Page 251 - Centrifugal Pumps Design and Application

224 Centrifugal Pumps: Design and Application

form. This is not possible with the diffuser-casing construction. The lat-
ter requires alternate assembly of impellers and diffuser casings and
therefore dismantling and reassembly of the impellers on the shaft after
dynamic balance. Exact restoration of dynamic balance after the rotor
has been dismantled cannot be assured.

Sag Bore. The double-volute design lends itself to machining of the
equivalent natural deflection of the rotating element into the bottom vo-
lute case half. The result is that all running clearances remain concentric
because the shaft will operate in its deflected position. This is especially
important when operating on turning gear in hot-standby condition. This
refinement, known as sag bore, cannot readily be effected with the diffu-
ser design with its multiplicity of concentric fits.

Running Clearance Check with Feeler Gauge. By placing the rotating
element, including all rotating and stationary wear parts, into the bottom
volute case half, all running clearances can be checked with a feeler
gauge to verify that a reconditioned rotor has proper clearances, or to
determine if wear has taken place in a used rotor.

Easier Rotor Replacement. A rotating element can be quickly removed
from its volute case and a spare one installed. Stocking of a spare inner
volute is optional because it is not considered a wearing part. Many users
stock a spare rotating element only. In order to expedite disassembly and
assembly time with a diffuser design, the user must purchase a complete
inner case assembly because field assembly of the rotor and diffuser
cases is a time-consuming procedure.

References
1. API Standard 610 Centrifugal Pumps for General Refinery Services,
7th ed., American Petroleum Institute, Washington, D.C., 1990.
2. Ashton, R. D. "Optimize b-f Pump Throttle Bushings by Close
Match to Feedwater Circuit," Power, Sept. 1980.
3. Feed Pump Hydraulic Performance and Design Improvement, Phase
I: Research Program Design, CS-2323 Research Project 1886-4,
Electric Power Research Institute, Palo Alto, CA, 1981.
4. Gopalakrishnan, S. and Husmann, J. "Some Observations on Feed
Pump Vibrations," Proceedings ofEPRI Symposium on Power Plant
Feed Pumps, Cherry Hill, N.J., June 1982.