Single-stage Centrifugal Pump Dismantling and Repair

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 Single-stage Centrifugal pump dismantling and repair

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• Once the pump is taken out for an overhaul, the observation process does not end but becomes even more focused.  A recommended procedure is to match and mark all parts prior to the dismantling of the pump and make the following checks. 

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PROCEDURE OF SINGLE STAGE CENTRIFUGAL PUMP DISMANTLING:

• Open the pump casing bolts and separate the pump casing or the volute. 

• Inspect the pump casing for corrosion and erosion after it is removed. The gasket face should be particularly inspected to insure that there is sufficient land area and the surface is smooth. In case damage is observed, these should be repaired in appropriate manner. 

• Similarly, inspect the impeller and the nut for wear, corrosion, and erosion. Impeller vanes should be inspected for pitting or cavities in the areas mentioned in look for signs of cavitation, suction, or discharge recirculation. 

• Inspect the impeller and casing wearing rings for any signs of rub and/or detachment. This can be an indication of excessive shaft deflection, improper assembly, or piping strain. 

• Open the seal flange nuts and check for the seal tension. Remove the impeller from the shaft after opening the nut. In case it has balancing holes, these should be unplugged. Wearing rings on the backside should be inspected carefully. 

• The pump shaft sleeve is then pulled out along with the rotary head of the seal. All these should be carefully dismantled and checked for weak signs on the primary seal face, as well as, the secondary seals like the O-rings and gaskets if any. 

• The bearings are checked for roughness. Shaft is also checked for any signs of corrosion of erosion.

• The endplay or axial float of a pump shaft is measured by placing the pointer of a dial gage against any step on the shaft. The magnetic base maybe fixed to seal housing or any other convenient location. The shaft end is then lightly pumped over to one end and the dial reading is set to zero. The shaft is then moved over to the other end and a reading is recorded again. The endplay should be in the region of 0.001–0.003 in. (0.02–0.07 mm). An axial float greater than this leads to pitting and fretting in the points of contact in the shaft packing or mechanical seal areas. On finding a higher float, one should ascertain whether this is due to an improper assembly of bearing or bearings in the housing or due to defective bearings. 

• Shaft runout is a measure of shaft roundness combined with a permanent bow or bend. The effect of shaft runout is greatest on a mechanical seal due to the orbital motion of the shaft. A shaft with a 3 mils runout (0.075 mm) will move the seal faces 3 mil per side or a total of 6 mils. In addition, a bent shaft tends to cause vibrations that have a large impact on the life of mechanical seals and bearings. This check can be carried out after the shaft is bare and the bearings have been removed. The shaft is placed on V-blocks that are resting on a flat machinist’s table.

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• Dial gage pointer is placed at different locations and readings are taken while the shaft is rotated by hand. Alternately, the shaft can be clamped at lathe centers and reading. The bearings journals are set at zero and runout at other locations is measured by keeping the dial gage on the tool post. 

• The next method, as shown in Figure 9.3, is recommended only when the pump bearings are in good condition. In this method, the dial gage is fixed on any part of the housing. The pointer is placed on the OD of the shaft at various locations. The shaft is rotated by hand. 

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• The runout should not be greater than 0.05 mm. When a pump sleeve is mounted on the shaft, then the runout should be measured on the sleeve OD. 

• The combined runouts of the shaft and the sleeve should be 0.05 mm. It is recommended to contain the maximum runout of the shaft and the sleeve to within 0.025 mm. Thus even in a worst case, the runout will not be more than 0.05 mm. In case the reading is higher, the shaft should be straightened. 

• Shaft radial movement check is next on the list and this can cause shaft whip, deflection, and ultimately, vibrations. This usually occurs when the bearing fits are loose due to corrosion, wear, or improper machining.

• Even defective bearings can cause this to happen. The method to check this condition is shown in Figure 9.4. A dial gage is fixed to the housing as close as possible to the inboard bearing housing and the pointer is placed on the shaft outside diameter (OD). The shaft is then lightly lifted and pressed downward and the dial reading is recorded. Any reading above the value of 0.07 mm is unacceptable and calls for corrective actions. 

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• Seal housing squareness is an important check to insure that the seal faces are parallel to each other when they are installed. A seal housing that does not sit square will cause an angularity between the seal faces and cause an unequal wear of the seal faces leading to shortened seal life (Figure 9.5). 

• This check is slightly different from the checks mentioned above. In this case, the magnetic base of the dial gage is placed on the shaft and the pointer is placed on the face of the seal housing as shown in Figure 9.6. 

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• The shaft is then rotated and facial reading is recorded. The allowable value is 0.05 mm. In case a higher reading is obtained, the seal housing should be put on a lathe and face should be squared with the mating face of the seal housing with the bearing housing. 

• The seal housing face should be free of any nicks, burrs, or any other surface defect. One type of seal assembly problem occurs due to the angularity of the seal housing as mentioned above. The next problem is the offset of the seal faces. This occurs when the seal housing is not concentric with the shaft axis (Figure 9.7). 

• The way to record the seal housing concentricity is shown in Figure 9.8: A dial gage is fixed to the OD of the shaft and the pointer sweeps the inside bore of the seal housing. The inside of the bore may have a rough surface because of corrosion or wear. In such a case, it should be cleaned with a sandpaper, washed, and dried with a solvent. 

• The allowable limit for concentricity is 0.05 mm. In case the reading is in excess of the allowable limit it should positioned and doweled until concentricity is achieved. 

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• In in-between bearing pumps which have two seal housing located at the two ends, this check is even more critical. In case the bearings are found to feel rough and/or there is excessive radial and axial float, the bearing housing should be dismantled to replace the bearings. 

• Usually, the outer race of the bearing has a transition fit with the housing and the inner races of the bearings have an interference fit on the shaft OD. 

• Thus, the pump shaft with the bearings can be drawn out of the bearing housing with slight tap of the hammer. 

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• The bearings can be removed from the shaft usually by using a hydraulic press. Once the bearings have been removed, the dimensions of the bore and the shaft OD need to taken and the bearing fits should be worked out. 

• The shaft at this stage can be checked for straightness as mentioned earlier. After this is checked, it is polished and kept ready for reassembly. If the bearing fits are found to be excessive, suitable repair procedures have to be worked out depending on the material of construction and size of the shaft. 

• The housings can be re-sleeved with rings of the same material of construction. The shaft diameter too can be subjected to weld build-up and re-machining. 

• However, this has to be done considering the material of construction and thickness of the section. Some materials may need a heat treatment and a smaller diameter shaft could warp if the weld procedure results in excessive heat build up. 

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