Flow measurement of open channel at the water stat

River side water station open channel flow measurement

1 Preface

the state collects resource fees according to the pump capacity of Jiangxin water station and Jiangbian water station for Wuhan Iron and Steel Corporation to quote Changjiang water. Therefore, measuring its water volume can not only make the water charge more reasonable, but also provide accurate data for the production of Wuhan Iron and Steel Corporation. According to the requirements of the technical center and the general energy plant, WISCO measurement and control company and youyou have also provided development opportunities for the lightweight of automotive materials. The scientific and technological personnel of relevant units have worked together to develop a multi-channel ultrasonic flow device, which is used in the open channel of WISCO river water station to measure the water flow. After half a year of trial operation, the system has achieved good results and achieved the expected goal

2. Basic theory of measurement

2.1 basic calculation formula of velocity

according to the basic theory of open channel, for the constant and uniform flow in open channel, it is known that there is the following calculation formula of point velocity u in logarithmic form along the direction of water depth

, U -- point velocity

u* -- flow velocity of rub Yin

c -- constant

h -- section water depth

h -- depth at point velocity

△ -- absolute roughness of channel wall

2.2 working principle of ultrasonic transducer

working principle of ultrasonic transducer measurement, measured by time difference method. That is, by measuring the propagation time difference of ultrasonic wave in "downstream" and "countercurrent", the flow velocity of the flow layer where the ultrasonic transducer is located can be obtained, and then the flow can be obtained according to the basic formula of flow velocity (1) and section data. As shown in Figure 1, a pair of ultrasonic transducers p1p2 are set obliquely in the channel, the distance between the two transducers is l, and the included angle between the sound path formed by them and the water flow direction is θ。

when P1 emits ultrasonic waves and P2 receives them, the propagation time of ultrasonic waves (forward propagation time)

in the formula, C -- ultrasonic velocity in still water

u -- velocity in the flow layer

similarly, when P2 emits ultrasonic waves and P1 receives them, the propagation time of ultrasonic waves (reverse propagation time):

the time difference between forward and reverse propagation is:

because the ultrasonic velocity in still water is related to temperature, according to (2) (3) In the two formulas, C is replaced by T1 and T2 to get

from formula (5), as long as the forward and reverse propagation times T1 and T2 of ultrasonic can be measured, the flow rate U of the flow layer where the transducer is located can be calculated, and then the flow rate can be calculated together with the measured flow rates of other flow layers

2.3 measurement method

according to the above basic measurement theory and in combination with the actual situation on site, it is decided to use four groups of ultrasonic transducers to measure the velocity of flow in each channel, and an ultrasonic water level gauge to measure the water level. See Figure 2 for the installation position of velocity transducer, water level well and water level gauge transducer in the channel. After measuring the water level and the flow velocity of each flow layer, the average flow velocity of the section can be calculated by the following formula

in the formula, UI -- flow velocity in a certain flow layer

ki -- weighting coefficient

calculate the flow from the average velocity

:

in the formula, B -- the channel should immediately open the scavenge pump to unload the sample section width

h -- section water depth

1, 2, 3, 4, 5, 6, 7, 8 in the figure show the installation position of the transducer in the channel. The number 9 refers to the installation position of the ultrasonic transducer of the water level well and water level gauge. 1～5； 2～6； 3～7； No. 4 ~ 8 transducers form four layer velocity measurement groups that transmit and receive each other to measure the velocity of different layers respectively. In the figure, H indicates the direction of water level and s indicates the direction of water flow

2.4 characteristics of this measurement method

(1) the flow can be measured in a large range. When the channel flow is small and the water level is low, it can be measured. When the channel flow is large and the water level is high, it can also be measured

(2) it can adapt to flow pattern distortion. When the flow changes or is disturbed, the cross-section velocity distribution curve will be deformed, that is, flow pattern distortion. Four groups of transducers can receive more information

(3) the measurement range is wide and the accuracy is high. Four groups of transducers are used in one channel to ensure that the measurement accuracy remains unchanged at different water levels and flow velocities in the channel

3. On site installation and commissioning

there are two canals at the riverside water station, on which there are dispatching control gates, gates in front of the embankment and gates behind the embankment. The canal is built underground and flows into the inland river through the levee. When the water level of the Yangtze River is lower than that of the inland river, 2 ~ 4 of the 9 units at the riverside water station pump the river water into two canals and discharge it into the inland river. When the water level of the Yangtze River is higher than that of the inland river, the river water is controlled by the rectangular equal cross-section strip sample size control gate through the two canals. The river water is directly discharged into the inland river through these two canals, and the water volume in the canal can be controlled by the unit or the dispatching control gate, The total length of the two canals is 187.9m, and the channel section is 2m × 4m × 3.5m, and the straight channel from the gate in front of the embankment to the gate behind the embankment is 85m. According to the specific conditions of the two channels, the ultrasonic flowmeter transducer is installed in the straight channel between the front gate and the back gate near the back gate. The flowmeter operation room is built next to the gate shaft behind the embankment. This choice meets the technical requirements of equipment installation, and also ensures the safe operation and maintenance of the equipment. See Fig. 3 for on-site installation of open channel flow

3.1 factors causing measurement errors during on-site installation

(1) inaccurate measurement of geometric parameters on the measurement section

① sound path length L measurement