Vibration problems of horizontal multi-stage pumps

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Our factory has a horizontal multi-stage centrifugal pump of the D48-50*12 series. The medium used is amine, the operating temperature is 80°C, the outlet pressure is 6.0 MPa, and the inlet pressure is 0.7 MPa. A few days ago, due to abnormal operation of the pump, it was disassembled and it was found that the guide vanes had been corroded by the amine and could no longer be used; therefore, new stainless steel guide vanes were installed. Friction occurred between the outer surface of the first-stage closed impeller and the guide vanes (the impeller is made of stainless steel), so the first-stage impeller was replaced, along with the guide vanes, balance disk, seal casing, and bearings. The runout of the pump shaft was measured to be less than 4 microns, and dynamic balancing was performed on the shaft together with the impeller and balance disk. After assembly and trial operation, the vibration at the front and rear bearings was 2.2 mm/s to 3.0 mm/s when the outlet was not opened. Once the outlet was opened, the vibration of the non-driving-end bearings increased gradually; after it was fully opened, the vibration of the driving-end bearings was 7.6 mm/s, while that of the non-driving-end bearings was around 15–18 mm/s. 1. The inlet pipeline was re-routed so that it was not under any stress, and the bolts connecting the outlet pipeline to the pump could pass through easily. 2. Elastic pin connectors were used for linking the drive wheels, and alignment was carried out using a vernier caliper on the end faces, with checks made in the horizontal, vertical, left, right, and upward directions – about 10 checks in total. Radial dial indicators were also used for alignment, with errors also being within about 10 units. 3. During pump installation, a level was used to ensure that the pump was level both horizontally and vertically; the anchor bolts were not loose, and the pump was firmly secured to its base. 4. The one-sided clearance between the guide vane throat ring and the impeller hub is 50–60 teeth. After removing the load from the support beneath the outlet gate valve on site, it was observed that the vibration of the pump changed significantly: the vibration at the drive end increased to 3.5 mm/s, while that at the non-drive end rose to 10–12 mm/s. Once the support beneath the outlet gate valve was subjected to stress again, the overall vibration of the pump increased; the vibration at the bends in the pump’s outlet pipe was 13–15 mm/s on the east and west sides, while it was 4.5 mm/s on the north and south sides. The vibration in the inlet pipeline is 3.2 mm/s. Two supports have been added in front of the outlet check valve, but it is these two areas that exhibit the highest vibration levels. It is suspected that resonance between the pump outlet and the pipeline is causing this issue, but it is not clear how to eliminate this resonance. One of the supports is fixed to the ground near the elbow, while the other is placed on the ground itself. Does anyone know what might be the cause?