Wrong air volume regulation of Roots blower

The air volume of Roots blower has always been a problem of great concern. When roots blower is used, the air volume will be too small or too small, and many friends will have wrong operation, which will lead to various faults of the fan. Let's take a look at the wrong posture first
Wrong posture 1:
The air volume can be adjusted by controlling the gate valve, so that the roots blower is easy to overpressure and burn the motor.
Wrong posture 2
When the air volume is increased and the pipe diameter is changed, the effect is 0.
Wrong posture 3
Without knowing the reason why the air volume becomes smaller, blindly adjust the various accessories of the fan.
Wrong posture 4
It is easy to make wrong judgment if only checking the fan and not checking other places.
In the case of unknown fault reasons, blind adjustment of Roots blower is most likely to cause such and such problems. In view of the above situation, what is the correct posture?
Correct posture 1: judge the reason of air volume change
After the installation of fresh air fan, the air volume and air pressure of Roots blower will change to a certain extent. This is often the case for new equipment in the running in period. After the running in period, the data of Roots blower will stabilize, which is a special case.
Check pipes and fans for insufficient air volume;
If the air volume is too large, check the fan. The disadvantage of large air volume is high power consumption, that is, if the model is selected as large, the air volume is large. If you don't want to change the model, you can install an overflow valve to reduce the air volume.
Correct posture 2: adjust the air volume
Remember the following three things:
1. Reduce the air volume: change the speed (frequency), increase the relief valve;
2. Increase the air volume: change the speed (frequency), clean the inlet and outlet pipes;
3. Change the fan model.
Correct posture 3
When we understand the main reason for the change of fan air volume, then according to the specific situation, adjust the air volume, do not operate blindly.
matters needing attention:
Belt drive, adjust the air volume, can be carried out by changing the pulley;
Direct drive can be equipped with frequency conversion control cabinet. Huadong motor recommends that customers who use non frequency conversion motor do not force frequency conversion.
Belt drive can also adjust the air volume by adding control cabinet, but it is not cost-effective.

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The difference between multistage pump and single stage pump in balancing device

The axial force of the segmented multistage centrifugal pump is the superposition of the axial forces of the impellers at all levels. The value of the axial force is very large, and it can not be completely borne by the bearings. Therefore, effective balancing measures must be taken.
① The symmetrical arrangement of impellers makes every two impellers of centrifugal pump symmetrically installed on the same pump shaft in the opposite direction, so that the axial force generated by each two impellers counteracts each other. This scheme is complicated and expensive. When the number of stages is large, the axial force can not be completely balanced due to different leakage conditions and different impeller diameters, and the auxiliary balancing device is often needed.
② Because the impeller of segmented multi-stage centrifugal pump is installed on the shaft along one direction, the total axial force of the balance plate device is very large. The balance plate is often installed behind the last stage impeller to balance the axial force. There is an axial clearance B between the balance disc 5 and the balance ring 4, and a radial clearance B0 between the balance disc 5 and the balance sleeve 3. The balance chamber behind the balance disc 5 is connected with the suction of the pump by pipes, so that the pressure before the radial clearance is the pressure P2 on the back of the last stage impeller, The pressure behind the balance plate is close to the pressure PL at the suction port. After the pump starts, the high-pressure liquid from the last stage impeller of the multistage pump flows through the radial clearance B0, and the pressure drops to P ｀. Due to the pressure P ｀ > PL, the pressure P ｀ - PL acts on the balance disc 5. This force is the balance force, and the direction is opposite to the axial force acting on the impeller.
When the centrifugal pump is working, when the axial force on the impeller is greater than the balance force on the balance disk 5, the rotor of the pump will move towards the suction direction, which reduces the axial clearance B0 of the balance disk 5, increases the fluid resistance of the liquid, and thus reduces the leakage. With the decrease of the leakage, the pressure drop of the liquid flowing through the radial gap B0 decreases, thus increasing the pressure P ｀ in front of the balance disc 5, that is, increasing the balance force on the balance disc 5. As the balance disc 5 moves to the left, the balance force increases gradually. When the balance disc 5 moves to a certain position, the balance force is equal to the axial force and reaches balance.
Similarly, when the axial force is less than the balance force, the rotor will move to the right, and after a certain distance, the axial force and balance force will reach a new balance. Due to the inertia, the moving rotor will not stop at the new balance position immediately, but continue to move, causing the balance to be destroyed, resulting in the condition that the rotor moves in the opposite direction.
When the water pump is working, the rotor will never stop at a certain position, but move axially at a certain balance position. When the working point of the pump changes, the rotor will automatically move to another balance position for axial movement. Due to the characteristics of balancing the axial force automatically, the balancing disc has been widely used.
Balancing device of single stage pump:
① The pressure on both sides of the impeller can be basically balanced by opening a balance hole on the impeller. But due to the liquid through the balance hole has a certain resistance, so there is still a small part of the axial force can not be completely balanced, and the efficiency of the pump will be reduced. The main advantage of this method is that the structure is simple, and it is often used in small centrifugal pumps.
② A balance pipe is installed on the pump body to connect the liquid at the back of the impeller with the liquid at the pump inlet through the balance pipe to balance the axial force. This method is superior to the opening of balance hole, it does not interfere with the liquid flow at the pump inlet, and the efficiency is relatively high.

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Working principle of centrifugal pump

 

The working principle of centrifugal pump is: relying on the high-speed rotating impeller, the liquid obtains energy under the action of inertial centrifugal force to improve the pressure. Before the pump works, the pump body and inlet pipe must be filled with water to prevent cavitation. When the impeller rotates rapidly, the blade makes the water rotate quickly. The rotating water flies away from the impeller under the action of centrifugal force. After the water in the pump is thrown out, the central part of the impeller forms a vacuum area. Under the action of atmospheric pressure (or water pressure), the original water is pressed into the inlet pipe through the pipe network. In this way, continuous pumping can be realized. It is worth mentioning that: before the centrifugal pump starts, it must be filled with water in the pump shell before it can be started, otherwise it will cause the pump body to heat, vibrate, reduce the water output, cause damage to the pump (referred to as "cavitation") and cause equipment accidents! The so-called cavitation refers to: when the centrifugal pump is started, if there is air in the pump, because the density of air is very low, the centrifugal force generated after rotation is very small, so the low pressure formed in the center area of the impeller is not enough to suck the liquid whose liquid level is lower than the pump inlet into the pump and cannot transport the fluid.
There are many kinds of centrifugal pumps, and the common classification methods are as follows
1. According to impeller suction mode: single suction centrifugal pump, double suction centrifugal pump;
2. According to the number of impeller: single stage centrifugal pump, multi-stage centrifugal pump;
3. According to impeller structure: open impeller centrifugal pump, semi open impeller centrifugal pump, closed impeller centrifugal pump;
4. According to the working pressure: low pressure centrifugal pump, medium pressure centrifugal pump, high pressure centrifugal pump;
5. According to the pump shaft position: horizontal centrifugal pump, vertical centrifugal pump.

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Slow closing check valve and safe operation of centrifugal pump

Pump is the core device of fluid industry, its function is like the heart of human body, providing power for the fluid in each device in the system, ensuring the normal operation of the process. Commonly used pumps are classified into vane pumps, positive displacement pumps and special purpose pumps. Among them, the centrifugal pump of vane pump is the most widely used.
During the operation of centrifugal pump, two problems need to be avoided: water hammer and lower than the minimum flow. There are many ways to solve these two problems, but the basic idea is how to configure a suitable system around the centrifugal pump. In these systems, valves play a very important role, and with the deepening of the understanding of these two kinds of problems, many special valves were born.
Water hammer protection and valve for pump shutdown
Water hammer is a series of water hammer phenomenon caused by the drastic change of medium velocity in the pressure pipeline. The harm of water hammer is very big, it can destroy the pump, pipeline and other equipment. There are many reasons for water hammer in pressure pipeline, such as quick closing of valve, abnormal pump stop and so on.
Mechanism of water hammer in pump stop
When the pump stops suddenly due to abnormal reasons, such as power failure: at the initial stage, the medium in the pipeline continues to move forward relying on inertia, but the speed gradually decreases to zero; at this time, if there is a high-low drop in the pipeline layout, the medium will flow backward to the pump under the action of gravity; When the backflow medium reaches a certain speed, the check valve at the outlet of the pump will close quickly. In this way, the speed of a large number of medium reaching here suddenly becomes zero, causing a sharp rise in the medium pressure - stop pump water hammer. A large number of literatures point out that the main cause of water hammer is the sudden closure of the check valve at the outlet of the pump. However, some studies have shown that although the check valve at the pump outlet can be cancelled in some cases, in most cases, the anti backflow setting at the pump outlet is necessary to prevent a large amount of medium from flowing back into the centrifugal pump.
Measures to avoid water hammer in pump stop
There are many measures to avoid the harm of water hammer, such as the installation of water hammer eliminator, pressure relief valve, pressure regulating tank, etc. the following only introduces two measures that are more widely used and related to general valves.
1. Setting slow closing check valve
Slow closing check valve is a kind of check valve which can close slowly by adding actuator and damper. The figure above shows the schematic diagram of the water hammer protection system of the centrifugal pump with slow closing check valve. At this time, the slow closing check valve needs to be used with the gate valve (on-off valve). When the medium flows backward under the action of gravity, the check valve closes slowly, which effectively avoids the water hammer caused by the sudden closure of ordinary check valve. Its disadvantage is that because the closing speed is slow, part of the medium inevitably flows back into the centrifugal pump, so the pump may have mechanical failure.
2. Setting slow closing check butterfly valve
This method is widely used and effective in large-scale water conservancy system. The slow closing check butterfly valve is composed of butterfly valve, executive structure and hydraulic control system. The figure above is the schematic diagram of the water hammer protection system for centrifugal pump with slow closing check butterfly valve. The system only needs to install slow closing check butterfly valve, which can not only play the role of check valve, but also play the role of on-off valve. When the centrifugal pump is started, it should be started slowly first and then quickly to ensure the low load start of the pump; when the pump stops suddenly, it should be shut down quickly first and then slowly, which can not only avoid the generation of water hammer, but also avoid too much medium backflow through the pump and cause the mechanical failure of the pump.
Avoid minimum flow conditions and valves
The minimum flow is the flow to ensure the normal operation of the pump. If the pump works under the condition of lower than the minimum flow rate, it will cause noise and vibration, the performance of the pump will become unstable, and even cause abnormal cavitation of the pump, reducing the service life of the pump. Therefore, measures must be taken to avoid the centrifugal pump working in the minimum flow condition. At present, the common practice is to set the minimum flow circuit for the centrifugal pump. But from the value of the pump itself, only for those large flow, high head, high power centrifugal pump set the minimum flow circuit is reasonable. The minimum allowable flow rate of centrifugal pump is determined by the pump manufacturer's calculation or experiment.
The simplest minimum flow circuit only needs to install an on-off valve in the circuit, such as gate valve. When the pump is about to be in the minimum flow condition, open the valve and connect the minimum flow circuit to avoid the pump working in the minimum flow condition.
summary
The basic structure of the valves described above is the same as that of general valves, but due to the corresponding improvements made according to the application conditions, these valves become special valves to solve these two problems.

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Product overview
Is type single-stage single suction (axial suction) centrifugal pump is an energy-saving pump jointly designed by the whole country. It is an update of Ba type, B type and other single-stage clean water centrifugal pumps. The advantages of single-stage single suction centrifugal pump are: reasonable layout of hydraulic performance of the whole series, wide selection range of users, convenient maintenance, and international average advanced level of efficiency and suction head. This type of pump is suitable for industrial and urban water supply and drainage, agricultural drainage and irrigation, as well as conveying clear water or other liquids with physical and chemical properties similar to clear water. The temperature is not higher than 80 ℃.
Is and IR pumps are Horizontal single-stage single suction water centrifugal pumps, which are used in industry, agriculture, city, drainage, fire water supply, etc. For suction and delivery of clear water and similar physical and chemical properties of water without solid particles of liquid. It is widely used in industry, agriculture and city, drainage, fire water supply, etc. The is and IR pumps are designed according to the performance and dimensions specified in the international standard is02858. Their technical standards are close to the international standards and reach the international advanced level. It is one of the energy-saving pump products promoted in China. The pump has the advantages of simple structure, reliable performance, small volume, light weight, good cavitation resistance, low power consumption and convenient maintenance. The is and IR pumps are widely used. There are 140 specifications in the whole series, but only four kinds of shafts are used. The shafts, bearings and shaft seals of the same specification can be interchanged. There are only four kinds of suspension in the whole series. Pump speed is divided into 2900 and 1450 rpm. The performance is as follows: 2900 RPM 1450 RPM large flow rate: 240m3 / min 400m3 / min high total head: 125m 55m high speed: 3500 rpm (when used for 60 wave power supply, impeller diameter is reduced) high working temperature: 80 ℃, allowable suction pipeline pressure 0.3MPa, high pump pressure 1.6Mpa.
Product features
1. The is single-stage single suction centrifugal pump is designed according to the performance and dimensions specified in the international standard ISO2858. It is mainly composed of pump body, pump cover, impeller, shaft, sealing ring, shaft sleeve and suspension bearing.
2. The pump body and cover of is single-stage single suction centrifugal pump are dissected from the back of impeller, which is commonly known as back door structure. It has the advantages of convenient maintenance. The pump body, suction pipeline, discharge pipeline and motor are not moved during maintenance. The rotor part can be removed for maintenance only by removing the intermediate connector of the extended coupling.
3. The housing of the pump (i.e. pump body and pump cover) constitutes the working chamber of the pump. Impeller, shaft and rolling bearing are the rotors of the pump. The suspension bearing supports the rotor part of the pump, and the rolling bearing is subjected to the radial and axial force of the pump.
4. In order to balance the axial force of the pump, the front and back of the impeller of most pumps are equipped with sealing rings, and there are balance holes on the back cover plate of the impeller. Because the axial force of some pumps is small, there are no sealing rings and balance holes on the back of the impeller.
5. The axial sealing ring of pump is composed of packing gland, packing ring and packing to prevent air intake or large amount of water leakage. If the impeller of the pump is balanced, the cavity with soft packing is connected with the suction port of the impeller. If the liquid at the inlet of the impeller is in a vacuum state, it is easy to enter the air along the surface of the shaft sleeve. Therefore, a packing ring is installed in the packing cavity. Through the small hole on the pump cover, the pressure water in the pump chamber is led to the packing ring for sealing. If there is no balance hole in the impeller of the pump, because the liquid pressure on the back of the impeller is greater than the atmospheric pressure, there is no air leakage problem, so the packing ring can not be installed.
6. In order to avoid the wear of the shaft, a shaft sleeve is installed at the position where the shaft passes through the packing chamber. There must be an O-ring between the shaft sleeve and the shaft to prevent air intake or water leakage along the mating surface.
7. The transmission mode of the pump is connected with the motor through the lengthened elastic coupling. The rotation direction of the pump is clockwise from the driving end.

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