Centrifugal pumps are most widely used in the water treatment industry due to their outstanding advantages, such as wide range of performance applications (including flow rate, head and adaptability to media properties), small size, simple structure, easy operation, uniform flow, long service life, and low purchase and operating costs.
Basic structure of centrifugal pump
The basic components of a centrifugal pump are a high-speed rotating impeller and a fixed volute-shaped pump casing. The impeller, which has several (usually 4 to 12) backward-curved blades, is fixed to the pump shaft and rotates at high speed along with the pump shaft by a motor.
The impeller is the component that directly performs work on the liquid inside the pump and serves as the power supply for the centrifugal pump. The suction inlet in the center of the pump casing connects to the suction pipe, and a one-way foot valve is installed at the bottom of the suction pipe. The discharge outlet on the side of the pump casing connects to the discharge pipe, which is equipped with a regulating valve.
Working principle of centrifugal pump
When the centrifugal pump is started, the pump shaft drives the impeller to rotate at high speed, forcing the liquid pre-filled between the blades to rotate. Under the action of inertial centrifugal force, the liquid moves radially from the center of the impeller to the outer periphery.
The liquid gains energy during its movement through the impeller, increasing its static pressure and velocity. As the liquid leaves the impeller and enters the pump casing, it decelerates due to the gradual expansion of the flow channels within the casing, converting some of its kinetic energy into static pressure energy, and finally flows tangentially into the discharge pipe.
Therefore, the volute pump casing is not only a component that collects the liquid flowing out of the impeller, but also an energy conversion device. As the liquid is thrown from the center of the impeller to the outer periphery, a low-pressure zone is formed at the center of the impeller. Under the influence of the total potential energy difference between the liquid surface in the reservoir and the center of the impeller, the liquid is drawn into the center of the impeller. Relying on the continuous rotation of the impeller, the liquid is continuously drawn in and discharged. The mechanical energy gained by the liquid in the centrifugal pump ultimately manifests as an increase in static pressure energy.
It is important to emphasize that if the centrifugal pump casing is not filled with the liquid to be pumped before starting the centrifugal pump, the low air density results in a small centrifugal force generated by the impeller's rotation. Consequently, the central area of the impeller is insufficient to create the low pressure required to draw the liquid into the storage tank, and the centrifugal pump cannot pump liquid even after starting. This indicates that the centrifugal pump lacks self-priming capability; this phenomenon is called air binding.
A one-way foot valve is installed in the suction line to prevent liquid that has been poured into the pump casing before startup from flowing out. Air entering the pump casing from the suction line will cause air binding.
Impeller and other components
1. Impeller of centrifugal pump
The impeller is a key component of a centrifugal pump.
(1) According to their mechanical structure, they can be divided into three types: closed, semi-closed and open.
Closed impellers are suitable for conveying clean liquids; semi-closed and open impellers are suitable for conveying suspensions containing solid particles, but these types of pumps are less efficient.
When closed and semi-closed impellers are running, some of the high-pressure liquid leaving the impeller can leak into the cavity between the impeller and the pump casing. Because the pressure at the liquid inlet on the front side of the impeller is low, the pressure of the liquid acting on the front and rear sides of the impeller is not equal, which generates an axial thrust pointing towards the impeller inlet side.
This force pushes the impeller towards the suction side, causing wear at the contact point between the impeller and the pump casing. In severe cases, it can cause pump vibration and disrupt normal operation. Drilling several small holes in the impeller back cover plate can reduce the pressure difference on both sides of the impeller, thereby mitigating the adverse effects of axial thrust, but it also reduces pump efficiency. These small holes are called balancing holes.
(2) According to the different liquid suction methods, impellers can be divided into two types: single suction type and double suction type.
Single-suction impellers have a simple structure, but liquid can only be drawn in from one side. Double-suction impellers can simultaneously and symmetrically draw in liquid from both sides of the impeller, which not only has a larger liquid suction capacity but also virtually eliminates axial thrust.
(3) Based on the geometry of the blades on the impeller, the blades can be divided into three types: backward-curved, radial, and forward-curved. Since backward-curved blades are conducive to the conversion of the kinetic energy of the liquid into static pressure energy, they are widely used.
2. Guide wheel of centrifugal pump
To reduce energy loss due to impact when liquid leaving the impeller directly enters the pump casing, a stationary guide wheel with blades is sometimes installed between the impeller and the pump casing. The blades in the guide wheel gradually change the direction of the liquid entering the pump casing and continuously expand the flow channel, effectively converting some kinetic energy into static pressure energy. Multistage centrifugal pumps are usually equipped with guide wheels.
The snail-shaped pump casing, the backward-curved blades on the impeller, and the guide wheel can all improve the conversion rate of kinetic energy to static pressure energy, and therefore can all be regarded as energy conversion devices.
3. Shaft sealing device
Since the pump shaft rotates while the pump casing remains stationary, there will inevitably be a certain gap at the contact point between the shaft and the pump casing. To prevent high-pressure liquid inside the pump from leaking out along this gap, or to prevent outside air from entering the pump from the opposite direction, a shaft sealing device must be installed.
Centrifugal pumps have two shaft sealing devices: stuffing boxes and mechanical (face) seals. A stuffing box is a sealing ring formed by the annular gap through which the pump shaft passes through the pump casing, and soft packing (such as oil-impregnated or graphite-coated asbestos rope) is filled into it.
A mechanical seal consists of a rotating ring mounted on a shaft and a stationary ring fixed to the pump casing. The end faces of the two rings are pressed together by spring force, causing them to rotate relative to each other and thus creating a seal. Mechanical seals are suitable for applications requiring high sealing levels, such as conveying acids, alkalis, flammable, explosive, and toxic liquids.
Types of centrifugal pumps
Centrifugal pumps are generally classified according to their structural characteristics, and there are several classification methods, including six categories such as working pressure, number of impellers, and impeller inlet method.
1. Based on work pressure
Low-pressure pump: pressure below 100 meters of water column;
Medium-pressure pump: pressure between 100-650 meters of water column;
High-pressure pump: Pressure higher than 650 meters of water column.
2. According to the number of working impellers
Single-stage pump: This means that there is only one impeller on the pump shaft.
Multistage pump: This refers to a pump with two or more impellers on the pump shaft. In this case, the total head of the pump is the sum of the heads generated by the n impellers.
3. According to the impeller water inlet method
Single-inlet pump: also called single-suction pump, which means that there is only one inlet on the impeller.
Double-inlet pump: also called a double-suction pump, meaning that there is an inlet on both sides of the impeller. Its flow rate is twice that of a single-suction pump, and it can be approximated as two single-suction pump impellers placed back to back.
4. According to the pump shaft position
Horizontal pump: The pump shaft is in a horizontal position.
Vertical pump: The pump shaft is located in a vertical position.
5. According to the form of pump casing joint
Horizontal split-case pump: This type of pump has a joint seam on a horizontal plane passing through its axis.
Vertical mating surface pump: that is, the mating surface is perpendicular to the axis.
6. The water from the impeller is directed to the discharge chamber.
volute pump: After the water exits the impeller, it directly enters the pump casing, which has a spiral shape.
Guide vane pump: After water exits the impeller, it enters the guide vanes installed outside the impeller, and then enters the next stage or flows into the outlet pipe.
7. According to the conveying medium
Centrifugal pumps are classified into clean water pumps, oil pumps, corrosion-resistant pumps, etc., depending on the medium they transport.
