Why Should the Discharge Valve Be Closed When Starting a Centrifugal Pump?

When starting a centrifugal pump, it is important to keep the discharge valve closed. This simple step helps protect the pump, motor, and piping system, ensuring a safe and smooth start-up. Below are the key reasons explained in detail.

? 1. Reduces Starting Load

If the discharge valve is open during startup, the pump must immediately overcome the full system resistance to deliver liquid. This causes:

• A high starting current for the motor;

• Heavy torque load that may lead to overheating or motor damage.

✅ By keeping the discharge valve closed, the pump operates under minimal flow and reduced load, allowing the motor to start easily and safely.

? 2. Prevents Water Hammer

When the discharge valve is open, the sudden flow of liquid can cause a pressure surge, known as water hammer, leading to:

• Vibration in the pipeline;

• Pressure shocks that may damage valves, pipes, or the pump itself.

✅ Starting with the discharge valve closed allows the liquid to accelerate gradually. Once the pump runs smoothly, the valve can be slowly opened to avoid water hammer.

⚙️ 3. Protects Mechanical Seal and Bearings

At startup, the internal flow in the pump is unstable. If the discharge valve opens too quickly:

• Pressure inside the pump fluctuates sharply;

• The mechanical seal may experience sudden impact;

• Bearings can suffer uneven loading.

✅ Keeping the discharge valve closed helps stabilize internal pressure, protecting the mechanical seal and bearings from damage.

? 4. Standard Operating Procedure

According to industrial operation standards, the typical startup sequence for a centrifugal pump is:

1. Fill the pump casing completely with liquid (to prevent dry running);

2. Keep the discharge valve closed;

3. Start the motor;

4. Once the pump runs smoothly, gradually open the discharge valve to reach normal flow.

✅ Summary