The 3,600-rpm motor drivers were 60 horsepower (hp). The oversized pumps considered for a conventional alternate design would require 125-hp motors.
The pumps also would need to be the more costly between-bearings type because of an impeller diameter exceeding the user’s 13-inch maximum impeller diameter criteria for overhung pumps at 3,550 rpm (see Figure 6).
A 2011 process revamp removed these pumps from service. The operators documented that the pumps satisfied all premised requirements, including reliability, during their 15 years of service.
Improvements to the Prototype Overhung Design
While 15 years of reliable service verified the viability of the split flow concept, there is almost always room for improvement.
The number of internal running clearances (at wear rings) was reduced from four (totaling approximately 250 percent internal pressure drop) to three (totaling approximately 150 percent pressure drop).
Deleting hub wear rings and balance holes from the primary impeller (a standard feature with most OH2 designs, including the original split flow prototype) enhances both hydraulic efficiency and net positive suction head (NPSH) margin by precluding bypass into the impeller eye.
This design enables hydrodynamic axial balance of the rotor assembly with fewer running clearances. It also allows a 20 percent shorter rotor cantilever and secondary impeller feed by internal rather than from external conduits.