Assembly and measurement requirements for multi-stage segmented centrifugal pumps

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Assembly and measurement requirements for multi-stage segmented centrifugal pumps 1. Assembly of each pump casing stage The burrs on the flanges of the various pump casing stages in a multi-stage segmented centrifugal pump must be removed prior to assembly. During assembly, the joints between various sections must be sealed in accordance with the requirements specified in the pump manufacturer’s instructions. If not required by the instructions, sealant can be applied to the joint surface to prevent leakage. When applying sealant, it is not necessary to cover the entire sealing surface; a narrow strip of sealant applied in one continuous line along the sealing surface is sufficient. To prevent changes in the axial dimension of the entire pump, the sealant layer must not be too thick. In a segmented multi-stage pump, the front, middle, and rear sections are held together and sealed by ensuring that the axial sealing surfaces between these sections make tight contact, which is achieved through the tension of the tightening bolts. Some manufacturers specify the tightening torque for the tension bolts in the instructions, allowing the bolts to be tightened to the specified value easily during assembly. The key aspect of assembling a segmented multi-stage pump is the axial centering of the rotor; this is crucial for ensuring that the working wheels are aligned with the guide vanes, as well as for maintaining the proper average clearance between the balance disc and the balance ring. In pumps equipped with a balance disc, the standard axial clearance on each side of the rotor is ±0.2 mm, which means a total of 0.4–0.5 mm on both sides. Since there are many identical components in each section of the pump and they have similar shapes, while the sizes of the casings, impellers, shaft sleeves, and sealing rings in each section, as well as the specified axial dimension tolerances, vary, it is necessary to number all the components during disassembly and record the axial dimensions of each component for reassembly purposes. II. Measurement and adjustment of axial play The axial play of an centrifugal pump refers to the axial gap between the rotor and the stator. The axial displacement of centrifugal pumps can be divided into total axial displacement and individual axial displacement. The play measured without the balance disc is the total play, while the play measured with the balance disc is the single play. During the final assembly of the pump, it is necessary to check not only the total axial displacement of the rotor but also to ensure its axial alignment, that is, to align the centerline of the impeller outlet flow channel with the centerline of the vane flow channel. The methods for measuring shaft play and the allowable values for such measurement vary depending on the structure of the centrifugal pump. The following describes the measurement and adjustment of axial play during the assembly of a multi-stage centrifugal pump in which both ends of the pump shaft are supported by sliding bearings and the rotor is equipped with a balance disk. ①Install the front section, front shaft sleeve, first-stage impeller, and middle section ； Tighten the large bolts to secure the inlet section and the intermediate section ； Push the rotor to one of its extreme positions, use a steel ruler to find a reference point at one end of the pump, and record the length of the shaft at that position ； Then push the rotor to the other extreme position; at this point, the amount of movement can be read on the scale of the ruler. This value represents the measured play. It is also possible to use a dial indicator to measure the amount of movement at both extreme positions of the rotor at the inlet end. ②Use the same method to measure the clearance once for each section assembled, and keep proper records. ③After installing the final stage impeller and the rear section, tighten the large bolts and measure the play – this represents the total play of the pump. ④After installing the balance disc, the rotor is pushed again to measure the axial displacement, and the measured value represents the pump’s single-axis displacement. Too small a clearance in a centrifugal pump can easily lead to wear between the impeller and the pump casing, while on the other hand it reduces the pump’s efficiency. The allowable amount of cross-talk for pumps with different structures generally varies. For hot oil pumps, considering the forward movement of the rotor due to thermal expansion, the clearance at the inlet side is 0.5 to 1 mm greater than that at the outlet side. If the total axial movement per stage of the centrifugal pump is too small, the length of the mouth ring can be shortened; if the total axial movement is too large, the mouth ring can be welded or replaced. The adjustment of the single thrust in a centrifugal pump can be achieved by shortening the balance disc wheel or by adding or removing shims in front of the balance disc hub in order to adjust the thrust at the outlet end. III. Measurement and adjustment of the coaxiality between the rotor of a segmented multi-stage centrifugal pump and its casing. The radial clearances between the rotor and the casing at various points must be equal; if these clearances are not uniform in all directions, it will result in the rotor’s axis being out of alignment with the casing’s axis both vertically and horizontally. This will cause dynamic and static friction as the rotor rotates, and in severe cases, the rotor may not be able to rotate at all. Therefore, it is necessary to adjust their coaxiality. The adjustment of coaxiality is achieved by tuning the three adjustment screws on the bearing seats at both ends of the pump to meet the requirements. The operation method is as follows. ①First, remove the upper and lower bearing shells at both ends of the pump, allowing the rotor to fall freely to the bottom of the pump casing; at this point, the gap between the rotor and the lower part of the pump casing is zero. ②Place dial gauges at the bearing locations before and after the pump shaft; ensure that the gauge heads point vertically upward toward the top of the shaft. Set the gauges to zero, then gently lift both ends of the rotor until they can no longer be lifted any further. Record the reading on the dial gauges – this reading represents the value when the bearings are not in place. ③Install the lower bearings at both ends of the pump, reset the dial of the dial indicator to its original position, and then gently lift both ends of the rotor until it can no longer be lifted. Check the reading on the dial indicator; if it is half of the reading obtained when the bearings were not installed, it means that the alignment of the rotor with the pump casing in the vertical direction has been adjusted properly. If the reading is not half, the required alignment can be achieved by adjusting the three set screws at each end of the shaft bearings. ④For adjustment in the left-right direction, it can be determined based on the distance from the shaft to the openings of the bearing seats on both sides; the method of adjustment follows that used for vertical adjustment. It is quite difficult to adjust both vertical and horizontal alignment simultaneously. Because when adjusting one of them and then adjusting the other, the data that has been adjusted for the first one may be compromised, so both need to be adjusted repeatedly until the rotor and the pump casing are concentric. Since the lubricating oil in the bearing shells creates an oil wedge while the pump is running, which lifts the rotor upward, when adjusting the concentricity in the vertical direction, the rotor center is often intentionally set at a position 0.03–0.05 mm below the center of the pump casing.