Abstract: In this paper, the problems of blocking, rusting, vibration and cleaning of trash rack in pumping station are studied. Combined with the cavitation and cavitation in Huaian No.2 Station of Jiangsu Province, the unit frequent on and off, energy and economic loss And other aspects of the trash rack on the impact of the pumping station, the research content of the South-to-North Water Diversion Project also has some reference to other pumping stations.
Keywords: pumping station trash rack obstruction energy loss 1 Introduction The South-to-North Water Diversion Project is based on the northward transfer of Jiangsu Jiang Shui-an and expand the scale of the north extension. Due to the low topography, the pumping method should be adopted. Apart from the existing Jiangdu Water Control Project and the pump stations in Huaian, the construction of pumping stations such as Liu Shan Station, Xie Station, Huai'an Station and Huaiyin Station will be started during the year. Pump station in the South-North Water Diversion Project plays a decisive role. As an important part of pump station, trash rack plays an irreplaceable role in preventing floats and sub-migrants from entering the pump station and ensuring the safe operation of the pump station. However, due to the relatively simple structure of the trash rack and the lack of understanding of its importance, the research on the trash rack has not reached the desired depth. In this paper, Jiangsu Huaian two stations of the trash rack in the pump operation in the operation of the problems and their impact on the pumping station for analysis, in order to cause trash rack attention, so that the pump station to achieve better economic efficiency.
2 trash in the use of the common problems 2.1 trash rack blocking trash rack role is to stop the water plants, roots, garbage and other foreign matter into the inflow channel, to prevent it from entering the pumping station structure, the hydraulic construction Material and hydraulic machinery to the detriment of the operation of the pumping station affected. However, the amount of debris brought by the water current can not be estimated and controlled. Therefore, the accumulated dirt in front of the trash rack can not be cleaned timely, which results in blockage of the trash rack and increases the water level difference before and after the trash rack. That is, increasing the force of the water flow on the trash rack, and even seriously, the deformation of the trash rack structure may even be damaged. Taking Doumen County Xi'an Pumping Station as an example, after the blockage of the trash rack occurs, the pressure exceeds the limit value of the bending stress of the trash rack material, resulting in buckling deformation and instability of the trash rack structure. The large pump is rolled into an iron cake [ 1]. Trash bar blocking deformation increases the pumping station operating energy consumption, causing pump cavitation, unit vibration, pumping station was forced to stop, etc., a great harm.
2.2 trash rack rust trash rack as a combination of space frame and platform plate structure, year-round immersion in the water will inevitably produce some rust. Although there is no blockage caused by the corrosion of the trash rack, the consequences of the blockage are serious. However, the corrosion can lead to the reduction of the trash rack over-water area, increase the head loss, reduce the over-current capacity of the pump and also affect the efficiency of the pump.
2.3 trash rack vibration problems In the trash rack use, due to trash rack dynamic characteristics of the ill-considered trash rack damage caused by the incident have occurred. There are two main reasons for grid vibration: (1) water exciting; (2) natural frequency. Water exciter is the main reason for trash rack damage. When the water flows through the trash rack, the Karman vortex shedding caused by the shedding of the grid at the tail end of the grid causes a lateral excitation force in the local area where the local damage occurs. The frequency of the exciting force increases with the increase of the flow rate. When the frequency of trash rack surface force and the trash rack's natural frequency of the same or close to, will cause trash rack resonance, resulting in trash rack damage. With the increase of Reynolds number of flow, there are three kinds of flow regimes of laminar flow, periodic flow and turbulent wake [2]. Periodic wake will stimulate periodic vibration of trash rack structure; turbulent wake will make the structure produce random vibration. The natural frequency of the grid depends on the size, shape and connection of the grid.
Impact of trash rack on pumping station The influence of trash rack on pumping station can be considered from different aspects such as hydraulic loss, working point change, cavitation cavitation, vibration, noise, energy and economic loss. In this paper, Jiangsu Huaian two stations for analysis.
Huaian Second Pumping Station, located at the intersection of Beijing-Hong Kong Grand Canal and North Jiangsu Irrigation Canal, is one of the second-stage water-lifting projects in Jiangsu Province's North-to-North Water Transfer Project. The project started construction in January 1975, completed in December 1978, with a total investment of yuan. The station and Huai'an pumping station, Huai'an pumping station jointly responsible for three Baihu Lake farmland drainage and farmland in Northern Jiangsu irrigation water and the region's industrial, shipping, urban and rural people's water for life task.
3.1 water flow through the trash rack to produce hydraulic losses Trash rack hydraulic loss consists of two parts: one is the inherent head loss, that is, when the water flow through the trash rack, the grid on the water flow has a partial impediment, resulting in local head loss . The other part of the additional head loss, which is due to the dirt trapped by the trash rack to partially block the gate hole, or corrosive water flow caused by corrosion, trash barriers to the original boundary conditions change, exacerbated the The obstruction of water flow, resulting in increased partial head gate loss [3].
Trash head loss [4] can be calculated as follows:
The generation of trash head local head losses leads to a number of undesirable effects, such as changes in operating points, vibration, and other consequent consequences. Table 1 shows the head loss caused by the blockage of the trash rack at the inlet and outlet of the Huai'an No.2 Station.
3.3 exacerbate the pump cavitation and air erosion trash rack obstruction to make the pump operating point changes, reduce pump operating point flow. The flow direction of the water entering the impeller is deviated from the design condition, and the backflow generated by the gap between the impeller and the impeller chamber and the secondary backflow generated by the blades cause the rotation of the water flow, and the center of the swirling flow pressure drops when the rotation reaches a certain intensity Vortex belt, when the inlet pressure of the impeller drops to a certain value, cavitation will occur on the back of the blade. When the pressure continues to decrease, cavitation will occur, which may lead to cavitation in the back of the blade from the inlet end of the blade to the middle of the blade .
When the pump cavitation, cavitation, the presence of a large number of bubbles changed within the flow passage, especially within the impeller flow area and flow direction, leaving the impeller and water flow between the stability of the energy exchange is destroyed, the energy loss increases, causing Flow, lift and efficiency decline, or even to cut off, seriously undermining the pump performance.
In addition, the pressure pulsation resulting from the continuous development and collapse of the bubble enters the rotor and the guide vane and causes the ripple of the rotor to fluctuate, causing the shaft power to fluctuate with a loud noise, causing the operation unstable and affecting Unit efficiency. When the pulsation frequency is close to the natural frequency of the pump or some parts, it causes a large amplitude. Field experiments at Huai'an No.2 Station show that when the pressure pulsation frequency of spontaneous oscillation in bubble area is close to the hydraulic frequency of the whole system, cavitation resonance will be caused [4].
3.4 Units frequent open stop In order to rule out the trash rack obstruction is necessary to trash rack cleaning, if the manual cleaning, to frequent on-off, which not only adverse to the basic operation of the pumping station and the main electrical performance and Insulation equipment poses a threat. For example, the insulation of the main motor of Huai'an No.2 Station has seriously deteriorated. In recent years, several times stator insulation has been punctured during the test and the rotor coil has been interrupted in operation. At the same time, due to each stop-stop caused by the transition of the pump device more harmful to the pumping station, due to changes in conditions caused by unsteady flow will cause the entire unit pressure changes, resulting in vibration damage.
3.5 Energy Loss and Economic Loss Due to blockage of the trash rack, the flow into the pump is reduced and the water head is also reduced. The unit is out of the high-efficiency zone and the efficiency is reduced, resulting in energy loss, resulting in some economic loss. Huai'an two stations in 1995 to 2001 because of congestion caused by the loss of 15,732,900 yuan, as shown in Table 2 [4].
4 trash cleaning problems In order to solve the problem of pumping station cleaning, pump cleaning stations have long been developed, but the basic goal is to study the mechanical cleaning function, did not fully consider the operation of irrigation and pumping stations and Management features. However, irrigation and drainage pumping stations generally have short average annual running time, about 80% of the time in one year is idle, and it is difficult to achieve good maintenance in the open air environment. Therefore, once the pumping station is switched on and running, Can not guarantee the normal work, emergency cleaning work can only be carried out manually [6]. This method needs to be carried out with a certain water velocity and can only catch surface floats. When the grid difference is more than 0.2 m, the upper part of the water flow accelerates, so that it is impossible to recover the trapped objects under 1.5 m water surface.
At present, the second station of Huai'an uses two methods of car-fishing and fishing, but due to the unreasonable design of the working bridge, these two kinds of cleaning methods have not achieved the desired results. Due to the adoption of the vacuum breaker, the instantaneous water pressure difference between the water pump and the water pump down to the lower water outlet of the vacuum breaker is reduced to 4.58 m, Inverted flow of water suddenly from the inside out against the erosion of trash rack, trash weeds on foreign bodies were sucked into the pool, not clean [6].
5 Conclusion (1) to strengthen the pump station trash rack structure and the relationship between the loss of head, gate design optimization efforts, making the gate flow regime is conducive to reducing cavitation, cavitation and vibration.
(2) To carry out experimental research and theoretical research on blockage of trash rack at pumping station and corrosive influence on head loss, and to seek a more efficient way of cleaning and to ensure timely and effective decontamination of trash rack.
(3) The South-to-North Water Diversion Project is a water conservancy project attracting worldwide attention in order to solve the shortage of water resources in our country. Its east-line project is based on a cascade pump station and carries water to the north step by step. The planned water-diversion scale is 14.8 billion m3 , To solve the water shortage problem in Huanghuai region. This reflects the important position of pump station in urban life and agricultural irrigation. Therefore, strengthening the research on pumping station will help to exert the positive effect of pumping station on people's life, therefore, trash rack is important as safe operation of pumping station. Factors more of its research value.
references:
[1] Zhang Yaoquan. Solutions to solve the instability of trash rack in Xi'an Big Pumping Station [J] .Guangdong Water Resources and Hydropower, 2003 (1): 37-39
[2] Yan Shiyu. Vibration of trash rack in hydropower station [J]. Journal of Water Conservancy and Hydroelectric Power, 2001 (2): 74-77
[3] GAO Jin-hua. Local head loss of trash rack at inlet of hydropower station and its research status [J]. Industrial Technology Economics, 1997. (4): 112-113
[4] Huaian second pumping station trash rack on the unit operation evaluation and countermeasures (R). Irrigation General Management Office of Jiangsu Province, Hohai University, October 10, 2003
[5] Liu Yuebo. Analysis of the current situation of trash rack plugging in Huaian No.2 pumping station [J] .Jiangsu Water Resources, 2003 (6): 1-2
[6] Hanyang Yuan. A new way of purifying mechanization of irrigation and drainage pumping stations [J]. China Rural Water and Hydropower, 2002 (6): 1
Jiangsu Province Irrigation Master Canal Administration Department Huai'an three trash trash blocking research, the project has passed the identification of Jiangsu Provincial Science and Technology Agency, Su Ke Jian Zi [2003] No. 1193
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