1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

Practical Analysis of Cleaning Water Supply Pipeline Using Air and Water Flushing Technology

Wenqi YUAN

Shanghai Yun Shui Water Engineering Design CO. LTD. blueround@163.com

ABSTRACT

To ensure the safety of water quality, pipe network cleaning and security management are put forward higher requirements. Traditional single-phase water flushing pipe technology requires high velocity and large flow rate of water, which does not suitable for large-diameter pipes. Air and water flushing pipes are new pipeline cleaning technology, which has high efficiency, good washing effect, and can save a lot of water. The principles of air-water flushing large-diameter, large-deviation-elevation, and many- ups-and-downs pipes are described in this paper. Based on the two-phase flow theories, it analyzes complex hydraulic characteristics and control parameters for flushing pipeline. Combine with engineering practice, air and water flushing diameter of 800 mm pipeline in Shanghai city and flushing pipe parameters are introduced. The air and water flushing effect and water resource consumption reduction are analyzed, as well. Keywords: Air and water two phase flow; Cleaning water pipeline, Engineering practice 1 Introduction Water supply pipe network likes a huge reactor. With water flowing through the pipe network, clear water reacts with the pipe wall, resulting in pollution of water quality. To ensure the safety of water quality, pipe network cleaning and security management are put forward higher requirements. Traditional single-phase water flushing pipe technology requires high velocity and large flow rate of water, which waste water resource, especially does not suitable for large-diameter pipes. Air and water flushing pipe technology is a new type of flushing technology in recent years. It has high efficiency, good washing effect, and can save a lot of water. After air is fed into the water supply pipe, the air-water two-phase flow is formed, and its flow is more complicated than the single-phase flow. In large diameter and undulating pipes, the flow becomes much more complex. Therefore, it is more difficult to control the flow cleaning water supply pipeline using air and water flushing technology. The principles of air-water flushing large-diameter, large-deviation-elevation, and many- ups-and-downs pipes are described in this paper. Based on the air and water two-phase flow theories, it analyzes complex hydraulic characteristics and control parameters for flushing pipeline. Combine with engineering practice, air and water cleaning diameter of 800 mm pipeline in Shanghai city and flushing pipe parameters are introduced. The flushing effect and water resource consumption reduction are analyzed, as well. 2 Theory and analysis

2.1 The principle of air-water flushing As air is fed into the water supply pipe, air and water two-phase flow is formed. Air-water two-phase flow is a very complex physical process. According to the analysis of characteristics of air-water flow in water supply pipeline, there are dispersion flow, intermittent flow and separation flow.

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

Fig.1 Air-water two-phase flow patterns

There are mainly two aspects of working principle of air-water flushing pipe: (1) The cleaning effect of air-water two-phase flow on the inner wall of tube. When air-water two phase flow in the pipe, the flow patten as intermittent flow, there is a obvious vibration and water shock. The pressure in the pipe has a great fluctuation, which causes impurities on the inner wall of the pipe to fall off. When the scattered bubbles flow in the pipeline, the function of the cleaning tube wall is similar to the filter gas and water backwashing. The vibration of the dispersed bubbles in the pipe can increase the shear force of the tube wall and break the dirt on the surface of the inner wall of the pipe to reach the washing effect. (2) Air and water intermittently flow in the pipe causes the fluctuation of velocity and pressure in the pipe, forming transient flow and cleaning the inner wall of the pipe.

When air is intermittently added into the pipeline, the velocity and pressure fluctuate in the pipe. The compressibility of the air in the pipe increases the fluctuation, causing transient flow, causing controllable water hammer. The pressure alternately produces air bubble compression and expansion, which acts on the flow velocity, with further affects the pressure in the tube. The pressure and velocity are coupled with each other, which aggravate the pressure fluctuation. The increase of pressure fluctuation enhances the inertia shear stress to pipe inner wall. Thus, the pipeline cleaning is more thorough. 2.2 Analysis of hydraulic influence of air and water flushing

Air and water flow is more complicated than the single-phase flow. It is more difficult to control the flow cleaning water supply pipeline using air and water flushing technology. With the increase of pipe diameter, the intake of air and water requirements are increased. However, the supply of air and water in the water supply system is limited. Thus, with the increase of pipe diameter, the flow velocity in the pipe is reduced, which is unfavorable to flushing effect. In descending pipe section, with the increase of pipe diameter, separation flow area expands. The decrease of fluid velocity reduces the ability of air to move downstream.

When fluid flow, the friction coefficient of air and tube wall is less than that of water and tube wall, so in the horizontal pipe, the presence of air reduces the head loss. In inclined downward pipeline, under the interaction of flow thrust and buoyancy, air drops slowly or even stays still. Because the density of air is far less than that of water, the static pressure of air is much smaller than that of water. The pressure energy recover in downward pipeline can not fully compensate the pressure eneryg consumed by the lifting fluid in the riser section, resulting in gravity pressure drop, which makes the head loss increase sharply. This is the main reason for the increase of head loss

dispersion flow intermittent flow separation flow

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

caused by air in water supply pipeline. When the inlet condition of the pipe unchanged, the increase of pipeline elevation difference makes head loss increasing, therefore, it is more difficult to rinse the pipe.

If there are many undulating sections in pipeline, the gravity pressure drop will overlap and the head loss will increase further.

Therefore, it is nesserary to improve the conditions of air-water flow, so that the air and water flushing can be carried out efficiently. 2.3 Analysis of air and water flushing control method The air and water flushing pipeline can be controlled by pipeline parameters and hydraulic parameters. (1) Pipeline parameters The pipeline parameters are mainly related to the pipes that need to be rinsed, including pipe diameter, pipeline undulation, pipe length, pipe valves, pipe exhaust valves and so on. The pipe length, pipe valves, and pipe exhaust valves are physical properties, but it can be decided or operated during the process of air and water flushing, which can be used as the operating factors. For an built pipeline to be rinsed, the length of pipe determines the number of pipe fluctuations and the total gravity pressure drop. Based on the calculation of total head loss,which including frictional head loss, local head loss and gravity pressure drop, it can be determined a reasonable length of flushing pipeline. The calculation method of total head loss of air and water has been presented in paper. Pipe valves, and pipe exhaust valves have an important influence on the control of flow velocity and air accumulation and release in the part of the pipe. It is necessary to combine the actual situation and carry out the operation. (2) Hydraulic parameters Hydraulic parameters are mainly related to fluid control factors. In the process of air and water flushing, the two phase flow pattern is very important. The decisive factors of flow pattern are air flow rate, water flow rate, intermittent flow rate of air and water. These factors need to be selected and operated according to the actual situation of pipeline flushing. Above all, through the rational selection of pipeline parameters and hydraulic parameters, the efficiency of the pipeline flushing is controlled in a high range, the washing effect is better, and the air and water flushing pipe has a wide range of applicability. 3 Case study 3.1 Basic introduction of case A engineering practice of case study has been carried on air and water flushing a water supply pipeline with diameter of 800 mm in Shanghai city. The total length of the pipeline is 6806 m, which crossing 11 rivers, including 7 horizontal directional drillings and 4 pipe bridges. The schematic of the flushing pipeline is shown in Figure 1, the way of crossing rivers and its size as shown in Table 1.

Fig.1 Schematic of flushing pipeline

Table 1 Flushing pipe sections through rivers way and dimensions

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

Sections Channel number Crossing way Elevation Difference (m)

Length (m)

S1

T1 horizontal directional drilling 15.0 252 Q1 pipe bridge 3.0 65 T2 horizontal directional drilling 13.0 200 Q2 pipe bridge 3.0 81

S2 T3 horizontal directional drilling 10.0 180 T4 horizontal directional drilling 8.0 450 Q3 pipe bridge 3.0 75

S3

Q4 pipe bridge 2.5 82 T5 horizontal directional drilling 11.0 250 T6 horizontal directional drilling 9.0 145 T7 horizontal directional drilling 20.0 188

3.2 Air and water flushing method and parameters Pipe valve, flow meter, pressure gauge and aerated pipe are installed upstream of the flushing pipe section. Valves and discharge port are installed downstream of the flushing pipe section. The downstream valve is closed, so that the flushing pipe section is relatively isolated and the flushing flow can be discharged from the outlet. The air pipe intake equipment composes air compressor, air storage tank, air supply pipe, air control valve, flowmeter, pressure gauge and central controller. The schematic diagram of air and water flushing equipment and process is shown in figure 2. Before air and water flushing, the single-phase water flow in the pipe section. The flow velocity of single-phase water in the pipe is adjusted through valve1. Then air is introduced into the pipe, and the air entry intermittence frequency is controlled by flowmeter 2 and pressure gauge 2. By controlling the opening and closing interval time by air control valve and valve 1. As to control air and water flow, the washing effect is realized. The air compressor and central controller are shown in figure 3.

Fig.2 Schematic diagram of air and water flushing equipment and process

Central controller

Air compressor

air storage tank

air control valve

Valve 1

Aerated pipe

The flushing pipe

Pressure gauge 1

Valve2（closed）

Discharge port

Water flow

Flowmeter 2

Flowmeter 1

Pressure gauge 2 Valve3（open）

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

(a) Air compressor (b) Central controller

Fig. 3 Equipments air compressor and central controller

The head loss and local head loss are calculated according to the full pipe flow, and the gravity pressure drop is calculated according to maximum pressure drop, as the most unfavorable condition, the hydraulic calculation of the pipeline is carried out. Combined with the actual conditions of pipeline, such as location of discharge port, the length of each flushing section is determined. The pipe is divided into three sections to be flushed. The length of first flushing section S1 is 3021 meters, and the flow comes from upstream connected pipe. The length of second flushing section S2 is 2553 meters, and the flow comes from upstream S1 connected section. And the length of third flushing section S3 is 1234 meters, and the flow comes from upstream S2 connected section. Each sections crossing rivers are shown in Figure1 and Table 1. The most unfavorable head loss and coming flow pressure of each section are shown in Table 2. It can be seen that the most unfavorable head loss of S1 and S2 sections are less than the coming flow pressure, and the process of air and water flushing can be carried out sucessfully. The most unfavorable water head loss of S3 section is greater than the coming flow pressure. It will be difficult to flushing with air and water, the phenomenon of air-water separation flow is formed in section. It is necessary to take measures to reduce gravity pressure drop and carry on the air and water flushing.

Table 2 The worst head loss for flushing pipe

Section The worst head loss (m)

Coming flow pressure (m)

S1 32.0 33.0 S2 25.0 31.0 S3 42.0 29.0

According to calculation of the pipe section most unfavorable head loss, combined with the outflow situation and real-time monitoring datas, the hydraulic parameters are adjusted properly, and the good washing effect is obtained. The hydraulic parameters of each pipe section, as shown in Table 3. The valve 1 is fully open during flushing each pipe section. Exhaust valve properly open to control the amount of air in pipe, thereby reducing the pressure drop. Among sections, the most unfavorable head loss is greater than the coming flow pressure in S3 pipe section, which makes it difficult to flushing with air and water.

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

Table 3 Hydraulic parameters for flushing pipe

Section Coming flow pressure (m)

Intake pressure (MPa)

Input time ratio of air and water (s)

S1 33.0 0.40-0.60 20:30 S2 31.0 0.40-0.60 20:20 S3 29.0 0.40-0.60 15:30

In the flushing parameters, the coming flow pressure is determined by water supply system, the flushing pipe location and other factors. Before flushing, hydraulic dispatching should be done by water supply department. Under the premise of ensuring the safety of pipe network, the coming flow pressure should be improved as much as possible to ensure the air and water flushing. The intake pressure of 3 flushing sections are the same as 0.40-0.60MPa, and the pressure value is greater than the coming flow pressure, therefore air can input into the pipe. Combined with the outflow situation and real-time monitoring datas, the input time ratios of air and water are obtained showing in Table 3. It can be seen that the ratio of S1 is slightly smaller than the one of S2, and the ratio of S3 is the smallest. The reason mainly due to the relationship between the most unfavorable head loss and the coming flow pressure. Special, the most unfavorable head loss of S3 is greater than the coming flow pressure. It is necessary to reduce the amount of air, reduce air and water separation length in inclined downward pipe, and reduce gravity pressure drop. At the same time, the exhaust valves at the top of downward pipe can effectively control the gravity pressure drop and achieve air and water flushing in S3section successfully. 3.3 The effect and analysis (1) Outflow situation As pipeline flushing parameters are adjusted, the air and water outflow at the outlet of the section are discharged intermittently. The outflow situations are shown in Figure 4. It can be seen that air and water in the outlet are fully mixed, and there are intermittent blasting noises during the flushing. This means that pipeline parameters and hydraulic parameters are selected reasonably. The outlet is intermittently accompanied by homogeneous outflow. The pipe flushing efficiency.

Fig.4 Outflow situation of air and water flushing pipe (2) Water quality The outflow water quality is sampled and tested every 15mins. The turbidity of water quality was analyzed by portable turbidimeter at the scene. When the turbidity meets the water quality requirement, the air and water flushing stopped, and the samples were taken to the water quality laboratory for detailed analys. The outflow water quality in S2 section is shown in Figure 5. It can be seen that the outflow water quality has increased rapidly in the early stage of flushing, and then the water quality has

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

improved rapidly in a short period of time, which will soon meet the water quality requirements of the flushing pipe.

0

200

400

600

800

1000

0 15 30 45 60 75 90 105 120 135 150 165 180 195

Turbidity / NTU

Time / min

Fig. 5 The outflow water quality in S2 section

(3) Flushing time Total time of air and water flushing for each section is 4.0 hour, 3.5 hour and 5.0 hour, respectively. The time is related to the efficiency of the flushing. Compared with S1 and S2 sections, the hydraulic condition of S3 section is relatively poor, which has been analyzed in the previous article. The efficiency of air and water flushing reduced, and total time for S3 section increased. The instantaneous single phase water flow rate in S3 section upstream is shown in Figure 7. The average flow rate in the pipe is 608.2 m3/h.

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00

Flow rate / m

3/h

Time / hour:min

Fig.7 Water flow rate of S3 section (4) Water consumption comparison According to engineering experience, the single phase water flushing diameter of 800 mm pipe is about 3 hours per kilometer. According to code for design of outdoor water cupply engineering, the velocity of flushing water is not less than 1.2m/s. The total water consumption between single phase water flushing and air-water flushing are shown in Table 4.

Table 4 Comparison of water consumption between single-phase water and air-water flushing pipe

Total flushing time

(hour) Flow rate

(m3/h) Total water consumption

(m3) Single phase water flushing 20.4 2170.4 44276.2

Air and water flushing 12.5 608.2 7602.5

average flow rate

1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, Canada – July 23-25, 2018

It can be seen that water requirement for air and water flushing is only 28.0% of the single phase water flow. The liquid-phase conversion velocity is 0.34 m/s, which greatly reduces the incoming water requirement. In the case, the project can save 36673.7 m3 water resource. The total water required for air-water flushing is only 17.2% of water consumption of the single phase water flushing, which greatly saves water resources. 4 Conclusions The principles of air-water flushing large-diameter, large-deviation-elevation, and many- ups-and-downs pipes are described in this paper.Based on the two-phase flow theories, it analyzes complex hydraulic characteristics and control parameters for flushing pipeline. Combine with engineering practice, air and water flushing diameter of 800 mm pipeline in Shanghai city and flushing pipe parameters are introduced. According to the analysis of the washing effect, water requirement for air and water flushing is only 28.0% of the single phase water flow, which greatly reduces the incoming water requirement. In the case study, the total water required for air-water flushing is only 17.2% of water consumption of the single phase water flushing, which greatly saves water resources.