Common failure symptoms and solutions for centrifugal pumps

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Main principles for diagnosing pump failures: (1) Adapt to local conditions and the timing of the situation. That is, in most cases it is necessary to consider the actual situation on site, such as whether maintenance has just been completed, whether the system is in a switching process, whether the backup pump has been shut down for a long time, or whether adjustments are currently being made. (2) Make a judgment by correlating the location where the fault occurred with its symptoms. Several common faults in mechanical pumps: 1. Leakage faults. Leaks can be either temporary or permanent; the former can be resolved through certain measures, such as evacuating the system to cause the stationary ring to move out of place, and with proper handling it can be returned to its original position. However, if there is cracking or damage, then the leak cannot be repaired. Damage to any component of a mechanical seal can lead to seal leakage; common issues include damage to the face friction pair, such as surface cracks, the formation of radial ring-shaped grooves, or cracks. Damage to the elastic components, such as failed bellows or stuck springs. Auxiliary sealing elements such as O-rings and V-rings may deteriorate and come loose. There are various reasons for seal failure: vibration, operational factors, inappropriate or defective flushing oil, and also situations where the balance parameters, or seal parameters, exceed the operating range of the mechanical seal. Types of mechanical seal failures:
(1) Issues such as overheating, smoking, formation of abrasive particles, and high power consumption: This is caused by an excessive gap between the rotor and the seal chamber, leading to friction and wear due to vibration. Solutions include enlarging the inner diameter of the seal chamber to increase the gap, checking the rotor’s balance, and adjusting its concentricity. If the issue is caused by friction between the shaft (or shaft sleeve) and fixed components, the solution is to correct the alignment of the gland and improve the precision of assembly. In cases where the seal surfaces are severely worn under high temperature and pressure, measures such as reducing spring pressure, increasing the balance coefficient, and improving the lubrication method can be taken. If the moving ring is stuck against the balance platform or experiences severe friction with the stationary ring, it is necessary to maintain a gap of 2–3 mm between them. When the medium vaporizes, causing dry friction, solutions include increasing the cooling flow rate and pressure. Insufficient cooling or poor lubrication can also lead to problems; in such cases, increasing the cooling flow rate and taking other corrective measures, such as cleaning, are necessary. If the rotor is unbalanced, resulting in wobbling, balancing the rotor and improving the precision of part fabrication are required.

(2) End-face leakage: This occurs when the pressure on the seal end faces is too low. Solutions include increasing the compression of the springs to raise the pressure. If the springs break or the moving and stationary rings crack due to heat, the seals need to be replaced, and the material and design of the seals should be improved. Impurities entering the end faces can cause wear; in such cases, using Y-Y type seal surfaces or dual-seal systems can help. If there is leakage due to low pressure in the sealing fluid used in dual-end-face seals, the pressure of the sealing fluid needs to be increased and maintained at a stable level. When the medium cokes, crystallizes, or deposits debris, it can prevent the moving ring from functioning properly; solutions include improving the design, enhancing external flushing to prevent the moving ring from getting stuck, or using soft water as a coolant.

(3) Axial leakage: This can be caused by the auxiliary seal ring being too tight or too loose; in such cases, selecting appropriate fitting dimensions is necessary. If the rubber seal ring is compressed into the shaft gap and damaged, the fitting gap should be reduced, and the seal ring replaced. If the sealing material lacks heat and corrosion resistance, it needs to be replaced with a better material. During installation, the seal ring may get curled or twisted; in such cases, ensure that the amount of interference fit is appropriate, and pay attention to the installation direction of the V-ring. If the surface of the seal ring is damaged, it must be inspected carefully before installation.

2. Excessive pump vibration and noise: This can be caused by misalignment between the motor and the pump; solutions include correcting this alignment. If the pump shaft is bent, it needs to be straightened. Corrosion or wear of the impeller, as well as imbalance of the rotor, can also cause problems; in such cases, the impeller needs to be replaced, and the rotor balanced. Friction between the impeller and the pump casing can also lead to issues; in such cases, the friction needs to be eliminated through inspection and adjustment. If the pump foundation is loose, the anchor bolts need to be tightened. Cavitation in the pump can also cause problems; in such cases, the pump outlet valve needs to be adjusted so that the pump operates within its specified parameters. There are many reasons for excessive vibration, some of which are gradual while others are sudden. Gradual issues are usually caused by wear of certain parts of the pump, such as wear of the sealing rings, excessive gaps, bent shafts, corroded impellers, worn balance components, or poor alignment – in other words, a loss of dynamic balance. Sudden occurrences generally include things like bearing damage or evacuation; there are also cases where nothing changes, such as loose foundation bolts. There are also many design-related reasons; inherent deficiencies play a role here. There are numerous manufacturers of centrifugal pumps, and some of these pumps have structural dimensions that are not up to standard, with clearance values that are not optimal. As a result of assembly errors, the components can be damaged (including the impeller, fasteners, shaft 7, and mechanical seal). Although the balance holes on the rear cover of the impeller reduce the efficiency of the centrifugal pump, they can decrease the pressure difference on both sides of the impeller and thus balance part of the axial thrust. Some manufacturers tend to overlook this issue, which inevitably leads to frequent damage of the bearings and reduces their service life. To extend the lifespan of bearings and seals, the following improvement measures can be taken: enhancing the standardization and normalization of centrifugal pumps and their components; reducing assembly errors; improving design characteristics such as reducing the shaft length while increasing the shaft diameter, using larger sealing chambers, employing larger-sized bearings, and enlarging the bearing housing to improve the lubrication conditions. Regarding installation, the assembly accuracy of the internal components of the centrifugal pump must meet relevant standards, including the impeller, seals, bearings, etc. During transportation, it is inevitable that the internal components of the centrifugal pump may become loose; therefore, after the pump is installed on its foundation, it is necessary to level and align it properly. After the inlet and outlet pipes of the centrifugal pump are connected, stress is generated, which causes a deviation from the original alignment, requiring re-alignment. If alignment is poor, it can easily lead to shock forces, causing radial movement of the shaft, shaft vibration, and shaft deviation during operation. This increases power consumption, accelerates wear of the bearings and seals, and shortens their service life. Studies have shown that when the axis separation and coaxiality result in a straightness of less than 0.005 mm every 25.5 mm, the service life of rotating machinery is around 100 months; when the straightness is 0.0076 mm per 25.5 mm, the service life is reduced to 10 months; and when the straightness is 1.27 mm per 25.5 mm, the service life is 2 months. In terms of selection, accurately choosing the flow rate and head ensures that the centrifugal pump operates at its optimal performance level during use. When a centrifugal pump operates at low flow rates, circular currents are formed within the pump, generating radial forces that cause the impeller to become unbalanced. This increases the load on the bearings, leading to damage to the seals and bearings. In severe cases of low flow, it can also result in an increase in the fluid temperature. In terms of maintenance, most centrifugal pumps use rolling bearings, and the components of these rolling bearings (the rolling elements, the inner and outer raceways, and the retainers) do not all experience pure rolling motion. Welcome to follow the Pump Friends Circle WeChat official account. Due to the elastic deformation of the parts under external loads, relative sliding occurs on the contact surface except at a few points. The contact areas between the various components of rolling bearings are small, which results in high pressure per unit area. If lubrication is inadequate, these components can easily stick together, or excessive friction can cause overheating, leading to tempering of the rolling elements and thus bearing failure. Therefore, the bearings must always be covered by a lubricating film. Bearing lubrication is usually carried out using oil baths or oil mist. To ensure that a oil film of a certain thickness is formed between the rolling elements and the raceways, turbine oil of medium viscosity (ISO 68) is used. In oil bath lubrication, the bearing parts are submerged in oil, with the oil level reaching 50% of the depth at the bottom of the bearing. Additionally, there are also many vibrations caused by coupling failures. 3. Causes and solutions for the pump failing to discharge liquid
Causes: Insufficient amount of liquid injected.
Solutions: Refill the pump with liquid.
Causes: Air trapped in the suction pipe or air leaks.
Solutions: Remove the air and fix any leaks.
Causes: The suction height exceeds the pump’s allowable limit.
Solutions: Reduce the suction height.
Causes: Excessive resistance in the piping system.
Solutions: Clean or modify the piping.
Causes: Blockages caused by debris in the pump or pipes.
Solutions: Inspect and clear out the blockages.

4. Insufficient flow rate or too low head – causes and solutions
Causes: Clogged suction valve or piping.
Solutions: Inspect and clean the suction valve and piping.
Causes: Impeller is blocked, severely worn, or corroded.
Solutions: Clean the impeller or replace it.
Causes: Severe wear on the impeller seal ring, resulting in excessive clearance.
Solutions: Replace the seal ring.
Causes: Air leaks in the pump body or suction pipe.
Solutions: Inspect and eliminate any air leaks.

5. No liquid discharge
The reasons for no liquid discharge are quite complex; they must be determined based on the specific site conditions. Based on years of experience, many cases are due to issues with the operation or the inlet and outlet pipelines; it is necessary to conduct a thorough analysis using the method of elimination. Common issues such as vacuum conditions or the presence of gas within the medium are quite obvious and easy to identify. The difficulty lies in the fact that there are no very obvious symptoms, so a thorough investigation is required. The problems with the pump itself may include corrosion and damage to the impeller, blockages in the flow channels, and damage to the pump’s sealing rings. Process-related reasons often include air evacuation within the medium, clogged inlet filters, blocked pipes, and backflow caused by poorly sealing check valves, among others. 6. Abnormal noises: This phenomenon **usually does not occur on its own; it often appears alongside vibrations, leaks, etc.** It can be determined based on the location from where the sound comes from; for example, abnormal noises are produced by damaged couplings, while noises from damaged bearings occur as well. Sometimes, seal leaks or dry friction can also result in squeaking sounds, and so on. Common issues also include loose oil seals, etc. The sound produced by damaged rolling bearings. Dry grinding of the primary seal after series sealing will produce noise. Faults in the pump can generally also be detected based on the noise. 7. Overload or excessive current. Cause: The packing is too tight. Solution: Loosen the packing gland. Cause: Friction between the rotating and fixed parts. Solution: Check the cause and eliminate the fault. 8. Stuck rotor. This includes situations where the entire rotor is stuck and cannot be rotated. And it mostly occurs before restarting the pump after it has been shut down. The causes are often dirt in the sealing surfaces and flow channels, blockages by foreign objects, the sealing gland being tightened too much, damaged bearings, or worn ring gaskets. Once this situation occurs, it is not possible to rotate the drive using the hard disk drive mechanism, nor can it be rotated through intermittent movements; it is necessary to contact a fitter or an experienced person promptly to handle the issue. 9. Methods for diagnosing faults: First, listen: check whether the sound of the pump’s operation is normal. Second, observe: check if the current flowing through the pump fluctuates or is abnormal, whether there are leaks in the seals, and whether parameters such as pressure are within normal ranges. Third, feel: check if the temperature and vibration of the pump are normal. Fourth, measure: determine if the level of vibration during pump operation exceeds the acceptable limits. Fifth, compare: use standard values to determine whether a fault exists. 10. Several common methods for fault diagnosis: One method is to distinguish between mechanical and electrical faults in a pump that is known to have a fault. To narrow down the scope of diagnosis, a simple approach is to disconnect the motor and see if the readings on the vibration meter drop rapidly to 0; if so, it’s an electrical fault, while if the drop is gradual, it’s more likely to be a mechanical fault. If the pump cannot be stopped, frequency analysis of the vibration signal can be performed for assessment. If there are prominent peaks at the 1x or 2x supply frequency, it indicates an electrical fault. Otherwise, it is a mechanical failure. II. Parameter direction characteristic identification: For different types of faults, the vibration amplitude at various positions at the measurement point varies. In many cases, if the horizontal vibration is large, it indicates imbalance; a large axial vibration value suggests misalignment of the axes. Of course, for a more detailed assessment, spectral analysis can be used – for instance, the presence of a second harmonic indicates parallel misalignment, and so on. I won’t go into further detail here. Large vertical vibrations are often due to loose footings. III. Location determination through isolation: Since the pump and the motor are connected together, vibration signals from different parts can interfere with each other. In the case of a faulty pump, to determine the location, the coupling can be removed if conditions permit. If the motor operates normally when running alone, then the fault lies with the pump. IV. Another method is temperature measurement; however, its sensitivity is far inferior to that of vibration monitoring. It only shows significant responses when there are serious lubrication problems with the bearing—such as insufficient or contaminated oil—or when there is severe damage to the bearing components. By then, major failures have often already occurred. Therefore, temperature measurement serves merely as an auxiliary monitoring method.