Centrifugal pump maintenance

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Centrifugal pump maintenance – Minor repair tasks: 1 Replace the packing seal. For 2-shaft support pumps, inspect and clean the bearings, bearing housings, oil shields, water shields, oil gauges, etc., and adjust the bearing clearance. 3 Check and repair the alignment of the coupling as well as that of the drive unit and the pump. 4 Address common defects that occur during operation. 5 Check and clean the cooling water, seal oil, lubrication, and other systems. 2 Major repair projects: 1 includes minor repair projects. 2 Check and repair the mechanical seal. 3 Inspect each component for wear, corrosion, and erosion during disassembly. Non-destructive testing is performed on the pump shaft and impeller when necessary. 4 Inspect and clean bearings, oil seals, etc., and measure and adjust the clearance of bearing oil seals. 5 Check the circular runout and clearances of various parts of the rotor, and perform dynamic balancing tests if necessary. 6 Check and correct the straightness of the shaft. 7 Measure and adjust the axial play of the rotor. 8 Check for any misalignment in the pump body, foundation, foot bolts, and inlet/outlet flanges, to prevent additional stress from being applied to the pump body; reconfigure the piping if necessary. When disassembling and maintaining a centrifugal pump, the three stages of disassembly, maintenance, and reassembly must be carried out in accordance with the specified requirements and procedures. During disassembly: 1. Before removing the thrust bearing, use a dial indicator to measure the clearance of the balance disc and record it ； 2. When disassembling a multi-stage pump, it is necessary to mark each component in its original assembly order to avoid confusion and incorrect installation during reassembly ； 3. Small components that are difficult to mark (such as keys) can be placed together with impellers or vanes of the same level (in the middle section), etc ； 4. During disassembly, it is possible to visually check for any abnormal parts, such as loose fittings. During maintenance: 1. Visually inspect the surfaces of all components to ensure they are in good condition; all mating surfaces must be free from dents, scratches, or rust ； 2. Use measuring tools to actually check whether the tolerances of the key mating parts are within acceptable limits ； 3. Check whether the clearances at the impeller seal ring, casing seal ring, guide vane seal ring, inter-stage shaft sleeves, etc., are within the allowable range; those that are excessively worn need to be replaced ； 4. Check whether the bearing is in good condition ； 5. It is best to replace all seals and gaskets with new ones. During reinstallation, 1. First, install the rotor and conduct a dynamic balance test again ； 2. Reassemble all components in the reverse order of disassembly. When reassembling, be sure to measure the clearance at each seal ring again to ensure accuracy ； 3. The total rotor string length should be measured before installing the balance disc ； 4. After installing the balance disk, measure the rotor’s half-string length ； 5. When compared with the total string length and half-string length specified in the manufacturer’s general assembly diagram, it should generally meet the requirements of the drawings. Under normal circumstances, the half-string quantity is roughly half of the total string quantity ； 6. Tighten all the main bolts evenly, making sure to do so diagonally ； 7. Attach a dial indicator to the shaft and rotate the shaft while using the indicator to check the balance disc; the tolerance should meet the requirements specified in the drawings, and generally should not exceed 0.06 ； 8. When installing the thrust bearing, attention should be paid to adjusting the clearance of the balance disk; the adjustment ring in front of the bearing should be used to set the balance disk clearance to the values specified in the drawings. There are corresponding standards for the maintenance of each component of a centrifugal pump, and these standards must be strictly followed during maintenance. Pump shaft 1: Clean and inspect the pump shaft; it should be free of cracks, severe wear, or other defects. If there is wear, cracks, erosion, etc., it should be recorded in detail, and the causes analyzed. 2. Check the straightness of the pump shaft of the centrifugal oil pump; this value should not exceed 0.05 mm over its entire length. The journal surface must be free of defects such as pitting and grooves; the maximum allowable value for surface roughness is 0.8μm, and the errors in roundness and cylindricity of the journal should be less than 0.02mm. 3. The parallelism error between the center line of the centrifugal pump’s keyway and the axis center line should be less than 0.03 mm/100. Impeller 1: Clean and inspect the surfaces of each stage of the impeller. The impeller surfaces should be free from defects such as cracks or wear; the flow channels on the impeller surface should be smooth, without any scaling or burrs. The blades should also be free from cracks or thinning due to erosion. 2. Check the sealing rings at the inlet and outlet of the impellers at all levels; they should not be loose, and their surfaces must be smooth without any burrs. The maximum allowable value for surface roughness Ra is 0.8 μm, while the assembly clearance between the impeller and these sealing rings should be between 0.05 and 0.10 mm. Taking the inner hole of the impeller as a reference, the radial runout of the impeller should not exceed 0.05 mm. The end face runout shall not exceed 0.04 mm. 3. The impeller and shaft are fitted with an interference fit, typically H7/h6. The interference fit between the key and the keyway is 0.09–0.12 mm, resulting in a clearance at the top of the centrifugal pump key after assembly of 0.04–0.07 mm. 4. The impeller must be statically balanced. Pump head, pump casing, and impeller: 1. Clean and inspect each stage of the impellers; there should be no defects such as wear, cracks, or erosion. 2. The anti-rotation pin of the impeller in the centrifugal pump should be free from bending, breaking, and loosening. The surfaces of the pump head and the seal ring of the pump casing should be free of pitting, scratches, or grooves. The maximum allowable value for surface roughness Ra is 0.8μm. The fit between the seal ring and the pump head as well as the pump casing should be between 0.05 and 0.10 mm, and the seal ring must not be loose. 3. Using the impeller of the centrifugal pump and the flange of its casing as references, measure the radial runout of the inner diameter of the sealing ring; this value should not exceed 0.50 mm, while the end-face runout should not be more than 0.04 mm. 4. Measure the gap between the seal ring of the centrifugal pump’s impeller and casing and its mounting seal ring; this value should be between 0.50 and 0.60 mm. Bearings 1, Sliding bearings: (1) The interference fit between the bearing and the bearing cover should be 0.02–0.04 mm; the lower bearing liner should make even contact with the bearing seat, with the contact area being at least 60% larger. (2) When replacing the bearing, the contact angle between the shaft journal and the lower bearing should be 60–90°; the contact area must be even, with no fewer than 2–3 contact points per square centimeter. (3) The bearing alloy layer must be firmly bonded to the bearing liner; the surface of the alloy layer shall be free from defects such as pores, inclusions, and peeling. (4) The clearance at the top bearing shall comply with the provisions in the table below. (5) The data for the bearing side clearance on the horizontal midplane is half of the top clearance. 2. Rolling bearings: (1) The fit between rolling bearings that bear axial and radial loads and the shaft is H7/js6. (2) The fit between a rolling bearing that is subjected only to radial loads and the shaft is H7/k6. (3) The fit between the outer ring of the rolling bearing and the inner wall of the bearing housing is Js7/h6. (4) For pumps that use rolling bearings for axial thrust support, an axial clearance of 0.02–0.06 mm should be provided in the outer ring of the rolling bearings. (5) When assembling or disassembling rolling bearings, the temperature used for hot installation shall not exceed 100°C; direct heating with a flame is strictly prohibited. (6) The rolling elements of the rolling bearing, as well as the oil and the surface of the raceways, should be free from corrosion, pits, and spots; the contact should be smooth with no abnormal noises. Coupling 1: The fit between the coupling and the shaft is H7/js6. 2. The axial clearance between the two end faces of the coupling is generally 2–6 mm. 3. When installing a toothed coupling, it is necessary to ensure that the outer teeth are located in the middle of the width of the inner teeth. 4. When installing an elastic ring pin coupling, the elastic ring and pin should be in an interference fit, with a certain degree of tension. The diameter clearance between the elastic pin and the coupling hole is 0.40～0.60 mm. Rotor 1: Circular runout of the rotor (1) The tolerance value for the circular runout of the rotor in single-stage centrifugal pumps shall meet the requirements specified in the table. (2) The circular runout of the rotor of multi-stage centrifugal pumps shall meet the requirements specified in the table. 2. For multi-stage pumps, the rotor should be dynamically balanced when necessary, and the requirements for this balancing must comply with the technical specifications. 3. The fit between the shaft sleeve and the shaft is H7/h6, with a surface roughness of ▽1.6. 4. The fit between the balance and the shaft is H7/js6. 5. Impeller: (1) The fit between the impeller and the shaft is H7/js6. (2) The impeller must be in static balance; for impellers operating at a speed of 3000 r/min, the allowable residual unbalanced weight on the outer diameter shall not exceed the values specified in the table. (3) The impeller is balanced using the weight-removal method; at appropriate locations, a portion of the material is removed such that the thickness removed does not exceed 1/3 of the wall thickness. (4) For hot oil pumps, when assembling the impeller and shaft, a clearance of 0.10–0.40 is left at the key top, and the axial clearance between the impeller and the front and rear partition plates should be no less than 1–2 mm. Sealing 1. Mechanical seal: (1) The diameter clearance between the gland and the shaft sleeve is 0.75–1.00 mm, and the thickness of the gasket in the cavity between the gland and the seal is 1–2 mm. (2) The roughness of the contact area between the sealing gland and the static ring seal is ▽3.2. (3) The shaft or sleeve at the location where the mechanical seal is installed must be free of defects such as rust spots or cracks, with a surface roughness of ▽1.6. (4) An axial clearance of 1–2 mm should be maintained between the root of the anti-rotation groove at the tail end of the stationary ring and the top of the anti-rotation pin. (5) The performance of the spring after compression shall meet the design requirements, with a deviation of ±2mm. (6) The rotation direction of the mechanical seal retainer spring should be opposite to that of the pump shaft. (7) The gland bolts should be tightened evenly to prevent the gland end face from tilting. 2. Packing seal: (1) The diameter clearance between the sealing oil ring and the shaft sleeve is generally 1.00–1.50 mm. (2) The diameter clearance between the oil seal ring and the packing box is 0.15～0.20 mm. (3) The diameter clearance between the filler gland and the shaft sleeve is 0.75～1.00 mm. (4) The clearance between the filler gland and the diameter of the filler box is 0.10～0.30 mm. (5) The diameter clearance between the filler bottom sleeve and the shaft sleeve is 0.70～1.00 mm. (6) The diameter clearance between the reduction ring and the sleeve is 0.50 to 1.20 mm. (7) The outer diameter of the packing ring should be 0.30–0.50 mm smaller than the bore diameter of the packing box, while its inner diameter should be 0.10–0.20 mm larger than the shaft diameter; the cut angle should be 45° to the axial direction. (8) During installation, the cuts of two adjacent layers of filling material should be offset by at least 90°. For spindle 1, the cylindricity of the neck portion is 1/4000 of the shaft diameter; the maximum value shall not exceed 0.025, and the surface must be free from defects, with a roughness level of ▽1.6. 2. Using the two journal surfaces as a reference, the radial runout tolerance for the coupling and the middle section of the shaft should be 0.04 mm. 3. The key and keyway must fit tightly together; no shims are allowed. The interference between the key and keyway shall meet the requirements specified in the table. The diameter clearances between the shell mouth ring and the impeller mouth ring, as well as between the intermediate thrust washer and the intermediate shaft sleeve, shall meet the requirements specified in the table. When performing maintenance on a multi-stage pump, if possible, it is best to first review the manufacturer’s maintenance instructions and overall assembly diagram to identify any special considerations. Packaging gland 1: The end face of the packaging gland must be perpendicular to the axis. 2. The clearance between the filler gland and the shaft sleeve diameter is 0.75～1.0 mm. 3. The clearance between the outer diameter of the packing gland and the packing box is 0.1～0.15 mm. 4. The gasket of the mechanical seal gland should be 1.50–2.50 mm above the contact surface. Oil seal ring 1: The clearance between the oil seal ring and the shaft sleeve is 1.00～1.50 mm. 2. The outer diameter of the oil-sealing ring is perpendicular to its end face. 3. The clearance between the filler box and the outer diameter of the seal oil ring is 0.15～0.2 mm. Coupling 1: Planar clearance of the coupling – 2.2～4.2 mm for cold oil pumps, and 1.55～2.05 mm greater than the aforementioned value for hot oil pumps. 2. The rubber ring for the coupling should be 0.15–0.35 mm smaller than the diameter of the hole. 3. Use special tools when removing the coupling to keep it smooth and avoid damage. For shafts and shaft sleeves, the allowable bending of the shaft diameter is not more than 0.013 mm; for pumps with low rotation speeds, this value is not more than 0.07 mm in the middle section of the shaft, while for pumps with high rotation speeds it is not more than 0.04 mm in the middle section of the shaft. 2. The shaft surface is smooth, free of cracks, wear, etc. 3. The surface of the shaft sleeve should maintain a Ra value of 1.6 um. 4. The shaft and sleeve use H7/h6.