Replacing Seven Vane Impellers With Eight Vane Impellers Leads To Electric Motor Overload In Nine-Stage Pipeline Pump

Abstract

This case study deals with a west Texas pipeline transfer pump with the following design specifications: Byron Jackson 3x6x9E DVMX; BEP at 650 gpm, 3560 rpm, Ns=1220; Originally 9 stages, 2 impellers were removed, utilizing remaining 7 stages; Directly coupled to a 450 HP electric motor drive. After de-bottlenecking the pipeline to obtain more flow, the motor driver for this pump began overloading at the new, higher flowrates. Horse power calculations suggested that the electric motor driver should have been adequate for the new hydraulic conditions. However, we began to suspect there was an internal pump design problem once it was found that the field performance diata did not match the original test data. We discovered that the pump was producing 20% more head than expected at the end-of-curve conditions (900 gpm). Upon disassembly, we found that 8 of the 9 impellers have 8 vanes instead of the expected 7 vanes. (The pump OEM provides both 7 and 8 vane impeller designs.) the OEM’s 4x6x9D 8 vane design produces more head past the best efficiency point than the corresponding 7 vane design and does not continuously rise to shutoff. To counteract the additional head producing capability of the 8 vane impellers, we decided to remove 2 stages. After de-staging, the motor overload condition was resolved. It is assumed that someone replaced the original 7 vane impellers with 8 vane impellers to obtain more head and hence more flow with the same pump. This case study clearly illustrates the danger of changing impeller designs without a thorough hydraulic analysis.