Impeller limits

Impeller limit. This was the subject of Chap. 23. Sometimes this is called a pump limit. 

Tip speed limits may be required to minimize erosion on an impeller, limit particle degradation, and control shear in the process. Other factors, such as mechanical stress limits on hardware, shafts, and other mechanical components, must also be considered. Normally, the equipment supplier will work with variables of power, speed, and impeller diameter to satisfy aU the constraints and try to pick an optimum selection. In the case of hydrofoil impellers, the tip chord angle (TCA) can provide an additional parameter for optimization. The TCA is defined as the angle of the chord of the blade tip to the horizontal. The optimum selection is typically based on either operating efficiency or initial cost. 

The Oldshue-Rushton column (Eig. 15d) was developed (162) in the early 1950s and has been widely used in the chemical industry. It consists essentially of a number of compartments separated by horizontal stator-ring baffles, each fitted with vertical baffles and a turbine-type impeller mounted on a central shaft. Columns up to 2.74 m in diameter have been reported in service (162—167). Scale-up is reported to be reliably predictable (168) although only limited performance data are available (169). A detailed description and review of design criteria are available (170). 

Flexible Impeller Pumps. Flexible impeller pumps are designed for general-use service. They can be utilized for pumping in either direction, have low pulsations, and are easy to service and maintain. These pumps are typically limited to low capacities and pressures. 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

The angular off-center position used with fluidfoil units is usually limited to impellers delivering 2.2 kW (3 bp) or less. The unbalanced fluid forces generated by this mounting can become severe with higher power. 

The fluidfoil impellers (shown in Fig. 18-2) usually give more flow for a given power level than the traditional axial- or radial-flow turbines. This is also thought to be an advantage since the heat-transfer surface itself generates the turbulence to provide the film coefficient and more flow should be helpful. This is true to a limited degree in jacketed tanks (Fig. 18-34), but in helical coils (Fig. 18-35), the... 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

Limit agitator power input and provide proper impeller design Limit shaft speed Monitor shaft speed Provide adequate cooling system Design system to accommodate maximum expected temperature, and pressure... 

Excessive mixing Limit agitator power input and provide proper of reactants or impeller design impurities which, Return process to pilot or development to rede-promotes process to eliminate or minimize this emulsification. problem Poor phase separa- tion resulting in L it shaft speed problems in subse- Monitor shaft speed quent processing, phase separation steps or in down- stream equipment. I" " de-emulsifiers CCPS G-29 Lees 1996... 

Propeller Yes No limitation 0.54 (Npe) - (Np,) --5 45 pitched, four-blade impeller. Fquation is based on limited data with regard to propeller pitch and vessel baffling. Divide hj obtained with this equation by a factor of about 1.3. 

Retreating blade Yes No limitation 0.33 (Npp) - (Np,) -33 Glassed-steel impeller. Three... 

Many reactions involve shear-sensitive materials, which severely limit the maximum mixing rate and make impeller and reactor design important. Mixing becomes the limiting factor. 

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