Mechanic’s Shop Equipment Maintenance Manual

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Chapter 1 Overview: As a chemical manufacturing facility, rotating equipment constitutes an essential part of the plant. Pumps and compressors play a vital mechanical role in the continuous production operation of the facility. In chemical plants, centrifugal pumps account for over 72.6% of all pumps. Since chemical pumps are used to transport flammable materials under extremely harsh temperature and pressure conditions, any failure of the equipment, especially that designed for special applications, can result in immeasurable economic losses. Therefore, it is extremely important to ensure the safe and long-term operation of such equipment. Chapter 2: Centrifugal Pumps Section 1: Maintenance of Single-Suction Centrifugal Pumps 1. Disassembly Steps 1. First, loosen the fastening bolts on the pump cover and the pump body, then remove the rotor assembly from the pump body. 2. Loosen the impeller nut in front of the impeller to remove it (the impeller key should be kept safely). 3. Remove the pump cover and shaft sleeve, and loosen the bearing gland to pull the shaft out of the suspension (be careful to wear an impeller nut when hitting the shaft end with a copper rod to prevent damage to the threads). II. Assembly sequence 1. Check that all components are free of damage and clean them thoroughly. 2. Tighten each connecting bolt, plug, etc., to the corresponding components respectively. 3. Place the “O”-ring and paper pad in their respective positions. 4. Install the seal ring, water seal ring, and packing gland into the pump cover in sequence. 5. After installing the bearing on the shaft, place it inside the suspension and close the cover to compress the bearing; then fit the water seal around the shaft. 6. Fit the shaft sleeve onto the shaft, then install the pump cover on the suspension. Next, insert the impeller, retaining washer, impeller nut, etc. in sequence and tighten them. Finally, assemble the aforementioned components into the pump body and tighten the bolts that connect the pump body to the pump cover. During the above process, special attention should be paid to small components such as flat keys, oil seals, water seals, and the \"O\"-shaped seals inside the shaft sleeves, as they are prone to being overlooked or installed incorrectly. III. Technical requirements: What is specified here are mainly the precision requirements for coupling alignment. After the pump and motor couplings are installed, a gap of 2–3 mm should be maintained between them. The vertical and horizontal deviations of the outer surfaces of the two couplings must not exceed 0.1 mm, and the difference between the maximum and minimum values of the end-face gap between the couplings must not exceed 0.08 mm. Section 2: Maintenance of double-suction pumps
I. Disassembly steps
1. Separating the pump casing
1.1 Remove the coupling pins to disconnect the pump from the motor. 1.2 Remove the bolts and pins that hold the pump together, in order to separate the pump cover from the lower pump body, and then remove the packing gland. 1.3 Disconnect the pipes connected to the system (such as air pipes, sealed water pipes, etc.), and cover the pipe fittings with cloth to prevent debris from falling in. 2. Lift out the pump cover: Once the above tasks have been completed, the pump cover can be lifted out. It should be lifted smoothly, and care should be taken to avoid rubbing against other components. 3. Lifting the rotor 3.1 Loosen and remove the bearing housing caps on both sides. 3.2 Lift it using steel wire ropes tied to the packing gland at both ends of the rotor, ensuring a stable and safe process. After being lifted out, the rotor should be placed on a dedicated support and secured firmly. 4. Disassembly of the rotor 4.1 Remove the coupling on the pump side and store the connecting key properly. 4.2 Loosen the end caps of the bearing housings on both sides and remove the bearing housings, then sequentially remove the bearing fastening nuts, the bearings, the bearing end caps, and the water seal. 4.3 Remove the sealing ring, packing gland, water seal ring, packing sleeve, etc., and check for wear or corrosion. 4.4 Loosen the shaft sleeve nuts on both sides, remove the shaft sleeves and check their wear level; replace them if necessary. 4.5 Check the wear of the impeller and the condition of cavitation; if it can continue to be used, there is no need to remove it. If removal is indeed necessary, use a specialized pulling tool to remove it while applying heat, in order to avoid damaging the pump shaft. II. Assembly Sequence 1. Rotor Assembly 1.1 The impeller should be installed in the correct position on the shaft; it must not be tilted to one side, as this will cause uneven axial clearance with the pump casing and lead to friction. 1.2 Install the shaft sleeve and tighten the shaft sleeve nut. To prevent water from leaking along the shaft, a sealing rubber ring must be inserted between the shaft sleeve and the nut. After assembly, it should be ensured that the rubber ring is pressed tightly by the shaft sleeve nut, and that the nut is firmly in contact with the shaft sleeve. 1.3 Install the sealing ring, packing sleeve, water seal ring, packing gland, and water baffle on the shaft. 1.4 Install the bearing end caps and bearings, tighten the bearing nuts, then install the bearing housing and secure it along with the bearing end caps. 1.5 Install the coupling. 2. Lowering the rotor 2.1 Gently lower the previously assembled rotor assembly into the pump body. 2.2 After positioning the sealing ring, rotate the rotor and check for any friction from the sealing ring; adjust it until rotating the rotor is smooth. 3. Fasten the pump cover: After attaching the pump cover, tighten the bolts at the pump’s joint surface as well as the gland caps on both sides of the bearings. Next, rotate the rotor to check if there are any differences from before; if no obvious abnormalities are detected, the air pipe, sealing water pipe, etc. can be connected, and the packing can be added. After that, it’s possible to align the couplings. III. Maintenance technical requirements 1. Here, only the precision requirements for coupling alignment are specified. The maximum and minimum gap difference between the two end surfaces of the coupling must not exceed 0.06 mm, and the difference between the centers of the two outer circles, either vertically or horizontally, must not exceed 0.1 mm. 2. If the impeller sealing ring remains unchanged after major maintenance, then the gasket at the pump casing joint can be kept at its original thickness ； If the sealing ring is raised upward, the thickness of the pump mating surface gasket must be measured using a lead wire compression method. Typically, the tightening force of the pump cover on the impeller seal ring is 0–0.03 mm. Once the new gasket is ready, it should be coated with lead black on both sides before being placed on the pump’s mating surfaces. Make sure the lead powder used is pure, with no lumps or impurities. At the filler box, the padding must be prepared with great care, ensuring that it is level with the edges of the filler box. If the gasket is not suitable, it will prevent the packing from sealing properly, resulting in excessive leakage and rework. Section 3: Maintenance of Multistage Centrifugal Pumps
1. Disassembly and assembly procedures
The DG-type high-pressure water pump is a multistage, segmented centrifugal pump. Before disassembling it, one must become familiar with the drawings in order to understand the pump’s structure and the sequence of disassembly and assembly, thereby preventing any damage to components due to mistakes. At the same time, as the disassembly proceeds, relevant data should be measured promptly for reference during assembly. Below is a sequential introduction to the disassembly of the pump. 1. Disassembly of the pump body: After removing the upper bearing shells on both sides and measuring their clearance and tightness, the oil baffle can be taken out. Next, remove the stuffing box cover, take out the packing and water seal ring, and then the bearing housing can be removed. For DG-type water pumps, disassembly should start from the discharge side. The basic sequence is as follows: 1.1 First, loosen the large nut and remove the bolts that hold the pump body together; then, remove the packing chamber on the discharge side as well as the rotating and stationary balance disk components, one after another. While removing them, it is necessary to measure the dimensions of the adjustment sleeves, toothed shims, and other components. 1.2 Remove the connecting bolts of the discharge section, then slowly lift out the discharge section axially. Next, remove the last-stage impeller along with its drive key and spacing sleeve. Subsequently, the impellers and guide vanes at each stage, as well as the intermediate section, can be removed one by one. Each impeller and spacing sleeve removed should be properly marked to prevent incorrect installation. 1.3 When removing the impeller, it is necessary to use a positioning piece to measure the distance between the outlet center of the impeller and the end face of its inlet side at the middle section; the flow channel of the impeller should be aligned with that of the guide vanes, otherwise the reason for this misalignment must be identified. During the disassembly of the pump body, the following points should be noted: (1) All removed components should be placed on clean wooden boards or rubber mats, and covered with clean white cloth or cardboard to prevent damage to the finely processed surfaces. (2) The removed rubber and asbestos gaskets must be replaced. If a copper gasket is used, it must be annealed before reinstallation ； If toothed gaskets are used, they can continue to be used as long as their condition is good and their thickness remains within the required range. (3) All components that may experience friction during installation or operation, such as the pump shaft and shaft sleeve, shaft sleeve nuts, impeller, and seal rings, should be coated with dry MoS2 powder (which must not contain any grease). (4) The axial position of the rotor should be recorded before disassembly (with the rotating and stationary balance discs in contact), so that after trimming the friction surfaces of the balance discs, the rotor can be reinstalled at the exact same position. 2. Inspection of stationary parts: After the pump body is completely disassembled, each component should be carefully inspected; any damage or defects found must be repaired or replaced. This section will cover the inspection and repair of stationary components. 2.1 Pump casing (mid-section) 2.1.1 Inspection of the fit gap Between adjacent pump casings in multi-stage pumps, there is a fit achieved through interlocking surfaces; if the gap between these surfaces is too large, it will affect the concentricity between the rotor and the stationary parts of the pump. The method for checking the clearance at the flange of the pump casing is as follows: Place the adjacent pump casings one on top of the other on a flat surface. Position a magnetic gauge holder on the upper casing, with a dial indicator clamped to it; the contacts of the indicator’s head should make contact with the outer surface of the lower casing. Subsequently, the pump casing above is pushed back and forth in a cross direction a second time for measurement; the difference in the readings on the dial indicator represents the gap between the shoulders. Typically, the fit clearance between the flanges is 0.04–0.08 mm; if this clearance exceeds 0.10–0.12 mm, it should be repaired. The simplest repair method is to uniformly weld 6–8 layers on the male and female fits of the pump casing where there is a large gap, and then machine them to the desired dimensions. 2.1.2 Crack inspection: Gently tap the pump body with a hammer; if a dull sound is heard from a certain area, it indicates that there is a crack in the casing. At this point, kerosene should be applied to the cracks; once it has penetrated, the oil on the surface should be wiped away with a cloth and a layer of chalk should be applied. Then, the pump casing should be tapped gently with a hammer, and the kerosene that has penetrated the cracks will wet the chalk, revealing the ends of the cracks. If the crack is located in a area that is not under pressure or does not serve a sealing function, a φ3mm circular hole can be drilled at each of the start and end points of the crack to prevent further propagation of the crack ； If cracks appear in the pressure-bearing areas, they must be rewelded. 2.1.3 Guide vanes: If the guide vanes of multi-stage pumps are made of stainless steel, they generally do not get damaged ； If tin bronze or cast iron is used, the erosion should be checked every 2 to 3 years, and new guide vanes should be replaced if necessary. For any newly cast guide vanes, the flow channels should be polished smooth with a hand grinder before use, which can increase efficiency by 2% to 3%. In addition, the wear condition of the guide vane bushings (which should fit together with the impeller) should also be checked, and it should be determined whether to repair or replace them based on the degree of wear. The radial fit clearance between the guide vane and the pump casing is 0.04–0.06 mm; if this value is too large, it will affect the concentricity of the rotor and the stationary components, and the part should be replaced. The fit between the positioning pin used to locate the vane and the pump casing should have an interference of 0.02–0.04 mm, and there should be an adjustment clearance of 1.0–1.5 mm at the junction between the pin head and the vane. The guide vanes should be properly pressed against the pump casing to prevent the guide vanes of high-pressure pumps from being eroded by the water flow against the plane of the casing partition. Typically, the method of compressing the guide vanes involves drilling holes in the ribs of the blades on the back side of the guide vanes, and installing 3 to 4 copper nails (placed as close as possible to the outer edge of the guide vanes and evenly distributed around the circumference). The interference fit provided by these copper nails ensures a seal between the guide vanes and the pump casing surface. The additional red copper nails should generally be 0.50–0.80 mm above the plane of the back guide vanes. 3. Balancing device: During the disassembly of the water pump, the pressure wire method is used to check the parallelism of the rotating and stationary balance surfaces. The procedure is as follows: 3.1 Place the shaft in its operating position, apply lubricant to the shaft, and ensure that the rotating disk can slide freely, with its keyway aligned with the keyway on the shaft. 3.2 Use butter to attach the lead wire to the four symmetric positions on the upper, lower, left, and right sides of the stationary disk’s end face, then push the movable disk forcefully against the stationary disk; remove the lead wire that has been deformed due to the impact and note its position ； 3.3 Rotate the moving disk another 180° and take another measurement, making a note of it. Use a micrometer to measure the thickness of the lead wire removed; the sum of the values measured for the upper and lower positions should be equal to the sum of the values measured for the left and right positions. The difference between the upper value and the lower value, or between the left value and the right value, should be less than 0.05 mm. Otherwise, it indicates that the stationary and moving discs are deformed or have some degree of skew, and this issue must be corrected. When the contact surfaces of the dynamic and static balance discs show only slight wear marks, fine grinding sand can be applied between their mating surfaces for lapping ； If the wear grooves are large and deep, they should be repaired on a lathe or grinder to ensure that the contact rate between the rotating and stationary balance discs is above 75%. 4. Sealing rings and guide vane bushings: Currently, sealing rings and guide vane bushings are generally made from two wear-resistant materials: stainless steel or tin bronze. Sealing rings and vane bushings made of stainless steel have a longer service life, but high requirements are placed on the quality of their processing and assembly; otherwise, slight gaps in the fit or minor shaft bending can lead to seizure during operation. If made of tin bronze, it is easy to machine, has low costs, and is less prone to seizure, but its erosion resistance is relatively lower. After the newly processed seal rings and vane bushings are installed in place, the concentricity deviation with respect to the impeller should be less than 0.04 mm. The radial clearance between the sealing ring and the impeller varies depending on the inner diameter of the sealing ring; details can be found in Table 2-3-1. The fit clearance between the sealing ring and the pump casing is generally 0.03–0.05 mm. 5. Removal of bearing shells and measurement of bearing shell clearance: When disassembling a multi-stage pump, it is first necessary to inspect the bearings at both ends (usually sliding bearings), and to measure the wear of the bearing shells after the pump has been in operation for a long period of time (i.e., after one major maintenance cycle). The measurement method usually uses the lead wire pressure method. The radial clearance of the bearing shells is generally 1‰ to 1.5‰ of D (where D is the diameter of the pump shaft). If the measured clearance exceeds the standard, the bearing shell alloy must be remelted and ground until it meets the required specifications. In addition, it is also necessary to check whether there is any peeling or cracking in the alloy layer of the bearing bush; if this affects performance severely, the alloy should be poured anew. After the shaft bearing inspection is completed, they can be removed in sequence, making sure to note the order and positions. Figure Table 2-1 Radial clearance between the sealing ring and the impeller (mm). The clearance between the vane bushing and the impeller hub is generally 0.40–0.45 mm. An interference fit is used between the blade bushing and the guide vane, with an interference amount of 0.015–0.02 mm, and it must be secured with set screws. II. Inspection and maintenance of rotor components The rotor components mainly include the pump shaft, impeller, and balance disk, etc. Whether a water pump can operate safely and reliably over the long term is closely related to the structure of the rotor, its balance accuracy, and the quality of its assembly. The maintenance procedures for these main components will be described below. 1. Pump shaft: The shaft is an important component of a water pump; it not only supports all the components on the rotor but also serves to transmit torque. 1.1 Inspection and replacement of the pump shaft: After disassembling the pump, the surface of the shaft should first be inspected visually; usually, a fine sandpaper is used to lightly polish the shaft to check for any grooves caused by water action, as well as any scratches or dents on the surfaces of the two shaft journals. If erosion is found on the surface of the shaft, special repair should be carried out. If any of the following conditions are found during inspection, the shaft should be replaced with a new one: 1.2 There are deep grooves on the surface of the shaft caused by high-speed water flow, especially in the keyway area. 1.3 Those with significant shaft bending that continues to occur during operation even after multiple straightening attempts. 2. Methods for measuring and correcting shaft bending 2.1 Place the pump shaft on a dedicated rolling bench; a lathe or V-block can also be used as a support for inspection. 2.2 Make permanent markings dividing the circumference into eight equal parts on the opposite end face of the pump shaft, usually starting from the keyway. The records of shaft bending in all maintenance files should correspond to the markings made. 2.3 When starting to measure shaft bending, the shaft should always be pressed against one end and not move back and forth (although the ends of the shaft must not be under stress) to ensure measurement accuracy. 2.4 The recorded values for each cross-section should be measured 2–3 times, and the reading error at each point must be kept within 0.005 mm. During the measurement, the angle of rotation should be consistent each time, and the direction of rotation of the disk should also remain the same. When rotating the rotor after installing the dial indicator, recording generally starts from the second point, and the value at the second point after one full rotation should be the same as the original value. 2.5 The measurement location should be chosen in an area without keyways, and 10 to 15 cross-sections are generally sufficient for measurement. The area where measurements are to be taken should be polished and cleaned to ensure it is free of defects such as burrs, irregularities, and dirt. 2.6 In any cross-section of the pump shaft, the maximum difference between the measurement readings of two points that are 180° apart is referred to as the \"runout\" or \"wobble\" of that cross-section; shaft bending is equal to half of this runout value. The deflection of each cross-section should be indicated by arrows, and it is determined whether the bending of the pump shaft occurs within the same longitudinal section by checking whether the directions of the arrows are consistent. 2.7 After the measurements are completed, identify the section with the maximum bending by examining the bending values for each cross-section. Then, a dial indicator can be used to further determine the section of the pump shaft with the greatest bending (this section is not necessarily the one that was measured earlier). Rotate the pump shaft back and forth to find the points of greatest convexity and concavity in this section, and make notes and markings of them. 8) Check that the maximum deflection of the pump shaft must not exceed 0.04 mm; otherwise, the “peening method” or “internal stress relaxation method” should be used to straighten it. The “local heating straightening method” should be avoided as much as possible. For specific operations related to the straight shaft, please refer to the relevant content later. 2. Impeller 2.1 Inspection and maintenance of the impeller and its sealing ring: After disassembling the water pump, check the degree of wear on the impeller’s sealing ring. If it is within acceptable limits, the worn areas can be repaired by using special fixtures on a lathe to expand the inner hole of the impeller; after repair, it is important to maintain the original level of concentricity and surface roughness. Finally, fabricate the corresponding seal rings and guide vane bushings to maintain the original seal clearance. After machining the impeller seal ring, in order to prevent deviations in concentricity caused by the movement of the fixtures during the processing, special fixtures are used for inspection. The specific steps are as follows: Insert a shaft with a shoulder into the inner hole of the impeller, fix this shaft on a vise at an angle of α, ensuring that the hub on the suction side of the impeller remains in contact with the shoulder of the fixture. Then rotate the impeller slowly; the variation indicated by the dial indicator at the location of the impeller seal ring should be less than 0.04 mm. If not, further machining is required. For the blades of the first-stage impeller, since they are prone to cavitation damage, if there are minor cavitation-induced holes, they can be repaired by weld patching or by using epoxy resin adhesives. Measure the clearance at the fit between the inner hole of the impeller and the shaft journal. If this clearance becomes too large due to wear from long-term use or repeated disassembly and assembly, in order to avoid affecting the rotor’s concentricity or causing rotor vibration as a result, it can be repaired by spot-welding certain areas of the impeller’s inner hole followed by machining, or by chrome plating followed by grinding. If the impeller still does not meet the quality requirements after being repaired using the above methods, it must be replaced with a new one. 2.2 Replacement of the impeller
For a newly replaced impeller, the following procedures must be carried out; it may be put into use only after passing the inspection:
2.2.1 The main geometric dimensions of the impeller—such as the runout of the impeller seal ring diameter relative to the shaft bore, the runout of the end faces relative to the shaft bore, the parallelism between the two end faces, the offset of the centerline of the keyway from the shaft axis, as well as the outer diameter D2, outlet width b2, and total thickness—must correspond to the dimensions specified in the drawings. 2.2.2 Clean the leaf rotation path thoroughly. 2.2.3 After finish machining, each new impeller undergoes a static balance test and passes it. The machining of the new impeller is primarily aimed at ensuring the concentricity between the outer circumference of the impeller’s sealing ring and its inner hole, as well as the perpendicularity and parallelism of the two end surfaces of the hub. 3. Trial assembly of the rotor
3.1 Purpose and prerequisites of the trial assembly
The trial assembly of the rotor is primarily aimed at improving the final assembly quality of the water pump. Through this process, the tight-state fluctuation of the rotor can be eliminated, and the axial distance between the impellers can be adjusted, thereby ensuring that the flow channel centers of all the impellers and vanes are aligned at the same time; this also makes it possible to determine the size of the adjustment sleeve. Before trial assembly, all dimensions of the various components should be measured to eliminate any significant deviations. The measurement method for the radial runout of various components can be referred to as described earlier. The inspection method for the end-face runout of each component is as follows: for the impeller, a special mandrel is inserted into its bore; this mandrel is fixed to a platform. By gently rotating the impeller, the reading on the dial indicator represents the end-face runout. This fluctuation value must not exceed 0.015 mm; otherwise, machining is required. The inspection of the end-face runout of components such as bushings can be carried out using a dial indicator on a flat surface, and this runout value must not exceed 0.015 mm. In summary, once it has been confirmed that the end faces of all rotor components are clean, that the clearance between the impeller’s inner hole and the shaft journal is appropriate, that the shaft’s bending is no more than 0.03–0.04 mm, and that the concentricity deviation of all assembled components is less than 0.02 mm with an end face runout of less than 0.015 mm, then trial assembly can begin on a specialized support that allows the rotor to rotate. 3.2 Steps for rotor trial assembly The rotor can be assembled following these steps: 3.2.1 Install all keys in their designated positions to prevent the shaft sleeve from coming into contact with the keys due to incorrect positioning of those keys. 3.2.2 Install all seals and the like in their proper positions, tighten the lock nuts, and note the distance from the lock nut on the outlet side to the shaft end; this value will serve as a reference for determining the tightness of the assembled components when the water pump is put together officially. 3.2.3 When tightening the lock nut of the shaft sleeve, it is necessary to always keep the pump shaft in the same orientation (for example, ensuring that the keyway on the shaft faces upward). Moreover, after each measurement of the rotor’s wobble, the lock nut should be loosened, and tightened again before the next measurement. The force applied each time the lock nut is tightened should be such that there is no gap between the assembled components and they remain secure; it should not be excessive. 3.2.4 When installing each set of components on the shaft, they should be arranged in a logical order based on their respective levels of wobble and orientation, to prevent the accumulation of wobble in any particular direction. When measuring rotor wobble, the rotor should not be able to move back and forth and should not be subjected to excessive forces in the axial direction. Finally, the runout of all parts of the assembled rotor should not exceed the values specified below: Figure 3.2.5 After installing all the components of the rotor and tightening the locking nuts, use a dial indicator to check whether the radial runout of each component is within acceptable limits. If it exceeds the standard, the end face runout values of all set components should be checked again until the requirements are met. 3.2.6 Check whether the center distances of the water outlets of the impellers at various stages are consistent, and measure the distance from the last impeller to the end face of the balance disk in order to determine the size of the adjustment sleeve. Once the trial assembly results meet the quality requirements and are properly documented, the individual components of each set can be disassembled for formal assembly. 4. Assembly and adjustment of the water pump: After cleaning, inspecting, and repairing all the components of the water pump, the assembly process can be carried out. Assembling the water pump is carried out in the reverse order of disassembly, and once reassembly is complete, the following adjustment tasks can be started: 4.1 Centering of the outlet of the first-stage impeller. Prepare a positioning piece (whose width K is determined through measurement), and insert this piece into the outlet of the first-stage impeller. Push the rotor until the positioning plate makes contact with the side of the water inlet section (at this point, the first-stage impeller must not lose contact with the retaining sleeve and shaft shoulder, so as to avoid any gaps); at this stage, the centerline of the impeller’s water outlet should be exactly aligned with the centerline of the guide vane’s water inlet. Use a scribing needle to mark lines on the outer circumference of the shaft sleeve at a level with the end face of the stuffing box on the inlet side, so as to check the alignment of the water outlet and the axial position of the impeller within the stator after the balancing device is reinstalled. 4.2 Measurement of total play: The method for measuring total play is as follows: Install the toothed washer; instead of a balance disc, use an old sleeve. After installing the shaft sleeve and tightening the lock nut, move the rotor back and forth. The difference between the two readings indicated by the dial indicator placed at the shaft end represents the total play of the shaft. Alternatively, it is also possible to use a setup that includes only the dynamic balance disc and the shaft sleeve; after tightening the locking nut of the shaft sleeve in the correct position, the rotor is moved back and forth, and the difference between the distances measured between the opposite end faces on two occasions gives the total play of the rotor. Regardless of the method used to measure the total displacement, while moving the rotor, use a scribe to mark lines on the outer circumference of the shaft sleeve, using the end face of the inlet side packing chamber as a reference. The line marked when moving towards the outlet side is denoted as a, and the line marked when moving towards the inlet side is denoted as b; the line c, which is used for aligning the outlet of the first-stage impeller, should be roughly located in the middle of the line ab. When adjusting the axial position of the rotor, this line (line c) should be used as a reference. 4.3 Assembly of the balance disk and adjustment of the rotor’s axial position: First, install the balance disk, adjustment sleeve, toothed washer, shaft sleeve, etc., and then tighten the lock nut. Move the rotor back and forth, and use a dial indicator to measure the thrust clearance. If the thrust clearance is greater than 4 mm, the length of the adjustment sleeve should be reduced to move the rotor position further toward the outlet side ； If the thrust clearance is less than 3 mm, a new toothed shimming piece should be installed to increase its thickness, thereby moving the rotor position forward toward the inlet side. Note: It is absolutely not allowed to use shims for adjustment. Finally, use a scribing needle to mark a line on the outer circumference of the shaft sleeve at a level with the end face of the filler chamber on the inlet side; this line should roughly coincide with the aforementioned line c. The axial position of the rotor is determined by the load-bearing surfaces of the dynamic and static balance discs. The maximum allowable wear for these two components is 1 mm; therefore, the allowable axial displacement of the rotor within the stator is as follows: 4 + 1 = 5 mm on the inlet side, and 4 – 1 = 3 mm on the outlet side. In this way, even when the balance disk wears out or the rotor expands due to heat beyond the extent of expansion of the stator, the relative position between the impeller and the guide vanes can still be maintained. 4.4 Adjustment of the concentricity between the rotor and the stationary parts: After the main components of the pump have been assembled, the bearings at both ends can be reinstalled. The steps are as follows: 4.4.1 Before removing the shaft bushings, support the rotor components on the stationary parts such as seal rings and guide vane bushings. Place a dial indicator on each of the bearing brackets at both ends. 4.4.2 Use a crowbar to lift both ends of the rotor smoothly at the same time (keeping the rotor as horizontal as possible), move it up and down, and record the difference in readings of the dial indicator during these movements; this difference is the radial clearance △d between the rotor and the stationary components. 4.4.3 Lift the rotor, place the lower bearing shell in position, then use a lever to move the rotor up and down, recording the difference δ in the dial indicator readings until δ = △d/2 is achieved. During adjustment, the adjustment bolts under the bearing frame can be moved up and down, or shims can be inserted between the bearing shell and the bearing frame at their mating surfaces. 4.4.4 During the adjustment process, it is necessary to maintain concentricity between the rotor and the stator; the method is the same as before (the lower bearing shell must be removed). During measurement, an inner dial can be used to determine whether the journal is in the center of the bearing housing. 4.4.5 At this point, the bearing housing bolts can be tightened, and the set screws can be installed. 4.4.6 After completing the above tasks, scrape the bearing shells and check their fit, then reinstall the bearings. The required bearing shell tightening force is generally around 0.02 mm, the clearance at the top of the bearing shell is 0.12–0.20 mm, and the clearances on both sides of the bearing shell are 0.08–0.10 mm. 5. Other tasks: After the maintenance of the water pump is completed, check that the pump rotor rotates properly, that all components are free of defects and exhibit minimal vibration during operation. Re-measure the various clearances between the rotor and stator, as well as the total axial movement of the rotor; ensure that these values meet the required standards. The parallelism deviation of the assembled rotor and stator should be less than 0.02 mm, and the tightening degree of the bolts used to secure the pump casing should not vary by more than 0.05 mm in any direction. If all these conditions are met, it can be concluded that the maintenance and installation of the water pump are of satisfactory quality. Section 4: Measurement and Calculation of Various Components of Centrifugal Pumps I. Measurement of Shaft Deflection When the pump shaft bends, it can cause imbalance in the rotor as well as wear on the moving and stationary parts; therefore, the deflection of the pump shaft should be measured during major repairs. 1. Place both ends of the shaft on V-blocks; the V-blocks should be positioned stably and firmly ； 2. Reinstall the dial indicator so that the measuring rod points toward the axis. Then, slowly rotate the pump shaft; when the shaft is bent, the dial indicator will show a maximum and a minimum reading with each full rotation, and the difference between these two readings indicates the degree of bending of the shaft. This measurement process actually measures the radial runout of the shaft, that is, the wobble. 3. Half of the deflection is the bending value of the shaft. Typically, the requirement for the radial runout of the pump shaft is: no more than 0.05 mm in the middle, and no more than 0.02 mm at both ends. II. Measurement of rotor runout The method for measuring rotor runout is similar to that used for measuring shaft bending. Generally, it is required that the radial runout of the impeller seal ring shall not exceed 0.08 mm, the wobble at the shaft sleeve shall not exceed 0.04 mm, and the wobble at both shaft journals shall not exceed 0.02 mm. III. Disassembly and assembly of the coupling 1. When removing the coupling, do not use a hammer directly; instead, use a copper rod as a padding. Hit the hub of the coupling rather than its outer edge, as the latter is very prone to damage. The ideal way is to use a wrench to remove the coupling. For small and medium-sized water pumps, since the interference fit is very small, the coupling can be removed easily. For larger water pumps, there is a significant interference fit between the coupling and the shaft; therefore, the coupling must be heated during disassembly. 2. When assembling the coupling, pay attention to the key numbers (for couplings with two or more keys). When hitting with a copper rod, it is necessary to pay attention to the area being struck. For example, hitting the end face of the shaft hole can easily cause it to shrink, preventing the shaft from passing through ； Hitting the outer edge of the wheel can easily damage the flatness of its end face, which will affect the accuracy of measurements when using a feeler gauge for alignment later on. For couplings with a large interference fit, they should be heated before installation. 3. The coupling pins, nuts, washers, and gaskets must all have consistent specifications and sizes to prevent disruption of the coupling’s dynamic balance. Appropriate markings should be made on the coupling bolts and the corresponding coupling pin holes to prevent incorrect installation. 4. The fit between the coupling and the shaft generally adopts a transitional fit; there may be a slight amount of interference or a slight amount of clearance. For couplings with longer hubs, a looser transitional fit can be used, as the shaft hole is longer and the uneven surface finish results in some interference occurring naturally after assembly. If it is found that the fit between the coupling and the shaft is too loose, affecting the concentricity of the holes and shaft, then welding repairs should be carried out. Marking the shaft with dots or using copper shims is a temporary solution and cannot be considered an ideal method. IV. Alignment of the water pump using the coupling: After the water pump has been repaired, to ensure its proper operation, it is necessary to keep the shafts of the pump and the prime mover aligned with one another during operation. This is to prevent additional stress on the bearings due to misalignment of the shaft centers, which could lead to heating and wear of the bearing surfaces as well as overloading of the prime mover. In severe cases, this might even cause intense vibrations that result in the shutdown of the pump unit. The alignment of the water pump after maintenance is carried out on the coupling. At the beginning, use a straight edge to compare the relative positions of the two couplings, one belonging to the prime mover and the other to the pump, around the coupling. After determining the direction of the deviation, make a rough adjustment to bring the centers of the couplings closer together so that their end faces are aligned. Usually, when the prime mover is an electric motor, it is necessary to adjust the center of the coupling primarily by adjusting the shims under the motor feet ； If the prime mover is a steam turbine, centering is mainly achieved by adjusting the water pump. During alignment, it is easier to achieve centering by adjusting the coupling end faces first and then the center. The following is an introduction step by step. 1. Preparations before measurement: Depending on the type of coupling, use a feeler gauge or dial indicator to directly measure the circumferential clearance α and the end-face clearance b. During the measurement process, the following points should also be noted: 1.1 Before alignment, the two couplings should be connected using special centering bolts. If it is a fixed coupling, the two should be inserted properly. 1.2 During the measurement process, the axial position of the rotor must remain constant to avoid errors caused by forward or backward movement as the rotor is rotated. 1.3 All anchor bolts should be properly tightened before measurement. 1.4 Timing adjustment must be carried out when the engine is cold; it is not possible to determine the center position when the engine is hot. 2. Basic requirements for alignment: 2.1 The radial deviation of the centerlines of the motor and pump shafts should be within the allowable range. 2.2 The end face of the motor’s back pulley should be parallel to that of the pump’s back pulley, and the axial clearance deviations at the four corners of the end faces should be within the allowable range. 2.3 There should be a certain gap between the two backrest wheels; the range of this gap varies depending on the pump. 3. Measurement process: Mark the two couplings and align them, with the marked side positioned at zero position (either vertically or horizontally). Install a special tool holder or dial indicator, and rotate the rotor in the direction of its rotation from the zero position by 90°, 180°, and 270° successively. At each position, measure the circumferential clearance α and the end-face clearance b, and record the obtained data one by one. Based on the measurement results, the values at each point in the two end faces are averaged. By analyzing the above data, it is possible to determine the tilt of the coupling and the direction in which adjustment is needed. 4. Analysis and Calculation: Analysis of the offset of the back wheel: 4.1 Four points are designated at the top of the back wheel and on its circular end face respectively (00, 900, 1800, 2700); on the top surface these points are a1, a2, a3, a4, while on the end face they are s1, s2, s3, s4 ； At zero degrees, set the large hand of the watch to zero with a certain amount of pressure. That is, at this time, the values of a1 and s1 are both 0. (Note: Pay attention to the position of the pointer on the small gauge.) In Figure 4.2, a positive value corresponds to the pointer moving in a clockwise direction, while a negative value corresponds to movement in a counterclockwise direction. Rotate the handwheel by one full turn, record the values at 4 o’clock, and ensure that a1 + a3 = a2 + a4, as well as s1 + s3 = s2 + s4. Figure 4.2.1: Case a1: s13, a1 < a3. 4.2.2: Case a2: s1 > s3, a1 > a3. 4.2.3: Case a3: s13, a1 > a3. 4.2.4: Case a2: s1 > s3, a1 < a3. All of these four cases involve deviations both radially and axially; in addition to these cases, there are also situations where the end faces are parallel or where the axial displacement is zero. Derivation of the calculation formula – Image (Figure 1): Let the length of the segment b1b2 be equal to b; thus, the value of b is s3 – s1, which represents the axial deviation of the end face. Let the length of segment c’l1 be equal to e; thus, the value of e is (a3 – a1)/2, which represents the radial displacement deviation. Since b1b3 is perpendicular to o l1, b2b3 is perpendicular to o l2, and b1b2 is perpendicular to l’1l’2, therefore Δb1b2b3 is similar to Δo; moreover, l1l2 and Δb1b2b3 are similar to Δol1l2. According to the theorem of similar triangles, the corresponding sides are proportional: b1b2 / b3b1 = l1l2 / o, hence l1 = l1l2 / ol1. Here, b1 and b2 are the diameters of the circle traced by the measurement point as it rotates one full circle, denoted by D. ol1 and ol2 represent the distances from the front and rear foot bolts to the center point of the motor’s back pulley, denoted as L1 and L2 respectively. By substituting b, e, D, L1, and L2 respectively, we get: b/D = l1l2/L1 = l1l2/L2. Therefore, the thickness of the shims under the front and rear foot bolts: The values shown in the image can be used directly in the calculations, but attention should be paid to the positive or negative sign of these values. If the resulting value is positive, shims should be added; if it is negative, shims should be removed. Pay attention to the measurement of the D value. The left-right deviation can be ignored; once it is measured, the motor can be adjusted to the correct position using the obtained data. Note: Since tightening or loosening the motor’s foot bolts has a significant impact on the measurement values, it is recommended to keep the motor’s foot bolts tightened when measuring radial deviation, and to loosen them when measuring axial deviation. 4.2 Allowable error during adjustment: When adjusting the shims, the measuring frame should be removed or loosened; any dirt on the feet of the shims as well as on the shims themselves must be cleaned off. Finally, when tightening the foot bolts, any external wedges or jacks used for support should be removed, and changes in the readings of the dial indicator should be monitored. As for the allowable error for aligning the couplings, it varies depending on the type of coupling; specific values can be found in the table provided. 4.3 The values for the alignment deviation of the coupling are shown in the attached table: Image. The values for the clearance between the couplings are given in the table below: Image. In addition, as operating conditions change, such as when the water pump transports water at high temperatures (above 60°C) or when the pump is driven by a turbine, it is necessary to take into account both the increase in the center position of the water pump and turbine rotors due to thermal expansion, as well as the calculated values for the center of the coupling. For example, in the case of a motor and a water pump installed on the same base, if the temperature of the water being pumped is 60°C, the motor needs to be raised by approximately 0.40–0.60 mm in order to ensure that the axis centers of the pump and the motor are perfectly aligned during operation. 4.4 Straight shaft operation: When the shaft bends, it is first necessary to measure the entire length of the shaft using a dial indicator at room temperature, following the method described earlier. A bending curve should be drawn to determine the location of the bend as well as the degree of bending (which is 1/2 of the difference between the maximum and minimum relative positions in any cross-section of the shaft). Secondly, the shaft should also undergo the following inspections: 4.4.1 Inspection for cracks – In the area where the maximum bending of the shaft occurs, cracks should be checked using methods such as dipping in kerosene followed by applying chalk powder, and these cracks should be eliminated before straightening the shaft. Before eliminating the crack, its depth must be determined using methods such as grinding, turning, or ultrasonic testing. Milder cracks can be repaired to prevent their expansion during straightening ； If the depth of the crack affects the strength of the shaft, it should be replaced. After the cracks are removed, a rotor balance test must be conducted to compensate for the shaft’s imbalance. 4.4.2 Checking hardness: Measure the hardness of the shaft surface at the crack site and in the normal areas surrounding it, in order to determine the extent of changes in the metal structure in the bent area and thus select the appropriate method for straightening the shaft. The quenched shaft should be annealed before straightening. 4.4.3 Material inspection: If the material of the shaft is uncertain, samples should be taken for analysis. Only after knowing the chemical composition of the steel can the straight-axis method and heat treatment process be determined more accurately. After all the aforementioned inspection tasks are completed, an appropriate straightening method and tools can be selected to carry out the straightening process. The methods for straightening shafts include mechanical compression, twisting, local heating, combined local heating and compression, and stress relaxation. 4.4.4 Twisting method (cold straightening method): The twisting method involves using a twisting rod to apply vibrations to the concave area where the shaft is bent. This causes the cohesive force between the metal molecules in that concave area – where the fibers are compressed and shortened – to decrease, thereby allowing the metal fibers to lengthen. At the same time, the metal on the surface of the shaft at the point of twisting undergoes plastic deformation, resulting in residual elongation of the fibers, and this achieves the goal of straightening the shaft. The basic steps for twisting and beating are: (1) Determine the position of the straight shaft based on the measurements of shaft bending, and mark it. (2) Select an appropriate twisting rod for twisting. The material for the twisting rod is usually 45# steel, and its width depends on the diameter of the shaft (typically 15–40 mm). The working end of the twisting rod must match the curvature of the shaft surface; its edges should be rounded to avoid sharp corners (R1=2–3 mm), so as to prevent damage to the shaft surface. After curling at the top of the twisting rod, it should be repaired or replaced promptly to prevent damage to the pump shaft. (3) For the straight shaft, place the concave side of the shaft facing up, support it with hardwood beneath the maximum bending section, and pad it with lead plates. Additionally, when using a straight shaft, it is best to place the shaft on a dedicated stand and press both ends of the shaft downward in order to accelerate the vibration of the metal molecules and thus cause the fiber to elongate. (4) The twisting range is 1/3 of the circumference (i.e., 120°), and this range should be marked in advance on the shaft. The axial length during twisting can be determined based on the degree of shaft bending, the material of the shaft, and the degree of surface hardening of the shaft; it is generally kept within the range of 50 to 100 mm. The twisting sequence is carried out alternately at symmetric positions, with more twists in the middle and fewer on the sides. (5) When twisting, a hand hammer weighing 1–2 kg can be used to strike the twisting rod; the center line of the twisting rod should be aligned with the marked range on the axis, and the force applied during striking should be moderate, not too strong. (6) After each shot, use a dial indicator to check for changes in bending. Generally, the straightening occurs rapidly at the initial stage, but then the speed of straightening slows down due to surface hardening of the shaft. If twisting at a certain bend no longer has a significant effect, the twisting should be stopped and the reason identified; a new appropriate location should then be found to proceed with twisting until correction is achieved. (7) After twisting and straightening the shaft, it should be bent slightly in the opposite direction to its original curvature by 0.02–0.03 mm, that is, straightened a bit too much. (8) Once the shaft bending has reached the required value, the twisting operation can be stopped. At this point, comprehensive and careful measurements of each cross-section of the shaft should be taken, with proper recording. (9) Finally, the twisting shaft is subjected to low-temperature tempering at 300–400°C to eliminate surface hardening and prevent the shaft from bending again after straightening. The aforementioned cold straightening method is the most commonly used straightening technique in practice, but it is generally suitable only for shafts with small journal sizes and a shaft bending of around 0.2 mm. The advantage of this method is high straightness of the shaft, easy control, low stress concentration, and no cracks occur during shaft alignment. Its disadvantage is that compressive stress remains within a small section of the shaft material behind the straight shaft, and the speed of the straight shaft is relatively low. 4.4.5 Local heating method: This method involves rapidly applying local heating to the convex surface of the pump shaft, thereby artificially inducing compressive stresses on the shaft that exceed the material’s elastic limit. Once the shaft is cooled, the metal fibers on the convex side are compressed and shortened, resulting in a certain degree of bending in order to achieve a straight shaft. The specific procedure is as follows: (1) Measure the shaft bending and plot the shaft bending curve. (2) Condition of cleaning and cracks around the entire circumference of the maximum bending section. Check and record it properly. (3) Place the shaft with its convex surface facing up on a specialized stand, and install dial indicators on both sides near the heating area to observe the changes after heating. (4) Wrap the area of maximum bending with asbestos cloth, and cut rectangular heating holes in the asbestos cloth centered on the point of maximum bending. The length of the heating hole (in the circumferential direction) is approximately 25% to 30% of the shaft diameter at that location, while the width of the hole (in the axial direction) depends on the degree of curvature and is about 10% to 15% of the diameter at that location. (5) Use smaller nozzles of size 5, 6, or 7 to heat the shaft surface at the heating hole. During heating, keep the welding nozzle at a distance of about 15–20 mm from the axis surface; start from the center of the hole and then move toward the sides, moving the nozzle in a uniform and rhythmic manner. When heated to 500–550°C (with the shaft surface appearing dark red), cover the heating hole immediately with asbestos cloth to prevent the shaft surface from hardening or cracking due to rapid cooling. (6) When correcting pump shafts with smaller diameters, the method of observing the thermal bending value is generally used to control the heating time. The thermal bending value is the difference in dial indicator readings of the axis before and after heating, at the area near the maximum bending section, when the protruding part of the axis is heated with a burner, causing the axis to bend upward even more. Generally, the value of thermal bending is 8 to 17 times the amount by which the shaft straightens; that is, when the shaft bulges by 0.08 to 0.17 mm due to heating, it can be straightened by 0.01 mm after cooling. The specific values depend on the length-to-diameter ratio of the shaft as well as its material. The relationship between the thermal bending value of a shaft after its first heating and the amount of elongation of the shaft should serve as a basis for straightening the shaft during the next heating. (7) After the shaft has cooled to room temperature, measure its bending using a dial indicator and plot the bending curve. If it does not fall within the allowable range, it should be realigned again. If reheating at the maximum bending point of the shaft proves ineffective, it should be moved axially to another position from the original heating site, while local heating for correction is carried out sequentially using two welding nozzles. (8) The shaft should have a slight bend for correction, namely a bending amount of 0.01–0.03 mm in the direction opposite to the original bend; this excess bend will disappear after annealing. When using the local heating method, the following points should be noted: ① Axial work should be carried out in a room with dim lighting and no air circulation. ②The heating temperature must not exceed 500–550°C, and colored glasses must not be worn when observing the color of the shaft surface. ③The stress required on the straight shaft can be adjusted in two ways: one is by increasing the heated surface area ； Second is to increase the depth of the metal layer of the shaft being heated. ④When the shaft has local damage, there is high surface hardness in certain areas of the straight shaft section, or the pump shaft is made of alloy steel, the local heating method should generally not be used to straighten the shaft. Finally, the straightened shafts should be heat-treated to prevent them from bending again in high-temperature environments, while shafts that operate at normal temperatures do not require heat treatment. Section 5: Starting and Commissioning of Centrifugal Pumps I. Operation Methods of Centrifugal Pumps 1. Checks before starting a centrifugal pump 1.1 After the motor has been overhauled, before connecting the coupling, check first whether the rotation direction of the motor is correct. 1.2 Check the pump inlet and outlet pipelines as well as the associated pipelines to ensure that the flanges and valves are installed properly, that the anchor bolts and grounding wires are in good condition, and that the couplings are properly installed. 1.3 Perform a spin test to check whether the rotation is normal. 1.4 Check whether the lubricating oil level is normal; add oil if it is low, and examine the quality of the lubricating oil (grease). 1.5 Open the various cooling water valves and check whether the pipelines are unobstructed. Note that the cooling water flow rate should not be too high or too low; too high a flow rate leads to waste, while too low a flow rate results in poor cooling efficiency. Generally, the cooling water flow in a linear pattern is sufficient. 1.6 Open the inlet valve of the pump, close the outlet valve of the pump, and open the pressure gauge handwheel. 1.7 Check the sealing condition of the pump and the opening degree of the oil seal. Note: The hot oil pump must be evenly preheated before starting. 2. Starting the centrifugal pump 2.1 Open the inlet valve fully, close the outlet valve, and start the motor. 2.2 When the pump outlet pressure is higher than the operating pressure, check that all components are functioning properly, and then gradually open the outlet valve. 2.3 When starting the motor, if it fails to start or abnormal noises are heard, the power supply should be cut off immediately for inspection; the fault must be resolved before the motor can be started again. 2.4 During startup, be careful not to face the coupling to prevent it from flying out and causing injury. 3. Shutdown procedure for centrifugal pumps 3.1 Slowly close the pump’s outlet valve. 3.2 Cut off the power to the motor. 3.3 Close the pressure gauge manual valve. 3.4 After parking, the cooling water should not be stopped immediately; it must be allowed to cool down to below 80 degrees before the water supply is turned off. 3.5 Close the inlet valve as needed, and vent the pump. 4. Operation and maintenance of centrifugal pumps 4.1 When a centrifugal pump is operating normally, the operator must carefully inspect the following aspects: 4.2 Check the outlet pressure, flow rate, current, etc. of the pump to ensure it is not operating under overload conditions, and accurately record parameters such as current and pressure. 4.3 Listen to the sounds to distinguish between the operating noises of pumps and motors, and determine whether there are any abnormalities. 4.4 Check the vibration of the pump, motor, and pump base; if the vibration is severe, replace the pump and conduct another check. 4.5 Check the temperature of the motor casing and the bearing housing of the pump; the temperature of the bearing housing should not exceed 65 degrees, while the temperature of the motor should not exceed 95 degrees. 4.6 Ensure normal quality of the lubricating oil and proper liquid level in the lubricating oil tank. Lubricating oil tank level; use the scale as a reference if one is available ； It has a sight glass (oil gauge) but no scale lines; the oil level should be maintained between 1/3 and 1/2. At the normal oil level, the lubricant leakage should not exceed 5 drops per minute. Oil should be added under pressure, in accordance with the instructions provided for the machine. 4.7 Check whether the machine pump seals, various flanges, plug valves, cooling water, and seal oil connections are leaking. 4.8 Check the readiness of the standby pump; turn it over once a day. 5. Switching operation of centrifugal pumps 5.1 To ensure that parameters such as flow rate and pressure remain essentially unchanged and without fluctuations when switching pumps, it is best for two people to carry out the operation simultaneously. 5.2 Carry out the preparatory work before starting the pump. 5.3 One person first starts the standby pump; once the pump is running smoothly, the outlet valve is gradually opened. As the outlet valve opens, the pressure at the pump’s outlet decreases slightly, but the motor current increases. Meanwhile, another person slowly closes the outlet valve of the pump that is to be shut down. Once the flow rate from the pump that is still in operation is high enough, the outlet valve of the pump to be shut down is completely closed, the power supply is cut off, and the pump is shut down properly. 6. Preheating of the hot oil pump 6.1 After maintenance is completed on the centrifugal pump, it is first pressurized with steam to check for any leaks in the pump, and at the same time, the condensate water inside the pump is removed. Slowly open the pump inlet valve (by turning it about 1–2 turns), keeping the pressure inside the pump below 0.2 MP. Once the pump is filled with the medium, fully open the pump inlet valve. Note that the opening degree of the outlet valve of the preheating pump should be adjusted to prevent the pump from running in reverse; at the same time, it is important to maintain a preheating rate of 50 degrees per hour. 6.2 During preheating, turn the pump every 15 minutes to ensure even preheating. 7. Precautions for operating centrifugal pumps 7.1 Avoid running the centrifugal pump at idle speed. 7.2 Avoid operating for extended periods with the outlet valve closed. 7.3 It is strictly prohibited to flush the motor with water. 7.4 Centrifugal pumps should be started with the outlet valve closed. II. Common Faults of Centrifugal Pumps and Their Solutions 1. The common faults of centrifugal pumps and the corresponding solutions are shown in the table below: Table of Common Faults of Centrifugal Pumps and Their Remedies