Analysis on the advantages and application of the

Advantages and application analysis of duckbill valve in sea discharge engineering

Abstract: the advantages and disadvantages of applying duckbill valve in sea discharge engineering are analyzed, and the applicable conditions of duckbill valve in sea discharge engineering are put forward. Combined with Shanghai Fengpu sea discharge project, the use of duckbill valve and multi nozzle nozzle are compared. It is considered that the use of duckbill valve in sea discharge project should be determined according to the comprehensive analysis of specific projects

key words: duckbill valve sea discharge project multi nozzle dilution

in recent years, with the development of science and technology and the emergence of new materials, a new type of valve, duckbill valve, has been successfully developed abroad. Duckbill valve is made of elastic neoprene and man-made fiber through special processing, and its shape is similar to duckbill. When there is no pressure inside, the duckbill outlet closes under its own elastic action; As the internal pressure gradually increases, the duckbill outlet gradually increases, so that the liquid can be discharged at a high flow rate. The duckbill valve has more advantages than the flap valve and gate valve commonly used in current projects. It has been widely used in urban, factory drainage, pump station outlet and other projects. It is also used in some sea drainage projects, such as the Weymouth outlet in the UK, the new territories outlet in the Hong Kong Special Administrative Region and other sea drainage projects. Based on domestic and foreign literature and certain physical model tests, combined with Shanghai Fengpu sea discharge project, this paper briefly analyzes the advantages and disadvantages of using duckbill valve in sea discharge project and the applicable conditions of duckbill valve in sea discharge project

1 advantages and disadvantages of using duckbill valve in sea discharge engineering

1.1 advantages

(1) maintain a high jet velocity. In the traditional sea discharge project, the nozzle is of fixed diameter, so the jet velocity at the nozzle increases linearly with the increase of flow. When the discharge at the outlet is small, the jet velocity at the nozzle is low; The outlet area of duckbill valve increases with the increase of discharge. Therefore, when the discharge is small, the jet velocity increases rapidly. When the flow increases to a certain value, the increase of jet velocity tends to be linear with the increase of flow. Figure 1 shows a comparison of the jet velocity of a fixed nozzle installed at a single hole discharge port and a duckbill valve with the increase of discharge under the same conditions [2]. It can be seen from Figure 1 that the jet velocity after installing the duckbill valve is significantly higher than that of the fixed nozzle. Under the condition of low discharge, the discharge outlet with duckbill valve can still have a higher jet velocity

Figure 1 Comparison of jet flow velocity between fixed nozzle and duckbill valve installed at single hole discharge outlet

(2) prevent seawater and sediment intrusion. There is a certain density difference between seawater and the discharged sewage. When the sewage discharge is small, the traditional outlet diffuser will cause seawater to invade the diffuser and bring sediment from the seabed environment into the diffuser due to the reduction of jet velocity at the nozzle [3]. The outlet of the duckbill valve changes with the change of the discharge. It closes when the discharge is zero, and can maintain a high jet speed when the discharge is low, so it prevents the invasion of seawater and environmental sediment

(3) it is conducive to the flushing of the discharge pipe. When the discharge outlet is operated for the first time or stopped using for a period of time, the diffuser is full of seawater. For the traditional discharge pipe, when the sewage flushing flow gradually increases, the sewage is generally discharged from the first riser near the shore, and the riser far from the shore will have seawater backflow, forming the seawater circulation in the diffuser; Only when the sewage discharge reaches a certain value, the riser gradually begins to discharge all sewage and remove the seawater in the diffuser pipe [4]. When a duckbill valve is installed on the discharge pipe, the sewage can be discharged from all the rising pipes when the discharge volume is very small under the same conditions. With the gradual increase of the discharge volume, the seawater at the bottom of the pipe is gradually sucked and discharged

(4) under certain conditions, a higher dilution can be obtained. The model test of single nozzle diffuser under the same environmental conditions found that under the condition of horizontal jet in the same direction of water flow, the installation of duckbill valve can obtain higher sewage dilution than the installation of fixed nozzle [1]

(5) no corrosion. For infrequently used discharge outlets (such as emergency discharge outlets), it is necessary to install movable flap doors or other backflow prevention facilities on the nozzle. However, if the metal components are submerged in the sea water for a long time, it is easy to produce corrosion and marine biological reproduction. It is necessary to make the jaws symmetrically clamp the sample as required; Secondly, check whether the jaw teeth are damaged, which makes the discharge port unable to operate normally and brings inconvenience to maintenance. Duckbill valve is processed by special treatment of rubber, avoiding the threat of corrosion

1.2 disadvantages

(1) increase the project cost. Because the processing technology of duckbill valve is complex, at present, only a few manufacturers in the world can produce products that meet the use requirements and customize them according to specific projects, so the price is expensive

(2) increase the head loss of the diffuser. Figure 2 shows the comparison of head loss between single hole discharge with fixed nozzle and duckbill valve under the same conditions. It can be seen from Figure 2 that the head loss after installing the duckbill valve is basically twice that of installing the fixed nozzle, which increases the energy consumption of the discharge outlet and increases the perennial operation cost

Figure 2 Comparison of head loss between fixed nozzle and duckbill valve at single hole discharge outlet

1.3 applicable conditions of duckbill valve in sea discharge engineering

the application of duckbill valve in sea discharge engineering should be determined through analysis according to the specific engineering characteristics. Based on the research at home and abroad and the model test results of Tongji University for many years [5 ~ 6], it is considered that the invasion and flushing difficulties of seawater and environmental sediment often occur under the following conditions: (1) the density difference between seawater and discharged sewage is large; (2) The height of the riser is large; (3) The Froude number of outflow density is small. For sea drainage projects with small density difference between sewage and seawater and low riser height, as long as the design is reasonable and can operate normally for a long time, seawater and environmental sediment intrusion generally will not occur, and there is no problem in flushing

based on the above analysis and considering the factors of project cost and long-term operation cost, the applicable conditions of duckbill valve in sea discharge project are: (1) deep water discharge outlet with large density difference between sewage and sea water and high riser height; (2) Discharge outlets that do not operate frequently, such as emergency discharge outlets

2 analysis on the application of duckbill valve in Fengpu sea drainage project

2.1 brief introduction of the original design

Fengpu sea drainage project is a tunnel type discharge outlet constructed by pipe jacking method. The diffuser is 200m long and 1.6m in diameter. There are 14 rising pipes with an inner diameter of 23.9cm, and the height from the rising pipe to the centerline of the diffuser is about 7.0m ~ 8.0m. The original design plan adopts a duckbill valve. Due to cost factors, the original design plans to install a duckbill valve on each riser; Because the cost of the duckbill valve with elbow elbow is about twice that of the duckbill valve without elbow, it is also a cost factor. The duckbill valve without elbow is used in the design, that is, the nozzle of the duckbill valve is vertically upward

the Hangzhou Bay Sea area at the discharge outlet has the characteristics of low salt. Salt LED lighting has gradually become a new trend of residential lighting and commercial lighting, with a degree of 10% - 20%. February is the highest salinity in the whole year, and August is the lowest in the whole year, but the annual variation range of salinity is small, 0.3% - 0.4%. From the salinity value, it can be seen that the density difference between sewage and seawater at the outlet is relatively small

2.2 comparison between the application of duckbill valve and multi nozzle nozzle

from the characteristics and applicable conditions of duckbill valve, it can be seen that the application of duckbill valve in emergency discharge outlet is necessary and reasonable. The following is only some analysis on the application of duckbill valve at the normal discharge outlet. In the analysis, the discharge outlet of the nozzle with multiple nozzles (the jet uses type I collagen adsorption and crosslinking angle is basically level) installed on the riser commonly used in similar sea discharge projects at home and abroad is compared. The application of duckbill valve scheme is considered according to the acceptable cost level of the project, that is, each riser is installed with a single vertical duckbill valve

(1) sewage dilution and diffusion performance. When the duckbill valve is installed vertically on the riser, due to the shallow environmental water depth, the duckbill valve is used to increase the jet flow rate, and the sewage reaches the water surface before it is fully diffused, which is not conducive to the dilution of sewage. According to the model test, it is found that the sewage jet will soon rush to the water surface, causing the instability of the vertical flow state and forming an obvious tumbling phenomenon (as shown in Figure 3). Due to the role of the vertical circulation, the mixed sewage will be brought into the plume to participate in new mixing, affecting the initial dilution. The physical model test and mathematical model calculation show that when the duckbill valve is installed vertically, the dilution of sewage when it rises to the water surface is only about 10. A multi nozzle nozzle is installed on the riser, and the water is discharged horizontally from the nozzle, which increases the mixing of sewage and environmental head, and is conducive to the dilution and diffusion of sewage. In recent years, it is considered that it is the best form suitable for shallow water sewage discharge through research. According to the physical model test and mathematical model calculation, when the multi nozzle nozzle is installed in the project, the dilution of sewage rising to the water surface can reach 200 ~ 250

Figure 3 rollover phenomenon formed on the water surface when the duckbill valve is installed vertically

(2) prevent seawater and environmental sediment intrusion. Duckbill valve can effectively prevent seawater and environmental sediment intrusion. In order to prevent the invasion of seawater and environmental sediment, the Froude number of jet density is required to be greater than 1 [6]. For this project, due to the small density difference between seawater and sewage and the small height of the riser, as long as the diffuser design is reasonable and the sewage discharge is normal, the general jet density Froude number is far greater than 1, there will be no seawater and environmental sediment intrusion, and there is no problem in the flushing of the discharge pipe

(3) sedimentation in the diffuser pipe. According to the experimental research, the use of duckbill valve can make the outlet flow of the riser more uniform along the way, which is beneficial for the deep-water discharge outlet and more in line with the general design requirements of deep-water discharge. However, when the sediment concentration in the sewage is high and the discharge is small, due to the small flow velocity in the pipe at the tail of the diffuser, it is easy to form serious sediment deposition, or even siltation. The diffuser in the form of multi nozzle nozzle can adopt flexible measures in design and operation management, such as uneven distribution of riser nozzles (the number of riser nozzles on the far bank is more than that on the near bank) or blocking the riser nozzles on the near bank, which can increase the flow velocity in the pipe section at the tail of the diffuser, reduce the siltation thickness, and prevent sediment siltation

(4) head loss. The discharge outlet with duckbill valve increases the head loss due to the increase of the local resistance coefficient at the outlet. Compared with the discharge outlet with multi nozzle nozzle, the increase of head loss will not reach twice that shown in Figure 2, but the increased head loss value is also considerable. Because of the increase of head loss, it is required to increase the height of the high-level well and the lift of the water pump, thus increasing the project cost and perennial operation cost

(5) repair and maintenance. According to the manufacturer's information, the duckbill valve can be used for a long time because it avoids corrosion, and generally does not need repair and maintenance. If the discharge port of multi nozzle nozzle is reasonably designed (considering anti-corrosion measures) and can maintain normal operation, it generally does not need repair and maintenance. For example, the Shanghai Zhuyuan discharge outlet (opened to water in December 1993) and the Shanghai Xinghuo Development Zone discharge outlet (operated in March 1993) carried out diving inspection and nozzle removal inspection respectively in 1998. It was found that the nozzle in normal operation had no corrosion and there was basically no marine parasitism at the nozzle [7]; Foreign data also show that the discharge outlet that can maintain normal operation does not have the problem of marine organism parasitic plugging the vent [8]

(6) project cost and operation cost. The duckbill valve itself is expensive, plus the additional head loss, which is very important in the project cost and