Summary Of Knowledge Points For Centrifugal Pumps

Jul 13, 2024

1. Working principle of centrifugal pump
When a centrifugal pump is working, it relies on the high-speed rotating impeller to increase the pressure energy of the liquid under the action of inertial centrifugal force. Before the centrifugal pump starts working, the pump body and inlet pipeline must be filled with liquid medium to prevent cavitation.
When the impeller rotates rapidly, the blades promote the medium to rotate quickly. The rotating medium flies out of the impeller under the action of centrifugal force, and the water inside the pump is thrown out, forming a vacuum area in the center of the impeller. Continuously inhaling liquid while continuously providing a certain amount of energy to the inhaled liquid to expel it. The centrifugal pump operates continuously like this.
2. Structure of centrifugal pump
There are many varieties of centrifugal pumps, and although the structures of each type of pump are different, the main components are basically the same.
The main components of a centrifugal pump include: impeller, pump shaft, pump casing, pump seat, packing box (shaft sealing device), leakage reducing ring, bearing seat, etc.

The impeller is the working component of a centrifugal pump, which relies on its high-speed rotation to do work on the liquid and achieve liquid transportation. It is an important component of a centrifugal pump.
The impeller is generally composed of three parts: the hub, blades, and cover plate. The cover plate of the impeller can be divided into front cover plate and rear cover plate. The cover plate on the impeller port side is called the front cover plate, and the cover plate on the other side is called the rear cover plate.
After 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 circumference.
The liquid gains energy during its movement through the impeller, resulting in an increase in static pressure energy and an increase in flow velocity. When the liquid leaves the impeller and enters the pump casing, it slows down due to the gradual expansion of the flow channel inside the casing. Part of the kinetic energy is converted into static pressure energy, and finally flows into the discharge pipeline along the tangential direction.
According to the structural form, impellers can be divided into the following three types.
(1)The closed impeller has cover plates on both sides of the impeller, with 4-6 blades between the cover plates. The closed impeller has high efficiency and is widely used, suitable for conveying clean liquids without solid particles and fibers.
(2)The open impeller has no cover plates on both sides of the blade, which is suitable for conveying liquids containing a large amount of suspended solids. It has low efficiency and the pressure of the conveyed liquid is not high.
The semi open impeller has only a rear cover plate and is suitable for conveying liquids that are easy to settle or contain solid suspended solids. Its efficiency is between open and closed impellers.

The main function of the pump shaft of a centrifugal pump is to transmit power and support the impeller to maintain normal operation in the working position. It is connected to the motor shaft through a coupling at one end and supports the impeller for rotational motion at the other end. The shaft is equipped with bearings, axial seals, and other components.
The commonly used materials for pump shafts are carbon steel and stainless steel.
The impeller and shaft are connected by keys. As this connection method can only transmit torque and cannot fix the axial position of the impeller, a shaft sleeve and locking nut are also used in the water pump to fix the axial position of the impeller.
After the impeller is axially positioned with a locking nut and shaft sleeve, in order to prevent the locking nut from retracting, it is necessary to prevent the water pump from reversing, especially for the initial installation of the water pump or the water pump after disassembly and maintenance, the steering inspection should be carried out according to regulations to ensure consistency with the specified steering.
The function of the shaft sleeve is to protect the pump shaft, transforming the friction between the packing and the pump shaft into the friction between the packing and the shaft sleeve. Therefore, the shaft sleeve is a easily worn part of the centrifugal pump.
The surface of the shaft sleeve can generally be treated with methods such as carburization, nitriding, chrome plating, spraying, etc. The surface roughness requirement is generally Ra3.2 μ m to Ra0.8 μ m. It can reduce the friction coefficient and improve the service life.
Bearings play a role in supporting the weight and load-bearing capacity of the rotor. Rolling bearings are commonly used on centrifugal pumps, with the outer ring and bearing seat holes using a base shaft system, and the inner ring and shaft using a base hole system. Bearings are generally lubricated with grease and oil.
When the pump shaft passes through the pump casing, there is a gap between the shaft and the casing. In a single suction centrifugal pump, if the shaft seal device is not used at this location, high-pressure water inside the pump casing will leak out in large quantities. The packing box is a commonly used shaft sealing device. The packing box is composed of five components: shaft seal, packing, water seal pipe, water seal ring, and packing gland.

A volute refers to a spiral flow channel with a gradually increasing cross-sectional area from the impeller outlet to the inlet of the next stage impeller or to the outlet pipe of the pump. The flow channel gradually expands and the outlet is in the shape of a diffusion tube. After the liquid flows out of the impeller, its flow rate can slowly decrease, converting a large portion of kinetic energy into static pressure energy.
The advantages of a volute are easy manufacturing, wide efficiency zone, and minimal efficiency changes in the pump after turning the impeller.
The disadvantage is that the shape of the volute is asymmetric, and when using a single volute, the pressure acting on the radial direction of the rotor is uneven, which can easily cause the shaft to bend. Therefore, in multi-stage pumps, only the first and last sections use volutes, while in the middle section, guide wheel devices are used.
The material of snail shells is generally cast iron. The volute of the anti-corrosion pump is made of stainless steel or other anti-corrosion materials, such as plastic, fiberglass, etc. Due to the high pressure, multi-stage pumps require high material strength, and their volutes are generally made of cast steel.
The guide wheel is a fixed disk with forward guide vanes wrapped around the outer edge of the impeller on the front, forming diffusion shaped flow channels. On the back, there are reverse guide vanes that guide the liquid towards the next stage of the impeller. After being thrown out of the impeller, the liquid slowly enters the guide vanes and continues to flow outward along the forward guide vanes. The velocity gradually decreases, and most of the kinetic energy is converted into static pressure energy.
The radial unilateral clearance between the impeller and guide vanes is approximately 1mm. If the gap is too large, the efficiency will decrease; If the gap is too small, it will cause vibration and noise. Compared with the volute, the segmented multi-stage centrifugal pump casing with guide wheels is easier to manufacture and has higher efficiency in energy conversion. But installation and maintenance are more difficult than snail shells.
In order to reduce internal leakage and protect the pump casing, replaceable sealing rings are installed on the casing corresponding to the impeller inlet. The radial clearance between the inner hole of the sealing ring and the outer circle of the impeller is generally between 0.1-0.2mm. After the wear of the sealing ring, the radial clearance increases, the pump's discharge volume decreases, and the efficiency decreases. When the sealing clearance exceeds the specified value, it should be replaced in a timely manner.
There are three structural forms of sealing rings:
Firstly, the flat ring type has a simple structure and is easy to manufacture, but the sealing effect is poor. Secondly, the right angle sealing ring provides a 90 ° channel for liquid leakage, resulting in better sealing performance than the flat ring type and is widely used. Thirdly, the labyrinth sealing ring has a good sealing effect, but its structure is complex and difficult to manufacture, which is rarely used in centrifugal pumps.
3. Working process of centrifugal pump
(1) Before starting the pump, fill the pump with the liquid to be conveyed.
(2) After starting the pump, the pump shaft drives the impeller to rotate at high speed together, generating centrifugal force. Under this action, the liquid is thrown towards the outer circumference of the impeller from the center, causing an increase in pressure and flowing into the pump casing at a high speed (15-25 m/s).
(3) In the volute pump casing, due to the continuous expansion of the flow channel, the flow rate of the liquid slows down, converting most of the kinetic energy into pressure energy. Finally, the liquid flows into the discharge pipeline at a higher static pressure from the discharge port.
(4) After the liquid inside the pump is thrown out, a vacuum is formed at the center of the impeller. Under the pressure difference between the liquid level pressure (atmospheric pressure) and the pump pressure (negative pressure), the liquid enters the pump through the suction pipeline, filling the position where the liquid is discharged.

4. Classification of centrifugal pumps
Centrifugal pump products are generally classified according to their structural characteristics, with multiple classification methods including working pressure, number of working impellers, and inlet method of impellers.
(1) According to 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: The pressure is higher than 650 meters of water column.
(2) According to the number of working impellers:
Single stage pump: refers to having only one impeller on the pump shaft.
Multi stage pump.: There are two or more impellers on the pump shaft, and the total head of the pump is the sum of the heads generated by n impellers.
(3) According to the impeller inlet method:
Single side inlet pump: also known as a single suction pump, which means there is only one inlet on the impeller.
Double sided inlet pump: also known as double suction pump, which means there is an inlet on both sides of the impeller. Its flow rate is twice that of a single suction pump, which can be approximated as two single suction pump impellers placed back-to-back together.
(4) According to the position of the pump shaft:
Horizontal pump: The pump shaft is located in a horizontal position.
Vertical pump: The pump shaft is located in a vertical position.
(5) According to the joint form of the pump casing:
Horizontal open type pump: refers to a joint seam opened on the horizontal plane passing through the axis.
Vertical joint surface pump: that is, the joint surface is perpendicular to the axis.
(6) The method of guiding the water from the impeller towards the pressure chamber is as follows:
Spiral case pump: After water comes out of the impeller, it directly enters the pump casing with a spiral shape.
Guide vane pump: After water comes out of the impeller, it enters the guide vanes set outside it, and then enters the next stage or flows into the outlet pipe.
(7) According to the different media conveyed by centrifugal pumps, it can be divided into clean water pumps, oil pumps, corrosion-resistant pumps, etc.
5. Cavitation and gas binding
According to the working principle of a centrifugal pump, when the liquid between the blades is thrown out of the high-speed rotating impeller, a low-pressure zone is formed near the inlet of the impeller. When the pressure at the inlet of the impeller is equal to or lower than the saturated vapor pressure pV of the transported liquid at the operating temperature, the liquid at that location will vaporize and produce bubbles. When bubbles flow with the liquid to the high-pressure zone, they quickly condense under pressure.
At the moment of bubble condensation, a local vacuum is generated, and the surrounding liquid rushes towards the space occupied by the bubble at a high speed, causing shock and vibration, resulting in a significant impact force. Especially when the condensation point of bubbles is located near the surface of the blade, numerous liquid particles impact the blade at a high frequency and pressure; At the same time, bubbles may also contain a small amount of oxygen, which can cause chemical corrosion to metal materials. Under the combined action of continuous impact and chemical corrosion, the surface of the blades is damaged, resulting in spots and cracks, which will lead to premature damage of the blades. This phenomenon is called cavitation in centrifugal pumps.
When a centrifugal pump is started, if there is air inside the pump, due to the low air density, the centrifugal force generated after rotation is small, and the low pressure formed in the center area of the impeller is not enough to suck in the liquid. Even if the centrifugal pump is started, it cannot complete the conveying task. This phenomenon is called air binding.
This indicates that the centrifugal pump has no self suction capacity, so the pump must be filled with the conveyed liquid before starting. Of course, if the suction port of the centrifugal pump is placed below the liquid level of the conveyed liquid, the liquid will automatically flow into the pump, which is a special case. The suction pipeline of the centrifugal pump is equipped with a bottom valve to prevent the liquid injected before starting from flowing out of the pump. The filter can block the solid suction in the liquid and block the pipeline and the regulating valve installed in the pump casing discharge pipeline is used for starting, stopping, and regulating the flow rate of the pump.
From the different causes of cavitation and gas binding:
Air binding refers to the presence of air in the pump body, which usually occurs when the pump is started and is mainly manifested as the air inside the pump body not being completely discharged; And cavitation is due to the liquid reaching its vaporization pressure at a certain temperature, which is closely related to the conveying medium and operating conditions.
There are the following methods to prevent the occurrence of gas binding phenomenon:
(1) Fill the shell with liquid before starting. Ensure proper sealing of the casing, and ensure that the valve and showerhead for filling water do not leak. Ensure good sealing performance.
(2) The suction pipeline of the centrifugal pump is equipped with a bottom valve to prevent the liquid injected before starting from flowing out of the pump. The filter can prevent the solid in the liquid from being sucked in. The discharge pipeline is equipped with a regulating valve for use when starting, stopping, and regulating the flow rate of the pump.
(3) Place the suction port of the centrifugal pump below the liquid level to be transported, and the liquid will automatically flow into the pump.
The main causes of cavitation are:
(1) The inlet pipeline has excessive resistance or the pipeline is too thin
(2) The temperature of the conveying medium is too high;
(3) Excessive flow, which means the outlet valve is opened too wide;
(4) The installation height is too high, which affects the suction capacity of the pump;
(5) Selection issues, including pump selection, pump material selection, etc
terms of settlement:
(1) Clean the foreign objects in the inlet pipeline to make the inlet unobstructed, or increase the size of the pipe diameter;
(2) Reduce the temperature of the conveying medium;
(3) Reduce installation height;
(4) Re select the pump or make improvements to certain components of the pump, such as using corrosion-resistant materials.