Bypassing characteristics

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

When only the No. 3 and No. 7 flows are present, gas bypasses from the draft tube side to the downcomer side at a high flow ratio and reverses the direction at a low flow ratio at a distance ofLI =L2 = 21.7 cm between the conical plate and the draft tube inlet (see Fig. 5). The angle of the conical plate (a = 45° and 60°) does not seem to affect the gas bypassing characteristics greatly. When the conical plate was moved closer to the draft tube inlet at LI = L2 = 14.1 cm, the gas bypassing direction was exclusively from the downcomer side to the draft tube side. 

Figure 5. Gas bypassing characteristics of conical distributor plates of different design configurations (No. 3 and No. 7 flows).

Effect of Distance Between Distributor Plate and Draft Tube Inlet. As expected, the closer the distance between the distributor plate and the draft tube inlet the lower the solids circulation rate as shown in Figs. 8 and 9. This is not only because of the physical constriction created by locating the distributor plate too close to the draft tube inlet but also because of the different gas bypassing characteristics observed at different distributor plate locations as discussed earlier. When the distance between the distributor plate and the draft tube inlet becomes large, it can create start-up problems discussed in Yang et al. (1978). 

Other specific requirements may be mandatory in certain hydraulic systems, such as bypass characteristics to maintain flow in the event of the element s becoming clogged, or alternatively visual or audible warning when the filter element requires replacement, or even automatic shut-down to ensure that unflltered oil is not circulated through the system. 

FIG. 29-59 Head-horsepower-capacity characteristics of a power-recovery turbine operating as the sole driver of a lean pump. If the total capacity of lean and semilean pumps exceeds the values indicated by available head limit, bypass must be used. Net recovery-pump head at 8.71 mVmin (2300 gal/min) is figured as follows ... 

Determining which accident sequences lead to which states requires a thorough knowledge of plant and process operations, and previous safety analyses of the plant such as, for nuclear plants, in Chapter 15 of their FSAR. These states do not form a continuum but cluster about specific situations, each with characteristic releases. The maximum number of damage states for a two-branch event trees is 2 where S is the number of systems along the top of the event tree. For example, if there are 10 systems there are 2 = 1,024 end-states. This is true for an "unpruned" event tree, but. in reality, simpler trees result from nodes being bypassed for physical reasons. An additional simplification results... 

Steam generator primary side (one steam generator model with bypass valve flow and thermal characteristics). 

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