Application of Flow Measuring Instrument in Water Transmission Pipeline
Time:2022-11-24 Read:719
Reading guide: This article will introduce the selection, characteristics, installation, use, and calibration of three types of flow meters respectively according to the practical application experience of water transmission pipelines in power plants.

 The water supply project is an important infrastructure for many industrial production projects, including power plants. In the process of water supply, it is necessary to ensure that the domestic water, production water, and other water used meet the requirements of water quality, quantity, and pressure. The flowing instrument of the water transmission pipeline plays an important role: the water flow measurement at the inlet and outlet of the water plant is the basis for the water plant to calculate the water supply, water production cost, pipe leakage in the water supply system and other important production indicators, and is an indispensable measurement link in the water supply industry.

Application of electromagnetic flowmeter in the water supply industry

Electromagnetic flow has been widely used in the water supply industry because of its good linearity, wide range ratio, strong reliability, and high accuracy.

(1) Selection of electromagnetic flowmeter. ① Caliber selection. The electromagnetic flowmeter can continuously measure the flow in a wide flow range and can be adjusted arbitrarily within the specified flow rate range (0.5~10m/s). In order to promote the flow instrument to work within the appropriate flow rate range, when the flow rate in the process pipe is low, the flow rate is relatively stable, and a certain pressure loss is allowed, the instrument diameter smaller than the diameter of the process pipe can be selected, and the internal flow rate of the instrument can be improved by strengthening reducers at both ends of the instrument diameter. In addition, to ensure the accuracy of instrument measurement, the center cone angle of the reinforced reducer shall not be greater than 15 °, and the upper section of the reducer shall have a straight pipe section at least 5 times the process pipe diameter. ② Setting of flow rate. According to the considerations of economy, practicality, and accuracy, the appropriate flow rate range for the use of an electromagnetic flowmeter is 1~5m/s; For the fluid medium containing solid particles in the liquid, the flow rate range should be controlled within 1~3m/s; If it is a fluid medium that may cause sediment on the pipeline, the best range is 2~5m/s.

(2) Characteristics of electromagnetic flowmeter. Because the measurement results of an electromagnetic flowmeter are not related to physical parameters such as water pressure, temperature, and conductivity, it has strong reliability and high accuracy. There is no blocking element in the measuring pipe, so there is no additional pressure loss in the flow meter. In the measurement process, only the pipeline and electrode are in contact with the measured liquid, so it is only necessary to select appropriate electrode materials to achieve the effect of corrosion resistance. In addition, the electromagnetic flowmeter has a long service life and low maintenance requirements. The electromagnetic flowmeter can measure the flow of positive water. Moreover, compared with other flowmeters, their output signal modes are more flexible and diverse.

(3) Use and installation of electromagnetic flowmeter. In the selection of diameter, we mentioned that the upper section of the reducer should have a straight pipe section at least 5 times the process pipe diameter, while its lower section needs a straight pipe section at least 3 times the length. The length of the front pipe of the flowmeter shall be at least 10 times the pipe diameter. During the installation of the electromagnetic flowmeter, the sensor grounding ring must be tightly grounded, and the grounding resistance shall not be greater than 10 Ω. At the same time, special attention should be paid to external electromagnetic interference, especially to avoid power frequency electromagnetic interference in the environment where an electromagnetic flowmeter is used for measurement. In addition, the best way to stabilize the flow pattern is to install a rectifier in a straight pipe with a diameter of 5-10 times on the electromagnetic flowmeter.

(4) Online calibration mode of electromagnetic flowmeter. There are two main methods for online calibration of electromagnetic flowmeter: electrical parameter method and standard meter method. The electrical parameter method checks the relevant parameters of the pipeline electromagnetic flowmeter through a certain measuring principle, including the resistance of the excitation coil, the insulation resistance of the excitation coil, the resistance of the electrode to the ground, etc. By detecting the change degree of these parameters and comparing them with the original factory data, if they are within the limited range, their accuracy will not be affected and they can be used with confidence; The standard meter method is to use the principle of the standard meter to let water (fluid) continuously pass through the calibrated meter and the standard meter at the same interval, compare the output flow value generated and determine the accuracy of the calibrated meter.

Application of electromagnetic flowmeter in the water supply industry

Application of ultrasonic flowmeter in the water supply industry

(1) Principle of the ultrasonic flowmeter. Time difference ultrasonic flowmeter measures the impact of water flow velocity on the two-way acoustic signal through advanced digital information processing technology and acoustic time difference detection method. Two sensing probes (T1 and T2) are installed at the outer side of the pipeline according to the limited interval distance to alternately transmit and receive signal waves. When the generated ultrasonic wave propagates in the static water flow, the upstream sensing probe T1 will send a signal to the downstream sensing probe T2. At this time, the downstream T2 probe also sends signals to the upstream T1 probe. Due to the effect of the water flow speed on the ultrasonic signal, the signal transmission speed from top to bottom is accelerated, and the signal transmission speed from bottom to top is slowed down. As a result, there is a time difference between the two signals, so the flow velocity of the water can be calculated, and then the corresponding flow can be calculated through the relevant calculation formula.

(2) Advantages of ultrasonic flowmeter application in the water supply industry. ① No additional technical processing is required. The ultrasonic flowmeter is a non-contact structure instrument, which can measure the flow through the external environment of the pipeline. In addition, all measuring components are installed outside the pipeline, which will not cause any pressure loss in the pipeline and thus will not affect the flow of water. ② The measurement method is convenient. The results of measurement with an ultrasonic flowmeter will not be affected by the viscosity of water flow and other characteristics of water quality, and the measurement of water flow can be completed during the process of water flow in the pipeline.

(3) The practical application of ultrasonic flowmeter. According to different applications, ultrasonic flowmeters can be divided into fixed ultrasonic flowmeters and portable ultrasonic flowmeters. ① Fixed ultrasonic flowmeter. If the installation is carried out in a fixed position, the fixed ultrasonic flowmeter shall be used. For long-term continuous measurement of water flow in a specific pipeline, a 220V AC power supply is required. At the same time, a fixed ultrasonic flowmeter usually has a signal output function for remote display. It's internal and can only store the parameters of one pipe. ② Portable ultrasonic flowmeter. Compared with the fixed ultrasonic flowmeter, the portable ultrasonic flowmeter has stronger mobility and is applied to the temporary measurement of water flow in different pipelines. The portable ultrasonic flowmeter can use either any AC power supply at the metering site or its built-in charging power supply. Because the portable ultrasonic flowmeter is mainly used for temporary measurement of pipeline water flow, it has no output function, but it can store the parameters of multiple pipelines.

(4) Online calibration method of the ultrasonic flowmeter. Select an appropriate ultrasonic flowmeter according to the actual situation of the pipeline, install the sensor on the pipeline to be tested in an appropriate way, record the initial flow value of the portable ultrasonic flowmeter and the fixed ultrasonic flowmeter, and then, after a period of time, calculate the final value of the two ultrasonic flowmeters to obtain the cumulative flow value, and then calculate the correction factor through the formula.

P correction factor=[1 - (S portable - S fixed)/S portable] × 100%

The fixed ultrasonic flowmeter shall be modified according to the modified parameters of the portable ultrasonic flowmeter compared with the fixed ultrasonic flowmeter, so as to ensure the accuracy of each data in the water supply measurement.

Application of ultrasonic flowmeter in the water supply industry

Application of vortex flowmeter in the water supply industry

(1) The principle of vortex flowmeter. A vortex flowmeter, also known as Karman Vortex Flowmeter, is a separate flowmeter developed based on the principle of the natural oscillation of water flow. When the water flow passes through an object perpendicular to the water flow direction at a high enough velocity, there will be vortex trains with opposite directions but arranged neatly on the two parallel lines behind the physics. By the formula F=S × V/d (where S is the Strouhal number, v is the average velocity of the water flow in the pipe, and d is the width of the cylinder facing), and then according to the principle that the frequency of the vortex street is proportional to the flow velocity, measure the frequency of the vortex street to deduce the flow velocity, and then measure the flow rate of the water flow.

(2) Structure and composition of vortex flowmeter. A vortex flowmeter consists of transmitter housing, vortex generation, signal detection, output amplifier, pulse shaper, and input amplifier. The transmitter housing is an integral part of the water flow pipeline. By selecting the appropriate shape and size of the vortex street sounder, the flow in the transmitter housing can generate a relatively stable vortex street signal within the Reynolds number range. The vortex street generator makes the flow generates a vortex train in the flow process, and the signal detector will convert the vortex train to a pulse signal. The output amplifier amplifies the weak and imperceptible electrical signal. The pulse shaper converts the irregular electric pulse into a square wave signal whose amplitude is proportional to its width. The output amplifier amplifies and converts the square wave signal into, and outputs the square wave pulse signal at the same time.

(3) Installation of vortex flowmeter. In order to ensure the measurement accuracy of the vortex flowmeter, straight pipe sections with appropriate lengths must be reinforced before and after the installation of the vortex flowmeter. If there is diameter-reducing resistance on the upstream side of the vortex flowmeter, a 15D long straight pipe section shall be reinforced; If there are elbows on the same side, the straight pipe section of 20D must be reinforced; The straight pipe section larger than 5D shall be reinforced at the downstream side of the vortex flowmeter. Vortex flowmeter can be installed at horizontal, vertical, and other positions. If the water flow is measured vertically, the flow rate must be ensured from bottom to top to ensure that the pipeline is fully filled with water.

(4) Application of vortex flowmeter. A vortex flowmeter is used in the water supply system. In order to prevent the instrument from losing due to water inflow in the rainy season, the instrument must be moved out of the instrument well in advance, or the separated sensor must be used to modify it. When there is a high-power generator around the vortex flowmeter, avoid power frequency interference affecting its accuracy. In this regard, the transmission cable can be shielded, and the signal output mode is changed to use a 4~20mA DC signal instead of the pulse signal.

Application of vortex flowmeter in the water supply industry

Conclusion

As for the selection of flow measuring instruments for water transmission pipelines, electromagnetic flowmeters, ultrasonic flowmeters, and vortex flowmeters have their advantages and disadvantages in different applications due to their different working principles. We should select appropriate instruments according to their characteristics, and fully use their experience feedback on installation, use, and calibration to ensure the accuracy, reliability, and economy of the instruments.