Impact of Closing the Discharge Valve on Centrifugal Pump Operation

Centrifugal pumps are widely used in various industrial applications due to their high efficiency and reliability. However, when the discharge valve of a centrifugal pump is closed, several critical changes occur. This article explores the consequences of such an action and highlights the potential risks associated with it.

What Happens When the Discharge Valve is Closed?

When the discharge valve of a centrifugal pump is closed, several important changes occur in the pump's operation:

Increased Pressure

When the discharge valve is closed, the flow of liquid is restricted, leading to a significant increase in pressure within the pump and the discharge line. This increase in pressure is a direct result of the pump's continued operation against a blocked discharge path.

Reduced Flow Rate

Since the discharge path is blocked, the flow rate decreases substantially. In extreme cases, the flow rate may approach zero, depending on how tightly the valve is closed. This reduction in flow rate is a critical parameter to monitor to prevent operational issues.

Increased Power Consumption

The motor driving the pump may draw more current as it tries to maintain its speed against the increased pressure. This phenomenon is often referred to as 'dead head operation.' Increased power consumption can lead to higher energy costs and potential overheating.

Risk of Overheating

If the pump operates in a dead head condition for an extended period, it can lead to overheating. Overheating can cause damage to the pump seals and bearings, potentially leading to catastrophic failure.

Potential for Cavitation

Cavitation can occur if the pressure in the pump drops too low due to the increased suction pressure required to maintain flow. Vapor bubbles can form and collapse within the pump, causing damage from mechanical impacts.

Operational Considerations When Closing the Discharge Valve

Even though the pump and motor have no protective devices, it is crucial to understand the potential risks associated with operating a centrifugal pump in a dead head condition. In some cases, closing the discharge valve is a procedure followed during the startup of very large centrifugal pumps to reduce the starting current.

Amperage and Rated Full Load Current

The amperage drawn by the pump motor may not exceed the rated full load current when the discharge valve is shut. However, this is the maximum permissible current, and prolonged operation in this condition can lead to serious damage.

Wheelie Effect of a Synchronous Motor

Centrifugal pumps are typically driven by synchronous motors. Synchronous motors have a fixed speed, independent of the load. As such, the current required by the pump motor increases to overcome the resistance created by the closed discharge valve.

Infinitely High Pressure and Current

When the fluid pressure tends to infinity, i.e., the discharge valve is completely closed, the current of the pump motor increases to overcome the infinite load resistance. This condition can result in serious damage to the pump and motor.

Conclusion

Closing the discharge valve of a centrifugal pump can have severe consequences on the pump's operation, including increased pressure, reduced flow rate, increased power consumption, and risks of overheating and cavitation. It is advisable to avoid operating a centrifugal pump with a closed discharge valve for extended periods to ensure optimal performance and safety.