On the shores of Lake Michigan, two stations contain cooling water pumps that feed water to BP’s Whiting Refinery. The No. 1 water station contains four IR 24 HV pumps—large, single-stage, double-suction, horizontal split case pumps. The four pumps in station No. 1 (P-11, P-12, P-13, and P-14 in Figure 1) are unique because they were designed with a bottom-suction configuration.
Image 1. Four IR 24 HV bottom-suction pumps and one IR 24 FV pump (P-15) in the refinery's water station on Lake Michigan Image courtesy of BP
The rotating equipment engineers at the refinery wanted to better understand the operating characteristics of these pumps, which were originally built by Cameron in 1928. Because they were installed so long ago, no net positive suction head (NPSH) data were available and the pumps’ best efficiency point (BEP) was not known.
Cameron performance pump, circa 1933 Image courtesy of BP
BP’s rotating equipment engineers contacted a reliable pump service provider with whom they had a long and positive relationship. Their initial inquiry for a pump performance test led to a review of the pumps’ operating environment. The service center engineers learned that one pump was a designated spare and three of the four pumps were being run at a much lower capacity. Block valves had been used to limit the discharge pressure for the three operating pumps in an effort to prevent leaks in the cooling water piping inside the refinery.
The service center engineers agreed with the refinery’s rotating equipment engineers that it would be beneficial to obtain the pumps’ BEP. Running the pumps too far back on their operating curves could create internal forces that would be harmful to the pumps and decrease their operating life. For this reason, the refinery decided to pull one of the bottom-suction pumps from service to be tested. However, before sending this pump to the service provider’s independent test lab in Chicago, they seized the opportunity to make modifications that would enable the vintage pump to meet current standards.
The pump was sent to the service center and a comprehensive engineering analysis was performed. The service center engineers communicated with the refinery’s rotating equipment engineers to determine the modifications and upgrades that could be made.
Engineered Modifications and Upgrades
The refinery’s rotating equipment engineers first asked the service center engineers about redesigning the bearing housings. The original bearing housings were 2.5 feet long with spherical shell journal bearings that were hard to machine and made proper alignment difficult. More important, redesigning the bearing housings would help reduce the weight on the external bearings.
Because the bearings were an obsolete design, the service center engineers evaluated new bearing configurations at the request of the refinery. Axial thrust and bearing loads were reviewed to determine the best bearing design. Modifying the bearing design gave the refinery another option to upgrade from packing to mechanical seals, which would provide an added benefit of reducing leakage and decreasing energy consumption. The evaluation of upgrading to mechanical seals prompted a review of shaft deflection, or bend in the shaft. Mechanical seals can accommodate a limited amount of shaft deflection, so the shaft would need to be redesigned to work with the mechanical seals.
Once the engineering review was complete and all the modifications were agreed upon, the service center went to work. The shaft was modified to a stepped design and the distance between the bearings was shortened to reduce deflection at the seal faces. The sleeve bearings were removed and ball bearings were put in to handle the axial thrust of the rotor, therefore stabilizing the shaft and lowering vibration.
New mechanical seals were installed along with newly manufactured bearing housings (see Figure 2), which were fabricated at the service center. The service center manufactured the new bearing housings from a steel plate using its five-axis computer numerical control (CNC) machine.
Original bearing housing Image courtesy of BP
Image 2. New bearing housing (fabricated at the service center), bearing and mechanical seal Image courtesy of Hydro, Inc.
Design changes were made to the impeller wear rings after a mechanical and metallurgical review was performed on the original cast iron wear rings. The service center manufactured new impeller wear rings from a more durable stainless steel. The size of one impeller wear ring was made larger to reduce axial shuttling by prescribing the direction of axial thrust away from the thrust bearing. Specialized processes were used to machine, grind and metalize the impeller wear rings. The applied metal is harder than the parent metal, increasing the erosion resistance and extending the life span of the wear rings.