Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
High Speed Pumps 187 As an aside, pump users should be aware that overly conservative statements of NPSHA in an application can work to their disadvantage. The pump manufacturer must meet the stated NPSHA, so understated suction conditions can force the design toward lower speed or more and larger stages, which can result in an efficiency penalty or higher initial cost. Inducers Need to improve suction performance becomes quickly apparent in the move toward exploitation of high speed advantage. Inducer development began more than 50 years ago to provide this improvement. An inducer is basically a high specific speed, axial flow, pumping device roughly in the range of N s = 4,000 to 9,000 that is series mounted preceding a radial stage to provide overall system suction advantage. Inducers are characterized by relatively few blades, shallow inlet blade angles, and generally sophisticated hydraulic design. The inducer must put up enough head to satisfy the needs of the radial impeller stage but in itself has a suction level requirement that establishes a new lower NPSHR for the system. Inducers are an important element in high speed pump design, and so have been and continue to be the subject of considerable interest and developmental work. Inducer design should be such that maximum suction performance is achieved, and such that cavitation erosion in the inducer itself is avoided in long-term operation. Inducer performance is generally taken as the suction specific speed which corresponds to 3% pump head depression as NPSH is decreased. Theory exists establishing optimum suction performance in an expression known as the Brumfield criterion. A form of the Brumfield criterion developed in a comprehensive document on inducer design developed by NASA is as follows: Where 4> is the inlet flow coefficient or the ratio of meridianal flow velocity to inducer tip speed: A plot of the Brumfield criterion is shown in Figure 11-4. It should be emphasized that this expression is theoretical but tempered by practical
188 Centrifugal Pumps: Design and Application Figure 11-4. Brumfield performance criterion, design considerations and that many details of inducer design are not addressed. The angle /3 is the fluid angle at inlet and differs from the blade angle, ft, by a positive incidence angle, a. Prerotation is assumed to be zero, and other considerations such as blade shape, blockage, hub geometry, and leakage are simply ignored. The intent here is primarily to show the fundamental influence of the inlet blade angle on suction performance potential. Optimal inducer design is distinctly a high-tech endeavor which must conform to hard-earned design guidelines and hydraulic disciplines. A well-designed inducer should possess a "sharp" breakdown characteristic as illustrated by the solid curve in Figure 11-5, rather than the "gentle" curve shown in broken line. The NPSHR disadvantage with gentle breakdown is evidenced by the NPSHR differential which exists at the 3% head depression level. It should not go unnoticed that the 3% head depression level refers to the inducer-pump combination, so the level of the 3% line on the inducer headrise curve will vary according to the head of the pump to which the inducer is coupled. Lower head units will suffer an NPSHR disadvantage with a gentle breakdown inducer compared to high head units equipped with the same inducer.
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- Page 188 and 189: 11 By Edward Gravelle Sundstrand Fl
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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.
- Page 238 and 239: Doubte-Case Pumps 223 Volute Casing
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- Page 242 and 243: Slurry Pumps 227 An approximate com
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High Spe<strong>ed</strong> <strong>Pumps</strong> 187<br />
As an aside, pump users should be aware that overly conservative<br />
statements of NPSHA in an application can work to their disadvantage.<br />
The pump manufacturer must meet the stat<strong>ed</strong> NPSHA, so understat<strong>ed</strong><br />
suction conditions can force the design toward lower spe<strong>ed</strong> or more <strong>and</strong><br />
larger stages, which can result in an efficiency penalty or higher initial<br />
cost.<br />
Inducers<br />
Ne<strong>ed</strong> to improve suction performance becomes quickly apparent in the<br />
move toward exploitation of high spe<strong>ed</strong> advantage. Inducer development<br />
began more than 50 years ago to provide this improvement. An inducer is<br />
basically a high specific spe<strong>ed</strong>, axial flow, pumping device roughly in the<br />
range of N s = 4,000 to 9,000 that is series mount<strong>ed</strong> prec<strong>ed</strong>ing a radial<br />
stage to provide overall system suction advantage. Inducers are characteriz<strong>ed</strong><br />
by relatively few blades, shallow inlet blade angles, <strong>and</strong> generally<br />
sophisticat<strong>ed</strong> hydraulic design.<br />
The inducer must put up enough head to satisfy the ne<strong>ed</strong>s of the radial<br />
impeller stage but in itself has a suction level requirement that establishes<br />
a new lower NPSHR for the system. Inducers are an important element<br />
in high spe<strong>ed</strong> pump design, <strong>and</strong> so have been <strong>and</strong> continue to be the subject<br />
of considerable interest <strong>and</strong> developmental work. Inducer design<br />
should be such that maximum suction performance is achiev<strong>ed</strong>, <strong>and</strong> such<br />
that cavitation erosion in the inducer itself is avoid<strong>ed</strong> in long-term operation.<br />
Inducer performance is generally taken as the suction specific spe<strong>ed</strong><br />
which corresponds to 3% pump head depression as NPSH is decreas<strong>ed</strong>.<br />
Theory exists establishing optimum suction performance in an expression<br />
known as the Brumfield criterion. A form of the Brumfield criterion<br />
develop<strong>ed</strong> in a comprehensive document on inducer design develop<strong>ed</strong> by<br />
NASA is as follows:<br />
Where 4> is the inlet flow coefficient or the ratio of meridianal flow velocity<br />
to inducer tip spe<strong>ed</strong>:<br />
A plot of the Brumfield criterion is shown in Figure 11-4. It should be<br />
emphasiz<strong>ed</strong> that this expression is theoretical but temper<strong>ed</strong> by practical