Working principle, working condition and structural characteristics of double suction centrifugal pump
First, the SOW open volute double suction centrifugal pump overview: Five, SOW open volute double suction centrifugal pump structural features:
SOW open volute double suction centrifugal pump , mainly for conveying water with a temperature not higher than 105 ° C or liquid with physical and chemical properties similar to water. Applicable to water supply plants, pumping stations, power stations, air conditioning circulating water, heating hot water circulation, industrial water supply systems, fire protection systems, ships, oil refining industry (general liquids) and other applications.
The centrifugal pump is generally driven by an electric motor, and the pump body and the suction line are filled with liquid before starting the pump. When the impeller rotates at a high speed, the impeller drives the liquid between the blades to rotate. Due to the centrifugal force, the liquid is swept from the center of the impeller to the outer edge of the impeller (the flow rate can be increased to 15 to 25 m/s ), and the kinetic energy is also increased. When the liquid enters the pump casing, the flow velocity gradually decreases as the flow path in the volute casing is gradually reduced, and a part of the kinetic energy is converted into static pressure energy, so that the liquid flows out along the discharge port with a higher pressure. At the same time, the center of the impeller forms a certain vacuum due to the liquid being pumped out, and the pressure Pa at the liquid level is higher than the center of the impeller. Therefore, the liquid in the suction line enters the pump under the pressure difference. The impeller rotates continuously, and the liquid is continuously sucked in and out. Since the centrifugal pump is capable of transporting liquid, mainly by the action of centrifugal force, it is called a centrifugal pump.
Centrifugal pumps are the most widely used in chemical production because of their wide range of properties (including flow, pressure head and adaptability to media properties), small size, simple structure, easy operation, uniform flow, low failure, and long life. Long, low purchase and operating costs are outstanding advantages. Therefore, this chapter focuses on the centrifugal pump as a typical example of the application of fluid mechanics.
I. The basic structure and working principle of the centrifugal pump . The basic structure and working principle of the centrifugal pump are discussed. It is necessary to firmly hold the theme of effectively converting kinetic energy into static pressure energy.
(1) The basic structure of the centrifugal pump
The basic components of a centrifugal pump are a high-speed rotating impeller and a fixed snail-shaped pump casing. An impeller having a plurality of (usually 4 to 12 ) back-curved blades is fastened to the pump shaft and driven by the motor for high-speed rotation with the pump shaft. The impeller is a component that directly works on the liquid in the pump and is an energy supply device for the centrifugal pump. The suction port in the center of the pump casing is connected to the suction pipe, and the bottom of the suction pipe is provided with a one-way bottom valve. The discharge port on the side of the pump casing is connected to a discharge line equipped with a regulating valve.
(2) Working principle of centrifugal pump
When the centrifugal pump is started, the pump shaft drives the impeller to rotate at a high speed, forcing the pre-filling liquid to rotate between the blades. Under the action of the inertial centrifugal force, the liquid moves radially from the center of the impeller to the outer circumference. The liquid gains energy during the movement through the impeller, the static pressure energy increases, and the flow rate increases. When the liquid leaves the impeller and enters the pump casing, some of the kinetic energy is converted into static pressure energy due to the gradual expansion of the flow passage in the casing, and finally flows into the discharge pipe tangentially. Therefore, the volute pump casing is not only a component that collects the liquid flowing from the impeller, but also a transducing device. When the liquid is from the center of the impeller to the outer circumference, the center of the impeller forms a low pressure zone, and the liquid is sucked into the center of the impeller under the action of the total potential energy difference between the liquid level of the sump and the center of the impeller. The liquid is continuously sucked in and discharged by the continuous operation of the impeller. The mechanical energy obtained by the liquid in the centrifugal pump ultimately manifests as an increase in static pressure energy.
[Animation] The basic structure and working principle of the centrifugal pump
It should be emphasized that if the pumped liquid is not filled into the pump casing before the start of the centrifugal pump, the centrifugal force generated after the impeller rotates is small due to the low air density, and the central region of the impeller is insufficient to form a low pressure of the liquid in the suction tank. Therefore, the liquid cannot be delivered even if the centrifugal pump is started. This indicates that the centrifugal pump has no self-priming ability, which is called air binding. The suction line is equipped with a one-way bottom valve to prevent the liquid poured into the pump casing from flowing out of the casing before starting. Air is trapped from the suction line into the pump casing.
(3) Impellers and other components of centrifugal pumps
1 . Centrifugal pump impeller
The impeller is a key component of the centrifugal pump.
(1) According to its mechanical structure, it can be divided into three types: closed type, semi-closed type and open type. Closed impellers are suitable for conveying cleaning fluids; semi-closed and open impellers are suitable for conveying suspensions containing solid particles, which are inefficient. When the closed and semi-closed impellers are in operation, a part of the high-pressure liquid leaving the impeller can leak into the cavity between the impeller and the pump casing. Since the pressure at the liquid suction port on the front side of the impeller is low, the liquid acts on the front and rear of the impeller. The pressure on the sides is not equal, and an axial thrust directed to the suction side of the impeller is generated. This force pushes the impeller to move toward the suction side, causing friction at the contact between the impeller and the pump casing, causing vibration of the pump in severe cases and damaging the normal operation of the pump. Drilling a number of small holes in the rear cover of the impeller reduces the pressure difference across the impeller, thereby reducing the adverse effects of axial thrust, but at the same time reducing the efficiency of the pump. These small holes are called balance holes.
(2) According to different liquid absorption methods, the impeller can be divided into single suction type and double suction type. The single suction type impeller has a simple structure, and the liquid can only be sucked from one side. The double suction impeller can simultaneously draw liquid symmetrically from both sides of the impeller, which not only has a large liquid absorption capacity, but also substantially eliminates the axial thrust.
(3) According to the geometry on the blade on the impeller, the blade can be divided into three types: back bend, radial and forward bend. Because the back bend blade is beneficial to the conversion of kinetic energy of liquid into static pressure energy, it is widely used.
2 . Centrifugal pump guide wheel
In order to reduce the energy loss caused by the impact when the liquid leaving the impeller directly enters the pump casing, a fixed and vaned guide wheel is sometimes arranged between the impeller and the pump casing. The vanes in the guide wheel gradually turn the liquid entering the pump casing and the flow passage continuously expands, so that part of the kinetic energy is effectively converted into static pressure energy. Multistage centrifugal pumps are usually equipped with guide wheels. The snail-shaped pump casing, the back-bending blade on the impeller and the guide wheel can all improve the conversion rate of kinetic energy to static pressure energy, so they can all be regarded as energy-transfer devices.
3 . Shaft sealing device
Since the pump casing is fixed and the pump casing is fixed, there must be a certain gap at the contact between the shaft and the pump casing. In order to avoid leakage of high pressure liquid in the pump along the gap or to prevent outside air from entering the pump from the opposite direction, a shaft seal device must be provided. The shaft seal of the centrifugal pump has a stuffing box and a mechanical (end) seal. The stuffing box is used to seal the pump shaft through the annular gap of the pump casing, and a soft packing (such as oil-impregnated or graphite-coated asbestos rope) is placed therein. The mechanical seal consists of a moving ring mounted on the rotating shaft and another stationary ring fixed to the pump casing. The end faces of the two rings are relatively rotated by the spring force to act as a seal. Mechanical seals are suitable for applications where the seal is high, such as the transport of acids, bases, flammable, explosive and toxic liquids. When the centrifugal pump is started, the pump shaft drives the impeller to rotate at a high speed, forcing the pre-filling liquid to rotate between the blades. Under the action of the inertial centrifugal force, the liquid moves radially from the center of the impeller to the outer circumference. The liquid gains energy during the movement through the impeller, the static pressure energy increases, and the flow rate increases. When the liquid leaves the impeller and enters the centrifugal pump casing, some of the kinetic energy is converted into static pressure energy due to the gradual expansion of the flow passage in the casing, and finally flows into the discharge pipe tangentially. Therefore, the volute pump casing is not only a component that collects the liquid flowing from the impeller, but also a transducing device. When the liquid is from the center of the impeller to the outer circumference, the center of the impeller forms a low pressure zone, and the liquid is sucked into the center of the impeller under the action of the total potential energy difference between the liquid level of the sump and the center of the impeller. By continuously operating the impeller, the liquid is continuously sucked in and discharged
Out. The mechanical energy obtained by the liquid in the centrifugal pump ultimately manifests as an increase in static pressure energy. The role of the impeller is to transfer the mechanical energy of the prime mover directly to the liquid to increase the static pressure and kinetic energy of the liquid ( mainly increasing the static pressure energy ) .
Impeller
The role of the impeller is to transfer the mechanical energy of the prime mover directly to the liquid to increase the static pressure and kinetic energy of the liquid ( mainly increasing the static pressure energy ) . The impeller has three types: open type, semi-closed type and closed type. The open impeller has no cover on both sides of the blade, which is simple to manufacture and convenient to clean. It is suitable for conveying materials containing a large amount of suspended solids. The efficiency is low and the liquid pressure is not high. The semi-closed impeller has no cover on the suction side. The plate has a cover plate on the other side, which is suitable for conveying materials that are easy to precipitate or contain particles, and the efficiency is also low; the closed impeller has a front and rear cover plate on both sides of the blade, and the efficiency is high, and is suitable for conveying impurities. Cleaning liquid.
The general centrifugal pump impeller is mostly of this type.
2. Pump casing
The function is to close the impeller in a certain space so that the liquid can be sucked in and out by the action of the impeller. The centrifugal pump casing is often made into a volute shape, so it is also called a volute. Since the cross-sectional area of ​​the flow passage is gradually enlarged, the high-speed liquid drawn from the periphery of the impeller gradually reduces the flow velocity, so that part of the kinetic energy is effectively converted into static pressure energy. The pump casing not only collects the liquid that is pumped out by the impeller, but is also an energy conversion device.
Centrifugal concept
Centrifugation is actually the expression of the inertia of an object . For example, the water droplets on the umbrella , when the umbrella rotates slowly , the water droplets will follow the umbrella . This is because the friction between the umbrella and the water droplets acts as a centripetal force to the water droplets . But if the umbrella rotates faster , this The friction is not enough to make the water droplets move in a circular motion , then the water droplets will move away from the outer edge of the umbrella . Just like pulling a stone with a rope to make a circular motion , if the speed is too fast , the rope will be broken and the stone will Fly out . This is called centrifugation. Centrifugal pump is based on this principle. High-speed rotating impeller blades drive the rotation of the water, the water out, so as to achieve the purpose of transportation. Centrifugal good variety from the civil and industrial use can be divided into pump, from the transport medium It can be divided into clean water pump, impurity pump, corrosion resistant pump and so on. Basic Structure of Centrifugal Pump The basic structure of a centrifugal pump consists of six parts: impeller, pump body, pump shaft, bearing, seal ring, stuffing box.
1. The impeller is the core part of the centrifugal pump. It has high output speed and large output force. The blade on the impeller plays a major role. The impeller should pass the static balance test before assembly. The inner and outer surfaces of the impeller are required to be smooth to reduce the frictional loss of water flow.
2 , the pump body is also called the pump casing, it is the main body of the pump. It acts as a support and is attached to the bracket on which the bearing is mounted.
3. The function of the pump shaft is to connect the coupling to the motor and transmit the torque of the motor to the impeller, so it is the main component that transmits mechanical energy.
4. The bearing is a member that supports the pump shaft on the pump shaft, and has two kinds of rolling bearings and sliding bearings. Rolling bearings use butter as a lubricant to refuel properly. Generally 2/3 to 3/4 of the volume will heat too much, too little and there is noise and heat! The sliding bearing centrifugal pump structure uses a transparent oil as a lubricant and refuels to the oil level line. Too much oil will ooze along the pump shaft and float * , too few bearings will overheat and burn out! During the operation of the pump, the temperature of the bearing is at most 85 degrees and generally runs at about 60 degrees. If it is high, it is necessary to find the cause (whether there are impurities, whether the oil is black or not, whether it is water) and deal with it in time!
5 , the seal ring is also known as the leak reduction ring. The gap between the impeller inlet and the pump casing is too large, so that the water in the high pressure zone of the pump flows to the low pressure zone through the gap, which affects the water output of the pump and reduces the efficiency! Too small a gap will cause friction between the impeller and the pump casing. In order to increase the backflow resistance and reduce the internal leakage, and delay the service life of the impeller and the pump casing, a sealing ring is installed at the inner edge of the pump casing and the outer joint of the impeller, and the sealing gap is preferably maintained between 0.25 and 1.10 mm .
6. The stuffing box is mainly composed of packing, water sealing ring, packing tube, packing gland and water sealing tube. The function of the stuffing box is mainly to close the air between the pump casing and the pump shaft, so that the water in the pump does not flow to the outside and the outside air does not enter the pump. Always keep the vacuum inside the pump! When the pump shaft and the packing friction generate heat, the water seals the water to the water seal ring to cool the packing! Keep the pump running properly. Therefore, it is especially important to check the stuffing box during the running inspection of the pump! The packing should be replaced after about 600 hours of operation.
Second, the technical parameters of the SWO open volute double suction centrifugal pump :
Exit diameter: DN=80 ～ 500mm
Flow rate: 50 ~ 4200m3 / h ( rated point )
Head: H=7.6 ~ 160m ( rated point )
Trial work pressure: 2.5Mpa;
Medium: Clear water or other liquid with physical and chemical properties similar to clean water (solid particles ≤ 80 mg / L) .
Temperature : ≤105°C
Third, SOW open volute double suction centrifugal pump features:
The optimized design of the hydraulic model not only makes the pump have excellent hydraulic performance, but also has no axial force (theoretical). This trapped pump, especially the large pump operation. SOW open volute double suction centrifugal pump body, SOW open volute double suction centrifugal pump cover, SOW open volute double suction centrifugal pump impeller is precision cast, smooth flow surface improves pump anti-steam Corrosion performance and efficiency.
Filler seals or mechanical seals are required depending on the transport medium or user requirements and are not subject to steering restrictions. For packing seals, there is plenty of room for replacement of the packing.
Fourth, SOW open volute double suction centrifugal pump working conditions:
1. SOW split volute double suction centrifugal pump working pressure of 2.5MPa trial; i.e. inlet pressure + pump head ≤2.5MPa. According to the maximum pressure of the pump casing, the pump body and pump cover are made of gray cast iron ( 1.6MPa ), ductile iron ( 2.0MPa ) and cast steel ( 2.5MPa ). Please specify the pressure or material of the pump when ordering.
2. Medium: water with a temperature of ≤105 ° C or other liquids with physical and chemical properties similar to clean water (solid particles ≤ 80 mg / L ).
The SOW open volute double suction centrifugal pump has compact structure and good stability. The pump is open in the pump casing, and the inlet and outlet are all on the pump body. It is only necessary to lift the pump cover during the inspection, and the rotor parts can be removed for maintenance work without disassembling the pipeline, which is very convenient.
The motor (driver) can be installed at the left or right end as needed. The axial dimension of the rotor component is shortened structurally, the rigidity of the shaft is better, and the operation is more stable and reliable. A pump body seal ring is provided, and the impeller seal ring is optional. The seal ring can be replaced after wear, reducing operating and maintenance costs.
1 . SOW open volute double suction centrifugal pump is a closed rolling bearing with grease lubrication, which is easy to maintain and has long running time.
2 . Oil cups with a constant oil level can also be lubricated with thin oil.
3 . Imported ( SKF , NSK , NTN ) high quality bearings are used for stable operation, low noise and long service life.