Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
High Speed Pumps 185 Figure 11-3. Partial emission efficiency comparison. Concentric bowl pump peak efficiency occurs at about N s = 800, and declines in efficiency at higher specific speeds as shown. This characteristic of RE. pumps exists in general because with high specific speeds the inlet and discharge passages enlarge toward overlap causing declining head coefficients and efficiency reduction. This characteristic establishes a boundary region delineating the suitability of P.E. and RE. pump types. Although very low specific speed design must inherently entail efficiency sacrifice, such design can have overall attraction. Efficiency is not the sole consideration in pump selection, and can be overridden by factors such as simplicity, low initial cost, and quick, easy maintenance. These alternative considerations tend toward dominance at modest power levels and for intermittent or low-usage services. Suction Specific Speed Another dimensionless parameter highly important in pump design is known as suction specific speed involving a parametric group nearly identical to the pump specific speed expression. This subject is discussed in detail in Chapters 2 and 8.
186 Centrifugal Pumps: Design and Application Suction specific speed ranges typically from S s = 7,500 to 10,000 for P.E, pumps, which is in about the same range as for higher specific speed pumps of single suction, overhung impeller design. High S s values translate into low NPSHR, or good suction performance. The range mentioned varies with flow rate where high S s values are associated with low flow rates. Solving the specific speed equation for NPSHR yields a suction expression in terms of speed and flow, which can alternately be converted into terms of head and specific speed: Contrary to what might be expected, the inlet radial blade geometry used in P.E. pumps achieves suction specific speed parity with the higher specific speed pumps utilizing more sophisticated inlet shapes. It is apparent, however, that high speed pumps will be demanding from the standpoint of NPSHR. The bracketed term in the latter expression is known as the Thoma cavitation parameter, usually designated by sigma: The Thoma parameter, then, states NPSHR as a fraction of pump head and is a function of the ratio of specific speed to suction specific speed. Low specific speed thus offsets to a degree the higher NPSHR associated with high speeds. The inlet eye size in the prior expressions is assumed to be generously sized, as is generally done so that only small NPSHR impact exists. The NPSHR expression expanded to include inlet eye size effect becomes: Inlet eye size has been found to have an influence on the efficiency potential of the pump, which as we have just seen, affects NPSHR. Availability of efficiency advantage via eye sizing then in reality hinges on NPSHA in the application. More will be said on this subject in the section "Partial Emission Design Evolution."
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- Page 192 and 193: High Speed Pumps 177 Figure 11-2. (
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- Page 204 and 205: High Speed Pumps 189 Figure 11-5. I
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- Page 210 and 211: High Speed Pumps 195 Figure 11-10.
- Page 212 and 213: High Speed Pumps 19? Figure 11-11.
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- Page 216 and 217: High Speed Pumps 201 Figure 11-13.
- Page 218 and 219: High Speed Pumps 203 nal bearings a
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- Page 222 and 223: Double-Case Pumps 207 jected to ext
- Page 224 and 225: Double-Case Pumps 209 Figure 12-3.
- Page 226 and 227: Double-Case Pumps 211 Figure 12-4.
- Page 228 and 229: Double-Case Pumps 213 ally by split
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- Page 236 and 237: Double-Case Pumps 221 Figure 12-12.
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- Page 242 and 243: Slurry Pumps 227 An approximate com
- Page 244 and 245: Slurry Pumps 229 Figure 13-2. Nomog
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High Spe<strong>ed</strong> <strong>Pumps</strong> 185<br />
Figure 11-3. Partial emission efficiency comparison.<br />
Concentric bowl pump peak efficiency occurs at about N s = 800, <strong>and</strong><br />
declines in efficiency at higher specific spe<strong>ed</strong>s as shown. This characteristic<br />
of RE. pumps exists in general because with high specific spe<strong>ed</strong>s the<br />
inlet <strong>and</strong> discharge passages enlarge toward overlap causing declining<br />
head coefficients <strong>and</strong> efficiency r<strong>ed</strong>uction. This characteristic establishes<br />
a boundary region delineating the suitability of P.E. <strong>and</strong> RE. pump types.<br />
Although very low specific spe<strong>ed</strong> design must inherently entail efficiency<br />
sacrifice, such design can have overall attraction. Efficiency is<br />
not the sole consideration in pump selection, <strong>and</strong> can be overridden by<br />
factors such as simplicity, low initial cost, <strong>and</strong> quick, easy maintenance.<br />
These alternative considerations tend toward dominance at modest power<br />
levels <strong>and</strong> for intermittent or low-usage services.<br />
Suction Specific Spe<strong>ed</strong><br />
Another dimensionless parameter highly important in pump design is<br />
known as suction specific spe<strong>ed</strong> involving a parametric group nearly<br />
identical to the pump specific spe<strong>ed</strong> expression. This subject is discuss<strong>ed</strong><br />
in detail in Chapters 2 <strong>and</strong> 8.
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