The generation of cavitation and impeller cutting in centrifugal pumps
Centrifugal pumps are prone to cavitation, which is related to many factors. What is the purpose of cutting the pump impeller and what role does it play?
1、 Cutting of Centrifugal Pump Impeller
1. Purpose of cutting:
A centrifugal pump has only one performance curve at a certain speed. To expand the working range of the pump, the method of cutting the outer diameter of the impeller is often used to change its working range from a line to a surface. When the cutting amount is small, it can be considered that the outlet angle and flow area of the blades before and after cutting remain basically unchanged, and the pump efficiency is approximately equal.
2. The expression for the cutting law:
Q '/Q=D2'/D2
H '/H=(D2'/D2) 2
N '/N=(D2'/D2) 3
In the formula, Q, H, N represent the rated flow rate, head, and shaft power of the pump
Corner marks' represent the corresponding parameters of the impeller after cutting
D2 represents the outer diameter of the impeller
2、 Cavitation and suction characteristics of centrifugal pumps
1. Cavitation phenomenon
According to the working principle of a centrifugal pump, the liquid flow enters the impeller under the pressure difference (Pa Pk) formed between the suction tank pressure Pa and the low pressure Pk at the impeller inlet. The lower the pressure Pk at the impeller inlet, the greater the suction capacity. But if Pk decreases to a certain limit value (currently, the saturation steam pressure Pt of the liquid at the conveying temperature is often used as the critical value for liquid vaporization pressure), cavitation phenomenon will occur.
2. Serious consequences caused by cavitation:
(1) Generate vibration and noise.
(2) It has an impact on the working performance of the pump: when cavitation develops to a certain extent, a large number of bubbles are generated, which will block the flow channel, causing a significant decrease in the pump's flow rate, head, efficiency, etc.
(3) The material of the flow channel will be damaged, mainly due to fatigue erosion of the metal near the blade inlet.
3. Suction characteristics of centrifugal pumps:
1) The basic condition for pump cavitation is that the low liquid flow pressure Pk at the blade inlet is ≤ the saturated vapor pressure Pt of the liquid at that temperature.
2) Effective cavitation margin: The excess energy head that is higher than the vaporization pressure after the liquid flows from the suction tank and reaches the pump suction port through the suction pipeline. use Δ Ha represents.
3) The necessary cavitation margin of the pump: the total energy loss of the liquid flow from the pump inlet to the low pressure point K in the impeller, calculated using Δ Hr represents.
4)· Δ HR and Δ Differences and connections between ha:
Δ Ha> Δ No cavitation of the hr pump
Δ Ha= Δ Hr pump begins to cavitate
Δ Ha< Δ Severe cavitation of the hr pump
5. For a pump, in order to ensure its safe operation without cavitation, a safety margin should be added to the necessary cavitation margin of the pump, usually 0.5 meters of liquid column. Therefore, the allowable NPSH of the pump is:[ Δ Hr]= Δ Hr+0.5.
The expression for the allowable geometric installation height of the pump is: [Hg1]=(Pa Pt)/r-hA-S-[ Δ Hr].
Pa - suction tank pressure
Pt - Saturated vapor pressure of liquid at delivery temperature
R - Liquid gravity
HA-S - Flow loss in the suction pipe
[ Δ Hr] - allowable cavitation margin
7. The methods to improve the anti cavitation performance of centrifugal pumps include:
A. Improve the pump structure and reduce Δ HR is a design issue with the pump.
B. The main commonly used method to increase the effective NPSH inside the device is to use a filling head suction device
In addition, efforts should be made to minimize the resistance loss of the suction pipeline and reduce the saturated vapor pressure of the liquid. When designing the suction pipeline, measures such as selecting larger pipe diameters, shorter lengths, fewer elbows and valves, and minimizing the temperature of the liquid being transported can all improve the effective cavitation margin of the device.
8. Balance device for axial force
① Causes of axial force generation
a. The axial force A1 caused by the different distribution of fluid pressure on the front and rear sides of the impeller (low pressure on the wheel cover side and high pressure on the wheel disc) is directed from the back side of the impeller towards the inlet of the impeller.
b. The dynamic reaction force A2 generated by the different directions and velocities of fluid entering and exiting the impeller is opposite to A1, so the total axial force A=A1-A2, and the direction is generally the same as A1 (generally A2 is smaller).
② Balance of axial force
a. Adopting a double suction impeller: the impeller is symmetrical on both sides, and the fluid is sucked in from both ends. The axial force is automatically counteracted to achieve balance.
b. Opening balance holes or installing balance pipes: A: Open several balance holes on the impeller disc relative to the suction port. B: To avoid increasing hydraulic loss due to disturbance of the main flow after opening the balance hole, a balance pipe can be installed instead of the balance hole, which uses a small pipe to introduce pressure to the back side of the wheel disc.
c: Using balanced blades: Several radial ribs are cast on the back of the impeller disc, which drive the fluid in the clearance on the back of the impeller to accelerate rotation, increase centrifugal force, and significantly reduce the pressure on the back of the impeller.
d: Use thrust bearings to withstand axial force. Generally, small single suction pumps can withstand all axial force and prevent pump shaft movement by stopping the thrust bearing.
③ Balance of axial force in multi-stage centrifugal pumps:
a. Same method as single stage centrifugal pump
b. Symmetrically arranged impeller
c. Using a balance drum to partially balance axial force
d. Adopting an automatic balancing disc, all axial forces are automatically balanced.
Precautions for inspection after assembly of fluorine lined centrifugal pump
When conducting maintenance and testing on fluorine lined centrifugal pumps after assembly, in addition to following normal maintenance procedures and standards, the following points should also be noted:
1. The concentricity of the pump rotor and volute. The degree of concentricity between the pump rotor and the volute is completely determined by the machining accuracy of the parts. It is required that the fluorine lined centrifugal pump rotor and the volute be concentric, in order to avoid friction between the dynamic and static components during operation;
2. Deviation between the centerline of the pump impeller flow passage and the centerline of the volute flow passage. The lower the deviation between the centerline of the impeller flow passage and the centerline of the pump casing flow passage, the higher the efficiency of the pump. The alignment deviation between the two flow channels should generally not exceed 0.5mm.
3. Is the tightening force of each bolt appropriate.
Fluorine lined centrifugal pump
4. Whether the internal clearances of the pump exceed the standard, and the data requirements of various pumps.
5. Whether the compression amount of the mechanical seal meets the requirements.
6. If necessary, conduct a hydraulic test on the casing of the high-pressure pump.
Can pipeline centrifugal pumps transport high-temperature hot water above 80 ℃?
Pipeline centrifugal pump is a type of centrifugal water pump suitable for industrial and urban water supply and drainage, pressurized water supply in high-rise buildings, garden sprinkler irrigation, fire pressurization, long-distance transportation, HVAC refrigeration circulation, bathroom and other cold and warm water circulation pressurization and equipment matching. The liquid that can be transported is also water below 80 ℃ or other liquids with physical and chemical properties similar to water.
Is it possible that pipeline centrifugal pumps cannot transport high-temperature liquids above 80 ℃?
No. Conventional pipeline centrifugal pumps cannot transport high-temperature liquids above 80 ℃. The improved IRG vertical hot water pipeline centrifugal pump and high-temperature pipeline centrifugal pump from Pump Manufacturing Co., Ltd. can achieve the transportation of high-temperature liquids. The IRG type hot water pipeline centrifugal pump produced by Pump Manufacturing Co., Ltd. can transport liquids with a temperature range of less than 120 degrees Celsius, while the high-temperature pipeline centrifugal pump can be used with a temperature range of less than 240 degrees Celsius.
If you are the procurement manager of an enterprise or public institution and want to purchase water pumps that deliver high-temperature hot water, you can contact our online customer service on the official website of Pump Manufacturing Co., Ltd. They will provide you with accurate pump selection and quotation services!
What factors can cause a decrease in the head of the sewage pump
(1) Motor reversal
Due to wiring reasons, the direction of rotation of the motor may be opposite to the actual requirement of the sewage pump. Therefore, when starting, it is generally necessary to observe the direction of rotation of the sewage pump. If the direction of rotation is reversed, any two wires on the motor terminal should be exchanged.
(2) Impeller channel blockage
If the flow channel of the impeller is partially blocked, it will affect the work of the impeller and cause a decrease in outlet pressure. Therefore, it is necessary to disassemble the pump for inspection and removal of foreign objects. To prevent the problem from recurring, a filtering device can be installed before the inlet of the sewage pump if necessary.
(3) Internal leakage occurs
When the gap between the rotating and stationary parts of the sewage pump exceeds the design range, it will cause internal leakage, which is reflected in a decrease in the discharge pressure of the pump, such as the clearance between the impeller mouth ring and the interstage gap of the multi-stage pump. At this point, corresponding disassembly and inspection should be carried out to repair or replace the components that cause excessive clearance.
(4) Cavitation occurs at the inlet
If the suction pressure of the sewage pump is too low, lower than the saturated vapor pressure of the pumped medium, cavitation will form. At this time, check whether the inlet Plumbing is blocked or whether the opening of the inlet valve is too small, or increase the liquid level of the suction pool.
(5) Speed decrease
The important factors affecting the head of sewage pump are the outer diameter of the impeller and the speed of the pump. When Ceteris paribus, the head of the pump is in direct proportion to the second power of the speed. It can be seen that the speed has a great impact on the head. Sometimes the speed of the pump decreases for some external reason, which will reduce the head of the pump accordingly. At this point, the pump speed should be checked. If the speed is indeed insufficient, the cause should be checked and a reasonable solution should be found.
(6) The operating point shifts towards high flow and low head
In general, centrifugal pumps have a continuous downward performance curve, and the flow rate gradually increases with the decrease of head. During the operation process, due to some reason, the back pressure of the pump decreases, and the working point of the pump passively shifts towards the low head and high flow point with the device curve, which will cause the head to decrease. In fact, this is caused by external factors such as changes in the device, and has no special relationship with the pump itself. At this point, simply increasing the pump back pressure, such as closing the outlet valve a little, can solve the problem.
What precautions should be taken when using a self priming magnetic pump
What situations should everyone pay attention to when using self priming magnetic pumps in order to better use them? Many people are curious about this issue. Today, centrifugal pump manufacturers have brought some precautions for the correct use of magnetic pumps. Friends who are not familiar with it should come and take a look.
1. When using, everyone must pay attention that the outlet pipeline of the pump cannot use a check valve. If a check valve must be used, a vent hole should be set at the lower end of the valve.
2. When using a self priming magnetic pump for conveying, the flow rate, head, and self priming height must be within the specified range. If there is a significant difference from the regulations, it is likely to cause the pump to operate improperly due to suction.
3. It is strictly prohibited to transport solid particles or crystalline media.
4. When restarting after a prolonged shutdown, it is necessary to check whether the liquid in the pump body is sufficient before self priming.
5. After the self priming magnetic pump is installed according to the requirements, the power can be switched on, and inching mode is used to check whether the pump rotates normally and flexibly.
6. Self priming magnetic pumps are strictly prohibited from experiencing vacuum suction, so it is necessary to have someone specially monitor the liquid when it is about to be delivered to prevent it from being sucked out.
Did you have some gains after listening to the editor's introduction? If you haven't noticed these issues in your daily life, quickly correct them now, so that you can better use the magnetic pump product and avoid unnecessary malfunctions.
Characteristics and classification of clean water pumps
Clear water pump is used for conveying clean water and other liquids with physical and chemical properties similar to clean water. It is suitable for industrial and urban water supply and drainage, pressurized water supply in high-rise buildings, garden sprinkler irrigation, fire pressurization, long-distance transportation, HVAC refrigeration cycle, farmland irrigation, bathroom and other cold and warm water circulation pressurization and equipment matching.
Characteristics of clean water pump:
Relatively good performance, with powerful motor device and reasonable body design, high efficiency and low noise. The location of the waterway is treated uniquely, which not only prevents corrosion and rust, but also has good wear resistance.
It has economic, practical, energy-saving design, and reasonable price.
Easy to set up and install, easy to use, with the use of new process design, promoting simple installation and convenient maintenance. It has a self priming function.
Common types of clean water pumps:
Self priming clean water pump: With good water resource quality and relatively fixed operating position, the suction head is within 3.5 meters, and the pipeline is stable. When used for daily water supply in wells and irrigation in plantations, it can reduce the inconvenience of water injection during pumping. The actual pump head is within 22 meters, and the working efficiency of chemical pumps is slightly weaker. When putting into operation for the first time, it is necessary to first inject circulating and emptying water before the water pump can start self priming.
Water injection type clean water pump: Water resources and water bodies have good mobility, and the pipeline is relatively not fixed. The actual pump lift within 2.5 meters is within 32 meters, and the pump flow can be adjusted according to demand. It is suitable for fruit planting professionals to spray pesticides and water supply, with high work efficiency. The suction pipe should ensure good sealing.
Submersible clean water pump: With abundant water resources, the pump has a low head and valves are more suitable for irrigation in fields and plantations that require a large amount of water. Due to the fact that the entire equipment of the submersible pump is placed in the water inlet and the axial water pushing increases, reducing the water flow rate will increase the pump load. Therefore, it is not necessary to install a valve in the drainage pipe of the submersible pump to avoid burning the motor due to excessive resistance when starting the pump due to valve shutdown.
Can a deep analysis of the magnetic pump idle or not?
With the increasing environmental protection situation, more and more chemical enterprises are required to use magnetic pumps to transport various corrosive materials. As is well known, magnetic pumps cannot idle. As long as they idle for a few minutes, the friction between the shaft and the sleeve inside them will generate extremely high temperatures, quickly wear out graphite bearings and shafts, and even burn out the inner rotor.
In recent years, domestic magnetic pump manufacturers have claimed to be able to produce magnetic pumps that are resistant to idling. Is this true? Is the idle magnetic pump really useful? Can it really idle for a long time?
Today, I will introduce several details to you, hoping to be helpful to users.
At present, there are two main types of idle magnetic pumps produced by some domestic manufacturers. One is to use ceramic rolling bearings to replace the original sliding bearings. When there is a lack of liquid inside the magnetic pump, the friction coefficient of the rolling bearings is lower, and the heat generated is relatively less. Therefore, it can idle without burning the bearings and rotors. The disadvantages of this structure are that it cannot bear high torque, materials with slightly higher density cannot be transported normally, and ceramic bearing balls are fragile.
Another type is the LHC idling fluorine lined magnetic pump developed by Green Ring. This type of magnetic pump is a complete upgrade of the main shaft and shaft sleeve on the basis of the original tradition, and is filled with nano carbon powder technology to achieve the function of idling self lubrication. Relatively speaking, the latter is more technically and theoretically in line with the requirements of idling.
