Structural Analysis and Comparison of Large Low Head Pump Units

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 low-head pumps with axial-flow impeller, only 6HL, 3HL, 1400HL216 and a few 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 structural analysis and comparison 2.1, safety and stability of the axial flow pump and mixed flow pump flow characteristics within the entire range. 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 Jiangsu Province Jiangdu pumping station incomplete statistics, all the big pump oil bearings are soaked in water, an average of more than 4.8 times per year, a single pump up to 18 times within 10 years as much as 4. Non-metallic water-lubricated guide bearing There is water damage problems, but the carrying capacity is low, for horizontal and inclined pumps, the work load, easily damaged due to partial wear and short life expectancy. 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 the vertical position of the motor ε high position, operating conditions 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 supplement a lot of dust in the air and the cylinder oil 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 pump guide vane 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 the 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. No hydraulic load shedding device