1. The main reasons for the decrease in centrifugal pump flow rate
1. Insufficient suction conditions
When the suction liquid level decreases, the inlet valve is not fully opened, the suction pipe is clogged, or the filter pressure difference is too large, there is insufficient liquid entering the impeller, and the pump flow rate will decrease.
If the pump inlet pressure is too low, cavitation may also occur. Common manifestations include:
Noise similar to the impact of sand and gravel in the pump;
Outlet pressure and flow fluctuations;
Increased vibration;
pitting corrosion on the impeller surface.
When selecting, it must be confirmed that the effective cavitation margin of the system meets:
NPSHa > NPSHr + Safety Margin
For high-temperature media, volatile liquids or long-distance suction systems, sufficient cavitation safety margin should be retained.
2. Impeller or flow channel is clogged
Media containing particles, fibers, crystals or high viscosity may block the impeller inlet and flow channel, reducing the effective flow area of the impeller.
High-temperature molten salt pumps, slurry pumps and chemical process pumps may also lose flow due to solidification, deposition or crystallization of the medium due to cooling. Therefore, media temperature, solids content, particle size, and purge or heat tracing conditions after shutdown should be checked.
3. Impeller wear or gap increase
Long-term transportation of corrosive or solid particle-containing media will cause wear of the impeller, orifice ring and pump body flow channel. When the mouth ring clearance increases, the return flow in the pump increases, and the external available flow rate and head decrease.
If the outlet valve status and system resistance do not change significantly, but the head and flow rate continue to decrease, you should check:
Impeller diameter and blade wear degree;
Actual clearance of mouth ring;
Pump body erosion or corrosion;
Bearing, pump shaft and impeller positioning status.
4. Abnormal speed or direction of rotation
The flow rate of a centrifugal pump is approximately proportional to the speed, and the head is approximately proportional to the square of the speed. The frequency converter frequency decreases, the motor fails, the coupling slips or the wiring causes reversal, which will significantly affect the output capability of the pump.
You can refer to the similarity law of centrifugal pumps:
Q₂/Q₁ ≈ N₂/N₁
H₂/H₁ ≈ (N₂/N₁)²
P₂/P₁ ≈ (N₂/N₁)³
Therefore, before adjusting the speed, the motor power, cavitation margin and the allowable operating range of the pump should also be evaluated. The insufficient flow rate cannot be solved by simply increasing the frequency.
5. System resistance changes
Pipeline scaling, insufficient valve opening, check valve failure, filter blockage or too small pipe diameter will increase the system resistance and move the operating point along the pump performance curve to the small flow area.
If the flow rate of the pump is normal in the initial stage of trial operation and gradually decreases after running for a period of time, priority should be given to comparing the filter pressure difference, valve status and pipeline pressure loss, rather than immediately determining that the pump model is inappropriate.
2. What parameters need to be checked to diagnose insufficient traffic?
| Check parameters | Judge purpose |
|---|---|
| Actual traffic Q | Confirm the deviation from the design flow |
| Inlet and outlet pressure | Calculate the actual head of the pump |
| Motor current and power< /td> | Judge load, blockage or speed problems |
| Operating speed | Check the status of the motor and inverter |
| Medium temperature and density | Correct cavitation conditions and shaft power |
| Viscosity and solid content | Judge performance correction and clogging risk |
| Vibration and noise | Judge cavitation, bearing or rotor failure |
| Filter pressure difference | Judge whether the suction or pipeline is blocked |
It is recommended to compare the current operating data with the pump factory performance curve and initial debugging records. With only pressure data without actual flow measurement, it is often difficult to accurately determine the source of the fault.
3. Common Procurement Questions
Can the flow be restored by replacing a motor with a higher power?
Usually not. Increasing motor power does not automatically change impeller diameter, pump speed, or system resistance. The real reason for the traffic drop should be confirmed first.
The outlet pressure is normal, why is the flow rate still insufficient?
Normal pressure does not mean normal flow. When the valve is throttled, the pipeline is blocked, or the system resistance increases, the pump may operate under high head and low flow conditions.
Is it possible to adjust the flow rate by cutting or replacing the impeller?
Yes, but the head, power, efficiency and NPSHr must be rechecked based on the performance curve. Changing the impeller size without calculation may result in reduced efficiency or overloading of the motor.
4. Feiyue Pump Industry’s Centrifugal Pump Solution
For chemical industry, petroleum, high-temperature molten salt, submerged transportation and particle-containing media working conditions, Jiangsu Feiyue Pump Industry can carry out pump type matching, material selection, cavitation check and performance point confirmation based on medium characteristics and system operation data.
Providing the medium, temperature, flow rate, head, inlet conditions, solid content and operation mode during the inquiry will help the engineering team determine whether the insufficient flow is caused by selection, pipeline or equipment wear, and provide a more targeted centrifugal pump solution.
