Pressure drop valve trays

For valve trays the effects observed only for the venturi (low dry pressure drop) valve. 

For a typical bubble-cap column, the following pressure drops per tray are considered reasonable, and they also would be order-of-magnitude values for sieve trays or valve trays ... 

This means that the average pressure drop per tray in the wash oil section was 6.74 in. of hot gas oil. Now the tray spacing (i.e., the vertical dimension between the tray decks) for the wash oil trays was 24 in. The pressure drop per tray as a percentage of tray spacing is an important measure of the capacity of a sieve tray (or common valve tray). To calculate this value, divide the observed pressure drop by the tray spacing ... 

Tray active panels installed with valves under downcomer of tray above. Limited capacity. High and unstable pressure drop at moderate rates. Installation and inspection error. 

Ao Same A valve, but with a contoured hole in the tray floor for lower operating pressure drop. 

To The same T valve and holddown with a contoured, low-pressure drop hole in the tray floor. 

The importance of the downcomer seal is to prevent vapor from the tray from bubbling into the downcomer (see Figure 8-63), whether the trays are bubble cap, valve or sieve types. If a seal weir is not included in the tray design, then operation problems to avoid flooding, weeping and unstable performance, including pressure drop, are increased, particularly during the start-up phase. 

Fv = valve tray F-factor, ft- /min/valve Fvm = valve tray F-factor at the beginning of the valve open region, fr /min/valve g = gravitational constant, ft/s he = clear liquid height, in. ho = dry tray pressure drop, in. 

For an operating tray the pressure drop profile is shown in Figure 8-148 [201]. The valves are closed at low hole vapor velocities, although, due to the design of the valves (see Figures 8-72 and 8-74), the metal tabs keep some styles of valves open sufficiently to allow some vapor and some liquid through, even at low flow rates. 

In the Figure 8-148 point A is where the valves on the tray are still closed but are just beginning to open. The pressure drop increases as the velocity increases from 0 to point A. ... 

Vh = vapor velocity through valve holes, ft/sec P = tray aeration factor, dimensionless AP = tray pressure drop, in. liquid pvm = valve metal density, tj = tray deck thickness, in. 

The pressure drop remains essentially constant as long as the liquid flow on tray remains steady during the period point A to point B on the diagram (the open balance point) [201]. At point B all valves are completely open off their seats, but are on the verge of closing and may be oscillating from open to closed. At point B the vapor velocity through the holes, opened balance point is ... 

Figure 8-148. Typical operating valve tray pressure drop profile. Valves start to open at A, the closed balance point. Used by permission, Klein, G. F. Chem. Eng. V. 89, No. 9 (1982) p. 81 all rights reserved.

Example 8-41 Procedure for Calculating Valve Tray Pressure Drop (after Klein [201])... 

Valve hole area 1.65 sq. ft. (separate calculation) = ho Tray pressure drop and froth height... 

Total number of actual trays in tower Number of caps per tray Number of slots per bubble cap Valve density, number of valves per ft or Number of valve units on a valve tray Depth of notches in weir, in or Exponent defined by Equations 8-288 and 327 Dry tray pressure drop for 50% cut baffles, in. liquid per baffle or... 

Actual tray pressure drop, in. liquid Prandtl number dimensionless Fractional opening in the circumference or a valve or. Pi... 

Eckert [125] provides some basic guidelines to good packing selection for various system performance requirements. Kunesh [126] illustrates the often-determined pressure drop advantage of random packed towers over the usual valve tray. See Figure 9-19 [126],... 

Figure 9-19. Comparison of typical valve tray and random packing showing that packing reduces pressure drop significantiy. Used by permission of Kunesh, J. G., Chemical Engineering, V. 94, No. 18 (1967) p. 101, ail rights reserved.

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. 

The plates may be any of several types, including sieve, bubble-cap, and valve trays. Valve trays constitute multiple self-adjusting orifices that provide nearly constant gas pressure drop over considerable ranges of variation in gas flow. The gas pressure drop that can be taken across a single plate is necessarily limited, so that units designed for high contacting power must use multiple plates. 

The tray as described is known as a sieve tray and it has perforations of up to about 12 mm diameter, although there are several alternative arrangements for promoting mass transfer on the tray, such as valve units, bubble caps and other devices described in Section 11.10.1. In all cases the aim is to promote good mixing of vapour and liquid with a low drop in pressure across the tray. 

There are several ways to account for variable pressures. If the total pressure of the column changes but not the pressure drop through the trays (the normal situation in heat-integrated columns, particularly with valve trays whose pressure drops are fairly constant), an approximate variable-pressure model can be used. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

Which of these two opposing effects dominates depends on the tray design and operating level. The pressure drops through valve trays change little with vapor rates unless the valves are completely lifted. Therefore the second effect is sometimes larger than the first. If this occurs, an increase in vapor boiiup produces a transient increase in liquid rates down the column. This increase in liquid... 

When vapor flows through a tray deck, the vapor velocity increases as the vapor flows through the small openings provided by the valve caps, or sieve holes. The energy to increase the vapor velocity comes from the pressure of the flowing vapor. A common example of this is the pressure drop we measure across an orifice plate. If we have a pipeline velocity of 2 ft/s and an orifice plate hole velocity of 40 ft/s, then the energy needed to accelerate the vapor as it flows through the orifice plate comes from the pressure drop of the vapor itself. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

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. 

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