Multistage Impellers

In thin and tall vessels H/T 1), multistage impeller systems are commonly used. The resulting shaft power is highly influenced by the clearance Ah between the impeller stages. In fully baffled systems and for Ah 1.2, the power for the single stages can be summed up. When multiphase systems are agitated, the power consumption can extremely differ from the one-phase system (see Section 13.6). 

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While the axial position of multistage impellers to their diffusers is not critical, they should line up reasonably well. Impellers are not extremely sensitive to leading-edge dings and minor damage, but anything, such as erosion on the exit tips, that tends to decrease the effective diameter of the impeller is more serious. Front shroud clearance on open impellers should be maintained close to the design values to minimize capacity loss. 

For double suction pumps, using the HI convention, is taken as the total pump flow, although some pubHcations use half-flow, ie, flow per impeller eye. For multistage pumps, the developed head must be taken per stage for the NS calculation. By definition (eq. 9), high head, low flow pumps have low specific speed low head, high flow pumps, such as turbine and propeller pumps, have high specific speed. 

These eompressors have impellers with a very low pressure ratio (1.1-1.3) and thus large surge-to-ehoke margins. Figure 6-44 shows a eross seetion of a typieal multistage eentrifugal eompressor used in the proeess industries. 

In-house correlation of pumps should be made using gpm vs. cost with head as a parameter. These should result in step functions, since one size pump with different impellers can serve several flow rates and heads. Different correlations should be made for each type of pump. The price of a vertical multistage pump may be quite different from a horizontal splitcase multistage pump. A sump pump with a 15-ft drive shaft would cost more than a single-stage horizontal pump with the same gpm and head. 

The radial-flow, or centrifugal compressor is a widely used comj sor and is probably second only to the reciprocating compressor in usage in the process industries. A typical multistage centrifugal compressor can he seen in Figure 1-11. The compressor uses an impeller consisting of... 

Although not universal, on most multistage compressors, the impellers are axially located by shaft sleeves. The sleeves form a part of the inter stage seal and are shrunk onto the shaft with a shrink level less than (Ik impeller. 

It is desirable to have additional axial-load control on the multistage compressor. A balance piston, also refeired to as the balance drum, can be located at the discharge end (see Figure 5-46). The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston (opposite the last stage location) is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston area to develop a thrust force opposite that generated by the impellers. The pressure on the... 

In the multistage compressor, the vanes are rectangular and located in a radial position ahead of the first impeller, with a linkage connecting the vanes to a power positioner. From that point, the control is affected in the same manner as the single-stage. 

Similar to multistage centrifugal pumps, centrifugal compressors, as shown in Figure 10-9, use a series of rotating impellers to impart velocity... 

Figures 12-46 illustrates the key styles of wheels for use on the rotating shafts of multistage centrifugal, single-stage compressors and fans, and axial flow compressors (to he discussed later in this chapter). The flow-handling capability of a specific impeller geometry is descrihed hy the flow coefficient, ...

The values of for high-pressure barrel compressors at the inlet are not to exceed 0.07 for high tensile strength steel impellers. For multistage (not barrel) compressors with flow coefficients... 

Figure 12-461, Part 2. The graph shows the influence of the flow coefficient, ch, on the efficiency. 1,2,3. . . are the individual impellers or stages and their efficiencies in a multistage compressor designed for optimum speed and overall efficiency (0,0,0) and alternatively, for reduced speed (V, V, V). (Used by permission Bui. 27.24.10.40-Bhi50. Sulzer Turbo Ltd.)...

Another centrifugal pump classification is whether the pump is a single-stage pump (the pressure head is developed by a single impeller) or a multistage pump (the pressure head is developed by two or more impellers). 

Double suction, 166, 167 Efficiency, 200 High speed, 169 Illustrations, 161, 164 Impellers, 164, 170 In parallel, 177, 178 In series, 176, 178 Liquid horsepower, 200 Materials of construction, 211 Mixed flow, 169 Multistage, 167 Operating curves, 180 Packing, shaft, 170... 

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