Repairing multistage pumps has its own nuances and challenges. While compromising quality on single stage smaller pumps is never a good idea, a careful and methodical approach to multistage pump repairs is even more critical. The entire process is complex, but it can be best summarized in seven fundamental steps (with accompanying photos to illustrate):
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Step 1 (above). Field analysis. A very important and critical step. All available records must be collected. It is not unusual for information (such as performance curves, sectional drawings, instruction manuals, and records of previous repairs) to be incomplete, incorrect, or nonexistent. Performing vibration analysis before and after completion of the job is a good way to assess the effectiveness of the repair.
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Step 2 (above). If the job involves emergency repair, Step 1 above is usually bypassed. In the example shown, a crushed rotor is compared to the spare rotor for possible parts utilization. As shown, the impellers of a spare rotor are different, but the shaft is of the same design and can be utilized, saving time and the need to make a new shaft.
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Step 3 (above). A rotor is disassembled and the impellers removed. The spare shaft is then inspected, dressed, and prepared for the assembly with either the new or, if possible, repaired old impellers. Fits and clearances are measured and documented. If incorrect fits are discovered, they often indicate a potential root cause of a problem.
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Step 4 (above). After the spare shaft is trued-up, the rotor is assembled and balanced to 4W/N (or often better), an API-610 balance criteria and a good way to ensure quality of balance and low vibrations.
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Step 5 (above). In the meantime, the casing is repaired, including the main flanges' surface restoration (milling). If the flanges' surfaces are not properly restored, the gasket may not seal well, resulting in leaks.
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Step 6 (above). Next, the casing halves are assembled, together with bearing housings, and line-bored to restore concentricity. Dowel pins ensure repeatability of assembly in the future. This assures perfect centering of the rotor within the bearings, as well as the impeller wear rings within concentricity of the casing wear rings. This also prevents potential crushing of the rings, metal galling and seizures of the unit.
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Step 7 (above). The pump is then reassembled (with data filled in proper forms and proper documentation assembled as a package to accompany the pump to the customer) and ready for a final paint before being sent on its way to the customer.
In this example, a spare set of bearings is added to the assembly. If this is not available, the repair procedure must include replacement of bearings, inspection and possible repair of bearing housing, and similar documentation of fits and clearances. As with the casing and rotor, bearing housing repairs must be documented and included with the quality package for the customer and for shop record keeping.
The photos and example described above illustrate an actual emergency repair of a multistage split-case pump that Pumping Machinery performed for a major pipeline company. Next month we'll look at a similar procedure with the repair process example of vertical turbine pumps.
Pumps & Systems, December 2007
