Valve trays, weights

Figure IW Weep point prewure balance for sieve and valve trays, (a) Sieve tray (6] well-designed valve tray, (c) valve tray with too many velvet or with valves that are too light (d) valve tray with too many valves, but fewer than in C (a) well-designed valve trey with two valve weights. (From w. h. Haifa, Chem. Eng. Png., 72 (B), p 43 (September 1876), reprinted courtesy of the American Institute of Chemical Engineers.) 305... 

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). 

Valve trays. Figure 6.216 illustrates the dry pressure drop of a typical valve tray as a function of vapor velocity. At low vapor velocities, all valves are closed (i.e., seated on the tray deck). Vapor rises through the crevices between the valves and the tray deck, and friction losses through these crevices constitute the dry pressure drop. Once the closed balance point (CBP) is reached, there is sufficient force in the rising vapor to open some valves. A further increase in vapor velocity opens more valves. Since vapor flow area increases as valves open, pressure drop remains constant until all valves open. This occurs at the open balance point (OBP). Further increases of vapor velocity cause the dry pressure drop to escalate in a similar manner to a sieve tray. When two weights of valves are used in alternate rows on the tray, a similar behavior applies to each valve type. The result is the pressure drop-vapor velocity relationship in Fig. 619e. 

A6. Valve trays are often constructed with two different weight valves. What would this do to Figure 12-21 What are the probable advantages of this design ... 

For valve trays, dry pressure Ad varies with valve positions, namely, partly open or fully open. At low and moderate vapor loading when the valve partly opens, dry pressure is more dependent on the valve weight than the vapor rate. In contrast, at high vapor rate when valve fully opens, the dry pressure follows the orifice correlation. Glitsch (1974) provides the following correlations of dry pressure for the valve tray... 

The above Fair s weep point method is applied to sieve trays. Bolles (1976) extended it to valve trays and details can be found in Bolles article. The weep rate is affected by minimum vapor loading in relation to weir loading, the number of holes for sieve tray, and the number of valves and weight of valves for valve tray. The experience shows that a well designed valve tray must maintain the weep point below the vapor loads at which the valve opens. To do this, a valve tray design should avoid use of too many valves. 

FIGURE R-10 A dual-weight cap for a valve tray enhances tray turndown ratio. 

Both the corrosion characteristics of the fluids involved and costs dictate the acceptable materials of construction for the shell and trays. Bubble caps, valves, and trays are usually made of a suitable metal to facilitate fabrication, but the shell material can be glass, plastic, impervious carbon, wood, glass-lined or resin-lined steel, or metal. Despite the additional weight involved, trays for bubble-cap units are often made of cast iron, usually 0.5 in. thick. With cast-iron trays, the risers may be fabricated as a permanent part of the tray. Lighter-gauge alloy metals may be cheaper than cast iron, and the final decision must be made on the basis of the situation for each individual case. 

To achieve an effective turndown operation, a manufacturer usually specifies two valve weights. When light valves open, the heavy ones still close, which reduces the active bubble area and thus avoids weeping. Furthermore, weeping usually occurs at the exit of the downcomer apron area. Thus, it is critical to maintain the level of the tray. 

Many towers equipped with valve or sieve trays do not operate efficiently at low feed rates. This is due to tray-deck leakage. As the pressure drop of the vapor flowing through the sieve holes falls below the weight of the liquid on the tray deck (as determined by the height of the weir), the tray will start to leak. 

Trays usually flood because of excessive vapor flow. The vapor flowing to tray 4 is essentially the heating steam flow. Certainly, most of this steam condenses on the trays, as the heating steam comes into contact with the 120°F softened water. It takes about 1 lb of 50-psig steam to heat 7 lb of water from 120 to 250°F. The uncondensed steam is then vented from the top of our tower through the atmospheric vent. But the full steam flow, from the pressure-control valve, does flow up through tray 4. Obviously, when this pressure-control valve is 100 percent open, the weight of vapor flow to tray 4 is at its maximum. 

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