L Centrifugal pump

The concept of centrifugal force Centrifugal force is actually a manifestation of an object’s inertia. For example, with the water droplets on an umbrella, when the umbrella rotates slowly, the droplets rotate along with it; this is because the friction between the umbrella and the droplets acts as a centripetal force that keeps the droplets in motion. However, if the umbrella rotates faster, this frictional force is no longer sufficient to keep the droplets in circular motion, and they will fall off the umbrella and move toward its edges. It’s similar to using a rope to make a stone move in a circular path – if the speed is too high, the rope will break and the stone will fly away. This is what is known as centrifugal force. Centrifugal pumps are designed based on this principle. The rapidly rotating impeller blades drive the water to move, pushing it outward and thus enabling the transfer of water. There are various types of centrifugal pumps. Based on their application, they can be classified as household or industrial pumps; based on the type of fluid they transport, they can be categorized as clean water pumps, pumps for handling debris, corrosion-resistant pumps, etc.

Basic structure of centrifugal pumps The basic structure of a centrifugal pump consists of six components, namely: the impeller, the pump casing, the pump shaft, the bearings, the sealing ring, and the stuffing box. 　　1. The impeller is the core component of a centrifugal pump; it operates at high speeds and generates significant force. The blades on the impeller play a key role, and the impeller must undergo a static balance test before being assembled. The inner and outer surfaces of the impeller must be smooth to reduce frictional losses in the water flow. 　　2. The pump body, also known as the pump casing, is the main component of a water pump. It serves a supporting and fixing function, and is connected to the bracket that holds the bearing. 　　3. The function of the pump shaft is to connect to the motor via a coupling, thereby transmitting the torque of the motor to the impeller; it is therefore the main component for transferring mechanical energy. 　　4. Bearings are components that are fitted over the pump shaft to support it; there are two types: rolling bearings and sliding bearings. Rolling bearings use butter as a lubricant, and the amount of lubricant applied should be appropriate – generally 2/3 to 3/4 of the bearing’s volume. Too much lubricant will cause overheating, while too little will result in noise and overheating as well! Sliding bearing centrifugal pumps use transparent oil as a lubricant, which is filled to the oil level line. Too much oil seeps along the pump shaft and leaks away, while too little oil causes the bearings to overheat and get damaged, leading to accidents! During the operation of the water pump, the temperature of the bearings reaches up to 85 degrees; under normal conditions it is around 60 degrees. If this temperature rises, it is necessary to identify the cause (such as the presence of impurities, whether the oil has turned black, or if water has entered the system) and take action promptly! 　　5. The sealing ring is also known as a leak-reduction ring. An excessive gap between the impeller inlet and the pump casing allows water from the high-pressure area inside the pump to flow through this gap to the low-pressure area, which affects the pump’s flow rate and reduces its efficiency! Too small a clearance can cause friction between the impeller and the pump casing, leading to wear. To increase backflow resistance, reduce internal leakage, and extend the service life of the impeller and pump casing, a sealing ring is installed at the junction between the inner edge of the pump casing and the outer periphery of the impeller; it is advisable to maintain the sealing gap between 0.25 and 1.10 mm. 　　6. The packing box is mainly composed of packing, a water seal ring, a packing barrel, a packing gland, and a water seal tube. The main function of the stuffing box is to seal the gap between the pump casing and the pump shaft, preventing the water inside the pump from leaking out and preventing outside air from entering the pump. Maintain a vacuum inside the water pump at all times! When heat is generated by the friction between the pump shaft and the packing, a water seal is used to direct water into the seal ring in order to cool the packing! Maintain the normal operation of the water pump. Therefore, special attention must be paid to the inspection of the stuffing box during the routine inspections of the water pump’s operation! The packing needs to be replaced after about 600 hours of operation.

Working principle of centrifugal pump: The impeller is installed inside the pump casing 2 and secured to the pump shaft 3, which is driven directly by the motor. In the center of the pump casing, there is a liquid suction port 4 that is connected to the suction pipe 5. The liquid enters the pump through the bottom valve 6 and the suction pipe. The liquid outlet 8 on the pump casing is connected to the discharge pipe 9. Before starting the centrifugal pump, the pump casing is filled with the liquid to be transported ； Once it is started, the impeller rotates at high speed driven by the shaft, and the liquid between the blades must also rotate along with it. Under the effect of centrifugal force, the liquid is thrown from the center of the impeller toward the outer edge, gaining energy as it exits the outer edge of the impeller at high speed and enters the volute pump casing. In the volute, the liquid slows down as the flow channel widens, converting some of its kinetic energy into static pressure energy; it then flows into the discharge pipe at a higher pressure and is sent to the desired location. As the liquid flows from the center of the impeller toward the outer edge, a vacuum is created at the center of the impeller. Since the pressure above the liquid level in the tank is greater than the pressure at the pump inlet, the liquid is continuously forced into the impeller. It can be seen that as long as the impeller keeps rotating, the liquid will continue to be drawn in and discharged.

2 Air entrapment phenomenon When there is air inside the pump casing, the lower density of air compared to that of the liquid results in a smaller centrifugal force. As a result, the pressure difference between the surface of the liquid in the tank and the pump’s suction inlet is not sufficient to force the liquid in the tank into the pump; in other words, the centrifugal pump does not have self-priming capability, which prevents it from pumping liquid. This phenomenon is known as the \"suction lock\" phenomenon. To fill the pump with liquid, a bottom valve equipped with a filter is usually installed at the bottom of the suction pipe. This bottom valve serves as a check valve, and the filter prevents solid substances from entering the pump and damaging the impeller or interfering with its proper operation.

The main components of a centrifugal pump are the impeller, the pump casing, and the shaft sealing device. 　　 1 Impeller The function of the impeller is to transfer the mechanical energy of the prime mover directly to the liquid, thereby increasing the liquid’s static pressure energy and kinetic energy, with an emphasis on increasing the static pressure energy. An impeller generally has 6 to 12 backward-curving blades. 　　Impellers come in three types: open, semi-closed, and closed, as shown in Figure 2-2. Open impellers have no cover plates on either side of the blades; they are simple to manufacture and easy to clean. They are suitable for transporting materials containing a large amount of suspended solids, but their efficiency is low and they cannot handle liquids at high pressures ； The semi-closed impeller has no cover on the suction side but one on the other side; it is suitable for transporting materials that tend to settle or contain particles, and its efficiency is relatively low ； A closed impeller has front and rear cover plates on both sides of the blades; it offers high efficiency and is suitable for transporting clean liquids free from impurities. Most centrifugal pump impellers are of this type. Impellers come in two types of suction methods: single-suction and double-suction. 　　One with an inlet is a single-suction type, while those that can take in water from both sides are double-suction types. 2 Pump casing Its function is to enclose the impeller within a certain space, so that the impeller can draw in and push out liquid. The pump casing is often designed in a volute shape, which is why it is also called a volute. As the cross-sectional area of the flow channel gradually increases, the high-speed fluid ejected from around the impeller sees its velocity decrease, allowing part of its kinetic energy to be effectively converted into static pressure energy. The pump casing not only collects the liquid thrown out by the impeller but also serves as an energy conversion device. 3 Axis seal device Its function is to prevent the liquid inside the pump casing from leaking out along the shaft, or to prevent outside air from entering the pump casing. The common shaft sealing devices are packing seal and mechanical seal. 　　The filler is generally oil-impregnated or graphite-coated asbestos rope. Mechanical seals achieve sealing primarily through the relative motion of the rotating ring mounted on the shaft and the stationary ring fixed to the pump casing at their end faces.

Types of centrifugal pumps I. Classification by the number of working impellers 1. Single-stage pump: That is, there is only one impeller on the pump shaft. 　　2. Multi-stage pumps: These are pumps that have two or more impellers on their shaft; the total head produced by such a pump is the sum of the heads generated by each of the impellers. 　　II. Classification by working pressure 　　1. Low-pressure pumps: Pressure below 100 meters of water column ； 　　2. Medium-pressure pump: Pressure ranging from 100 to 650 meters of water column ； 　　3. High-pressure pump: Pressure higher than 650 meters of water column. 　　III. Classification by the way water enters the impeller 1. Pumps with water inlet on one side: Also known as single-suction pumps, meaning there is only one water inlet on the impeller ； 　　2. Double-sided inlet pump: Also known as a double-suction pump, it features an inlet on each side of the impeller. Its flow rate is twice that of a single-suction pump; it can be considered as two single-suction pump impellers placed back to back. 　　IV. Classification by the type of joint in the pump casing 1. Horizontal split-case pumps: Those with a joint located in the horizontal plane that passes through the axis of the pump. 　　2. Vertical joint surface pump: that is, the joint surface is perpendicular to the axis line. 　　V. Classification by pump shaft position 1. Horizontal pumps: The pump shaft is in a horizontal position. 　　2. Vertical pump: The pump shaft is in a vertical position. 　　VI. Classification by the way in which the water exiting the impeller is directed to the discharge chamber 1. Volute pump: After exiting the impeller, the water enters the pump casing, which has a spiral shape. 　　2. Vane pump: After exiting the impeller, the water enters the vanes located outside it, and then proceeds to the next stage or flows into the inlet/outlet pipe. 　　When we say that a certain water pump is a multi-stage pump, it refers to the number of impellers it has. Based on other structural features, it could also be a horizontal pump, a pump with vertical joint surfaces, an impeller-type pump, a high-pressure pump, or a pump with water intake on one side. Therefore, the names vary depending on the criteria. Additionally, they can also be classified according to their purpose, such as oil pumps, water pumps, condensate pumps, ash discharge pumps, circulation water pumps, etc.
**Classification methods and characteristics of centrifugal pumps:**
- **By suction method:**
Single-suction pump: Liquid flows into the impeller from one side, resulting in an axial force.
Double-suction pump: Liquid flows into the impeller from both sides, eliminating axial force; the flow rate is almost twice that of a single-suction pump.
- **By number of stages:**
Single-stage pump: There is only one impeller on the pump shaft.
Multi-stage pump: Two or more impellers are mounted on the same pump shaft; liquid flows through each stage sequentially. The more stages, the higher the head.
- **By orientation of the pump shaft:**
Horizontal pump: The pump shaft is placed horizontally.
Vertical pump: The pump shaft is perpendicular to the horizontal plane.
- **By casing type:**
Split-case pump: The casing is divided along a plane perpendicular to the shaft, with sections connected by long bolts.
Half-open case pump: The casing is split along a plane passing through the axis of rotation.
Volute pump: A centrifugal pump equipped with a spiral-shaped water chamber, such as the commonly used end-suction cantilever centrifugal pump.
Turbomolecular pump: A centrifugal pump with guide vane-type water chambers.
**Special structures:**
- Pipeline pump: Functions as part of the pipeline, so no changes to the pipeline are required during installation.
- Submersible pump: The pump and motor are integrated and submerged in water.
- Underwater pump: The pump body is submerged in liquid.
- Shielded pump: The impeller and motor rotor are combined within the same sealed enclosure; no sealing mechanism is needed, making it a leak-free pump.
- Magnetic drive pump: The entire pump body is enclosed except for the inlet and outlet; the pump and motor are connected through magnetic attraction.
- Self-priming pump: No priming is required when starting the pump.
- High-speed pump: The speed of the pump shaft is increased using a gearbox; speeds can exceed 10,000 rpm. It can also be called a partial-flow pump or tangential acceleration pump.
- Vertical cylindrical pump: The inlet and outlet connections are at the same height at the top. It has inner and outer casings: the inner casing consists of the rotor and guide vanes, while the outer casing serves as an inlet guide channel; liquid is drawn in from below.
There are many types of centrifugal pumps, and common classification methods include the following:
**Southern Pump 1: Classified by impeller suction method:** Single-suction centrifugal pump, Double-suction centrifugal pump ； 　　2. Classified by the number of impellers: single-stage centrifugal pumps, multi-stage centrifugal pumps ； 　　3. Classified by impeller structure: open impeller centrifugal pump, semi-open impeller centrifugal pump, closed impeller centrifugal pump ； 　　4. Classified by operating pressure: low-pressure centrifugal pumps, medium-pressure centrifugal pumps, high-pressure centrifugal pumps ； 　　5. Classified by pump shaft position: horizontal centrifugal pumps and vertical centrifugal pumps. 　　ISG domestic water supply pumps, pumps for domestic use, community water pumps, and domestic water supply and drainage equipment. These products are designed by combining the performance parameters of IS and IR type centrifugal pumps with the unique structure of vertical pumps, and they are manufactured in strict accordance with ISO2858 standards. Designed using high-quality domestic hydraulic models, they represent the ideal new generation of horizontal pump products. This product always uses a cemented carbide mechanical seal. Application range: The ISW type pump is suitable for industrial and municipal water supply and drainage applications, such as pressurizing water supply in high-rise buildings, garden irrigation, fire protection systems, long-distance water transport, HVAC systems, as well as for pressurization in bathrooms and other related equipment. Its operating temperature should not exceed 85°C. The ISWR type of pump is widely used for pressurizing and transporting boiler feed water in industries such as metallurgy, chemicals, textiles, and papermaking, as well as in restaurants, as well as in urban heating systems. The SGWR type can be used at temperatures up to 120°C. 　　Key installation techniques for pipeline centrifugal pumps: Selection of the installation height of the centrifugal pump, i.e., the suction head. I. Key installation techniques for centrifugal pumps The key to installing pipeline centrifugal pumps lies in determining their installation height, that is, the suction head. This height refers to the vertical distance from the water surface to the centerline of the centrifugal pump impeller. It should not be confused with the allowable suction vacuum level. The allowable suction vacuum level indicated in the pump’s manual or nameplate refers to the vacuum value at the pump’s inlet section, and it is determined through tests conducted under 1 standard atmosphere and a water temperature of 20 degrees Celsius. It does not take into account the water flow conditions after the installation of the water absorption pipes. The installation height of the water pump should be the value remaining after deducting the head loss in the water intake pipeline from the allowable suction vacuum height; this height is necessary to overcome the actual topographical constraints related to the water intake height. The installation height of the water pump must not exceed the calculated value; otherwise, the centrifugal pump will not be able to draw water. Furthermore, the resistance loss head of the water intake pipeline affects the magnitude of the calculated value; therefore, it is advisable to use the shortest possible piping layout and minimize the use of fittings such as elbows. It is also possible to consider using pipes with a larger diameter in order to reduce the flow velocity inside the pipes. 　　It should be noted that when the elevation and water temperature at the installation site of the pipeline centrifugal pump differ from those in the test conditions, such as when the local altitude is above 300 meters or the temperature of the water to be pumped exceeds 20 degrees Celsius, the calculated values need to be adjusted. That is, the atmospheric pressure at different altitudes and the saturated vapor pressure at water temperatures above 20 degrees Celsius. However, when the water temperature is below 20 degrees Celsius, the saturated vapor pressure can be ignored. 　　From the perspective of pipeline installation techniques, water intake pipelines require strict sealing to prevent air or water leakage; otherwise, it will disrupt the vacuum level at the inlet of the centrifugal pump, resulting in a reduced flow rate of water from the pump, and in severe cases, the pump may even fail to draw in water. Therefore, it is necessary to carry out pipeline joint work carefully to ensure the construction quality of pipeline connections. 　　II. Calculation of the installation height Hg for centrifugal pumps The allowable suction vacuum height Hs refers to the maximum degree of vacuum that can be achieved at the pump inlet pressure p1. 　　The actual allowable suction vacuum height Hs value is not the one calculated using the formula, but rather a value determined through experiments by the pump manufacturer; this value is included in the pump manual for users’ reference. It should be noted that the Hs value given for the pump samples applies when clean water is used as the working medium, under operating conditions of 20°C and a pressure of 1.013×105 Pa; conversions are required when the operating conditions or the working medium differ. 　　1. For transporting clean water, but when the operating conditions differ from those in the experiment, conversion can be carried out using the following formula: Hs1 = Hs + Ha – 10.33 – Hυ – 0.24. 2. When transporting other liquids, and when both the properties of the liquid being transported and the operating conditions differ from those in the experiment, two steps of conversion are required: the first step involves using the formula above to determine Hs1 based on the values found in the pump specifications ； In the second step, Hs1 is converted to H΄s using the following formula. 2. Cavitation head Δh: For oil pumps, the cavitation head Δh is used to determine the installation height; it represents the vacuum level that the pump can tolerate when drawing in liquid, and thus it indicates the maximum allowable installation height of the pump, with the unit being meters. The net positive suction head Δh is obtained from the oil pump data sheet, and its value is also determined using water at 20°C. If other liquids are to be transported, corrections are also required; consult relevant books for details. 　　Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters). The standard atmospheric pressure can create a vacuum in the pipeline up to a height of 10.33 meters. 　　For example: If a pump requires a net positive suction head of 4.0 meters, what is the suction lift Δh? 　　Solution: Δh = 10.33 – 4.0 – 0.5 = 5.83 meters. For safety reasons, the actual installation height of the pump should be less than the calculated value. When the calculated Hg value is negative, it indicates that the pump’s suction inlet should be located below the liquid level in the tank. 　　Example 2-3: For a certain centrifugal pump, the allowable suction vacuum height Hs as determined from the specifications is 5.7 m. It is known that the total resistance of the suction pipeline is 1.5 mH2O, the local atmospheric pressure is 9.81×104 Pa, and the dynamic head of the liquid in the suction pipeline can be neglected. Try to calculate: 1. Installation of a centrifugal pump for transporting water at 20°C ； 　　2 is changed to the installation height of the centrifugal pump when conveying water at 80°C. 　　Solution: 1. Installation height of the pump when transporting water at 20°C. Given: Hs = 5.7 m, Hf0-1 = 1.5 m, u12/2g ≈ 0. The local atmospheric pressure is 9.81×10^4 Pa, which is roughly consistent with the conditions under which the pump was tested at the factory; therefore, the installation height of the pump is Hg = 5.7 – 0 – 1.5 = 4.2 m. 　　2 Installation height of the pump when transporting water at 80°C When transporting water at 80°C, it is not possible to use the Hs value given in the pump specifications to calculate the installation height; instead, Hs must be adjusted using the following formula: Hs1 = Hs + Ha – 10.33 – Hυ – 0.24. It is known that Ha = 9.81×10^4 Pa ≈ 10 mH2O. The saturated vapor pressure of water at 80°C is 47.4 kPa, as stated in the appendix. 　　Hv = 47.4×10³ Pa = 4.83 mH2O 　　Hs1 = 5.7 + 10⁻¹⁰·³³ – 4.83 + 0.24 = 0.78 m 　　The installation height can be determined by substituting the value of Hs1 into the relevant formula: 　　Hg = Hs1 – Hf0 – 1 = 0.78 – 1.5 = –0.72 m 　　Since Hg is negative, it indicates that the pump should be installed below the water surface of the tank, at least 0.72 m below it. 　　Single-stage double-suction centrifugal pump 　　The single-stage double-suction centrifugal pump is a new type of efficient and energy-saving water pump; under the same energy consumption conditions, its operating efficiency can be nearly 20% higher than that of conventional pumps. 　　1. It has a compact structure, an attractive appearance, good stability, and is easy to install. 　　2. Smooth operation: The optimally designed double-suction impeller minimizes axial forces, features blade shapes with excellent hydraulic performance, and is manufactured through precision casting; as a result, the inner surface of the pump casing and the impeller surface are extremely smooth, providing significant cavitation resistance and high efficiency. 　　3. SKF and NSK bearings are used for the shaft bearings to ensure smooth operation, low noise, and a long service life. 　　4. For shaft sealing, BURGMANN mechanical seals or packing seals are used. It can ensure 8,000 hours of operation without leaks. 　　5. Installation method: No adjustment is required during assembly, and it can be adapted to the on-site usage conditions. Can be installed in a discrete or horizontal configuration. 　　6. By installing a self-priming device, automatic water suction can be achieved; thus, there is no need for a bottom valve, a vacuum pump, or backflow prevention, and the pump can still start operating. 　　Methods to extend the service life of centrifugal pumps 1. Selection and installation of centrifugal pumps Centrifugal pumps should be selected based on the liquid to be pumped, and their required performance characteristics must be evaluated; factors such as the conditions for suction and discharge, as well as whether the pump will operate intermittently or continuously, also need to be taken into account. Centrifugal pumps should generally operate at or near the pressure and flow conditions specified by the manufacturer. The following checks should be carried out when installing the pump: ① The dimensions, position, and elevation of the foundation must meet the design requirements; the foot bolts must be properly and correctly fixed in the concrete foundation, and the machine should not have any missing parts, damage, or rust ； 　　②Depending on the properties of the medium pumped by the pump, it is necessary to verify the materials of the main components, shaft seals, and gaskets as required ； 　　③The leveling and alignment of the pump shall comply with the provisions in the equipment’s technical documents; in the absence of such provisions, it shall conform to the requirements of the current **standard \"General Specifications for the Construction and Acceptance of Mechanical Equipment Installation Projects\" ； 　　④All pipes connected to the pump body, the installation of pipe fittings, and the cleaning of lubrication oil pipelines must comply with the provisions of relevant **standards. 　　2. Use of centrifugal pumps The trial operation of the pump shall meet the following requirements: ① The rotation direction of the drive mechanism shall be the same as that of the pump ； 　　②Determine the rotation direction of the pipeline pump and the co-axial pump ； 　　③All fixed connections should be secure, and the type and quantity of lubricant applied to all lubrication points must comply with the specifications stated in the equipment’s technical documents ； 　　④Areas that require pre-lubrication should be pre-lubricated as specified ； 　　⑤All indicating instruments and safety protection devices must be sensitive, accurate, and reliable ； 　　⑥The turntable should operate smoothly, without any abnormal phenomena ； 　　⑦Before trial operation, the pump body of a high-temperature pump should be preheated, with the temperature rising evenly; the temperature increase per hour should not exceed 50℃ ； The temperature difference between the surface of the pump body and the process pipeline with the working medium inlet should not exceed 40℃ ； 　　⑧Install connection devices to eliminate the effect of temperature rise, and set up bypass connection devices to provide a cooling water source. 　　The following points should be noted when operating a centrifugal pump: ① Operation without water is prohibited; do not adjust the suction inlet to reduce the flow rate, and it is forbidden to operate at excessively low flow rates ； 　　②Monitor the operation process to completely prevent leaks in the packing box; use new packing when replacing it ； 　　③Ensure that the mechanical seal has an adequate flow of water for flushing; excessive water flow is not allowed for water-cooled bearings ； 　　④Do not use too much lubricant ； 　　⑤Conduct inspections at the recommended intervals. Maintain operation records, including hours of operation, adjustments and replacements of fillers, addition of lubricants, as well as other maintenance actions and the times at which they were carried out. The suction and discharge pressures, flow rate, input power, as well as the temperatures of the fluid and bearings of the centrifugal pump, along with its vibration levels, should all be measured and recorded regularly. 　　⑥The main unit of a centrifugal pump relies on atmospheric pressure to pump water from lower levels to higher ones, and atmospheric pressure can support at most a water column of about 10.3 meters; therefore, the main unit of a centrifugal pump cannot operate when it is 12 meters above the water surface. 　　3. Maintenance of centrifugal pumps 　　3.1. Analysis of mechanical seal failure in centrifugal pumps 　　The shutdown of centrifugal pumps is mainly caused by the failure of their mechanical seals. The most common symptom of failure is leakage, and the reasons for leakage include the following: ① Leakage at the sealing surfaces of the stationary and rotating rings; the main causes are that the flatness and roughness of these surfaces do not meet the required standards, or there are scratches on the surfaces ； There are particulate substances between the end faces, preventing the two end faces from operating in a consistent manner ； It was not installed properly; the method used was incorrect. 　　②The leakage of the compensation ring seal is mainly caused by: gland deformation and uneven preload ； Incorrect installation ； The quality of the sealing ring does not meet the standards ； The wrong sealing ring was selected. 　　Actual usage experience shows that the areas where sealing elements fail most frequently are the end faces of the rotating and stationary rings. Cracking in the end faces of these rings is a common failure phenomenon in centrifugal pump seals. The main reasons for this include: ① An excessive gap between the sealing surfaces during installation, which prevents the flushing fluid from removing the heat generated by the friction between the components ； The flushing fluid leaks through the gaps in the sealing surfaces, causing overheating of the end faces and resulting in damage. 　　②The vaporization and expansion of the liquid medium cause the two end surfaces to separate due to the forces resulting from this expansion. When the two sealing surfaces are pressed together tightly, the lubricating film is destroyed, leading to overheating of the end surface. 　　③The liquid medium has poor lubricity, and coupled with excessive operating pressure, the two sealing surfaces do not rotate synchronously. For example, in a high-speed pump with a speed of 20,445 rpm and a center diameter of the sealing surface of 7 cm, the linear velocity of this surface can reach as high as 75 m/s once the pump is in operation. If one of the sealing surfaces lags behind and fails to keep up with the rotation, the resulting instantaneous high temperature can cause damage to that sealing surface. 　　④The orifice plate or filter screen of the sealing flush fluid is clogged, resulting in insufficient water flow and thus causing the mechanical seal to fail. 　　Furthermore, surface grooves on the sealing surface, as well as gaps that occur when the end faces come into contact with each other, can lead to the failure of the sealing element. The main reasons for this are: ① The liquid medium is not clean; it contains small, hard particles that move at high speeds across the sealing surface, causing scratches on its surface and thus leading to failure. 　　②The coaxiality of the pump’s driving components is poor; once the pump starts operating, the end face comes into contact and rubs against something once for each rotation. The path taken by the moving ring is not concentric, which leads to vaporization of the end face and excessive wear. 　　③Frequent changes in the hydraulic properties of the liquid medium cause vibration in the pump set, leading to misalignment of the sealing surfaces and their failure. 　　Corrosion of the sealing elements by liquid media, stress concentration, the combination of soft and hard materials, erosion, auxiliary sealing O-rings, V-rings, and concave rings that are incompatible with the liquid medium, as well as deformation, can all cause damage and failure to the surface of mechanical seals. Therefore, it is necessary to conduct a comprehensive analysis of the forms of damage in order to identify the root causes and ensure the long-term proper operation of mechanical seals. 　　3.2 Requirements after the centrifugal pump stops operating ① After the centrifugal pump stops, the inlet valve of the pump should be closed; once the pump has cooled down, the valves of the auxiliary systems should be closed one by one. 　　②The shutdown of high-temperature pumps shall be carried out in accordance with the provisions of the equipment’s technical documents. After shutdown, the pump shaft should be turned half a turn every 20 to 30 minutes until the temperature of the pump body drops to 50°C. 　　③When a cryopump is shut down, unless there are special requirements, the pump should always be filled with liquid ； The intake valve and exhaust valve should remain in an open state ； In cryopumps equipped with double-end face mechanical seals, the liquid level controller and the sealing fluid in the pump’s seal chamber must maintain the pump’s filling pressure. 　　④Pumps used to transport media that are prone to crystallization, solidification, or precipitation should be prevented from clogging after shutdown, and the pump and pipelines should be promptly rinsed with clean water or other appropriate media. ⑤Drain the liquid accumulated in the pump to prevent rust and freezing damage. 　　3.3 Storage of Centrifugal Pumps 　　① For pumps that have not yet been installed, a suitable rust inhibitor should be applied to their unpainted surfaces. Bearings that are lubricated with oil should be filled with the appropriate oil, while bearings lubricated with grease should be filled with only one type of grease; mixed greases should not be used. 　　②Pump in clean liquid for a short period to flush the suction line, discharge line, pump casing, and impeller, then drain the flushing liquid from the pump casing, suction line, and discharge line. 　　③Drain the oil from the bearing housing, refill it with clean oil, thoroughly clean the grease, and then fill it with new grease. 　　④Seal the intake and exhaust ports, store the pump in a clean, dry place to protect the motor windings from moisture, and spray the inside of the pump casing with rust preventive and anti-corrosion fluids. 　　⑤The pump shaft is rotated once a month to prevent freezing and to lubricate the bearings. 　　Working principle of centrifugal pumps The main flow-passing components of a centrifugal pump are the suction chamber, impeller, and discharge chamber. The water intake chamber is located in front of the inlet of the impeller, and its function is to direct the liquid toward the impeller ； Pressurized water chambers mainly come in three forms: spiral pressurized water chambers (volute type), guide vanes, and spatial guide vanes ； The impeller is the most important working element of a pump; it is the heart of the flow-through component, and consists of shrouds and blades in between. 　　Before a centrifugal pump starts operating, the pump must first be filled with liquid. Once the pump is started, the impeller rotates rapidly; the blades of the impeller propel the liquid to move. As the liquid moves, it flows toward the outer edge of the impeller due to inertia. At the same time, the impeller draws in liquid from the suction chamber. During this process, the liquid circulates around the blades, and as it does so, it exerts an upward force on the blades. In turn, the blades exert a force on the liquid that is equal in magnitude but opposite in direction to this upward force. This force does work on the liquid, giving it energy so that it can flow out of the impeller. As a result, both the kinetic energy and pressure energy of the liquid increase. 　　A centrifugal pump transfers the mechanical energy of the prime mover to the liquid by means of the action of the rotating impeller on the liquid. Due to the action of the centrifugal pump, as the liquid flows from the inlet to the outlet of the impeller, both its kinetic energy and pressure energy increase. The liquid discharged by the impeller passes through the discharge chamber, where most of its kinetic energy is converted into pressure energy; this pressurized liquid is then sent along the discharge pipeline. At the same time, a vacuum or low pressure is created at the impeller’s inlet as a result of the liquid being discharged. Under the effect of the surface pressure (atmospheric pressure) in the liquid reservoir, the liquid is forced into the impeller’s inlet. Thus, the rotating impeller continuously draws in and discharges liquid.

Analysis of the main reasons why centrifugal pumps fail to draw water. Centrifugal water pumps are the most widely used type in agriculture due to their simple structure, ease of use and maintenance, and high efficiency; however, they can also cause significant problems when they fail to draw water. Now, an analysis will be conducted on the intentional cause of the failure to supply water. 　　There is air in the water inlet pipe and inside the pump. (1) Some users fail to fill the pump with enough water before starting it ； It appears that the water has spilled out through the vent holes, but the pump shaft was not rotated to completely expel the air, resulting in some air remaining in the inlet pipe or within the pump body. 　　(2) On the horizontal section of the inlet pipe in contact with the water pump, a slope of 0.5% or more in the opposite direction to the flow of water should be applied; the end connected to the water pump inlet should be at the highest point, and it should not be completely horizontal. If it curves upwards, air will remain in the water inlet pipe, reducing the vacuum level in the pipe and pump and affecting water suction. 　　(3) The pump packing has worn out due to long-term use, or it is not compressed tightly enough, which causes a large amount of water to spray out from the gaps between the packing and the pump shaft sleeve. As a result, external air enters the inside of the pump through these gaps, affecting its ability to lift water. 　　(4) Due to being submerged for a long time, the pipe walls of the water inlet pipe corroded and developed holes; as the water level dropped continuously after the pump started operating, air entered the water inlet pipe through these holes once they came above the water surface. 　　(5) Cracks at the bend of the water inlet pipe, as well as small gaps at the connection between the water inlet pipe and the water pump, can both allow air to enter the water inlet pipe. 　　The water pump speed is too low. (1) Human factors. A considerable number of users, due to the damage of the original motor, simply install another motor to drive the pump, which results in low flow rates and low head pressure, preventing the pump from pumping water. 　　(2) Wear of the drive belt. Many large-scale water pump extractors use belt drives; over time, the belts wear out and become loose, resulting in slippage and a decrease in the pump’s speed. 　　(3) Improper installation. A too small center distance between the two pulleys, or axes that are not parallel enough, result in the tight side of the belt being installed on top, which leads to a too small wrap angle. Errors in calculating the diameters of the pulleys, as well as a large eccentricity between the axes of the coupling-driven water pump, can all cause changes in the pump’s rotation speed. 　　(4) Mechanical failure of the water pump itself. Loose fastening nuts on the impeller and pump shaft, or deformation and bending of the pump shaft, can cause the impeller to shift significantly and come into direct contact with the pump body; in addition, damaged bearings can also lead to a decrease in the pump’s rotation speed. 　　(5) Repair of power engines is not recorded. The motor loses its magnetism due to burned-out windings, and changes in the number of windings turns, wire diameter, or wiring method during repairs, as well as factors resulting from incomplete elimination of faults during repairs, can also cause a change in the pump’s speed. 　　The suction lift is too high. In some cases, the water source is deep, or the area surrounding the water source is relatively flat; as a result, the allowable suction lift of the pump is not taken into account, leading to insufficient water being drawn up or no water being drawn at all. It should be understood that there is a limit to the degree of vacuum that can be created at the water intake of a pump; at absolute vacuum, the suction height is approximately 10 meters of water column, and it is impossible for a pump to create an absolute vacuum. Excessive vacuum can easily cause the water inside the pump to vaporize, which is detrimental to the operation of the water pump. Each centrifugal pump has its maximum allowable suction lift, which is generally between 3 and 8.5 meters; when installing a water pump, one must not prioritize convenience and simplicity. 　　Excessive resistance loss when water flows in and out of the pipes. Some users have found that the vertical distance from the reservoir or water tower to the water surface in the source is actually slightly less than the pumping head of the water pump, yet the amount of water pumped is low or no water is pumped at all. The reason is often that the pipeline is too long, there are many bends in the water pipes, resulting in excessive resistance loss in the water flow path. The reason is often that the pipeline is too long, there are many bends in the water pipes, resulting in excessive resistance loss in the water flow path. Under normal circumstances, a 90-degree elbow creates more resistance than a 120-degree elbow; each 90-degree elbow results in a head loss of about 0.5 to 1 meter, while resistance over every 20 meters of pipe causes a head loss of approximately 1 meter. Furthermore, some users arbitrarily choose the diameters of the water pump inlet and outlet pipes, which also has a certain impact on the head pressure. 　　Other influencing factors: (1) The bottom valve cannot be opened. Usually, it is because the water pump has been left unused for too long, causing the gasket of the bottom valve to stick; without a gasket, the bottom valve may rust. 　　(2) The bottom valve filter screen is clogged ； Or the bottom valve can cause clogging of the filter screen in the sludge layer underwater. 　　(3) The impeller is severely worn. The impeller blades wear out over time, affecting the performance of the water pump. 　　(4) Faults or blockages in the gate valve or check valve can cause a reduction in flow rate, preventing water from being pumped up. 　　(5) Leaks in the outlet pipeline can also affect the water lifting capacity. 　　The flow-through components of a centrifugal pump include three parts: the suction chamber, the impeller, and the discharge chamber. The impeller chamber is the core of the pump, as well as the core of the flow components. The pump increases the energy of the liquid by doing work on it through the impeller. Impellers are classified into three types according to the direction in which the liquid flows out: (1) Radial impellers (centrifugal impellers): The liquid flows out of the impeller in a direction perpendicular to the axis. 　　(2) In an inclined-flow impeller (mixed-flow impeller), the liquid flows out of the impeller in a direction inclined to the axis. 　　(3) In an axial-flow impeller, the direction of fluid flow is parallel to the axis. 　　Impellers are classified into two types based on the way they draw in fluid: (1) Single-suction impellers (i.e., impellers that draw in fluid from one side). 　　(2) Double-suction impeller (i.e., the impeller draws in liquid from both sides). 　　Impellers are classified into three types according to the cover design: (1) Enclosed impellers. 　　(2) Open impeller. 　　(3) Semi-open impeller. 　　Among them, closed impellers are widely used; the single-suction and double-suction impellers mentioned earlier both belong to this type. 　　Cavitation in Centrifugal Pumps and Its Solutions
Phenomena:
1. In sewage pumps, operating at around 80 degrees Celsius, the pump pressure rises to normal levels after the pump is started, and the pressure at the pump outlet is also normal. However, after about 10 minutes, the pump pressure drops rapidly, accompanied by noise and vibration; cavitation occurs. Upon inspection, it was found that the valves at the sewage treatment station were closed.
2. In material pumps used for transporting organic substances that vaporize easily at around 90 degrees Celsius, the pump pressure rises to normal levels after startup. Due to a low flow rate, the opening of the pump outlet is small, so the pressure remains normal. After about 30 minutes, however, the pump pressure drops again, along with noise and vibration; cavitation occurs.
After observing these phenomena, we identified two possible causes: first, the opening of the outlet valves was not sufficient; second, the temperature of the material entering and leaving the pump increased significantly compared to before. It was determined that cavitation occurred in these cases due to insufficient or closed outlet valve openings. When the outlet valve is closed or not fully open, the energy obtained by the material from the pump cannot be discharged promptly. As a result, the kinetic energy of the material is converted into heat, raising its temperature. When this temperature reaches a certain level, cavitation occurs within the pump. 　　Once the cause is identified, it’s easy to resolve the issue. By adding a return line at the pump outlet and opening the return valve appropriately after starting the pump, cavitation no longer occurs. 1·Check whether there is any looseness in the piping and connections of the centrifugal pump. Turn the centrifugal pump by hand to check whether it moves smoothly. 　　2. Add bearing lubricating oil to the bearing housing; ensure that the oil level is at the center line of the oil gauge. The lubricating oil should be replaced or topped up promptly. 　　3·Unscrew the priming plug on the centrifugal pump casing and fill it with priming water (or slurry). 　　4·Close the gate valve on the outlet pipeline, as well as the outlet pressure gauge and the inlet vacuum gauge. 　　5. Turn on the motor manually to check whether its rotation direction is correct. 　　6. Start the motor; once the centrifugal pump is operating properly, open the outlet pressure gauge and the inlet vacuum gauge to ensure that appropriate pressures are displayed, then gradually open the gate valve, while simultaneously checking the motor load. 　　7. Try to keep the flow rate and head of the centrifugal pump within the ranges specified on its label, so that it operates at its highest efficiency point and maximum energy savings can be achieved. 　　8· During operation, the bearing temperature of a centrifugal pump must not exceed the ambient temperature by 35°C, with the maximum temperature not exceeding 80°C. 　　9· If any abnormal noise is heard from the clean water pump, stop the machine immediately to check for the cause. 　　10·When stopping a centrifugal pump, first close the gate valve and pressure gauge, then stop the motor. 　　11· Within the first month of operation, the centrifugal pump’s lubricating oil should be changed after 100 hours; thereafter, it should be changed every 500 hours. 　　12·Regularly adjust the packing gland to ensure normal dripping in the packing chamber (dripping in drops is appropriate). 　　13·Regularly check the wear of the shaft sleeves; replace them promptly if the wear is significant. 　　14·When using a centrifugal pump in cold winter conditions, after shutting it down, it is necessary to loosen the drain plug at the bottom of the pump body to drain all the fluid from it. Prevent frost cracking. 　　15· When a centrifugal pump is not in use for an extended period, it is necessary to disassemble the pump completely, dry off any moisture, apply grease to the moving parts and joints, and then reassemble it properly. 1· Check the pipelines and joints of the centrifugal pump to ensure there is no looseness. Turn the centrifugal pump by hand to check whether it moves smoothly. 　　2. Add bearing lubricating oil to the bearing housing; ensure that the oil level is at the center line of the oil gauge. The lubricating oil should be replaced or topped up promptly. 　　3·Unscrew the priming plug on the centrifugal pump casing and fill it with priming water (or slurry). 　　4·Close the gate valve on the outlet pipeline, as well as the outlet pressure gauge and the inlet vacuum gauge. 　　5. Turn on the motor manually to check whether its rotation direction is correct. 　　6. Start the motor; once the centrifugal pump is operating properly, open the outlet pressure gauge and the inlet vacuum gauge to ensure that appropriate pressures are displayed, then gradually open the gate valve, while simultaneously checking the motor load. 　　7. Try to keep the flow rate and head of the centrifugal pump within the ranges specified on its label, so that it operates at its highest efficiency point and maximum energy savings can be achieved. 　　8· During operation, the bearing temperature of a centrifugal pump must not exceed the ambient temperature by 35°C, with the maximum temperature not exceeding 80°C. 　　9· If any abnormal noise is heard from the clean water pump, stop the machine immediately to check for the cause. 　　10·When stopping a centrifugal pump, first close the gate valve and pressure gauge, then stop the motor. 　　11· Within the first month of operation, the centrifugal pump’s lubricating oil should be changed after 100 hours; thereafter, it should be changed every 500 hours. 　　12·Regularly adjust the packing gland to ensure normal dripping in the packing chamber (dripping in drops is appropriate). 　　13·Regularly check the wear of the shaft sleeves; replace them promptly if the wear is significant. 　　14·When using a centrifugal pump in cold winter conditions, after shutting it down, it is necessary to loosen the drain plug at the bottom of the pump body to drain all the fluid from it. Prevent frost cracking. 　　15·When a centrifugal pump is not in use for an extended period, it must be completely disassembled, all moisture removed, the moving parts and joints lubricated with grease, and then stored properly.

Increasing the inlet pressure of the pump can improve its NPSH performance. Prevent the liquid inside the pump from reaching a pressure lower than its saturated vapor pressure and thus vaporizing, which could cause cavitation in the pump. As for why energy consumption can be reduced, it is unknown.

During operation of a centrifugal pump, the impeller is subject to an axial thrust directed toward the inlet. The pump must do work to overcome this axial force; increasing the inlet pressure creates a counterforce to this axial thrust, thereby reducing it. As a result, the amount of work the pump needs to do to overcome this thrust decreases, which in turn reduces energy consumption. (This is purely my personal opinion; I’m not sure if it’s reasonable or not.)

As the inlet pressure of the pump increases, the outlet pressure rises as well, and the pump’s flow rate automatically increases; this is what is known as the self-regulating property of centrifugal pumps. Obviously, an increased pumping volume leads to energy savings.