When starting a centrifugal pump, should you turn on the pump first or open the valve first?

The starting sequence of a water pump and the opening/closing status of the valves directly affect its operational safety and efficiency. Different pump types have drastically different starting methods due to differences in their working principles.

I. Centrifugal Pump: Valve-Closed Start-up

Centrifugal pumps are the most widely used type of water pump in industrial and civil applications. Their startup must follow the "valve-closed priming" principle. The specific operation is as follows:

Priming and Air Exhausting: Before starting, fill the pump casing with liquid, ensuring the impeller is completely submerged. This is to expel air from the pump and create a vacuum environment. If insufficient priming is performed, the impeller cannot generate sufficient suction through centrifugal force, leading to dry running and damage.

Closing the Outlet Valve: The outlet valve must be completely closed during startup. Under zero-flow conditions (valve closed), the centrifugal pump has the highest head and lowest shaft power (typically 30%~50% of rated power). At this time, the motor load is low, avoiding a sudden increase in current due to high flow (starting current can be reduced to 20%~30% of full-load current), protecting the motor and power supply lines.

Gradual Valve Opening: After the motor starts, the outlet valve should be opened slowly to allow the flow rate to gradually increase to the rated value along the pump performance curve (usually taking 10-30 seconds). Opening the valve too quickly may cause the flow rate to momentarily exceed the design value (e.g., reaching 150% of the rated flow), leading to motor overload or pump vibration.

Centrifugal pumps rely on the centrifugal force generated by the high-speed rotation of the impeller to transport liquids. Closing the valve during startup limits the flow rate, reduces impeller resistance, and ensures a smooth transition of the motor to operating conditions. Experimental data shows that starting with the valve closed can improve the mechanical efficiency of a centrifugal pump by approximately 15%-20%.

II. Axial Flow Pumps: Fully Open Valve Start-up

Axial flow pumps are suitable for high-flow, low-head applications (such as farmland irrigation), and their starting characteristics are the opposite of centrifugal pumps.

Necessity of Fully Open Valve: Axial flow pumps have maximum shaft power at zero flow, reaching 140%-200% of the rated power. If the valve is closed during startup, the motor will have to withstand extremely high loads, potentially causing the windings to overheat (temperature rise can reach over 60°C) or burn out completely.

Start-up Operation: Before starting, the outlet valve must be fully opened to minimize pipeline resistance. At this time, the pump operates at maximum flow, and the shaft power drops to 60%~80% of the rated value, significantly reducing the starting current (approximately 50% of the closed valve state).

Abnormal Operating Condition Handling: If the valve is not fully open, the pump may vibrate (amplitude exceeding 0.1mm) or produce abnormal noise (sound pressure level > 85dB), requiring immediate shutdown and inspection.

Technical Principle: The blade design of the axial flow pump is similar to that of an aircraft propeller, relying on the axial flow of the liquid to generate thrust. At high flow rates, the impact force of the fluid on the blades is evenly distributed, resulting in lower mechanical losses (efficiency can reach 80%~90%).

III. Starting Characteristics of Mixed-Flow Pumps and Vortex Pumps

Mixed-Flow Pumps: Combining the characteristics of centrifugal pumps and axial flow pumps, with moderate head and flow rate. At zero flow, the shaft power is 100%~130% of the rated value. Fully open valve startup is recommended to balance the load (starting current approximately 70% of the rated current) and avoid impeller cavitation risk (cavitation margin must be > 3m). Vortex pump: Suitable for high-head, low-flow applications. In zero-flow operation, shaft power reaches 130%~190% of the rated value, requiring full valve opening for startup. If the valve is closed for more than 30 seconds, the pump body temperature may rise above 80℃, leading to seal failure.