The structure and function of the large low-lift pump and its associated motor are briefly described. Various structural functions are compared comprehensively in terms of safety, stability, reliability, durability, installation, maintenance and system efficiency. The failure mechanism of the failure of the key components is analyzed, and the design of the unit structure is proposed.

In recent years, China has developed a series of high-performance low-lift pump hydraulic model, its performance indicators have reached the international advanced level. However, the research on the structure and function of large pump units has been neglected. In some pump units, the functional design of the unit structure is unreasonable and often fails. Some individual pumping stations have not been able to operate normally for several years, affecting their functions. In the forthcoming construction of the South-North Water Diversion Project and other key projects in the construction of pumping stations and the 1960s to 1970s construction of large-scale pumping station renovation, special attention should be paid to the structural function of the pump unit, it is necessary for China's large low-lift pump unit Structure and function of the analysis and comparison.

1, the basic structure of large low-lift pump unit

1.1, the structure of the pump structure Most of the large low-head pumps with axial impeller, only 6HL, 3HL, 1400HL216 and a few other pump-type Francis impeller. Impeller diameter D = 1.20 ~ 5.70m, single pump flow Q = 5.0 ~ 97.5m3 / s, supporting power N = 500 ~ 7000kW, rated speed n = 250 ~ 75r / min, single station number 2 ~ 20 units.

Large low-lift pump unit structure in various forms. In the form of axis, there are vertical (bent pipe, shaft, pincers, double volute, box culvert and core pulling), inclined (15 °, 30 °, 45 °) and horizontal S-type, horizontal axis stretching, cat-style back and cross-flow pump units) and so on.

Vertical pipe bending water pump is characterized by a 60 ° metal elbow, elbow in the upper and guide vane body with two guide bearings for impeller diameter D = 1.20 ~ 2.00m of the pump. The pouring pipe of vertical pumped type water pump is cast on-site by reinforced concrete. The pump shaft is pierced with a seal. The pumps used are D = 2.80,4.00,4.50m axial flow pump, D = 2.85,5.70m double scroll Shell mixed flow pump. Vertical wellbore pump installed in the cast-in-place concrete wellbore, all components can be suspended from the wellbore installation, the upper metal pump cover and wellbore to form a bend. Among them, the impeller shell, water inlet and expansion joints and the lower part of the guide vane body exposed in the pump layer structure called semi-shaft cylinder, there are currently D = 2.00,3.10 m axial flow pump; water pump surrounded entirely within the wellbore structure, Weigh the whole well. Vertical box-type pump developed in recent years, it applies to frequent changes both inside and outside the water level, high and low intertwined along the polders special low-lift two-way two-way runner pumping station, the structure of the upper and lower runner only separated by a plate , Can raise irrigation and lifting, can also use the bottom runoff gravity irrigation and drainage. The pump shaft pierced the upper runner with a pump cover and seal, similar to the wellbore.

SEZ series mixed flow pump uses a vertical core-type lifting overall structure, this structure is suitable for D≤2.20m medium-sized vane-type mixed flow pump.

Large inclined and horizontal pump housing parts are generally cast steel or cast iron, often made upside down, in order to facilitate the installation and maintenance. Taking into account the structure of outlet runoff and other station structures, the motor size is not allowed too large, only small size, fewer poles, high speed synchronous motor, gearbox gear change required.

1.2, the motor structure in China using synchronous motor, the type of vertical motor, vertical reversible two-speed motor, vertical two-speed motor, inclined or horizontal motor. Inclined and horizontal motors are small in size and high in rotational speed. They are integral structures and are easy to install. There is a transmission between the pump and the pump, and the motor can not withstand axial thrust. Vertical direct motor with large volume, out of the decentralized structure, the need to pump components in the installation of combination, high installation requirements, technical and strong. [2]. Above 2800kW vertical motor in the lower frame with a brake and the top car Device, 5000kW and above vertical motor thrust bearing hydraulic load shedding device, and the use of air cooler enclosed ventilation structure.

2, large low-lift pump unit structure and function analysis and comparison

2.1, safety and stability Axial flow pump and mixed flow pump flow characteristics within the entire range vary widely. Certain blade angle, Francis pump high efficiency area wider than the axial pump. Axial flow pump close to zero flow, the head and power is large, about the design value of 2 to 3 times, and the small flow area (high lift) unstable operation, high starting lift, power, vibration, gate or fast When the gate is cut off, it is difficult to control the timing and speed of the downtime of the shut-down door, which may easily result in overload of the valve closing operation or reverse fly-away, affecting the safety of generating units and pumping stations. Q-N mixed-flow pump curve is relatively smooth, the unit start-up and shutdown cut off relatively easy. In the head, the flow area, the same flow rate, the impeller vane pump impeller axial thrust is smaller than axial flow impeller, motor thrust bearing load is small. Using the door to stop the flow of the pump station, the door was immersed in water for a long time, by the impact of water swing, the door hinges easily rust and fatigue damage caused by the door knocked down, resulting in shut down the reverse flow; Large; sediment deposition in the reservoir hinders the opening of the door, which will easily cause the unit to start overloading (such as Jiangxi Bali Lake pumping station). These will affect the engineering and crew safety.

Inlet flow channel (especially two-way inlet flow channel) design is irrational, the flow disorder, easy to form vortex, increase cavitation and vibration; two-way outlet also easily lead to vibration [3]; Siphon outlet runway gas Insufficient capacity, easy long-term survival, vibration more than.

2.2, reliable durability (1) single station unit number and capacity. Pumping station installed capacity of a certain number of units depends on the stand-alone capacity. Large unit, high efficiency, but deep foundation excavation station. Large parts, manufacturing, installation, maintenance and transportation difficulties, some of the key components of the technical performance but off. The number of single stations is too small, the operation guarantee rate is low. If only 2 units, 1 after the failure, the pumping capacity is only 50%; if a spare, the unit is too large, resulting in waste. On the contrary, the pump station installed capacity is small, the number of stations is too much, the operation and management inconvenience, the station is too long, covers an area of ​​large, the former pool and the pool excavation length, increasing civil investment.

(2) pump guide bearing failure. Babbitt bearing guide bearing capacity, but the structure is complex, especially the water seal structure is not reliable, a large amount of water leakage into the guide vane body hub cavity, easy access to oil bearings, often the unit has not yet reached the overhaul time, or even just installed and running soon, Due to flooding, the bearings and the journal are severely napped and peeled off and the gap increases, causing the blade to touch the shell. According to the incomplete statistics of Jiangdu Pumping Station in Jiangsu Province, all the big pump oil bearings are soaked in water that the average single station has occurred 4.8 times a year or more, and the single pump can occur as many as 18 times in 10 years [4].

Non-metallic water-lubricated guide bearings do not suffer from the problem of water immersion damage, but have low load carrying capacity. They are used for horizontal and inclined pumps with large working load, easily damaged by partial grinding and short service life. Such as the river under the salt River Guanxiang 15 ° P24 p24 Bakelite plastic bearings running life of only hundreds of hours; Jiangsu Xinshagang oblique 30 ° pump P23 phenolic plastic bearings clean water lubrication, running 300h, bearing and journal obvious eccentric wear and dents , Run 500h, wear 0.5mm, stop running, to prevent the blade touch the shell. For large inclined and horizontal pumps, the metal guide bearing is easily immersed in water and the non-metallic bearing has a short service life, so its reliability and durability are poor.

Vertical pump guide bearing although the work load is small, non-metallic guide bearing water lubrication, or the use of water lubrication, but the seal structure is not reliable, the river of sediment into the bearing clearance, the journal squeeze embedded bearings, wear shaft Neck, rough grinding shaft journal bearings in turn accelerate the wear and tear, a vicious cycle, increased bearing wear, journal surface layer soon peeling, blade gap increases, causing the blade touch the shell.

(3) pump cavitation. nD is too large, the installation elevation is too high, influent channel design is not reasonable, will increase the pump cavitation. Airfoil cavitation and interstitial cavitation serious, overcurrent surface into a honeycomb, the blade edge flaking, the gap increases, the efficiency is reduced, and even the shell perforation, cavitation treatment frequently affect the durability of the unit.

(4) motor failure. Thrust bearing is the most critical vertical wear-resistant motor wear parts, bearing bushings are common faults, accounting for 70% of motor failure to 80%. Poor installation quality, high lift is the direct cause of the fundamental reason is the bearing overload and Structure is not reasonable. According to statistics, the bearing load rate ε> 019 easy to burn, and ε <018 rarely occur shingling accident [5].

Vertical unit motor position is high, the operating environment and ventilation and cooling conditions. The use of natural air by the rotor yoke up and down the fan caused by the pressure or the formation of a vacuum fan forced hot air from the stator out of the wind tunnel outside the cold air through the upper and lower vents into the replenishment of a large amount of dust in the air and oil in the cylinder due to temperature rise High and turbulent and vaporized oil mist mixture, accumulated in the stator core coil and the ventilation slot, a serious impact on heat and insulation, often forcing the motor overhaul in advance, cleaning a lot of work.

Tubular pump unit immersed in water for a long time, bulb body wall condensation is difficult to solve, environmental humidity, high requirements for sealed insulation, poor reliability.

2.3, installation of maintenance of large-scale water pump unit installation elements are elevation, level, center, concentric, swing, bearing force, clearance, etc. 6LZ13.5-8 with 1600kW motor, 2.8ZLQK-7.0 with TDL325 / 56- 40 motor and other unit structure is not reasonable, the unit can not be installed according to the normal procedure, the installation quality can not be guaranteed. Inclined, horizontal unit is difficult to install crank, the axis of the swing is difficult to measure and adjust. Tubular pumps, box-type pumps, full-bore pump external space is small, difficult to install and operate, pump failure and cavitation difficult to find, maintenance and repair of the need to dismantle most of the motor and pump components, heavy workload. Vertical pumped type guide vane pump body and the following components from the pumping station dedicated hanging hole hoisted by the pump layer transport channel moved to the pit installation, installation and maintenance of poor.

Vertical half-shaft pump all parts can be hoisted by the wellbore, the unit around the operating space, installation is good; open wellbore and pump cover into the hole can guide bearing repair or replacement. Open vertical split impeller shell, the pump impeller is completely visible. Vulnerable parts work load disassembly, maintenance costs less, good maintenance. Large vertical units without hydraulic load shedding equipment require 25- to 35-person lathes and high labor intensity. Before installing and starting a vertical motor without a roof loading device, four jack jacks should be placed on the lower frame, so that the space is small. Operation difficult.

2.4, the working condition is adjustable The direct drive is easy to set the blade adjustment mechanism, good adjustability. Large pump variable conditions for three purposes: to achieve economic operation, reduce the starting power to shorten the start-up time (limited to run full regulation), regulate the flow. Semi-adjustment is only to increase the scope of the pump can not be used to adjust the conditions. Full regulation of hydraulic pressure, work reliably, but the hydraulic device system is complex; mechanical fully adjustable structure is compact, auxiliary equipment is simple, is the development direction, but more failures and poor reliability. The pumping station should choose the blade adjusting mode according to the actual situation [7] .With the two-speed motor, the pump has two high-efficiency zones with different heads, and the pumping station can change the motor wiring according to the lift of different seasons and change the rotation speed to achieve the purpose of economic operation.

2.5, economic rationality Vertical unit station height; inclined and horizontal unit station height is small, but the width increases. Vertical pump runner turn more flow head loss; inclined and horizontal pump runner turn less flow direction changes small, especially the cross-flow pump, the flow direction is basically the same, the hydraulic loss is small. Considering single energy point (system efficiency), when the motor efficiency is the same as that of the pump pump section, the cross-flow pump device is the best, the other horizontal and inclined devices are the second, and the vertical device is less efficient. Considering the non-vertical device indirect transmission efficiency, the efficiency of the vertical pump system is 1% ~ 3% lower than that of the non-vertical device. It is difficult to set the full adjustment mechanism for the non-vertical device, and the blade adjustment mechanism is set for the pumping stations and vertical devices with frequent head changes. Through the variable angle to ensure that the system is always efficient operation, the average operating efficiency of the system is not less than oblique, horizontal device. Axial form, the vertical structure of the pump, respectively, as shown in Table 1,2.

3. Large-scale low-lift pump unit structure design selection 3.1, a reasonable choice Pump-type mixed-flow pump compared with the axial flow pump, the power of the dead point is small, the small flow area is relatively stable operation, gate flow is not easy to overload, therefore, the same priority Using mixed flow pump, can improve the safety and reliability of units and operational stability. At present, there are fewer mixed-flow hydraulic models for large pumping stations. Recently, Jiangsu University of Jiangsu Fluid Machinery Research Center and Jiangsu Provincial Department of Water Resources, developed a high ratio of the number of revolutions of the vane pump 211 hydraulic model. The maximum efficiency of the model is 85.3%, and the efficiency range is more than 83%: Q = 0.26-0.47m3 / s, H = 4.6-9.2m, ns = 650-950. The results show that the model 211 and axial flow pump model Compared with the three major characteristics: (1) a very wide range of high efficiency; (2) significantly reduce the instability of small flow areas; (3) adapt to large changes in head occasions. Considering the factors of manufacture, installation, management, mature technical conditions and pumping unit cost per thousand tons of rice water, impeller diameter D = 2.8 ~ 3.1m or so, stand-alone power N = 1000 ~ 3000kW or so, the number of single station 3 to 8 More appropriate, important projects should consider setting up a spare unit per station.

3.2, Reasonable design, selection of unit structure Unit type, structure, size design and selection should take into account the reliability and durability, installation and maintenance of economic rationality. For an important pumping station with a long running time such as the East Route of South-to-North Water Diversion Project with an average annual running time of 5000-6000h, a pump station will have problems that will affect the normal operation of the entire water-diversion line. Therefore, the safety and reliability of the unit is One's Vertical half shaft pump unit has good reliability, durability, installation and maintainability and adjustability of working conditions. The use of pipe-type water lubrication bearings [9], can ensure that sediment does not enter the bearing, the service life can be more than 40000h.

Through the analysis of the bearing load to improve the quality of the unit manufacturing and installation, as much as possible during installation to make the sub-load caused by various factors offset each other, can reduce the bearing load and prolong the service life and improve the reliability and durability [10]. Considering a unit overhaul 1 need 2 to 4 million, compared with other programs, the program saves the unit maintenance costs, flexible operating conditions to adjust the operating costs savings and improve the equipment availability rate of economic and social benefits far exceed Increased investment in equipment and engineering.

Pulling vertical Francis pump unit installation is simple, suitable for pump head higher, but its maintenance of the general. As a result of the use of newer ceramic bearings, high speed, impeller nD value larger, reliable durability and cavitation performance to be tested over time. Drainage pump stations with extremely short running hours may also adopt inclined or horizontal structures in consideration of saving construction, construction and equipment investment. However, at least one inspection or replacement of the pump guide bearings is required every year, and a water pump Risk of bearing failure. So do not make recommendations.

Axial flow pump is best to use siphon water flow vacuum valve broken off flow; to save the project volume, mixed flow pump should adopt fast gate to stop flow, or use simple winch gate flow, but subject to verification. Reasonable design of the inlet channel and pretreatment of the surface of the relevant overcurrent component such as powder coating [11] can improve the cavitation resistance; stainless steel skin is applied on the blade surface, stainless steel strip is embedded in the middle of the inner surface of the shell, Performance, replacement and convenient, economical. Vertical motor air duct closed air duct ventilation, can greatly reduce the accumulation of stator grease to ensure good heat dissipation and insulation. The thrust bearing load ratio ε <018 should be ensured, and elastic metal plastic tiles may be used if necessary [12]. The use of hydraulic load shedding devices and roof loading devices can greatly improve the installation and maintenance performance of the units.

3.3 Reasonable selection of adjustment function The full control structure cost of the pump accounts for about 1/4 ~ 1/3 of the equipment cost of the pump unit. For the unit with large blade angle starting difficulty and frequent change of the pump head, a full adjustment mechanism should be set up. Some pumps realize variable angle economic operation, and the efficiency of the pump device can be increased by 5% -10%. [13] Mechanical adjustment mechanism is compact, but needs to be improved and perfected, the available adjustment force is increased, the adjustment force of the water pump is reduced, reliability.

Pump blade adjustment, equivalent to the basic line with the Q axis parallel, more suitable for flow control. Frequent blade adjustment, root and hub seal easily failure and leakage or water. In recent years, the efficiency curve of mixed flow pump with adjustable leading vanes manufactured by German KSB company is basically parallel with Q-H curve [14], which is more suitable for the economical operation of pump stations with large lift changes. The mechanism is simple and reliable with low cost , Is the direction of development.

Drainage pumping station with less running hours, running costs and equipment investment, especially drainage compared to social benefits as a secondary factor, as long as there is no problem starting the unit, you can use semi-or not adjust the impeller to save equipment costs. For example, the phoenix neck drainage pump station in Anhui Province, although the pump is large (D = 3.1m), still adopts the semi-adjustment structure; the other drainage pump station originally adopted the hydraulic full-regulation. Since the actual operation is not used, Maintenance, the results of oil spills into the water, serious corrosion, has now been changed to a fixed blade [15] .If the pumping station changes particularly frequent headway and a long average number of hours, such as the East Route Project of South-to-North Water Transfer pumping stations, , Twice a day long down, you can use frequency conversion device to achieve speed economic operation. Although the inverter device cost is high, the economy is still reasonable, technically feasible, but also low-speed start to reduce starting power, low-speed shutdown to reduce the risk of gate overload shedding. Such as 2000kW frequency conversion device equipment and installation and commissioning costs about 1.3 million yuan, according to the annual run 5000h calculation, 3 to 4 years of savings in electricity to recover the increased investment in equipment [16].

4 Conclusion

(1) Large low-lift pump unit axis, the structure of the form, complex and diverse functions, affecting the safety and stability of the unit, reliable durability, installation and maintenance, working conditions and economic rationality adjustable. The key factors are: pump type, axis form, water pump guide bearing, water pump cavitation, stop flow, motor thrust bearing, motor ventilation and so on.

(2) pumping station should be preferred mixed flow pump. Vertical and semi-shaft pump units have good overall performance; pump blades and the shell made of stainless steel, anti-cavitation performance was significantly enhanced, the number of operating hours should be given priority to the pump station priority use.

(3) axial flow pump siphon water outlet vacuum breaker valve is relatively safe; mixed flow pump gate flow, can reduce project investment and improve device efficiency.

(4) vertical motor should be set up the necessary auxiliary installation, the use of closed air duct, air cooler ventilation.

(5) many years of operating hours, pumping frequent changes in the pump station, you can consider the use of variable frequency speed economic operation. On the contrary, drainage pump stations with few running hours should adopt semi-regulated pump units with simple structure and low cost.

references:

[1] Liu Chao and so on. Development and application of hydraulic model of high-performance low-lift axial-flow pump [J] .Pump Technology, 2001, (3): 3 ~ 61

[2] Du Ganghai large pumping station installation and maintenance [M] Beijing: Water Power Press, 19951

[3] Luo Shihong and other large two-way X-type runner pumping station vibration damping technology research [D] irrigation and drainage machinery, 2001,19 (4): 15 ~ 201

[4] Wei Qianglin, Huang Haitian, Qiu Baoyun P23 phenolic plastic bearings in the application of large vertical water pump guide [J] pump technology, 1999, (2): 26 ~ 281

[5] QiuBoYun Pumping Station Motor Babbitt Thrust Tile Burning Analysis [J] Yangzhou University, 2000,3 (1): 62 ~ 641

[6] QiuBoYun large vertical pump unit installation elements analysis and control [J] pump technology, 1998, (6): 19 ~ 231

[7] Qiu Baoyun, Liu Chao pump pump adjustment method summary [J] pump technology, 1997, (4): 29 ~ 331

[9] Qiu Bao-yun, Wei Qiang-lin, Tang Zheng-jun Guanshui clear water lubrication bearings in a large vertical pump [J] Journal of Agricultural Engineering, 1999,15 (3): 256 ~ 2581

[10] Qiu-Yun Qiu large vertical axial-flow pump guide bearing load analysis and calculation [J] Agricultural Machinery, 2000,31 (1): 58 ~ 611

[11] Chu Xun, Ma Yundong pump anti-cavitation alloy powder spray welding technology [J] agricultural machinery, 2001,32 (5): 96 ~ 991

[12] Huang Haitian, Tang Zhengjun, Ma Xiaozhong application of flexible metal plastic bearing in pumping station [J] China Rural Water and Hydropower, 1999, (6): 21 ~ 221

[13] Qiu Baoyun full regulation variable angle economic operation of pump station [J] Pump Technology, 1993, (4): 25 ~ 281

[14] Huang Jingguo with adjustable guide vane large mixed flow pump [A] National Pump and Pumping Station 2001 Academic Annual Conference [C], 2001,111

[15] Wang Huanping, Gu Hesheng full adjustment of a pumping station in Huainan City [J] irrigation and drainage machinery, 2002,20 (1): 35 ~ 361

[16] Qiu Bao-yun, Gao Gang Pumping Station Unit 1 ~ 3 Unit Frequency Control Benefit Analysis [J] irrigation drainage, 2000,19 (4): 56 ~ 601

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