Experience-rich technician’s summary: Analysis of the causes of pump vibration

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I. Reasons for pump vibration: There are many reasons that cause vibration in the power unit and the building housing the pump room. Some of these factors are interconnected and interact with one another; generally speaking, there are mainly the following four categories of reasons. 1. In terms of electricity, the motor is the main component of the unit; imbalances in the magnetic field within the motor and faults in other electrical systems often cause vibration and noise. For example, in an asynchronous motor during operation, the radial alternating magnetic pulling force between the stator and rotor generated by the interaction of the harmonic fluxes from the stator and rotor teeth, or in a large synchronous motor during operation, situations such as misalignment of the magnetic centers of the stator and rotor or air-gap differences in various directions exceeding the allowable limits, can all cause periodic vibration of the motor and the generation of noise. 2. Mechanically, issues such as uneven mass in the rotating parts of motors and pumps, poor manufacturing quality, inadequate installation, asymmetry in the axis of the unit, vibration amplitudes exceeding allowable limits, low mechanical strength and stiffness of components, wear and damage to bearings and sealing elements, as well as resonance caused by the critical speed of the pump coinciding with the natural frequency of the unit, can all lead to severe vibrations and noise. 3. In terms of hydraulics, uneven flow velocity and pressure distribution at the pump inlet, pressure fluctuations of the working fluid at the pump inlet and outlet, fluid bypassing, deviation from the normal flow path, and fluid separation from the pump surface, as well as operating conditions that are not within the specified parameters and pump cavitation caused by various factors, are all common causes of vibration in pump units. Dynamic transition processes such as the start-up and shutdown of water pumps, the opening and closing of valves, changes in operating conditions, and emergency shutdowns in case of accidents, which cause sudden pressure changes in the water conveyance pipes and water hammer effects, often also lead to vibrations in the pump house and the pumping units. 4. Unreasonable design of the water inlet channels for the turbines and other related components, or a mismatch between these channels and the turbines; inappropriate submersion depth of the pumps; as well as improper sequencing of turbine startup and shutdown – all of these can deteriorate the water inlet conditions, lead to the formation of vortices, cause cavitation, or increase vibration in the turbines and pump houses. In units that use vacuum interruption by breaking the siphon effect, during startup, if it is difficult for air to be carried in the hump section, the siphon formation time becomes excessively long ； The gate design of the units with gate-driven flow shutdown is unreasonable; it opens and closes repeatedly, constantly striking the gate seat ； Reasons such as uneven settlement of the foundation supporting the water pump and motor, or poor rigidity of the foundation, can also cause vibration in the unit. II. Technical measures to eliminate the hazards of pump vibration: In rotating equipment and flowing media, low-intensity mechanical vibration is inevitable. Therefore, during the manufacturing and installation of the unit, interference caused by vibration should be avoided as much as possible in terms of its design, operation, and management, in order to minimize the hazards associated with vibration. When vibration occurs in the pump house or the machine set, it is necessary to analyze the possible causes of the vibration on a case-by-case basis. Once the root cause of the problem is identified, effective technical measures should be taken to eliminate it. Some measures are relatively simple, while others are quite complex. If a large amount of funding is required, a technical and economic comparison of the available options should be conducted, taking into account the technical upgrades of the units. III. Common causes of motor vibration and countermeasures 1. Uneven bearing wear: Misalignment of the unit or worn bearings. Remedial measures: Realign the concentricity of the unit, and adjust or replace the bearings. 2. Stator-rotor friction: uneven air gap or bearing wear. Remedial measures: Readjust the air gap, adjust or replace the bearings. 3. The rotor must not stop at any arbitrary position or have unbalanced power. Remedial action: Re-check the static and dynamic balance of the rotor. 4. Axial looseness: Loose screws or improper installation. Remedial action: Tighten the screws and check the installation quality. 5. Foundation vibration: Poor foundation stiffness or loose corner screws. Remedial measures: Reinforce the foundation or tighten the bottom corner screws. 6. Unstable three-phase current: reduced torque, faults in the rotor bars or end rings. Remedial measure: Inspect and repair the rotor bar or end ring. IV. Common Causes of Water Pump Vibration and Remedial Measures 1. Difficulties in manual shaft turning: bent pump shaft, worn bearings, misalignment of the unit, and the impeller hitting the pump casing. Remedial measures: Straighten the pump shaft, adjust or replace the bearings, realign the concentricity of the unit, and readjust the clearance. 2. Excessive pump shaft runout: Wear of bearings and journal surfaces or excessive clearance. Remedial measures: Repair the journal, adjust or replace the bearing. 3. Hydraulic imbalance: Imbalance of the impeller, or blockage or damage in certain vanes of the centrifugal pump. Remedial measures: Re-check the static and dynamic balance of the impeller, remove blockages, and repair or replace the impeller. 4. Excessive shaft power of the axial flow pump: The water level in the inlet tank is too low, the immersion depth of the impeller is insufficient, debris gets entangled around the impeller, the degree of cavitation damage to the pump varies, and the impeller is damaged. Remedial measures: Raise the water level in the inlet tank, lower the installation height of the pump to remove debris, install trash racks, and repair or replace the impeller. 5. Foundation vibration: poor foundation stiffness, loose corner screws, or resonance. Remedial measures: Reinforce the foundation, tighten the foot screws. 6. The efficiency of the centrifugal pump unit drops sharply, or the efficiency of the axial flow pump unit decreases slightly, accompanied by cavitation noise. Remedial measures: Change the pump speed to avoid the resonance range, identify the cause of cavitation, and take actions to eliminate it. V. Vibration of the unit caused by other reasons and remedial measures 1. Blockage of the trash rack, resulting in a decrease in the water level in the inlet tank. Remedial measures: Clean the trash racks, and install devices for cleaning the trash racks. 2. The design of the pre-tank and inlet tank is unreasonable, and the mismatch between the water inlet channel and the pump deteriorates the water inlet conditions. Remedial measures: Clean the trash racks, install appropriate trash rack cleaning devices, and optimize the design of the forebay, intake basin, and inlet channels. 3. The time required to form a siphon is too long, causing the unit to operate under conditions other than those designed for an extended period. Remedial measures: Install a vacuum extraction device, and design and improve the siphonic water outlet channel appropriately. 4. The inlet pipe is not securely fixed, causing resonance. Remedial measures: Add pipeline pier caps and supports, reinforce pipeline supports, and adjust operating parameters to avoid the resonance zone. 5. Repeated knocking on the door causes it to strike the door frame, or the force used to close it is too great. Remedial measures: Install exhaust vents before the outlet of the flow channel (or pipeline), and design the check valves properly to take control measures and reduce the impact force when the check valves close. 6. Sudden pressure changes in the water outlet pipe and water hammer effect. Remedial measures: check valves and pressure regulation chambers, among other measures to prevent water hammer. 7. The sequence of unit startup and shutdown is unreasonable, resulting in deteriorated water intake conditions for the pump. Remedial measure: Optimize the startup and shutdown sequence.