High-frequency failures in high-power magnetic pumps

Abstract: This paper examines the P-106 circulating benzene pump from three perspectives—structural design, operating principles, and practical operation—highlighting frequent operational failures during its service that led to multiple shutdowns, resulting in emergency equipment halts, significant operational complexity, high maintenance frequency, and difficulties in scheduled switching. The study conducts a root cause analysis and proposes corresponding solutions to these issues.

Keywords: Magnetic pump; Failure; Production halt; Maintenance

1 Device Overview

The ethylbenzene unit’s process package employs Sinopec’s gas-phase dry gas-to-ethylbenzene technology, encompassing raw material pretreatment, reaction, separation, heat carrier systems, and utility processes. The unit’s circulating benzene pump P-106 divides the reaction medium benzene from tank V-105 into two streams: one for alkylation reactions and the other for alkyltransfer reactions, serving as a dedicated feed supply pump for these processes.

The magnetic pump shares the same structural components as a conventional centrifugal pump—namely the motor, pump housing, impeller, and coupling—but also features an internal magnetism not present in ordinary centrifugal pumps.

The components of a magnetic pump include the rotor, outer magnetic rotor, and isolation sleeve; consequently, its manufacturing cost is higher than that of a centrifugal pump. Additionally, the structural characteristics of a magnetic pump impose stricter operational requirements compared to a centrifugal pump. Ensuring long-term reliable operation of a magnetic pump is therefore a critical aspect of equipment management.

2 Fault Description

The circulating benzene pump P-106 serves as the supply unit for ethylbenzene reaction feedstock. Since commissioning, the equipment has undergone multiple emergency repairs and caused three unplanned shutdowns during operation, resulting in significant impacts. Equipment failures have been categorized into three main types: (1) Abnormal shutdowns: Occurring during normal operation (with cycles of 3–4 months), characterized by DCS current exceeding the motor’s rated current and abnormal equipment noise; (2) Turning jam after pump switching: When the originally operating pump stops after periodic pump switching, turning becomes impossible, accompanied by fragmentation of thrust discs and thrust bearings, as well as wear on the internal magnetic cylinder and isolation sleeve; (3) Power outage-induced shutdowns: Caused by thunderstorms or fluctuations in external power supply, leading to damage to thrust discs, thrust bearings, and wear on the internal magnetic cylinder and isolation sleeve. Detailed fault descriptions are presented in Table 1.

3 Working Principle and Structure of the Device

A magnetic drive pump, commonly referred to as a magnetic pump, typically consists of an electric motor, a permanent magnet drive unit, and a centrifugal pump. Its primary feature is the utilization of magnetic induction principles to transmit torque. The system comprises a pump head, external magnets, an internal magnetic cylinder, and an isolation sleeve. When the electric motor drives the external magnetic rotor to rotate, magnetic flux lines pass through the gaps and the isolation sleeve, acting on the internal magnetic rotor to synchronize its rotation with that of the motor, thereby transmitting torque without mechanical contact. At the power input end of the pump shaft, the liquid is contained within the stationary isolation sleeve without any dynamic seals, ensuring complete leak prevention; this design makes it widely applicable in various petrochemical industries. The pump shaft and internal magnetic rotor are fully enclosed by the pump housing and isolation sleeve, effectively eliminating issues such as leakage, seepage, dripping, or spillage. The circulating benzene pump model GSP6x4x13 FAI ESS+ induction wheel is an imported unit designed for benzene transportation, with a density of 733 kg/m³ at operating temperature, requiring a cavitation margin of 2.5 m, powered by a 380 V, 160 kW drive motor operating at 3140 rpm. Relevant parameters are listed in Table 2.

4 Cause Analysis

Reason 1: Axial force imbalance; Due to uneven axial forces within the rotor, the axial force exceeds the capacity of the thrust bearing, causing the silicon carbide thrust ring to fracture due to friction. The resulting fragments enter the gap between the inner magnetic rotor and the isolation sleeve, leading to localized wear and high-temperature oxidation with discoloration. Causes of excessive axial force include: variations in seal ring clearance, changes in the gap between the impeller cover and pump chamber end, and alterations in the clearance between the impeller bore and shaft. Reason 2: Cavitation; The working medium is benzene at a normal operating temperature of 152°C and inlet pressure of approximately 0.5 MPa, deviating from design specifications (design temperature: 149°C; design inlet pressure: 0.55 MPa). Actual operating conditions approach benzene’s vaporization pressure, making cavitation highly likely. Reason 3: Operational challenges; As imported equipment, the magnetic pump features expensive components. To prevent dry friction, low-current underload protection is implemented. During startup/stop cycles, the outlet valve must be operated within specified timeframes to maintain minimum flow rate and undercurrent protection thresholds, resulting in significant operational fluctuations, unstable fluid delivery, and increased internal mechanical stress.

5 Rectification and Improvement

(1) Repair the clearance of the sealing rings; both the front and rear sealing rings of the impeller as well as the pump body sealing ring are worn. Since the sealing clearance significantly affects axial force,

Actual measurements indicate that the clearance of the sealing ring is approximately 50%–60% larger than the value specified by the original manufacturer. A newly designed matching sealing ring has been developed. The fit between the new impeller sealing ring and the base body adopts an interference fit, while the fit between the new housing sealing ring and the base body uses a clearance fit to accommodate thermal expansion. The clearance control parameters are shown in Table 3.

The impeller inlet (front) should be slightly larger to account for factors such as thermal deformation of the pump body, while the rear cover plate should be slightly smaller due to its proximity to the bearing housing.

It is determined based on the principle that it facilitates axial force balance. The assembly clearance is controlled as follows: the operating clearance of the impeller inlet sealing ring is 0.6 mm, and the clearance of the rear cover plate sealing ring ranges from 0.59 to 0.6 mm.

(2) Repair the clearance between the pump cavity and the impeller front cover plate. Considering that friction had previously occurred between the original impeller and the front end of the pump cavity with a small gap, adversely affecting axial force balance, and to prevent contact between the impeller and the end of the housing cavity, the clearance between the impeller front cover plate and the housing was appropriately increased to balance axial forces and reduce axial impact during startup. The originally worn area at the pump cavity end was machined and adjusted. After assembling the rotor components, they were installed in the pump cavity; the clearance between the impeller cover plate and the pump cavity was measured to ensure no contact with the housing occurs at any position during axial movement of the rotor, with a minimum clearance of no less than 2 mm. The housing end was machined down by 1 mm; after assembly of the rotor assembly, measurement showed a clearance of 5 mm between the impeller front cover plate and the housing end under pressure testing conditions, with no abnormal friction observed during rotation. This ensures prevention of recurrence of contact and facilitates axial force balance.

(3) Repair the clearance between the impeller and the shaft; the inner bore of the impeller had worn out due to repeated disassembly and assembly of the pump, resulting in increased clearance with the shaft end, measured at Φ40.4 mm, which compromised its positioning function. Both ends were repaired using 304 argon arc welding wire; after welding, they were realigned and precision-machined to achieve a normal fit clearance of Φ40 mm H7. The shaft end was adjusted to be parallel to the thrust plate, and the impeller underwent dynamic balancing according to G2.5 standards, meeting all requirements.

(4) Entrance pressure boosting to prevent cavitation: Due to the high cavitation margin requirements under the design operating conditions of the circulating benzene pump, the pump model ordered was a magnetic pump with an inlet impeller; however, the actual equipment lacked this impeller, resulting in inadequate cavitation resistance and performance compared to the design specifications. To eliminate cavitation caused by vaporization of the working medium (benzene) at the pump inlet, nitrogen pressurization was added to the circulating benzene tank V-105 for staged pressure control, increasing the pressure from the designed value of 0.5 MPa(G) to 0.55 MPa(G). This measure prevents overheating of internal components and potential equipment damage during low-flow operation.

(5) Implementation of variable frequency operation: The frequency converter cabinet incorporates an additional 25-second lockout mechanism to protect the motor during the transition from zero load to rated load, ensuring stable pump operation. Variable frequency drives serve two primary functions: First, utilizing the delayed start function of the frequency converter to increase the pump’s rotational speed to achieve the required actual head; second, enabling the motor or pump’s speed to reach the desired operating speed over an extended period, thereby allowing a gradual increase in pump load. This approach addresses the torque matching issue between the pump and the motor. Since direct motor startup generates significant torque, the soft-start function of the frequency converter enables the motor to deliver high power output with minimal torque.

(6) Add an anti-voltage fluctuation module; install an anti-voltage fluctuation module in the ethylbenzene frequency converter cabinet to prevent voltage fluctuations caused by thunderstorms or grid instability, ensuring timely restoration of pump operation for restart.

6 Summary of Effect Evaluation and Testing

(1) Commissioning after maintenance: After 30 minutes of operation, perform performance tests at load levels of 80%,85%,90%,95%, and 100%. Upon restoring the normal rated flow rate, continue operation for another 2 hours before shutting down the pump. Disassemble and inspect all internal components for wear or damage; if no issues are found, reassemble them. The equipment operated smoothly upon reinstatement.

(2) Maintenance data summary: Collection of all internal coordination parameters of the equipment shall be preserved as primary maintenance documentation to serve as a reference for future maintenance activities and ensure maintenance quality (see Table 4).

7 Solving Complex Problems

Regarding the clearance between the impeller front cover and the pump housing end, international suppliers generally advise against increasing this clearance. During this maintenance, expanding this clearance serves two purposes: first, to prevent further contact or friction; second, to facilitate axial force balance. Based on the current operating conditions of the pump, repairing this area is deemed appropriate.