During the flow process of fluids, some mechanical energy is lost due to the flow resistance. Therefore, to transport the fluid from one place to another, whether it is to transfer the fluid from a location with lower total specific energy to a location with higher total specific energy, or merely to overcome the flow resistance, mechanical energy must be provided to the fluid. The machinery used for transporting liquids is called a pump (Pump). Pumps are mainly classified into three categories based on their structural characteristics and working principles:
I. Vane-type pumps: These pumps work by having the rotating vanes do work on the fluid, thereby increasing the mechanical energy of the liquid. Examples include various centrifugal pumps, vortex pumps, and axial flow pumps, etc.
II Positive Displacement Pumps: These pumps utilize the reciprocating motion of pistons or the rotating motion of rotors to change the volume of the working chamber, compressing the liquid and doing work on the liquid, thereby increasing the mechanical energy of the liquid. Examples include reciprocating pumps, gear pumps, and screw pumps, etc.
III Jet Pump: It works by using the high-speed jet generated by the working fluid to eject the fluid, and then through momentum exchange, the energy of the ejected fluid is increased.
Due to its simple structure, ease of manufacture, stable flow, strong adaptability and convenient operation, centrifugal pumps are widely used in chemical production. Therefore, in this article, we will focus on introducing centrifugal pumps.
The working principle of a centrifugal pump
When a centrifugal pump is in operation, it relies on the high-speed rotating impeller to enable the liquid to acquire energy and increase its pressure potential under the effect of inertial centrifugal force. Before the centrifugal pump starts working, the pump body and the inlet pipeline must be filled with liquid medium to prevent the occurrence of cavitation.
When the impeller rotates rapidly, the blades cause the medium to rotate quickly. The rotating medium is thrown out of the impeller under the action of centrifugal force. After the water inside the pump is thrown out, a vacuum area forms in the center of the impeller. At the same time, it continuously sucks in liquid and continuously gives certain energy to the sucked-in liquid, then discharges the liquid. Thus, the centrifugal pump works continuously in this way.
The structure of a centrifugal pump
There are many types of centrifugal pumps. Although the structures of various types of pumps are different, the main components are basically the same.
The main components of a centrifugal pump include: impeller, pump shaft, pump casing, pump base, packing box (sealing device), sealing ring, bearing housing, etc.
1. Impeller
The impeller is the working component of a centrifugal pump. It achieves the pumping of liquids by rotating at high speed and doing work on the liquids. It is an important part of the centrifugal pump.
The impeller is generally composed of the hub, the blades and the cover plate. The cover plate of the impeller is divided into the front cover plate and the rear cover plate. The cover plate on the inlet side of the impeller is called the front cover plate, and the other side's cover plate is called the rear cover plate.
When the centrifugal pump is started, the pump shaft drives the impeller to rotate at high speed together. This forces the liquid that has been pre-filled between the blades to rotate. Under the action of the inertial centrifugal force, the liquid moves radially from the center to the periphery of the impeller.
During the flow process through the impeller, the liquid acquires energy, with its static pressure increasing and the flow velocity increasing. When the liquid leaves the impeller and enters the pump casing, it slows down due to the gradually expanding flow channels inside the casing. Some of the kinetic energy is converted into static pressure energy, and finally flows tangentially into the discharge pipeline.
According to their structural forms, the impellers can be classified into the following three types.
(1) The closed impeller has cover plates on both sides. There are 4 to 6 blades between the cover plates. The closed impeller has a high efficiency and is the most widely used type. It is suitable for conveying clean liquids without solid particles or fibers.
(2) The open-type impeller has no cover plates on both sides of the blades. It is suitable for transporting liquids containing a large amount of suspended solids. However, its efficiency is relatively low and the pressure of the transported liquid is not high.
(3) The semi-open type impeller has only a rear cover plate. It is suitable for conveying liquids that are prone to sedimentation or contain solid suspended matter. Its efficiency lies between that of the open and closed type impellers.
2. Pump shaft
The main function of the pump shaft of a centrifugal pump is to transmit power and support the impeller to keep it in the working position and operate normally. One end of the shaft is connected to the motor shaft through a coupling, and the other end supports the impeller for rotational movement. The shaft is equipped with components such as bearings and axial seals.
The common materials for pump shafts are carbon steel and stainless steel.
The impeller and the shaft are connected by a key. Since this connection method can only transmit torque but cannot fix the axial position of the impeller, in the pump, an axial sleeve and a locking nut are used to fix the axial position of the impeller.
After the impeller is axially positioned with the locking nut and the shaft sleeve, to prevent the locking nut from loosening, it is necessary to prevent the pump from reversing. Especially for newly installed pumps or pumps that have undergone disassembly and repair, a turning direction check should be conducted as per regulations to ensure consistency with the specified direction.
3. Sleeve
The function of the shaft sleeve is to protect the pump shaft, converting 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 wear-prone component of the centrifugal pump.
The surface of the shaft sleeve can also undergo treatments such as carburizing, nitriding, chrome plating, and spraying. The surface roughness requirement is generally required to reach Ra3.2μm - Ra0.8μm. This can reduce the friction coefficient and increase the service life.
4. Bearings
The bearings play the role of supporting the weight and load of the rotor. In centrifugal pumps, rolling bearings are mostly used. The outer ring of the bearing is in a base shaft system with the bearing housing hole, while the inner ring is in a base hole system with the rotating shaft. The matching category national standards have recommended values, and they can be selected according to specific circumstances. Bearings are generally lubricated with grease and lubricating oil.
5. Filler box
When the pump shaft protrudes from the pump casing, there is a gap between the shaft and the casing. In single-suction centrifugal pumps, if no shaft sealing device is used at this part, the high-pressure water inside the pump casing will leak out in large quantities. The packing box is one of the commonly used shaft sealing devices. The packing box is composed of five components: the shaft sealing sleeve, the packing, the water seal pipe, the water seal ring and the packing cover.
⒍蜗壳
The volute is a spiral-shaped flow channel that gradually increases in cross-sectional area from the outlet of the impeller 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 form of a diffuser tube. After the liquid flows out of the impeller, its flow velocity can be smoothly reduced, converting a large part of its kinetic energy into static pressure energy.
The advantages of the volute are that it is easy to manufacture, has a wide efficiency zone, and the efficiency of the pump changes little after the impeller is machined.
The drawback is that the volute shape is asymmetrical. When using a single volute, the pressure acting on the rotor radially is not uniform, which is likely to cause the shaft to bend. Therefore, in multi-stage pumps, only the first and last sections use volutes, while the middle section adopts a guide wheel device.
The material of the volute is usually cast iron. The volute of the anti-corrosion pump is made of stainless steel or other anti-corrosion materials, such as plastic, fiberglass, etc. For multi-stage pumps, due to the high pressure, the material strength requirements are higher, and their volutes are generally made of cast steel.
⒎ Drive Wheel
The guide wheel is a stationary disc with forward guide vanes wrapped around its outer edge on the front side. These guide vanes form a series of diffuser-shaped flow channels. On the back side, there are reverse guide vanes that direct the liquid to the inlet of the next stage impeller. After the liquid is ejected from the impeller, it flows smoothly into the guide wheel and continues to flow outward along the forward guide vanes, with its speed gradually decreasing and most of its kinetic energy being converted into static pressure energy.
The radial unilateral clearance between the impeller and the guide vanes is approximately 1mm. If the clearance is too large, the efficiency will decrease; if it is too small, it will cause vibration and noise. Compared with the volute, the pump casing of the segmented multi-stage centrifugal pump with guide wheels is easier to manufacture and has a higher efficiency of energy conversion. However, its installation and maintenance are more difficult than that of the volute.
16. Sealing ring
To reduce internal leakage and protect the pump casing, a replaceable sealing ring is installed on the shell corresponding to the inlet of the impeller. The radial clearance between the inner hole of the sealing ring and the outer circle of the impeller is generally between 0.1 and 0.2 mm. After the sealing ring wears out, the radial clearance increases, resulting in a decrease in the pump's liquid discharge volume and a reduction in efficiency. When the sealing gap exceeds the specified value, it is necessary to replace it in time.
The structural forms of the sealing ring are three types:
Flat-ring type, with a simple structure and easy manufacturing, but poor sealing effect;
The right-angle type sealing ring allows liquid leakage to pass through a 90° channel, resulting in a better sealing effect compared to the flat-ring type. It is widely used.
The labyrinth seal ring has a good sealing effect, but its structure is complex and its manufacturing is difficult. Therefore, it is rarely used in centrifugal pumps.
The working process of a centrifugal pump
Before starting the pump, fill the pump with the liquid to be transported first.
2. After the pump is started, the pump shaft drives the impeller to rotate at high speed, generating centrifugal force. Under this force, the liquid is thrown from the center of the impeller to the periphery of the impeller, its pressure increases, and it flows into the pump casing at a very high speed (15-25 m/s).
3. In the蜗形 pump casing, as the flow channel continuously expands, the flow velocity of the liquid slows down, causing most of the kinetic energy to be converted into pressure energy. Finally, the liquid flows out from the discharge port with a relatively high static pressure and enters the discharge pipeline.
4. After the liquid inside the pump is expelled, a vacuum is formed at the center of the impeller. Under the pressure difference between the liquid surface pressure (atmospheric pressure) and the pressure inside the pump (negative pressure), the liquid enters the pump through the suction pipeline and fills the position where the liquid was expelled.
Classification of centrifugal pumps
Centrifugal pump products are generally classified according to their structural characteristics. There are various classification methods, including six types: classified by working pressure, by the number of working impellers, by the way the impeller takes in water, etc.
⒈ According to work pressure:
Low-pressure pump: The pressure is lower than 100 meters of water column.
Medium-pressure pump: The pressure ranges from 100 to 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: This refers to a pump where there is only one impeller on the pump shaft.
Multi-stage pump: This type of pump has two or more impellers on its shaft. In this case, the total head of the pump is the sum of the heads generated by each of the n impellers.
3. According to the water intake method of the impeller:
Single-side water-intake pump: Also known as single-suction pump, it means that there is only one water intake port on the impeller.
Bidirectional suction pump: Also known as double-suction pump, it has an inlet port on both sides of the impeller. Its flow rate is twice as large as that of a single-suction pump. It can be roughly regarded as two single-suction pump impellers placed back-to-back.
4. According to the position of the pump shaft:
Horizontal pump: The pump shaft is in a horizontal position.
Vertical pump: The pump shaft is in a vertical position.
5. According to the form of the pump casing joint:
Horizontal split pump: It is one where the joint seam is opened on the horizontal plane passing through the axis.
Vertical joint surface pump: This refers to a pump where the joint surface is perpendicular to the axis line.
6. The method of directing the water exiting the impeller towards the discharge chamber:
Casing pump: After 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 set outside the impeller, and then proceeds to the next stage or flows into the outlet pipe.
⒎ According to the different media being transported, centrifugal pumps can be classified as: water pumps, oil pumps, corrosion-resistant pumps, etc.
Cavitation and vapor lock
Erosion phenomenon
From the working principle of the centrifugal pump, it can be known that after the liquid between the blades is ejected from the high-speed rotating impeller, a low-pressure area is formed near the impeller inlet. When the pressure at the impeller inlet is equal to or lower than the saturated vapor pressure pV of the transported liquid at the operating temperature, the liquid in this area will vaporize and form bubbles. When the bubbles travel with the liquid to the high-pressure area, they will rapidly condense due to the pressure.
At the moment of bubble condensation, a local vacuum is formed. The surrounding liquid rushes towards the space previously occupied by the bubble at a high speed, causing impact and vibration, resulting in a significant impact force. Especially when the condensation point of the bubble is near the surface of the blade, numerous liquid particles impact the blade at a high frequency and pressure; at the same time, the bubble may also contain a small amount of oxygen and other substances that have a chemical corrosive effect on metal materials. Under the combined action of continuous impact and chemical corrosion, the surface of the blade is damaged, forming spots and cracks, which will lead to the premature damage of the blade. This phenomenon is called cavitation in centrifugal pumps.
The phenomenon of gas binding
When a centrifugal pump is started, if there is air in the pump, due to the low density of air, the centrifugal force generated after rotation is small. As a result, the low pressure formed in the center area of the impeller is insufficient to suck in the liquid. Even if the centrifugal pump is started, it cannot complete the transportation task. This phenomenon is called "air lock".
This indicates that the centrifugal pump has no self-priming capability. Therefore, before starting the centrifugal pump, it must be filled with the liquid to be conveyed. Of course, if the suction inlet of the centrifugal pump is placed below the liquid level of the conveyed liquid, the liquid will automatically flow into the pump. This is a special case. The suction pipeline of the centrifugal pump is equipped with a bottom valve to prevent the liquid that was filled before starting from flowing out of the pump. The filter screen can prevent solid substances in the liquid from being sucked in and blocking the pipelines and the discharge pipe of the pump housing. The regulating valve installed in the discharge pipe is used for starting the pump, stopping the pump, and regulating the flow.
From the perspective of the different causes of cavitation and vapor lock:
Air binding refers to the presence of air inside the pump body. It usually occurs when the pump is started. The main manifestation is that the air inside the pump body has not been completely removed. While cavitation is caused by the liquid reaching its vaporization pressure at a certain temperature. It can be seen that it is closely related to the transported medium and the working conditions.
The following methods can be used to prevent the occurrence of the air lock phenomenon:
1. Before starting, fill the shell with liquid. Ensure a tight seal on the shell. The water filling valve and the shower head must not leak. The sealing performance should be good.
2. The suction pipeline of the centrifugal pump is equipped with a bottom valve to prevent the liquid that was pumped in before startup from flowing back into the pump. The filter screen can prevent solid particles in the liquid from being sucked in. The discharge pipeline is equipped with a regulating valve which is used for starting and stopping the pump and regulating the flow rate.
3. Place the suction inlet of the centrifugal pump below the liquid level where the liquid needs to be transported. The liquid will automatically flow into the pump.
The causes and solutions of cavitation occurrence
The main causes of cavitation are:
1. The resistance of the inlet pipeline is too high or the pipeline is too thin.
2. The temperature of the conveyed medium is too high;
3. Excessive flow, that is to say, the outlet valve is opened too wide;
4. The installation height is too high, which affects the liquid intake capacity of the pump.
5. Selection issues, including the selection of pumps and the selection of pump materials, etc.
Solution:
1. Remove foreign objects from the inlet pipeline to ensure smooth flow, or increase the diameter of the pipeline.
2. Reduce the temperature of the transported medium;
3. Reduce the installation height;
4. Replace the pump or make improvements to certain components of the pump, such as using materials resistant to cavitation.
