Page 245 - Centrifugal Pumps Design and Application

218 Centrifugal Pumps: Design and Application

5,400 rpm. For larger shafts and higher speeds, tilting pad radial bear-
ings are favored. These bearings require force-feed lubricating oil sys-
tems.

Thrust Bearings

Ring-oiled sleeve-type radial bearings generally use anti-friction thrust
bearings. When force-feed lubricated radial bearings are used, thrust
bearings should be of the tilting pad type.

Baseplates and Foundations

Baseplates and foundations must be designed so that misalignment be-
tween the pump, the driver, and other drive-train elements (such as gear-
boxes or variable-speed devices) is within allowable limits and so that
they provide optimum dynamic support to minimize vibration. The tradi-
tional solution is to make the baseplate very stiff and to secure it rigidly
to the foundation by bolting and grouting. A variation of this design tech-
nique is the use of sole plates under the pump and driver that are rigidly
attached to the foundation and the use of very stiff concrete pedestals for
the centerline mounted pump.
An alternative solution is to mount a fully rigid baseplate on springs or
other flexible members to isolate the pumping unit from the foundation.
This system has been used for many years in installations that include
very large boiler feed pumps, and the experience gained is now being
used to isolate pumping units from the potentially large motions of flexi-
ble offshore oil production platforms. Because weight is critical, honey-
comb structures that are very light and stiff have been used for such base-
plates. A sophisticated three-point mounting system can make the
rotating equipment almost insensitive to large motions of the platform be-
cause no in-plane bending or torsion is generated by the deck motions.
In either case, large or high-speed pumping systems should be sub-
jected to modal analysis and detailed finite element dynamic analysis to
avoid mechanical and fluid (acoustic) resonances. Such analyses are es-
pecially important for variable-speed units. Some of the analysis methods
used are described in Chapter 19.

Mounting of the Barrel

Double-case pumps for hot service are mounted at the pump centerline
and the barrel is restrained from horizontal movement. This will assure
that the horizontal and vertical position of the shaft axis is maintained
during unit heatup and cooldown. The barrel is secured on the baseplate