Valve unit

Fig. 16. Individual valve units used in valve plates (a) Koch Flexitray valve, courtesy of Koch Engineering Co. (b) GHtsch Ballast valve, courtesy of...

Diameter of valve unit at narrowest opening, mm Diameter of circular weir, in. 

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... 

Hand valves are used to reduce the load and to maintain reasonably good efficiencies. In general, one hand valve closed allows operation at 60-75% of rated load for a single valve unit. When the turbine has two hand valves, with both valves closed, the unit gives 50-60% of rated load with one... 

Modem designs fitted with non-retum valves through which the plasticised rubber compound passes into the injection cylinder portion are capable of giving precise shot volume. The non-retum valve is activated by the injection pressure. The injection ram/non-retum valve unit ensures complete purging of the injection chamber. 

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. 

Required Capacity Per Valve Units 36. Lifting Lever Plain [ ] Packed [ ] None [ ]... 

In valve trays, the perforations are equipped with valve units (Fig. 19). At high gas rates, the gas force opens the valves, thus providing area for gas flow. At low gas rates, there is insufficient force to keep many of the valves open, and these close, preventing the liquid from weeping. Sieve and valve trays show comparable capacity, efficiency, and... 

FIGURE 19 Flexitray valve unit (courtesy of Koch Engineering Company, Inc.). 

Tray layout. The preliminary tray and downcomer layout is prepared in the column-sizing phase and refined during the hydraulic design phase. In addition to the parameters previously set, such parameters as hole diameter or the type of valve unit are determined. 

The best type of valve unit depends on the corrosive and fouling tendencies of the service, as some valve units tend to pop out of their seats in corrosive services, while others tend to stick to their seats in fouling services. 

Figures 6.3 and 6.4 show different types of valve trays and valve units. Valves can be round or rectangular, with or without a caging structure. A detailed description is available elsewhere (1), and in manufacturers literature (7-9). The valve disks rise as vapor rate is increased (Fig. 6.2c), The upper limit of opening is controlled by a caging structure or by restrictive legs at the bottom of the valve unit. As vapor rate falls, the disk openings are reduced, or they may settle intermittently over the holes. This stops the liquid from...

Figure 6.4 Types of valve units, (a) An uncaged, mind valve unit. (6) A caged, round valve unit, (c) A caged, round valve unit featuring contoured orifice, fd) A rectangular valve unit it) A futed, round valve unit, if) A fixed, triangular v hre unit, (forts a and e courtesy of Gtiisch Inej parts b, c and f courtesy of Koch Engineering Company, Inc. pari d courtesy of Nutter Engineering.)...

Open slot area, ASo (valve trays) the slot area when all the valve units are fully open. 

In some cases, after careful evaluation, other valves may be considered for EBVs such as spring-loaded control valves that fail closed, back flow check valves (these are not normally considered reliable enough for EBVs by many engineers), and excess flow valves. Excess flow is the loss of material from the confined environment of a vessel or pipeline. Two approaches are available for the detection and valve action of excess flow valve systems (1) External, where excess flow is detected outside the valve itself, and (2) internal, which is within the valve unit and has limited applications. Excess flow conditions are detected more readily because of loss of resistance to flow than because of loss of pressure. All excess flow detection systems are based on product physical properties as well as flow rate. A change of products or process conditions may require a change in the excess flow detection system. For example, a number of excess flow valves are rated for low pressure and are made of materials not suitable for hydrocarbons. 

Basically, the overall model for valve trays is similar to that for sieve trays with the exceptions that the vapor flow area in the contactor can vary for valve units depending on the extent of the valve opening, and the number of kinetic... 

Tji. . Weir length, in. - t N Number of valve units on a fra f f -Qj Liquid flow from tray, gaFmin Qy Vapor flow to tray, ft /s 5 Tray submergence, in. of liquid., ... 

Finally, in the 1990s, Pfizer under a new CEO initiated major divestitures. In 1990, it sold off its citric acid business to Archer-Daniels-Midland. Then, in 1992, there followed the spinning off of its other specialty chemicals and refractory products, followed quickly by the sale of the Coty cosmetic and fragrance divisions for 440 million and the overseas portion of its Plax mouthwash business (purchased in 1988) to Colgate-Palmolive for 105 million, and its Shirley heart-valve unit to a subsidiary of Italy s Fiat. ... 

Business Week, July 1, 1991, pp. 86-87 New York Times, Dec. 12, 1991, Mar. 17, 1992, and Aug. 18, 1992. See a so Moody s Industrial Manual (1991), pp. 3297-3299. Pfizer sold its Shirley heart-valve unit in 1992 after a defective prodnct forced the company to set aside 300 miUion to meet claims from resulting lawsuits. This accounts for the company s dechne in revennes noted in Table 1.2. 

---