Page 247 - Centrifugal Pumps Design and Application

220 Centrifugal Pumps: Design and Application

Requirements of new supercritical power plants, new oil refinery pro-
cesses, and high pressure oil field water injection facilities have in-
creased 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 pro-
cess. They also may be subjected to a rotor response analysis. At the
present time, computer programs for rotor response analysis are avail-
able [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 bush-
ings), and impellers. The magnitudes of hydraulic excitation forces (es-
pecially those generated by impellers) are less well known, but are be-
lieved to be much greater than mechanical excitation forces in large
double-case pumps (which are precision manufactured to minimize me-
chanical 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 impel-
lers 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 subsynchro-
nous 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 stabiliz-
ing Lomakin effect is generated. This advantage is reduced as the inter-
nal 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.