Hydraulically Rerating Centrifugal Pumps To Improve Reliability

Abstract

The authors present two case studies where improperly applied hydraulics resulted in poor mechanical reliability, due to chronic flow instability problems. A successful hydraulic rerate of these pumps, which required minimal modifications to their original pump casings and no modifications to the piping, drivers, or baseplates, dramatically improve their reliability and reduced annual repair costs. The first case study describes the rerate of a 4 X 6 X 13, 3560 rpm, 300 hp, single suction overhung pump and its spare, which were averaging between seven and 12 failures per year from 1990 to 1995. The second case study involves the rerate of a 12 X 14 X 23, 1780 rpm, 600 hp, double suction, between bearings pump and its spare, which were experiencing three to five failures per year. Both rerates involved selecting an impeller pattern with a best efficiency point closer to actual operating conditions and an acceptable suction specific speed (Nss < 11,000), and then modifying the pump casing to obtain the proper volute throat areas for the desired performance. The pumps in both case studies realized marked improvements in reliability as a result of the hydraulic rerates. The authors will take the readers through the rerate process, illustrating the key steps involved, and explain which types of problematic pumps qualify for this conversion. They will also clearly explain the hydraulic principles employed to give the readers insight into the design process. Finally, the authors will explain the economic benefits that can be derived from a hydraulic rerate and how this type of work can be economically justified.