Reasons for the decrease in the flow rate of the magnetic pump and the corresponding solutions

The decrease in the flow rate of aate of a magnetic pump is a common issue, usually caused by changes in the pump itself, the piping system, the properties of the medium, or the operating conditions. The main causes and corresponding solutions are as follows:

Main Cause Analysis

I. Entry Issues (Most Common Causes):

1. Entry Blockage: The entry filter, filter screen, bottom valve, pipeline, or impeller flow channel is clogged with impurities, particles, crystalline substances, or fibers.

2. Insufficient Entry Pressure/ Cavitation:

The entry liquid level is too low or the suction height is too high;

The resistance of the entry pipeline is too large (small pipe diameter, many elbows, valves not fully opened);

The medium temperature is too high, resulting in an increase in saturated steam pressure and a decrease in available cavitation margin;

The viscosity of the medium increases.

3. The entry valve is not fully opened or faulty.

4. Entry pipeline leakage: Air enters, preventing the pump chamber from being fully filled with the medium.

II. Issues with the pump body and impeller:

1. Wear or corrosion of the impeller: When transporting media containing solid particles, abrasive, or corrosive substances, the impeller blades, front/back cover plates may wear or corrode, resulting in a decrease in hydraulic efficiency.

2. Wear of the impeller sealing ring: Excessive gap between the sealing ring leads to an increase in leakage within the pump and a reduction in the effective output flow.

3. Blockage of the pump chamber: The medium crystallizes, deposits, or accumulates in the pump chamber, reducing the effective area of the flow passage or increasing the flow resistance.

4. Deformation or damage of the isolation sleeve: It affects the gap between the inner and outer magnetic steel and the efficiency of magnetic coupling, and may even cause friction with the impeller.

III. Issues with Magnetic Couplings

1. Magnetic Slip:

Overload: If the actual operating power of the pump exceeds the designed torque transmission capacity of the magnetic coupling (such as prolonged closure of the outlet valve, significant increase in medium viscosity, or blockage of the impeller), it causes a slip between the inner and outer magnetic steel, resulting in a decrease in pump speed and flow rate.

Demagnetization: High temperature (exceeding the allowable temperature of the magnetic steel), severe vibration, strong reverse magnetic field, or quality issues with the magnetic steel itself lead to a reduction in magnetic force and a decrease in torque transmission capacity, making a slip more likely to occur.

2. Breakage or Damage of the Isolation Sleeve: This causes the medium to leak into the magnetic steel cavity, which not only affects the magnetic force transmission but also may damage the magnetic steel.

3. Excessive Eddy Current Loss in the Isolation Sleeve: For high conductivity media or under high rotational speed, the eddy current generated inside the isolation sleeve results in excessive heat loss, consuming part of the power and leading to a decrease in pump efficiency. In severe cases, it may also cause the isolation sleeve to overheat and be damaged.

IV. Outlet Issues

1. The outlet valve is not fully opened or malfunctioning.

2. The outlet pipeline is blocked: The pipeline, valve, flowmeter, or other equipment is clogged with foreign objects or severely fouled.

3. The outlet pressure increases: Downstream equipment (such as a filter blockage, reactor pressure increase, or liquid level rise) or system backpressure increases beyond the pump's head capacity at the current speed, causing the flow to decrease along the characteristic curve.

4. Inappropriate pipeline configuration: Excessive elbows, valves, or too small pipe diameters result in excessive system resistance.

V. Changes in Medium Characteristics:

1. Increased viscosity: Decreased temperature or changes in the medium composition lead to a significant increase in viscosity, significantly increasing flow resistance and reducing flow rate.

2. Density change: An increase in density (although it has a relatively small impact on volumetric flow rate) is sometimes accompanied by other issues (such as a change in viscosity).

3. Increased gas content: The medium contains entrained gas or air is drawn in at the inlet, resulting in a decrease in pumping efficiency.

4. Increased solid content: Excessive solid particle concentration increases flow resistance and impeller wear.

VI. Deviation from Design Conditions during Operation:

1. Decrease in rotational speed (rare, except in variable frequency control): The frequency of the drive motor decreases or there is a transmission problem, causing the pump's rotational speed to be lower than the designed value. The rotational speed of a magnetic pump is usually synchronized with the motor (direct connection).

2. Change in system resistance curve: Changes in process conditions result in an increase in the actual required head of the system, causing the pump's operating point to shift to the left and the flow to decrease.

Solution and troubleshooting steps

Troubleshooting should follow the principle of "from easy to difficult, from outside to inside":

I. Check the inlet side:

1. Confirm the inlet liquid level/pressure: Ensure it meets the minimum net positive suction head requirement of the pump.

2. Check and clean the inlet filter/filters: This is the most common solution.

3. Check the inlet valve: Confirm it is fully open and without any faults.

4. Check the inlet pipe: Are there leaks (especially in the negative pressure area), blockages, is the pipe diameter appropriate, and are there too many elbows? Try briefly opening the inlet exhaust valve to vent.

5. Check the medium temperature: Is it too high causing cavitation? Consider cooling or reducing the suction height.

6. Check the medium viscosity: Is it significantly higher than the design value? Consider heating (if the process allows) or using a pump with higher power.

II. Inspection of the outlet side

1. Check the outlet valve: Ensure it is fully open and free from any faults (such as valve plate detachment).

2. Check the outlet pressure: Is it abnormal? Check if the downstream equipment (filters, heat exchangers, reactors, high-level tank liquid level, etc.) is clogged or has increased pressure.

3. Check the outlet pipeline: Is there any blockage (especially at the valve and instrument locations)? Is there severe scaling? Check if the system back pressure is normal.

4. Check the system configuration: Are the pipe dimensions and the number of elbows reasonable? Try to minimize unnecessary resistance.

III. Check the operating status of the pump

1. Listen for sounds: Are there any abnormal noises (such as the popping sound of cavitation, frictional sounds, or bearing damage sounds)?

2. Measure the current:

If the current is significantly lower than the rated value: It may be due to an inlet blockage/insufficient liquid level (the pump is emptying), the outlet valve is closed, the impeller is severely clogged/loose, or there is severe slip.

If the current is significantly higher than the rated value: It may be due to an outlet blockage (the valve is not open), high viscosity/high density of the medium, impeller friction, bearing jamming, or mechanical failure causing overload. At this time, there is a high risk of magnetic slip or demagnetization!

If the current is normal or slightly higher/slightly lower: It may be caused by impeller wear, wear of the sealing ring, or changes in system resistance, resulting in a decrease in efficiency.

3. Check for vibration: Has the vibration increased abnormally? This may indicate cavitation, bearing damage, unbalanced impeller, improper alignment (if there is a base) or magnetic coupling problems.

4. Temperature Measurement:

Isolation sleeve temperature: Is it abnormally high? It could be due to slippage, excessive eddy current loss, high medium temperature, or poor cooling. High temperature is the main cause of demagnetization of the magnetic steel!

Bearing area temperature: Is it too high? It might be due to poor lubrication or damage.

5. Check for any leaks: Especially at the isolation sleeve, connection flange, and exhaust/liquid discharge ports.

IV. Consider the characteristics of the medium:

Confirm whether the composition, viscosity, density, solid content, and gas content of the medium are consistent with the design. If there are any changes, assess the impact on the pump performance.

V. Inspect the magnetic coupling (usually requires stopping the pump and disassembly):

1. Check for slip marks: Disassemble and inspect the surfaces of the inner and outer magnetic steel for any scratches or marks caused by relative friction or high-temperature discoloration. If present, it indicates a severe slip.

2. Check the magnetic force: Feel if the magnetic force has significantly weakened (require experience or specialized tools for measurement).

3. Inspect the isolation sleeve: Are there any cracks, deformations, or severe wear? Is there any high-temperature discoloration on the inner surface due to eddy current?

4. Measure the gap between the magnetic steel: Is it within the allowable range? An excessive gap affects the torque transmission.

VI. Disassembly and Inspection of Pump Body (Requires Professional Maintenance):

1. Inspect the impeller: Check for wear, corrosion, and blockage. If necessary, clean, repair, or replace.

2. Inspect the sealing rings: Check if the wear gap exceeds the standard. Replace if necessary.

3. Inspect the pump chamber/flow path: Check for blockages, crystallization, or scaling. Clean thoroughly.

4. Inspect the bearings: (Sliding bearings) Check for wear and if the clearance exceeds the standard. Replace if necessary.

5. Inspect the shaft sleeve: Check for wear. Replace if necessary. preventive measure

Regular maintenance:

Clean the inlet filter strictly in accordance with the procedures.

Regularly check the wear condition of the bearings (usually replace them based on the running time or monitoring status).

Regularly inspect the vulnerable parts such as the impeller and the sealing ring.

Regularly monitor the operating parameters (current, pressure, flow rate, temperature, vibration).

Correct operation:

Before starting, ensure that the inlet valve is fully open, and the outlet valve is closed or slightly open (as per the pump requirements). After starting, slowly open the outlet valve to the required working condition.

Do not operate with the outlet valve closed for a long time! This is the most common incorrect operation that leads to magnetic slip and demagnetization.

Avoid dry running.

Prevent the inlet from drawing in gas.

Selection and Design:

Ensure that the pump selection (flow rate, head, suction lift margin, material, magnetic torque) meets the actual process requirements and leaves an appropriate margin, especially when dealing with variable media or media prone to clogging.

The inlet pipeline design is reasonable to reduce resistance.

Monitoring and Protection:

Consider installing dry-run protectors, temperature sensors (to monitor the temperature of the isolation sleeve), overload protection, etc.

Summary: A decrease in the flow rate of the magnetic pump is a systemic problem. The key to solving it lies in systematic troubleshooting, starting from the simplest inlet filter, valve status, and gradually delving into the pump interior and the magnetic coupling. Paying close attention to changes in operating parameters (current, pressure, temperature) is an important basis for quickly locating the problem. For issues involving the magnetic coupling (slip demagnetization) and wear of pump internal components, it is usually necessary to stop the pump and have it inspected and repaired by professionals. Preventive maintenance and correct operation are the fundamental ways to avoid a decrease in flow rate.