Centrifugal pumps have become indispensable core equipment in all areas of industrial production due to their efficient and reliable fluid transfer capability. Petrochemical, electric energy to water supply and drainage systems, centrifugal pumps bear the key task of transporting liquids from one place to another. However, during long-term operation, centrifugal pumps will inevitably encounter a variety of failures, which will not only affect production efficiency, and may even cause safety accidents. Therefore, accurate diagnosis of common failures of centrifugal pumps and take timely and effective measures to eliminate, to ensure the stable operation of industrial production is of great significance. In this paper, we will introduce in detail the diagnosis method and elimination strategy of centrifugal pump common faults, to provide practical solutions for the relevant technical personnel.
一.Centrifugal Pump Overview and Working Principle
Centrifugal pump is mainly composed of impeller, pump casing, pump shaft, sealing device and bearings. Its working principle is based on the role of centrifugal force: when the prime mover drives the pump shaft and the impeller to rotate, the blades in the impeller force the liquid to rotate, and the liquid is thrown to the outer edge of the impeller from the centre of the impeller under the action of centrifugal force and flows into the pump casing at a high speed. In the pump casing, due to the gradual expansion of the flow path and deceleration, and part of the kinetic energy into static pressure energy, and finally along the tangential flow into the discharge pipeline. At the same time, the impeller is thrown out of the centre of the liquid to form a certain vacuum, and the pressure Pa at the liquid surface is higher than the impeller centre, so the liquid in the suction line under the action of differential pressure into the pump. As the impeller keeps rotating, the liquid is continuously sucked in and pressed out to achieve continuous liquid conveying.
二.The centrifugal pump common fault diagnosis and troubleshooting
1.The pump can not start or start difficult
Fault phenomenon: after pressing the start button, the centrifugal pump does not respond, the motor does not rotate; or although the motor shows signs of rotation, it can not drive the pump to run normally. Failure cause analysis
Electrical failure: the power supply is not connected, fuse blown, motor winding short circuit or broken circuit, control line failure, thermal relay protection action.
Mechanical failure: pump shaft and motor shaft is not concentric, bearing damage jammed, impeller and pump casing between the foreign body jammed, packing pressure is too tight.
Other reasons: the viscosity of the conveying medium is too high, resulting in excessive starting resistance; the pump is not filled with liquid, forming an air-binding phenomenon.
Solution
Check the electrical system: firstly, check whether the power supply voltage is normal, make sure the power cord is firmly connected and not loose or broken. Use a multimeter to check whether the fuse is blown, if blown, replace the fuse with the same specification. Further test the resistance value of the motor winding, judge whether there is a short circuit or broken circuit, if there is any problem, the motor needs to be repaired or replaced. Check the switch, contactor, relay and other components in the control line, repair or replace the faulty parts. If the thermal relay protection action, you need to check whether the motor is overloaded, to be cooled down after the motor to reset the thermal relay, and troubleshoot the cause of overload.
Examine the mechanical failure: Correct the concentricity of the pump shaft and the motor shaft so that the deviation is controlled within the permissible range. Disassemble the pump body and check whether the bearings are damaged. If the bearing balls are worn, jammed or poorly lubricated, replace the bearings and add suitable grease in time. Clean the foreign matter between the impeller and pump casing to ensure that the rotating parts are not stuck. Properly adjust the tightness of packing gland to ensure good packing seal and not affect the rotation of pump shaft.
Handle other problems: if the viscosity of the conveying medium is too high, take heating, dilution and other methods to reduce the viscosity; make sure that the pump is full of liquid before starting, and exclude the air to avoid the phenomenon of air binding.
2.Insufficient pump flow
Trouble phenomenon: the actual flow rate of the centrifugal pump is lower than the design flow rate, which cannot meet the requirements of the production process.
Failure cause analysis
Suction pipeline problems: suction pipeline blockage, such as filter clogging, debris accumulation in the pipeline; suction pipeline leakage, resulting in air into the pump, the formation of cavitation or air binding; suction height is too high, exceeding the pump’s allowable suction height.
Impeller failure: the impeller is badly worn, the vane is damaged or broken, resulting in a reduction of the effective work area of the impeller; the key between the impeller and the pump shaft is damaged or loose, so that the impeller can not rotate normally.
Poor sealing of the pump body: the sealing gasket between the pump casing and the pump cover is damaged, resulting in leakage; the packing seal or mechanical seal fails, resulting in liquid leakage.
Other reasons: the outlet valve is not fully opened; the nature of the conveyed medium changes, such as increased viscosity, density changes, etc.; the pump speed is insufficient.
Solution
Check the suction line: clean the filter and debris in the suction line to ensure that the line is smooth. Check the connection parts of the suction line, use soapy water to smear the interface, if there are bubbles, it means that there is air leakage, need to re-tighten or replace the seals. According to the performance parameters of the pump, adjust the suction height to ensure that it is within the permissible range.
Inspect impeller: Disassemble the pump body and inspect the impeller for wear and damage. If the wear of the impeller is light, it can be repaired; if the vane is damaged or broken seriously, it is necessary to replace the impeller with a new one. Check the key between the impeller and the pump shaft, if the key is damaged or loose, the key should be replaced and the impeller should be reinstalled to ensure that the installation is firm. Repair pump body seals: Replace damaged pump casing and pump cover sealing gaskets, and make sure the gaskets are in the correct position and seal well when installing. For packing seals, adjust the packing gland and add or replace the packing; for mechanical seals, check the wear of the sealing surface and replace the mechanical seal assembly if necessary.
Dealing with other problems: Make sure the outlet valve is fully open; if the nature of the transport medium changes, adjust the pump’s operating parameters according to the actual situation or take appropriate measures to deal with the problem. Check whether the motor speed is normal; if the speed is insufficient, check the motor and transmission device to eliminate the problem.
3.Insufficient pump head
Failure phenomenon: The actual head of the centrifugal pump is lower than the designed head, and it is unable to transfer the liquid to the specified height or pressure.
Failure Cause Analysis
Impeller Problem: The impeller reverses, resulting in the liquid not being able to obtain sufficient centrifugal force; the impeller is badly worn, and the shape and size of the blades change, affecting the head.
There is air in the pump: air leakage in the suction pipeline or failure to exhaust air before the pump starts, so that air exists in the pump, affecting the normal delivery of liquid and energy conversion.
The speed of the pump is reduced: the speed of the pump is lower than the rated speed due to motor failure, loose or slippery drive belt, improper parameter setting of the frequency converter and so on.
Excessive resistance of the outlet pipeline: The outlet pipeline pipe diameter is too small, the pipeline is too long, there are too many elbows, the valve is not completely open or there are sundries blocked in the pipeline, etc., which increases the resistance of liquid flow and leads to an increase in head loss.
Solution
Check the impeller: confirm whether the impeller rotation direction is correct, if reversed, adjust the wiring phase sequence of the motor, so that the impeller positive rotation. For the worn impeller, repair or replace it according to the methods described in the previous section.
Remove air from the pump: check the sealing of the suction line and repair the air leakage. Before starting, open the exhaust valve on the pump body to exhaust the air inside the pump and make sure the pump is full of liquid.
Adjust the pump speed: check the running status of the motor, repair or replace the faulty motor. Adjust the tension of the transmission belt to ensure that the belt transmission is normal; if you use a frequency converter to control the speed, check the parameter settings of the frequency converter so that it meets the operating requirements of the pump.
Optimise the outlet pipeline: according to the performance of the pump and the actual demand, reasonably select the pipe diameter and length of the outlet pipeline, and reduce unnecessary elbows and valves. Clean up the debris in the outlet pipeline, ensure that the valve is completely open, and reduce the resistance of the pipeline.
4.Excessive pump vibration
Failure phenomenon: centrifugal pump in the operation process produces abnormal vibration, vibration amplitude exceeds the prescribed standard, and may be accompanied by noise.
Failure cause analysis
Mechanical reasons: pump shaft and motor shaft is not centrifugal, resulting in unbalanced force when rotating; bearing wear, damage or poor lubrication, so that the bearing parts of the vibration intensified; impeller imbalance, such as local wear and tear of the impeller, corrosion, or foreign objects attached; pump body foundation is not solid, loose ground bolts or insufficient strength of the foundation. Hydraulic reasons: pump cavitation phenomenon, air bubbles rupture in the pump to produce impact, causing vibration; pump flow instability, such as improperly adjusted outlet valves, piping system with blockage or leakage.
Other reasons: motor failure, such as motor rotor imbalance, motor winding short circuit, etc.; the connection between the pump and the pipeline is unreasonable, and the stress of the pipeline acts on the pump body.
Solution
Dealing with mechanical problems: recalibrate the concentricity between the pump shaft and the motor shaft, and use tools such as a percentage table to make precise measurements and adjustments. Replace worn or damaged bearings and add or replace suitable grease regularly. Perform dynamic balance test and calibration on the impeller, clean the foreign matter on the impeller, repair or replace the unbalanced impeller. Reinforce the foundation of the pump body, tighten the ground bolts to ensure the strength and stability of the foundation.
Solve hydraulic problems: reduce the installation height of the pump, increase the pipe diameter of the suction pipeline, reduce the resistance of the suction pipeline, and prevent cavitation.
Stabilise the flow rate of the pump, check the opening of the outlet valve, clear the blockage in the piping system and repair the leakage parts. Examine other causes: overhaul the motor, check the balance of the rotor and the insulation of the windings, and repair or replace the faulty motor. Optimise the connection between the pump and the pipeline to avoid excessive stress on the pump body from the pipeline, and measures such as flexible connection or bracket support can be used.
In summary, the diagnosis and elimination of common faults of centrifugal pumps require technicians to have solid professional knowledge and rich practical experience. Through careful observation of the failure phenomenon, in-depth analysis of the causes of the failure, and take targeted solutions to effectively troubleshooting, safeguard the normal operation of centrifugal pumps, for the stable operation of industrial production to provide a strong guarantee.
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