Mobil Oil Corporations Joliet Refinery once had a constant problem with cavitation in its cooling tower pumps. Suffering from NPSH and vortex cavitation, original cast iron impellers were subject to severe damage. There was mass destruction of both the vanes and material between them. In addition to diminishing pump performance and efficiency, this progressively deteriorating situation unbalanced the 600 lb impellers causing extra loading on the bearings, rings and motors of the 1250 hp pump units.
     Cooling tower pumps move water that is open to the atmosphere. The make-up of the water, the design of the piping system, the location of the pumps and the general atmosphere where a cooling tower is are all factors that can contribute to cavitation problems. Because many of these systems are arranged the same way, cavitation problems are common to many cooling tower systems.
     The original impellers in the Mobil Oil cooling tower pumps were cast iron, which is particularly vulnerable to cavitation damage. Maintenance and engineering personnel tried repairing the damaged impellers by welding the vanes. This only resulted in a slight improvement. They also tried impellers cast in other metallic materials, with only limited success.

 With this history, maintenance and engineering personnel of the plant were looking for a material that would hold up better against cavitation damage. SIMS Pump Company was selected because it could manufacture the impellers out of SIMSITE®, a structural graphite composite that holds up extremely well against the effects of shock and cavitation. SIMS also had the unique

   ability to redesign the impellers to slow down cavitation damage and rebuild the rotating elements to extend the time between pump overhauls.
    SIMS engineers approached the problem from two sides. First, they decided to change the hydraulics of the impeller to guarantee the minimum NPSH requirements. Second, the impeller material was changed to reduce the effects of cavitation.
     The new impeller was designed on a CAD/CAM system, then precision machined from solid blocks of SIMSITE®. Known in the pump industry for the last 40 years, this proprietary material is recognized for its effectiveness in combating corrosion, electrolysis and cavitation problems in centrifugal pumps. Several grades of SIMSITE have been tested and qualified by the United States Navy for use in its centrifugal pumps. SIMSITE is six times lighter than cast iron or bronze, but it is much more resistant to cavitation. SIMSITE impellers decrease the bearing load and shaft deflection and have a long life in a pump even when subjected to cavitation.
     An analysis of the original cast iron impellers showed that the vane entrance angles were too  low and the channels too narrow. Because the pumps were installed with a limited NPSH available, cavitation problems were unavoidable. In anticipation of this problem, the new SIMSITE impellers were designed with five vanes instead of the six on the originals. Entrance angles were also changed to minimize the effects of cavitation, and impeller diameters were increased to optimize performance.