[Q&A Question 219] 2016.11.25

Prevent excessive flow and motor overcurrent when the pump starts

The starting current is minimal when the switch is closed, which prevents the motor from being damaged by excessive current

If the outlet valve is not closed, when the pump starts, its head is very low while its flow rate is high. Since the power of the pump is proportional to the flow rate, the pump’s power becomes very high, which inevitably results in a large starting current； On the other hand, the water in the pump inlet pipe is at rest before the pump starts operating; if its velocity increases suddenly, the water behind it cannot keep up, which leads to a sharp drop in inlet pressure. This causes the water to vaporize, resulting in the centrifugal pump becoming empty of fluid.

At zero flow rate, centrifugal pumps consume the least power. When the motor goes from a stationary state to its operating speed, the current required during startup (i.e., the motor’s power consumption) is 5 to 7 times higher than during normal operation. In such cases, if the outlet valve is not closed and thus mechanical load is increased, it may lead to accidents such as the motor tripping. To enable the motor to start under the lowest possible load, the outlet valve must be closed.

The power of a centrifugal pump is proportional to its flow rate. When the outlet valve is closed while starting the pump, the flow rate is at its minimum, and thus the starting power is also at its minimum, which helps to prevent overcurrent when the motor starts.

Centrifugal pumps have an important characteristic: when the pressure (head) is very low, their flow rate is high, as can be seen from the pump’s performance curve. The power of the pump is proportional to its flow rate. When the pump starts, there is no pressure in the pipeline, which results in a high flow rate and thus high pump power. Additionally, the rotating parts of the motor and pump need to accelerate from a stationary state to high speed, which inevitably leads to a high starting current. Therefore, the outlet valve is closed to prevent water from being discharged by the pump during startup, thereby minimizing the pump’s power consumption. Once the pump reaches its rated speed, the outlet valve is gradually opened to increase the water flow rate, allowing the motor current to rise steadily to the rated value. On the other hand, the water in the pump inlet pipe is at rest before the pump starts operating; if its velocity increases suddenly, the water behind it cannot keep up, which leads to a sharp drop in inlet pressure. This causes the water to vaporize, resulting in the centrifugal pump becoming empty of fluid. (Because a centrifugal pump uses the pump impeller to drive water to rotate, thereby generating centrifugal force; since the mass of gas is very low, its centrifugal force is small according to F=ma, which prevents the pump from pumping water out.)

Prevent excessive motor current at startup, which could damage the motor or cause a trip

Closing the outlet valve is equivalent to having no load, which prevents excessive starting current and prevents the pump from tripping

When a centrifugal pump is started, there is no water in the pump’s outlet pipeline; as a result, there is no pipeline resistance nor head loss. After the pump starts, its head is very low while the flow rate is high. At this point, the motor driving the pump generates a large amount of power (according to the pump’s performance curve), which can easily lead to overloading and damage to the pump’s motor and electrical wiring. Therefore, it is necessary to close the outlet valve at the time of startup in order to ensure the proper operation of the pump.