operation under variable loads About 4-5 1. Some special designs achieve (or claim) 10 1 or more Excellent, better than valve trays. Good at extremely low liquid rates Low, even lower than sieve trays (10). Unsuitable for variable load operation... 

For most other services, either sieve or valve trays are the best choice, Sieve trays are at an advantage when the service is fouling, or corrosive, or when turndown is unimportant, while valve trays are preferred when turndown is important. With high energy costs, the energy saved during even short turndown periods usually justifies the relatively low cost difference between valve and sieve trays. This has made valve trays most popular. 

Several experiences of severe weeping from valve trays have been reported (1,71,75). A well-designed valve tray is unlikely to have too many valves, but trays with light valves are common in an effort to reduce pressure drop. To avoid the turndown problems, manufacturers often specify a valve tray with two valve weights (Fig. 6.19e). When the light valves open, the heavy ones are still shut, which reduces the ratio of slot to active area and avoids weeping. This practice is discussed in detail elsewhere (1,71). 

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). 

Turndown. Valve and bubble-cap trays normally give better turndown than packings. Unless very expensive distributors are used, packed tower turndown is usually limited by distributor turndown. With random packings, dewetting may also limit turndown. 

When conparing tray designs the turndown ratio is inqjortant because it is a measure of the flexibihty of a column in dealing with a change in flow rate. The turndown ratio is defined as the ratio of the maximum to minimum operating flow rate. For bubble cap and valve trays, the turndown ratio is about ten whereas for sieve trays it is only about three. 

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