Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Double-Case Pumps 219 so that the thermal expansion is away from the coupling. This maintains the axial gap at the pump-to-driver coupling. Design Features for Pumping Hot Oil with Abrasives One of the most difficult double-case pump applications is the pumping of hot oil (500°F and above) with substantial quantities (2% or more) of entrained abrasive solids. Surface Coating. The key to prolonged periods of operation without maintenance is the application of a hard surface coating, which may extend service life by a factor of 4 or more. The coating should have a minimum hardness of 60 Rockwell C. It should be applied to all wear surfaces, to all accessible hydraulic passages in impellers and inner cases, and to the outside of the impeller shrouds. The coating is typically applied with a high-velocity spray process that produces a strong mechanical bond. High coating density and proper coating thickness are critical, Impeller and Case Wear Rings. Impellers designed with extra stock on the integral wear ring surfaces are generally preferable to replaceable impeller wear rings. Worn impeller wear ring surfaces can be re-coated and ground to size. They run against case wear rings that are coated in the bores and on the ends, and are sized to match the impeller running surfaces. Key ways. Unless special design precautions are observed, rapid erosion occurs in keyways that are subjected to more than one stage of differential pressure. The two center stage impellers of opposed-impeller type pumps should be welded together at the hubs, and the key (or keys) terminated blind in a relief. A shrink-fit land is provided between the impeller bore and the shaft at the high-pressure end to seal against leakage and prevent erosion. Similar construction should be used for the sleeve under the throttle bushing of an opposed-impeller pump, or under the balancing drum of a pump with inline impellers. Double-Case Pump Rotordynamic Analysis The rotordynamic analysis requirements for a double-case pump depend on the size, rotational speed, and horsepower of the specific pump. Dry and wet critical speed analyses are adequate for small and mediumsize pumps running at 5,500 rpm and below. This type of analysis is described in Chapter 19. Most current specifications only require critical speed analyses, and a full scale test with specified vibration limits.
220 Centrifugal Pumps: Design and Application Requirements of new supercritical power plants, new oil refinery processes, and high pressure oil field water injection facilities have increased the demand for predictably reliable, high speed, high horsepower double-case pumps. In addition to critical speed analyses, these pumps should be subjected to a rotor stability analysis as part of the design process. They also may be subjected to a rotor response analysis. At the present time, computer programs for rotor response analysis are available [7]. Response analysis includes consideration of excitation forces from both mechanical and hydraulic origins. Mechanical excitation forces from sources such as dynamic unbalance, misalignment, and shaft bow are well known. Hydraulic excitation forces are generated at the wear rings, long annular seals (such as balance drums or throttle bushings), and impellers. The magnitudes of hydraulic excitation forces (especially those generated by impellers) are less well known, but are believed to be much greater than mechanical excitation forces in large double-case pumps (which are precision manufactured to minimize mechanical forces). Some cutting edge research on hydraulic excitation forces has been conducted and is ongoing [7], The Effect of Stage Arrangement on Rotordynamics The opposed-impeller stage arrangement of volute-type pumps offers greater rotordynamic stability than the inline arrangement with all impellers facing in the same direction, which is common to difftiser-type pumps. This has been shown for a number of years [4] by critical speed analyses. A recent comparison, based on the stability analysis of an 8,000 rpm pump [6], is given in Table 12-1. Here an analysis of a pump with opposed-impellers is compared with the analysis of an "equivalent" inline impeller arrangement. Identical impeller forces and annular seal coefficients were used. With design clearances and smooth (not grooved) annular seals, the analyses showed stable operation and no subsynchronous whirling up to 14,000 rpm for opposed impellers, but only up to 8,000 rpm for inline impellers. The difference is attributed to the extra center bushing in the opposed-impeller design, where a strongly stabilizing Lomakin effect is generated. This advantage is reduced as the internal annular seals wear and clearances increase. The Effect of Impeller Growth from Centrifugal Forces Radial growth of high-speed pump impellers caused by centrifugal forces is significant [6]. The unsymmetrical impeller deformation caused by centrifugal forces, pressure loading, and shrink fit to the shaft for a four-stage, 8,000-rpm boiler feed pump is shown in Figure 12-12.
- Page 184 and 185: Pipeline, Waterflood and COa Pumps
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- Page 210 and 211: High Speed Pumps 195 Figure 11-10.
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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 232 and 233: Figure 12-11. pump for 4,000 psi In
- 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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- Page 258 and 259: Slurry Pumps 243 ing the pump speed
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Double-Case <strong>Pumps</strong> 219<br />
so that the thermal expansion is away from the coupling. This maintains<br />
the axial gap at the pump-to-driver coupling.<br />
<strong>Design</strong> Features for Pumping Hot Oil with Abrasives<br />
One of the most difficult double-case pump applications is the pumping<br />
of hot oil (500°F <strong>and</strong> above) with substantial quantities (2% or more) of<br />
entrain<strong>ed</strong> abrasive solids.<br />
Surface Coating. The key to prolong<strong>ed</strong> periods of operation without<br />
maintenance is the application of a hard surface coating, which may extend<br />
service life by a factor of 4 or more. The coating should have a minimum<br />
hardness of 60 Rockwell C. It should be appli<strong>ed</strong> to all wear surfaces,<br />
to all accessible hydraulic passages in impellers <strong>and</strong> inner cases,<br />
<strong>and</strong> to the outside of the impeller shrouds. The coating is typically appli<strong>ed</strong><br />
with a high-velocity spray process that produces a strong mechanical<br />
bond. High coating density <strong>and</strong> proper coating thickness are critical,<br />
Impeller <strong>and</strong> Case Wear Rings. Impellers design<strong>ed</strong> with extra stock on<br />
the integral wear ring surfaces are generally preferable to replaceable impeller<br />
wear rings. Worn impeller wear ring surfaces can be re-coat<strong>ed</strong> <strong>and</strong><br />
ground to size. They run against case wear rings that are coat<strong>ed</strong> in the<br />
bores <strong>and</strong> on the ends, <strong>and</strong> are siz<strong>ed</strong> to match the impeller running surfaces.<br />
Key ways. Unless special design precautions are observ<strong>ed</strong>, rapid erosion<br />
occurs in keyways that are subject<strong>ed</strong> to more than one stage of differential<br />
pressure. The two center stage impellers of oppos<strong>ed</strong>-impeller type<br />
pumps should be weld<strong>ed</strong> together at the hubs, <strong>and</strong> the key (or keys) terminat<strong>ed</strong><br />
blind in a relief. A shrink-fit l<strong>and</strong> is provid<strong>ed</strong> between the impeller<br />
bore <strong>and</strong> the shaft at the high-pressure end to seal against leakage <strong>and</strong><br />
prevent erosion. Similar construction should be us<strong>ed</strong> for the sleeve under<br />
the throttle bushing of an oppos<strong>ed</strong>-impeller pump, or under the balancing<br />
drum of a pump with inline impellers.<br />
Double-Case Pump Rotordynamic Analysis<br />
The rotordynamic analysis requirements for a double-case pump depend<br />
on the size, rotational spe<strong>ed</strong>, <strong>and</strong> horsepower of the specific pump.<br />
Dry <strong>and</strong> wet critical spe<strong>ed</strong> analyses are adequate for small <strong>and</strong> m<strong>ed</strong>iumsize<br />
pumps running at 5,500 rpm <strong>and</strong> below. This type of analysis is describ<strong>ed</strong><br />
in Chapter 19. Most current specifications only require critical<br />
spe<strong>ed</strong> analyses, <strong>and</strong> a full scale test with specifi<strong>ed</strong> vibration limits.