Standpipes

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

Whereas standpipes have been ia operation for many years, many aspects of their operation are not well understood. The purpose of a standpipe is to transfer soHds from a region of low pressure to a region of higher pressure. In a properly operating standpipe of Group A soHds, the soHds are moving downward at a relative gas—soHds velocity greater than /e. The relative gas soHds velocity, H, is defined as... 

Fig. 22. Standpipe and standpipe pressure profiles showiag (—) fluidized flow and (--------) packed bed or defluidized flow.

Note that the soHds density used ia this equation should be the tme soHds, ie, skeletal, density, because the gas ia the pores is also compressed. For Group A soHds the aeration gas should also be added evenly along the standpipe. 

Group B soHds have higher minimum fluidization velocities than Group A soHds. For best results for Group B soHds flowing ia standpipes, standpipe aeration should be added at the bottom of the standpipe, not uniformly along the standpipe. 

Steel tank cats, often lined to minimize iron contamination, are usually employed for high concentrations of sulfuric acid. Bottom outlets or valves are not allowed, nor are internal steam coils. Tank contents must be unloaded via standpipe. Using air pressure to unload is not recommended for safety reasons, but if air pressure is used, gauge pressures should be held at <0.21 MPa (30 psi). 

Fig. 5. (a) By-product coke oven of the Koppers-Becker type and (b) a section therein, where A represents the coke oven chamber containing coal B, oven wall heating flue C, sole heating flue D, air preheating E, standpipe F, collecting main and G, relationship between the two sections. 

American Water Works Association (AWWA) The association has many standards deahng with water handling and storage. A list of its publications is given in the AWWA Handbook (annually). AWWA DlOO, Standard for Steel Tanks—Standpipes, Reservoirs, and Elevated Tanks for Water Storage, contains rules for design and fabrication. 

Spheres, spheroids, and toroids use steel or concrete saddles or are suppoi ted by columns. Some may rest directly on soil. Horizontal cyhndrical tanks should have two rather than multiple saddles to avoid indeterminate load distribution. Small horizontal tanks are sometimes supported by legs. Most tanks must be designed to resist the reactions of the saddles or legs, and they may require reinforcing. Neglect of this can cause collapse. Tanks without stiffeners usually need to make contact with the saddles on at least 2.1 rad (120°) of their circumference. An elevated steel tank may have either a circle of steel columns or a large central steel standpipe. Concrete tanks usually have concrete columns. Tanks are often supported by buildings. 

Standpipes, Solids Feeders, and Solids Flow Control. 17-10... 

The pressure is higher at the bottom of the sohds draw-off pipe due to the relative flow of gas counter to the sohds flow. The gas may either be flowing downward more slowly than the solids or upward. The standpipe may be fluidized, or the solids may be in moving packed bed flow with no expansion. Gas is introduced at the bottom (best for group B) or at about 3-m intervals along the standpipe (best for group A). The increasing pressure causes gas inside and between... 

Seal legs are frequently used in conjunction with solids-flow-control valves to equ ize pressures and to strip trapped or adsorbed gases from the sohds. The operation of a seal leg is shown schemati-caUy in Fig. 17-19. The sohds settle by gravity from the fluidized bed into the seal leg or standpipe. Seal and/or stripping gas is introduced near the bottom of the leg. This gas flows both upward and downward. Pressures indicated in the ihustratiou have no absolute value but are only relative. The legs are designed for either fluidized or settled solids. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

From the regenerator, the regenerated eatalyst flows down a transfer line eommonly referred to as a standpipe. A standpipe provides the neeessary pressure head needed to eireulate the eatalyst around the... 

Instrumentation level instrumentation for MLO (standpipe level and ultrasonic) has limited use in a shutdown accident. Standpipe level is correct in the absence pressure m the system Ulirti sonic is correct only when the level is within the reactor coolant loops. 

The spray bank consists of a series of standpipes with nozzles connected to a horizontal header. The nozzles are arranged to ensure that the spray gives good coverage of the spray chamber without causing any interference with the adjacent nozzles. The pressure through these nozzles is normally between 140 kPa and 280 kPa. 

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