Common faults, causes and solutions in centrifugal pump operation

Centrifugal pumps are among the most widely used rotating equipment in industrial processes, found almost everywhere from petrochemicals and power generation to metallurgy, from urban water supply to agricultural irrigation. However, many pumps fail to reach their designed lifespan-a standard centrifugal pump, operating near its optimal efficiency point and properly maintained, could last 15 to 20 years, or even more than 25 years. In reality, a large number of centrifugal pumps fail prematurely, require frequent repairs, and cause significant production losses due to unplanned downtime.

This paper systematically identifies common faults in centrifugal pump operation, analyzes their mechanisms, failure paths, and destructive characteristics, and, based on standards such as API 610 and engineering practice, proposes a full lifecycle management strategy, from design selection and operation/maintenance to condition monitoring. This paper aims to help equipment managers identify hidden fault signs, establish a predictive maintenance system, and fundamentally extend the service life of centrifugal pumps and improve unit reliability.

 

 

• Radial force and shaft deflection

Radial force is the resultant lateral force acting on the pump shaft when the impeller operates outside of its design conditions. When the pump deviates from its optimal efficiency point (BEP), the pressure distribution within the volute becomes uneven, and the pressure around the impeller becomes asymmetrical, generating a radial force pointing in a certain direction. The magnitude of this force varies with the flow rate – the lower the flow rate, the greater the radial force; the higher the flow rate, the greater the radial force; only at the BEP is the radial force minimal.

 

The danger of radial force lies in its "hidden" nature. Under the influence of radial force, a pump shaft that is perfectly straight when the pump is stopped may bend during operation; once stopped, the shaft returns to its straight position. This means that conventional static alignment checks cannot detect the problem. A pump shaft operating at 3,550 rpm bends (deflects) 7,100 times per minute, 426,000 times per hour, and over 3.7 billion times per year. This high-frequency alternating stress is the root cause of mechanical seal failure and premature bearing failure. High radial forces causing shaft deflection make it difficult for the sealing faces to maintain contact, disrupting the fluid layer required for sealing. Simultaneously, they overload the radial bearing, and bearing life is exponentially related to misalignment – ​​a 1.5 mm misalignment will cause bearing failure within three to five months, while a 0.025 mm misalignment can allow the same bearing to operate for over 90 months.

 

The solution: Operating the centrifugal pump near its BEP (Boundary Effect Point) is the primary principle for controlling radial forces. Impeller width is one of the main factors affecting radial forces – the wider the impeller, the greater the radial force. Choosing a reasonable volute geometry and sufficient shaft diameter can reduce radial forces through design. For pumps already in operation, avoid prolonged operation below the minimum flow rate.

 

• cavitation

When the local absolute pressure near the pump inlet or impeller blade inlet drops below the liquid's saturated vapor pressure at that temperature, the liquid vaporizes, forming numerous tiny vapor bubbles. These bubbles, carried by the liquid flow into the high-pressure zone, collapse rapidly or "implode," generating high-frequency (600~25,000 Hz) shock waves with local pressures reaching up to 49 MPa. This impact repeatedly acts on the metal surface, causing mechanical erosion; simultaneously, the chemical corrosion generated by the heat of vaporization further exacerbates the damage. Cavitation also interferes with energy exchange within the impeller, leading to a comprehensive decline in the Q-H, Q-P, and Q-η curves, and in severe cases, even interrupting the liquid flow and rendering the pump inoperable.

 

There are various types of cavitation. The most common is vaporization cavitation (NPSHa deficiency type), where the fluid velocity increases and the pressure decreases as it passes through the impeller inlet, causing the liquid to boil and vaporize. Turbulent cavitation is caused by eddies and pressure differences generated by components such as bends and valves in the pipeline. Blade syndrome cavitation occurs when the gap between the impeller and the pump casing is too small, resulting in a sharp increase in flow velocity and a drop in pressure. Internal recirculation cavitation occurs when the pump outlet valve is closed or the flow rate is too low, causing the fluid to circulate around the impeller, generating heat and bubbles. Additionally, air entering the pump through faulty valves or loose connections can also cause air entrainment cavitation.

 

The most vulnerable areas for cavitation in centrifugal pumps include: the front cover plate where the impeller curvature is greatest, the low-pressure side near the blade inlet edge, the discharge chamber volute tongue, and the low-pressure side of the guide vane inlet edge.

 

Solutions: Ensure the system's Net Positive Suction Head (NPSHA) is always greater than the Required Net Positive Suction Head (NPSHR), generally recommended to have a margin of at least 0.5–1.0 m. Improving cavitation resistance can be approached from two aspects: first, improving the pump design itself – increasing the flow area, increasing the radius of curvature of the impeller cover plate inlet section, and using a front-mounted inducer or double-suction impeller; second, improving system conditions – increasing the liquid level pressure in the upstream reservoir, reducing the pump installation height, changing from top suction to backflow, and reducing pipeline losses, etc.

 

• vibration

The sources of vibration can be categorized into the following main types:

1. Motor-related issues: Rotor eccentricity or bending leads to excessive static and dynamic balance; broken squirrel cage bars cause magnetic field imbalance; unbalanced stator winding resistance generates uneven electromagnetic force.
2. Foundation and support: Loose foundations or flexible foundations reduce constraint stiffness; loose anchor bolts exacerbate vibration.
3. Couplings: Uneven bolt spacing, eccentric extension sections, poor static and dynamic balance, improper fit clearances, etc.
4. Impeller: Substandard casting or machining quality leads to eccentricity; corrosion of the flow channel causes eccentricity; worn wear rings increase friction.
5. Shaft system: Insufficient shaft stiffness, excessive deflection; excessive balance disc clearance or improper axial movement.
6. Operating conditions: Deviating from design conditions leads to increased radial force; improper selection or mismatched parallel connections.
7. Bearings and lubrication: Insufficient bearing stiffness, excessive clearance, poor lubrication.

 

In addition, the fluid itself is also a source of vibration – water flow impacting the guide vane and impeller leading edge, pressure pulsation within the pump body, and cavitation can all induce vibration. Excessive vibration can lead to rotor-stator friction or even seizing, shaft seal failure and leakage, and may also affect pipe fittings, valves, and foundations, causing secondary hazards to operators and the environment.

 

Solution: Vibration diagnosis requires a systematic approach. Through time waveform, spectrum, and phase analysis, combined with continuous monitoring, the vibration source can be accurately located. It is recommended to establish a vibration health baseline for equipment, referring to vibration evaluation standards such as ISO 10816-7. During routine inspections, an abnormal increase in vibration values ​​is often the earliest warning signal.

 

• Bearing and lubrication contamination

Bearings are among the most vulnerable components of centrifugal pumps, and over 85% of bearing failures are caused by contaminants – whether dirt, foreign matter, or water. The destructive power of contaminants is staggering: just 250 parts per million (ppm) of water can shorten bearing life by four times.

 

Lubricating oil contamination comes from various sources: it could be due to process media seepage after seal failure, impurities introduced during maintenance, or deterioration of the lubricating oil itself over long-term operation. Common bearing failure modes include adhesive wear, fatigue wear, and abrasive wear, often caused by improper contact surface clearance, poor surface roughness, or deterioration of the lubricating oil's physicochemical properties.

 

In addition to contamination, bearings are subjected to additional loads from radial forces, axial forces, misalignment, and piping strain. These forces cause bearings to operate far beyond their design load, accelerating the failure process.

 

The solution: Maintaining clean lubricating oil is the most effective measure to extend bearing life. Establish a regular oil testing system (at least quarterly is recommended), and strictly control moisture (≤100 ppm) and particulate matter content (not lower than NAS Class 9 cleanliness standard). At the same time, ensure that the pump operates near the BEP to reduce radial force, ensure precise alignment of the pump and drive, and eliminate pipeline strain.

 

• Minimum flow rate operation

Many operators believe that "as long as the pump is running, it's fine," unaware that operating at low flow rates is the most insidious form of chronic damage to centrifugal pumps.

 

Every centrifugal pump has a specific operating range. Exceeding this range can trigger a series of problems: significantly increased vibration and noise, decreased performance, shortened component lifespan, and even mechanical damage. The API 610 standard defines two minimum flow rates: the minimum continuous thermal limit flow rate (the lowest flow rate at which the pump can maintain operation without being damaged by temperature rise) and the minimum continuous steady flow rate (the lowest flow rate that does not exceed the specified vibration limits).

 

Operating below the minimum flow rate increases the pump's radial load and vibration, intensifies fluid recirculation, and shortens the lifespan of bearings and mechanical seals. Fluid separation can occur within the pump, further increasing vibration and noise. In high-power centrifugal pumps operating with the outlet valve closed, the fluid inside the pump will become increasingly hot, potentially burning out the bearings. At low flow rates, the pump shaft also experiences additional radial forces.

 

The minimum continuous steady flow rate is affected by factors such as suction specific speed and energy density. Generally, the minimum continuous steady flow rate increases with increasing suction specific speed. Low-flow operation can also induce internal recirculation cavitation, further accelerating damage.

 

The solution: Setting a minimum flow rate line is the most reliable protective measure. When the operating flow rate is lower than the pump's minimum flow rate, a low-flow rate line (i.e., a minimum recirculation line, or bypass) should be installed. In addition, variable frequency drives, automatic recirculation valves, and power or vibration monitoring can be used. For pumps conforming to API 610 standards, operation must strictly adhere to the specified minimum continuous stable flow rate.

 

• Shaft seal leakage

Mechanical seals are the most common shaft sealing method for centrifugal pumps, but they are also frequently prone to failure. Leakage points in mechanical seals are mainly concentrated at the dynamic and static ring end faces and the auxiliary seal.

 

The causes of seal failure are varied: substandard flatness or roughness of the sealing end faces; particulate matter between the end faces; negative pressure in the sealing cavity during pump start-up and shutdown, leading to dry friction of the end faces; prolonged pressure buildup in the pump damaging the seal; insufficient pump flow causing the medium to circulate, heat up, and vaporize; aging or damage to the auxiliary seal. Furthermore, shaft deflection caused by radial force makes it difficult for the sealing end faces to maintain contact, a major cause of mechanical seal failure.

 

Solutions: Mechanical seal maintenance requires meticulous management. Routine inspections can use an infrared thermometer to monitor the sealing cavity temperature (normal ≤ ambient temperature +25℃), and weekly scanning of the flange interface with an ultrasonic leak detector. During assembly, ensure that the axial movement of the shaft is less than 0.1 mm. For special media, high-temperature and corrosion-resistant sealing materials should be selected, such as hard alloy to hard alloy friction pairs and perfluoroelastomer O-rings.

 

• Axial force imbalance

When a single-stage, single-suction centrifugal pump is running, the rotor experiences an axial thrust directed towards the suction inlet. For multi-stage centrifugal pumps, the axial force problem is more severe – most failures stem from improper balancing mechanism settings or balancing system malfunction.

 

Axial force imbalance increases the workload on the thrust bearing and causes the rotor to move towards the suction inlet, resulting in vibration. In severe cases, this can lead to impeller ring friction or even pump body damage. Common axial force balancing structures or devices include balancing holes, balancing discs, or balancing drums (with balancing return pipes). Eccentric installation of the balancing disc can cause rotor vibration, potentially damaging the bearings and pump shaft.

 

Solutions: Regularly check the balancing holes and return pipes for blockages. For balancing disc devices, strictly control the installation eccentricity, generally requiring it to not exceed 0.05 mm. Before starting a multi-stage pump, thorough priming and venting are essential to avoid excessive instantaneous axial thrust. Online monitoring of thrust bearing temperature rise should be implemented, as sudden temperature changes in the bearing are often a precursor to axial force imbalance.

 

• Broken shaft

Pump shaft fracture is one of the most serious failures of centrifugal pumps, often caused by rotational tension and bending fatigue. In almost all rotating machinery, shaft fracture is preceded by intense vibration.

 

Common causes of shaft fracture include: improper pump shaft design, misalignment of the unit shaft center, inappropriate bearing selection, improper installation of the outlet pipe; stress concentration at the shaft end relief groove; and excessive main shaft span leading to increased bending stress. Fatigue cracks initiate at the weakest point, gradually propagate under repeated alternating stress, and eventually fracture suddenly at a certain moment. Because the pump shaft often shows no abnormalities when shut down, this fatigue accumulation process is extremely insidious.

 

Solutions: Avoid stress concentration in the design; appropriately set the relief groove radius (recommended not less than 5% of the shaft diameter). Strengthen vibration monitoring during operation; abnormal increases in vibration values ​​are often a precursor to shaft fracture. Regularly check the shaft's coaxiality and runout; magnetic particle or ultrasonic testing of the shaft system is recommended during major overhaul cycles. For units that have experienced shaft breakage, a comprehensive analysis of the cause of failure is essential; simply replacing the shaft and putting it back into operation is not permitted.

 

Prevention is better than repair, and monitoring is better than guesswork. Establishing a systematic system for vibration monitoring, temperature monitoring, lubricant analysis, and operating parameter recording is the fundamental way to extend the life of centrifugal pumps and ensure continuous production.