The pump is a fluid machine. It converts the mechanical energy of the prime mover into the energy of the fluid being transported, giving the fluid kinetic or potential energy. Since the pump is widely used in various sectors of the national economy, the variety of the series is varied, and its classification method is also different. According to its principle of action, it can be divided into the following three categories:
(1) Blade type: The vane type pump has an impeller, and the impeller is provided with blades. Its pressure-feeding of the fluid is accomplished by high-speed rotation of the impeller with blades. The vane pump is divided into centrifugal, axial flow and mixed flow.
(2) Volumetric type: Its pressure supply to the fluid is accomplished by the change of the volume of the pump body working chamber. There are two types of reciprocating motions and swinging motions in which the volume of the working chamber is generally changed. Reciprocating pumps mainly include piston type and plunger type; rotary pumps mainly include gear pumps and screw pumps.
(3) Other types: This type refers to special types other than vane type and volume type. In this category, there are mainly screw pumps, jet pumps (also known as ejector), water hammer pumps, water wheel pumps, and air lift pumps (also known as air pumping machines). In addition to the screw pump is to use the principle of spiral propulsion to improve the potential energy of the liquid, the above various pumps are characterized by the use of high-speed liquid flow or kinetic energy or momentum of the air flow to transport the liquid. In the water supply and drainage project, when special water pumps are used to transport water or chemicals (coagulants, disinfectants, etc.) in combination with specific conditions, they often have good effects.
The range of use of the various types of pumps described above is quite different. Figure 3-1 shows the general spectrum of several commonly used pumps, which can be used as a reference when selecting a pump. It can be seen from the figure that the range of use of various types of vane pumps currently produced is quite extensive, and the range of use of centrifugal pumps, axial pumps, mixed flow pumps and reciprocating pumps has different performances. The range of use of reciprocating pumps is focused on high lift and low flow. The range of axial and mixed-flow pumps is focused on low lift and high flow. The use range of the centrifugal pump is between the two, the working range is the widest, and the product variety, series and specifications are also the most.
In the case of urban water supply projects, the general water plant's head is between 20 and 100 m, and the single pump flow rate is generally between 50 and 10000 m3/h. To satisfy such a working range, it can be seen from the total spectrum that the use of a centrifugal pump device is quite suitable. Even in some large water plants, multiple centrifugal pumps can be used in parallel in the pumping station to meet the water supply requirements. From the perspective of drainage engineering, urban sewage and rainwater pumping stations are characterized by large flow and low lift. The lift is generally between 2 and 12 m, and the flow rate can exceed 10,000 m3/h. Such a working range is generally suitable for axial flow pumps.
Figure 3-1 General spectrum of several commonly used pumps
In summary, it can be considered that in the water supply and drainage projects of urban and industrial enterprises, a large number of commonly used pumps are centrifugal and axial flow.
At present, the general trend of pump development can be summarized as:
(1) Large capacity and high lift. Increased pump capacity reduces equipment and construction costs, saving energy. It is easy to manage and adopt automation, and it can also improve the technical and economic indicators and operational reliability of the unit.
The domestic 300MW unit is equipped with two boiler feed pumps, each with a drive power of 5500kw. The foreign 1300 MW unit uses only one feed pump, its driving power is 5000kw; the power of the 1800MW unit feed pump is 55000kw; and even the feed pump with driving power up to 75000kw. The outlet pressure of the feed pump is also from ultra-high pressure 13.7~15.7MPa, subcritical pressure 17.7~20MPa, and supercritical pressure 25.6~29.4MPa. A few days ago, foreign countries are preparing to develop the next generation of high-efficiency supercritical units, and their feed pump outlet pressure will be as high as 50 MPa or more.
(2) High speed. In the 1960s, due to steam turbidity and material problems, the pump speed was generally only 3000r/min. In recent years, with the continuous development of science and technology, the pump speed has become higher and higher. For pumps, increasing the speed increases the pump's single-stage head. Therefore, when the total lift is the same, the number of stages of the pump can be reduced, the length of the pump shaft can be shortened, the volume can be reduced, the weight can be reduced, the raw materials can be saved, and the manufacturing cost can be saved. For example, the feed pump of the US 660MW unit, when the speed is increased from 3000r/min to 7500r/min, the single-stage head can reach 1143m, the number of stages is reduced from 5 to 2, and the weight is reduced by 3/4. It can be seen that the economic benefits brought by the increase in rotational speed are very significant.
(3) High efficiency. The pump is a general-purpose mechanical product, and its power consumption is considerable. To this end, improving pump efficiency is of great significance for energy conservation. As early as in the 1970s, China began to technically reform and update low-efficiency pumps, such as centrifugal pumps and axial-flow pumps with an efficiency of less than 60%, so that the improved feed pump efficiency reached about 79%. In the 1980s, China introduced the technologies of KSB (KSB), WEIR (Well) and SUIZER (Sur), and produced the third generation of high-pressure boiler feed pumps. The efficiency is 82. %the above.
It is worth noting that in 1995, the results of the national pump operation survey showed that the actual operating efficiency of centrifugal pumps in China is 10% to 30% lower than that of developed countries. To this end, in addition to improving the efficiency of the pump itself, it is also necessary to improve its operating efficiency in the system.
(4) High reliability. As the pump develops toward large capacity and high speed, the reliability requirements are getting higher and higher. Because only the pursuit of high efficiency and neglect reliability, the energy saving costs in operation can not offset the economic losses caused by pump accident shutdown. To this end, while improving efficiency, reliability should be at the forefront.
The reliability of the pump should be guaranteed from design and manufacture to installation and operation.
(5) Low noise. Noise pollution, like air pollution and water pollution, is harmful to people's health.
At present, many countries have done a lot of research on the mechanism of noise control, noise detection technology, and noise limit standards, and formed an emerging discipline.
(6) Automation. With the development and application of computer technology and network technology, DCS (Distributed Control System), which is a distributed computer control system or a distributed control system, has been realized in the above 300MW units. Foreign countries have already incorporated the monitoring and control of the electrical part of the thermal power plant into the DSC system, thereby realizing the monitoring and control of the computer network system of the entire thermal power plant and becoming an automated thermal power plant. In the DCS system, the pump is no longer a single-stage control, but a network monitoring control. It can realize automatic start and stop of the pump, real-time monitoring, display and control of parameters such as flow, pressure and temperature, as well as automatic fault diagnosis, automatic interlocking and protection.
Working principle and basic structure of centrifugal pump
In hydraulics, we know that when an open cylinder rotates at an equal angular velocity around the central axis, the water surface inside the cylinder is a parabolically rising concave concave surface, as shown in Figure 3-2. The larger the radius of the cylinder, the faster the liquid will rise along the cylinder wall as it rotates faster. Centrifugal pumps work on this principle (see Figure 3-2), except that the impeller and pump casing of the centrifugal pump are specially hydraulically calculated and designed.
Figure 3-3 shows the basic structure of a single-stage single-suction centrifugal pump commonly used in water supply and drainage projects. The water pump includes a volute-shaped pump casing 1 and an impeller 3 that is mounted on the pump shaft 2. The suction port of the volute-shaped pump casing is connected to the suction pipe 4 of the water pump, and the water outlet is connected to the water pressure pipe 5 of the water pump. The impeller of the water pump is generally composed of two circular cover plates with a plurality of curved blades between the cover plates, and the channels between the blades are water-passing groove grooves, as shown in Figure 3-4. The front cover of the impeller has a large round hole, which is the water inlet of the impeller. It is installed in the suction port of the pump casing and communicates with the water suction pipe of the pump. Before starting the centrifugal pump, the pump casing and the suction pipe should be filled with water, and then the motor is driven to make the impeller and water rotate at a high speed. At this time, the water is pulled out of the impeller by the centrifugal force, and is passed through the volute casing. The pressure pipe that flows into the water pump through the flow path is poured into the pipe network by the pressurized water pipe. At the same time, the center of the pump impeller is vacuumed due to the water being pumped out. The water in the suction pool flows into the impeller suction port along the suction pipe under the action of atmospheric pressure, and is also subjected to the high-speed rotation of the impeller. The impeller is fed into the pressurized water pipe. In this way, continuous water delivery of the centrifugal pump is formed.
It can be seen from the above that the working process of the centrifugal pump is actually an energy transfer and conversion process, which converts the mechanical energy of the high-speed rotation of the motor into the kinetic energy and potential energy of the pumped liquid. In this transfer and conversion process, there is a lot of energy loss. The greater the energy loss, the worse the performance of the centrifugal pump and the lower the work efficiency.
The overall construction of the centrifugal pump is described below by two representative centrifugal pump configurations.
1) Single stage single suction cantilever pump
The structural characteristics of this pump can be known from its name. Single-stage means that the pump has only one impeller, and the single-suction water flow can only enter from one side of the impeller, that is, there is only one suction port. The so-called cantilever refers to the support bearing of the pump shaft is installed at one end of the pump shaft, and the other end of the pump shaft is equipped with an impeller, which is like a cantilever. Single stage single suction cantilever pumps are generally horizontal. The types of single-stage single-suction cantilever pumps designed and produced in China mainly include: BA type, B type, IS type and so on. The B-type pump is an improved version of the BA-type pump. The B-type pump is currently used in China. The IS-type pump was designed and manufactured according to international standards in the early 1980s and will be used to replace the B-type and BA-type pumps. The basic structure of the B-type pump and the IS-type pump are respectively described below.
Figure 3-5 shows the structure of a B-type single-stage single-suction horizontal axis cantilever pump. This kind of pump is produced earlier in China. Its impeller is fixed to the right end of the pump shaft by impeller nut, retaining washer and key. The left end of the pump shaft is connected to the shaft of the power machine through the coupling, and the pump shaft is set at the pump casing. There are shaft seals to prevent liquid leakage in the pump. The shaft seals of these pumps are generally sealed. The pump shaft is supported by two single row radial ball bearings. As can be seen from the figure, the pump foot of the pump is integrally molded with the bracket, and the pump body cantilever is mounted on the bracket, so the pump belongs to the bracket type cantilever pump. The advantage of such a pump is that the pump body can have different mounting positions relative to the bracket so that the pump outlet faces up, down, forward or backward depending on actual needs. However, when the pump is overhauled, it is necessary to separate the suction line and the extrusion line from the pump body, which is troublesome. In addition, the full weight of the pump is mainly supported by the bracket, and the bracket is relatively bulky. Therefore, the single-stage single-suction centrifugal pump produced in recent years in China has not used the bracket type cantilever structure.
There are 17 types of B-type pumps, 39 kinds of specifications, 6 kinds of caliber (maximum inlet diameter of 200mm), the applicable range is: head 10~100m, flow rate 45~360m3/h.
Figure 3-6 shows the structure of an IS-type single-stage single-suction horizontal axis cantilever pump. The general structure of the pump is similar to that of the B-type pump. The difference is that the bracket type is changed to a suspension type, that is, the pump foot is integrally molded with the pump body, and the bearing is placed in the suspension. At the same time, the thickness of each component is correspondingly reduced, which reduces the weight of the pump. The weight of the whole pump is mainly absorbed by the pump body, and the bracket only serves as an auxiliary support. In addition, an extension coupling is added (ie, a short shaft with a coupling at both ends between the pump shaft and the motor shaft end flange).
Since the pump cover of the IS pump is located at the right end of the pump body (see 6 in Figure 3-6), and the suspension type is used in the structure, and the extension coupling is added, it is only necessary to remove the bolt connecting the pump body and the pump cover. Parts such as the impeller, pump cover and suspension can be removed from the pump body together. In this way, it is not necessary to disassemble the suction line and the extrusion line during the inspection, and it is not necessary to move the pump body and the power machine. The intermediate part of the extension coupling can be removed, and the pump rotor component can be removed. The disadvantage is that the length of the unit is increased and the strength is reduced.
Compared with the B-type pump, the efficiency of the IS-type pump is about 2% to 4% higher; the parts of the IS pump are standardized and generalized; the range of the IS pump is large, there are 29 varieties, 51 specifications; The water diameter is 50~200m, the lift is 5~125m, and the flow rate can reach 6.3~400m3/h.
Because of the small flow and high lift, the single-stage single-suction centrifugal pump is mostly used in hilly areas where the terrain is high and the water source is insufficient.
In the single-stage single-suction centrifugal pump, there is also a direct-coupled system, which is characterized in that the pump is coaxial with the power machine or a connecting shaft (see Figure 3-7). Because the pump adopts the direct connection type, the pump structure is simple and compact, the outer shape is small, the quality is small, the disassembly and assembly is convenient, and the utility model is suitable for the situation that the work site is frequently replaced.
2) Single stage double suction pump
The single stage double suction pump has one impeller and two suction ports. Such a pump generally adopts a double support structure, that is, bearings supporting the rotor are located on both sides of the impeller and close to both ends of the shaft. The S-type pump shown in Figure 3-8 is called a single-stage double-suction horizontal axis double-support pump. The double suction impeller is fixed to the shaft by a key, bushing and bushing nut to form a rotor, which is a separately assembled component. When assembling, the bushing nut can be used to adjust the axial position of the impeller on the shaft. The pump rotor is double supported by two bearings located in the bearing body at both ends of the pump body. When the radial force is applied to the pump shaft at the coupling, the left end bearing away from the coupling is subjected to a small radial load, so the bearing periphery should be axially fastened so that it can withstand The axial force of the rotor.
The S-type pump is sucked in and out laterally. The suction port and the pressure outlet of the pump are integrally molded with the pump body, and the horizontal open-type pump casing is used, that is, the pump casing is cut into the upper part along the horizontal plane passing through the axial line. The pump cover and the lower part of the pump body, the spiral pressurized water chamber and the two semi-spiral suction chambers are formed by the pump body and the pump cover. This kind of structure can be opened only when the pump cover is opened, and it is very convenient to disassemble the inlet and outlet pipes and the power machine. The top of the pump cover is provided with a screw hole for installing an air suction pipe, and a screw hole for discharging water is provided at a lower portion of the pump casing. A shaft seal is provided on both sides of the impeller suction port. The shaft seal is also a packing type seal, which is composed of a packing sleeve, a packing, a packing ring and a packing gland. The water seal pressure water used for the shaft seal is a groove which is opened through the opening surface of the pump cover, from the pressurized water chamber. Lead to the packing ring. Some medium-open double-suction pumps send water-sealed pressure water into the packing ring through a special water-sealing tube.
Compared with the cantilever pump, the double-bearing pump has a slight influence on the hydraulic performance due to the pump shaft passing through the impeller inlet, and the pump parts are more complicated, the shape of the pump body is complicated, and the process is poor, but the double-bearing pump shaft The rigidity is much better than that of the cantilever pump; in addition, for the double suction pump, the double support structure can make the shape of the suction port on both sides of the impeller symmetric, which is beneficial to the balance of the axial force. Therefore, in order to improve the reliability of the pump operation, the size is better. The large double suction pumps are double supported.
At present, the domestic single-stage double suction pump models are: SH type, SA type, S type. Among them, the SH type is used more, the diameter is 150~800mm, the head is 10~140m, and the flow rate is 0.35~1.5m3/s. The SA type is an improved version of the SH type, with a total of 13 varieties, 45 specifications, and a diameter of 150 to 800 mm. The S-type was developed in the 1980s to replace the SH-type and SA-type products, with a total of 41 specifications. Currently only some specifications are put into production. Compared with SH type and SA type pumps, the S type pump has improved and improved in structure and performance. For example, the peripheral water sealing tube is removed, and the water sealing groove is changed; the thickness of the pump casing is thinned to reduce the quality; the material is saved, and the suction stroke and efficiency are improved.