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Flow measurement

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Flow measurement

Flow measurement: at present, most countries in the world adopt the control method of combining pollutant emission concentration control and total amount control. Therefore, when understanding the sewage quality, we should also measure the water level (m), flow rate (M / s), flow rate (m3 / s) and other hydrological parameters to calculate the environmental capacity, control the emission of pollution sources, and estimate the pollution control effect.
（1） Measurement of surface water flow
For larger rivers, hydrological departments generally have hydrological monitoring sections, so the measured parameters should be used as much as possible. If there is no hydrological section in the monitored river section, the section with stable hydrological parameters and representative discharge should be selected for measurement
one Velocity area method
First, divide the measuring section into several small pieces, measure the area and velocity of each small piece, calculate the corresponding flow, and then add up the flow of each small section, that is, the water flow on the section
（ 3-1）
Where: Q -- water flow, m3 / S;
——The average horizontal velocity on each small section, M / S;
F1, F2... FN -- the area of each small section, M2.
The flow velocity is mostly measured by current meter, which includes single point current meter (such as mechanical current meter, electric current meter, etc.), three-dimensional current meter (such as Doppler acoustic current meter, etc.) and multi-functional intelligent current meter. The flow velocity range of 0.020-10m / s can be measured by using different settlement devices.
two Buoy method
Buoy method is a simple method to measure the velocity of water in small rivers and canals. During the survey, a straight river section is selected to measure the three cross-sectional areas of the starting point, the middle point and the end point within 2m interval of the river section, and the average cross-sectional area is calculated on this basis. Put the buoy in the upstream to measure the time required for the buoy to flow through the determined reach (L). Repeat the measurement for several times to calculate the average time (T) to calculate the flow velocity (LGT), and then calculate the flow rate according to the following formula:
（ 3-2）
Where: Q -- water flow, m3 / min;
——Average velocity of buoy, M / s, i.e. L / T;
S -- cross section area of discharge, M2.
K is the coefficient of buoy, which is related to the air resistance and the uniformity of velocity distribution on the cross section. Generally, it needs to be calibrated with a current meter, and its range is 0.84-0.90.
（2） Waste water flow measurement
one Flowmeter method
At present, more than ten kinds and hundreds of kinds of sewage flow meters have been developed at home and abroad, mainly including volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic and overflow weir flowmeter. The suitable flowmeter can be selected according to the flow range and test accuracy of the actual water flow.
two Volumetric method
The sewage is introduced into a container or sewage pool with known volume, the time of full flow of the container or sewage pool is measured, and then the volume of the container or sewage pool with its receiving volume is divided, and the flow can be calculated. This method is simple and suitable for measuring the continuous or intermittent discharge of sewage with small sewage flow.
three Overflow weir plate method
Overflow weir plate method is the main method to measure water flow. This method is suitable for the measurement of water flow in irregular sewers. In this method, triangular or rectangular or trapezoidal weir plates are used to block the flow and form overflow weir. The water head and water level before and after the weir plate are measured and the flow is calculated.
Figure 3-3 is a schematic diagram of flow measurement with triangular weir method. The flow calculation formula is as follows:
（ 3-3）
Where: Q -- water flow, m3 / S;
H -- weir head height, m; K -- discharge coefficient; D -- height from flow bottom to weir edge, m;
B -- upstream flow width of weir, M.
Under the following conditions, the error of the above formula is ＜ ± 1.4%：
0.5m≤ B≤ 1.2m
0.1m≤ D≤ 0.75m 0.07m≤ h≤ 0.26m H≤ B/3

Flow measurement: at present, most countries in the world adopt the control method of combining pollutant emission concentration control and total amount control. Therefore, when understanding the sewage quality, we should also measure the water level (m), flow rate (M / s), flow rate (m3 / s) and other hydrological parameters to calculate the environmental capacity, control the emission of pollution sources, and estimate the pollution control effect.

（1） Measurement of surface water flow

For larger rivers, hydrological departments generally have hydrological monitoring sections, so the measured parameters should be used as much as possible. If there is no hydrological section in the monitored river section, the section with stable hydrological parameters and representative discharge should be selected for measurement

one Velocity area method

First, divide the measuring section into several small pieces, measure the area and velocity of each small piece, calculate the corresponding flow, and then add up the flow of each small section, that is, the water flow on the section

（ 3-1）

Where: Q -- water flow, m3 / S;

——The average horizontal velocity on each small section, M / S;

F1, F2... FN -- the area of each small section, M2.

The flow velocity is mostly measured by current meter, which includes single point current meter (such as mechanical current meter, electric current meter, etc.), three-dimensional current meter (such as Doppler acoustic current meter, etc.) and multi-functional intelligent current meter. The flow velocity range of 0.020-10m / s can be measured by using different settlement devices.

two Buoy method

Buoy method is a simple method to measure the velocity of water in small rivers and canals. During the survey, a straight river section is selected to measure the three cross-sectional areas of the starting point, the middle point and the end point within 2m interval of the river section, and the average cross-sectional area is calculated on this basis. Put the buoy in the upstream to measure the time required for the buoy to flow through the determined reach (L). Repeat the measurement for several times to calculate the average time (T) to calculate the flow velocity (LGT), and then calculate the flow rate according to the following formula:

（ 3-2）

Where: Q -- water flow, m3 / min;

——Average velocity of buoy, M / s, i.e. L / T;

S -- cross section area of discharge, M2.

K is the coefficient of buoy, which is related to the air resistance and the uniformity of velocity distribution on the cross section. Generally, it needs to be calibrated with a current meter, and its range is 0.84-0.90.

（2） Waste water flow measurement

one Flowmeter method

At present, more than ten kinds and hundreds of kinds of sewage flow meters have been developed at home and abroad, mainly including volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic and overflow weir flowmeter. The suitable flowmeter can be selected according to the flow range and test accuracy of the actual water flow.

two Volumetric method

The sewage is introduced into a container or sewage pool with known volume, the time of full flow of the container or sewage pool is measured, and then the volume of the container or sewage pool with its receiving volume is divided, and the flow can be calculated. This method is simple and suitable for measuring the continuous or intermittent discharge of sewage with small sewage flow.

three Overflow weir plate method

Overflow weir plate method is the main method to measure water flow. This method is suitable for the measurement of water flow in irregular sewers. In this method, triangular or rectangular or trapezoidal weir plates are used to block the flow and form overflow weir. The water head and water level before and after the weir plate are measured and the flow is calculated.

Figure 3-3 is a schematic diagram of flow measurement with triangular weir method. The flow calculation formula is as follows:

（ 3-3）

Where: Q -- water flow, m3 / S;

H -- weir head height, m; K -- discharge coefficient; D -- height from flow bottom to weir edge, m;

B -- upstream flow width of weir, M.

Under the following conditions, the error of the above formula is ＜ ± 1.4%：

0.5m≤ B≤ 1.2m

0.1m≤ D≤ 0.75m 0.07m≤ h≤ 0.26m H≤ B/3

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