Should The Outlet Valve Be Closed When Starting A Centrifugal Pump?
Why should the outlet valve be closed when starting a centrifugal pump?
When the centrifugal water pump is started, there is no water in the outlet pipe of the pump, so there is no pipe resistance and lifting height resistance. After the centrifugal pump is started, the centrifugal pump head is very low and the flow rate is very large. At this time, the pump motor (shaft power) output is very large (according to the pump performance curve), which is easy to overload, which will damage the pump motor and circuit. Therefore, the outlet valve must be closed when starting to make the pump operate normally.
No matter which type of pump, the blades must be immersed in water before starting to establish the head. Since centrifugal and mixed flow water pumps have suction head, the suction pipe must be filled with water before starting. At this time, the water inlet valve needs to be set at the water inlet to ensure that there is water in the pipe. Axial flow pumps do not have suction head. At any time, the blades must be immersed in water. So there is no problem with the water inlet valve.
Before starting a centrifugal pump, two points must be ensured.
1. Fill the pump casing with water to form a vacuum.
2. The gate valve on the outlet pipe must be closed so that the pump does not form a flow, which can reduce the motor starting current and facilitate the smooth start of the pump. The gate valve should be opened slowly and in time as the pump starts smoothly. The centrifugal pump relies on the centrifugal force of the impeller to form a vacuum suction to lift the water. Therefore, when starting the centrifugal pump, the outlet valve must be closed first and water must be filled. The water level must exceed the impeller part, and the air in the centrifugal pump must be discharged before it can be started. After starting, a vacuum is formed around the impeller, sucking the water upward, and it can automatically open to lift the water. Therefore, the outlet valve must be closed first.
About Centrifugal Pumps
A centrifugal pump is a vane pump that relies on the rotating impeller. During the rotation process, due to the interaction between the blades and the liquid, the blades transfer mechanical energy to the liquid, which increases the pressure of the liquid and achieves the purpose of conveying the liquid. The operation of a centrifugal pump has the following characteristics:
① The head generated by a centrifugal pump at a certain speed has a limited value. The flow rate and shaft power at the working point depend on the conditions of the device system connected to the pump (position difference, pressure difference and pipeline loss). The head changes with the flow rate.
② Stable operation, continuous delivery, and no pulsation in flow and pressure.
③ Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline needs to be evacuated before it can start working.
④ The centrifugal pump is started with the discharge pipeline valve closed, and the vortex pump and axial flow pump are started with the valve fully open to reduce the starting power.
Before the pump is started, the pump casing is filled with the liquid to be transported; after starting, the impeller is driven by the shaft to rotate at high speed, and the liquid between the blades must also rotate. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge and gains energy, leaving the outer edge of the impeller at high speed and entering the volute pump casing.
In the volute, the liquid slows down due to the gradual expansion of the flow channel, and part of the kinetic energy is converted into static pressure energy, and finally flows into the discharge pipe at a higher pressure and is sent to the required place. When the liquid flows from the center of the impeller to the outer edge, a certain vacuum is formed in the center of the impeller. Since the pressure above the liquid surface in the storage tank is greater than the pressure at the pump inlet, the liquid is continuously pressed into the impeller. It can be seen that as long as the impeller keeps rotating, the liquid will be continuously sucked in and discharged.
Centrifugal pump structure
Centrifugal pumps are generally driven by electric motors. Before starting the pump, the pump body and the suction pipe are filled with liquid. When the impeller rotates at high speed, the impeller drives the liquid between the blades to rotate together. Due to the centrifugal force, the liquid is thrown from the center of the impeller to the outer edge of the impeller (the flow rate can be increased to 15-25m/s), and the kinetic energy increases accordingly. After the liquid enters the pump casing, the flow channel in the volute-shaped pump casing gradually expands, the liquid flow rate gradually decreases, and part of the kinetic energy is converted into static pressure energy, so the liquid flows out along the discharge port at a higher pressure. At the same time, a certain vacuum is formed at the center of the impeller due to the liquid being thrown out, and the pressure Pa at the liquid surface is higher than that at the center of the impeller. Therefore, the liquid in the suction pipe enters the pump under the action of the pressure difference. The impeller keeps rotating, and the liquid is continuously sucked in and pressed out. Since the centrifugal pump can transport liquid mainly by the action of centrifugal force, it is called a centrifugal pump.
Start-up of other centrifugal pumps
The above are all centrifugal pumps. For other types of pumps, the situation is as follows:
1. Large flow starting characteristics of axial flow pumps - fully open valve starting. The shaft power of axial flow pumps is maximum at zero flow conditions, which is 140%~200% of the rated shaft power. The power is minimum at maximum flow. Therefore, in order to minimize the starting current, the starting characteristic of the shaft power should be large flow starting (i.e. fully open valve starting).
2. Starting characteristics of mixed flow pumps - fully open valve starting. The shaft power of the mixed flow pump at zero flow conditions is between the above two pumps, which is 100%~130% of the rated power. Therefore, the starting characteristics of the mixed flow pump should also be between the above two pumps, and it is best to start with a fully open valve.
3. Starting characteristics of vortex pumps - fully open valve starting. The shaft power of the vortex pump is the largest under zero flow conditions, which is 130%~190% of the rated shaft power. Therefore, similar to axial flow pumps, the starting characteristics of vortex pumps should be large flow starting (i.e. fully open valve starting).
