The reasons for the water grabbing phenomenon in centrifugal pumps operating in parallel, and how to avoid this phenomenon.

"Water grabbing" is a typical operational instability phenomenon in parallel centrifugal pump systems. Essentially, it occurs when the outlet pressures of the pumps in the system become unbalanced, causing pumps with higher performance or steeper head curves to "steal" flow from the shared suction or discharge header. This not only reduces system output efficiency but can also cause severe vibrations, cavitation, and even equipment damage, seriously threatening operational safety.

• Analysis of the reasons for water grabbing by parallel centrifugal pumps

Mismatched pump characteristic curves: When pumps of different models or with varying degrees of wear are connected in parallel, differences in their head-flow curves can lead to uneven flow distribution. For example, a high-efficiency pump may suppress a low-efficiency pump, preventing the latter from discharging water normally.

Uneven pipeline resistance: Differences in branch pipeline length, number of bends, or pipe diameter create localized resistance differences, affecting flow distribution. Experimental data shows that when the pipeline resistance difference exceeds 10%, the risk of water leakage increases significantly.

Control system defects: Lack of flow feedback adjustment when constant-speed pumps are connected in parallel, or failure of variable-frequency pumps to adjust their frequency synchronously, can cause operating conditions to deviate from the design point.

• How to avoid the phenomenon of "water grabbing" when centrifugal pumps are operating in parallel?

Design Selection: Eliminating Imbalance Risks at the Source

1. Performance Curve Matching: This is the most fundamental preventative measure. When operating in parallel, the flow rates of each pump are superimposed at the same head. Therefore, pumps of the same model and specifications should be prioritized to ensure their flow-head curves are as consistent as possible. The greater the performance difference, the higher the risk of water contention.
2. Ensure identical suction conditions: All parallel pumps should draw water from the same suction header, and the length, diameter, and resistance characteristics of the suction pipeline should be basically the same. Uneven suction conditions will lead to different inlet pressures for each pump, resulting in insufficient effective net positive suction head (NPSH) for some pumps, thus causing cavitation and "water grabbing".

System layout: Optimize piping and processes

1. Optimize suction pipeline design: Use symmetrical and smooth pipeline layouts whenever possible, reducing unnecessary bends, valves, and other resistance components. Asymmetrical pipelines will cause pumps closer to the main pipe to have higher flow rates due to lower resistance, while pumps further away will have lower flow rates. If necessary, the diameter of the parallel main pipe can be appropriately increased to reduce the impact of the main pipe resistance on flow distribution.
2. Adopt a reasonable suction flow path: Studies have shown that using a specific flow path arrangement at the suction end (such as drawing pipes from both sides of the main pipe) can significantly improve the suction conditions of each pump, fundamentally avoiding water competition.

Operation: Fine control and adjustment

Follow a scientific start-up and shutdown sequence:

• During startup: Start one pump first, and only start the next pump after it is running smoothly. If pumps of different sizes are connected in parallel, start the larger pump first, opening its outlet valve to 1/4 to 1/3 for throttling. After the system pressure stabilizes, start the smaller pump, and finally slowly open the outlet valve of the larger pump fully.
• During switching: During the switching process, closely monitor the outlet flow of each pump to ensure that the outlet pressure of the newly added pump is basically consistent with that of the operating pump.
• Maintain consistent outlet pressure: During operation, the outlet pressure of each pump should be adjusted to the same level. This is the core control method to prevent "water grabbing". For variable frequency pumps, the outlet pressure can be fine-tuned by setting a speed offset.
• Use recirculation valves for auxiliary adjustment: For pumps equipped with recirculation pipelines, if "water grabbing" occurs, try closing the recirculation valve of the pump being "grabbed" to increase its outlet pressure, or opening the recirculation valve of the "grabbing" pump to decrease its outlet pressure, so that the pressures of the two pumps reach equilibrium.
• Valve throttling: In systems with small outlet pipe diameters, flow can be balanced by finely adjusting the opening of each pump outlet valve. In large pumping stations, valves can be pre-calibrated to establish a correspondence between opening and flow rate for operators' reference.

Monitoring and Maintenance: Real-time monitoring and proactive maintenance

1. Installing a single-pump flow meter: Monitoring only the main pipe flow is insufficient. Installing an independent flow meter for each pump is the most effective way to monitor the real-time operating status of each pump, promptly detect flow deviations, and issue early warnings of water shortages. If a flow meter cannot be installed, flow can be estimated using motor power and the differential pressure between the pump inlet and outlet.
2. Implementing an intelligent control system: Variable frequency speed control is an advanced solution. The system uses the main pipe pressure as feedback and automatically distributes the speed of each pump proportionally through a master-slave communication mode to achieve flow balance.
3. Establishing a regular inspection and maintenance system: As operating time increases, impeller wear, pipe scaling, and other factors gradually alter the performance of each pump, disrupting the original balance. Therefore, a regular inspection system must be established to promptly repair or replace worn parts and fine-tune control system parameters online to maintain long-term stability.

Preventing "water competition" among centrifugal pumps operating in parallel is a systematic project covering the entire lifecycle of the equipment. Its core lies in ensuring consistent pump performance, maintaining balanced outlet pressure, and supplementing this with meticulous operation management and real-time monitoring. Only by implementing measures effectively in every aspect of design, layout, operation, and maintenance can the long-term, safe, and efficient operation of the parallel units be guaranteed.