The application of pipeline electromagnetic flowmeter in the water supply industry

As a flow meter with good linearity, wide range ratio, high reliability and high precision, electromagnetic flowmeter has been widely used in the water supply industry, especially in the measurement of source water and factory water at the inlet/outlet of water plants. occupies an important position. Although the design and process technology of electromagnetic flowmeters are constantly improving, many factors will directly affect the performance of the instrument during the actual application at the user's site, and it cannot meet the user's expectations. During the entire process of instrument design and selection, installation, debugging and maintenance, users need to try their best to ensure the basic conditions for the normal operation of the instrument so that the instrument runs in good working condition.

1 Design selection 1.1 Necessary conditions to ensure the measurement accuracy of electromagnetic flowmeter The measured fluid medium must be conductive; the measured fluid medium must be filled with the pipe; the flowmeter measurement system must be well grounded; the flowmeter should meet The length of the straight pipes before and after it is required; strong electromagnetic field interference should be avoided near the flow meter.

1.2 General Selection Principles Determination of Caliber The electromagnetic flowmeter can continuously measure flow in a wide flow range, and the measurement range can be adjusted arbitrarily within the specified flow (flow rate) range (0.5~10m/s). Under normal circumstances, the diameter of the flow meter is selected to be equal to the diameter of the process pipeline, which can meet the needs of the working conditions, and is easy to install without pressure loss. The relationship between flow rate, flow rate and diameter is as follows: Q=0.*D2*vv=Q/(0.*D2) where: Q=flow rate (m3/h) D=nominal diameter of flow meter (mm); V =fluid medium flow rate (m/s).

Recommended flow rate a. From the aspects of accuracy, economy and durability, the recommended flow rate range is between 1 and 5m/s. Within this range, the flowmeter has high measurement accuracy, good linearity, small power loss, and small wear and tear on the flowmeter lining and electrodes caused by the fluid medium. b. For fluid media containing solid particles, the recommended flow rate range is between 1 and 3m/s. This choice helps avoid excessive wear of the meter lining and electrodes from suspended solids caused by excessive flow rates. c. For fluid media that may cause sediment in pipelines, the recommended flow rate range is between 2 and 5m/s. If the requirements are not met, if the pressure loss is allowed, a flowmeter smaller than the pipe diameter can be selected and a reducer can be installed. Higher flow rates tend to remove excess sediment. Install the flowmeter vertically or in a V-shaped elbow to easily eliminate excess sediment. The selection of corrosion-resistant flowmeter electrode materials and lining materials should be based on the corrosiveness of the fluid medium being measured. For fluid media with complex components such as mixed acid, a coupon test should be performed.

The instrument caliber is different from that of the process pipeline. a. In order to make the instrument work in a suitable flow rate range, when the flow rate in the process pipeline is low, the process flow is relatively stable, and a certain pressure loss is allowed, the instrument can be selected If the diameter is smaller than the diameter of the process pipeline, add reducers before and after the instrument to increase the local flow rate in the instrument. b. For large-diameter process pipelines, when the flow rate in the pipeline is low and the process flow is relatively stable, you can choose an instrument with a smaller diameter.Adding reducers before and after the meter can not only reduce the cost of the meter, but also enable the meter to operate in a linear flow rate range. c. In order to ensure the measurement accuracy of the instrument, the central cone angle of the reducer should not be greater than 15°, and the upstream side of the reducer joint should have a straight pipe section of at least 5 times the diameter of the process pipe.

2 Installation 2.1 Installation site requirements The installation site and location of the electromagnetic flowmeter can be installed horizontally, vertically and tilted according to the actual needs of the user. In order to make the electromagnetic flowmeter work stably and reliably, the following requirements should be paid attention to: the flowmeter measuring tube must be filled with fluid medium (that is, no empty tube or full tube is allowed). The electrode axis of the flowmeter should be approximately horizontal. The straight pipe section upstream of the inlet of the flow meter (measured from the electrode axis) is at least 5D long, and the straight pipe section downstream of the outlet is 2D long. The flow direction of the measured fluid should be consistent with the direction pointed by the flow direction mark of the flow meter. For the convenience of installation, maintenance and upkeep, sufficient space for operation and maintenance should be ensured near the pipe flange. When the diameter of the pipe is inconsistent with the diameter of the flowmeter, gradually expanding or reducing pipes can be installed at both ends of the flowmeter. The cone angle should be less than 15°. The installation site of the pipeline electromagnetic flowmeter should avoid strong magnetic fields and strong vibration sources. There should be fixed supports on the pipes on both sides of the flow meter. The separately installed flow meter converter should be installed in a ventilated and dry place, and should be protected from rainwater and flooding. This prevents the electrical components of the instrument from getting wet, causing insulation performance degradation and damage.

2.2 Grounding requirements In order to make the electromagnetic flowmeter work stably and reliably and ensure that its measurement accuracy is not interfered by external electromagnetic fields, the flowmeter should have good independent grounding. If the pipe connecting the flow meter is coated with an insulating layer or is a non-metallic pipe, the flow meter should be equipped with a grounding (liquid) ring.

3 Measurement accuracy and error curves The flow meter measurement accuracy and error curves given by the electromagnetic flowmeter manufacturer refer to technical indicators under reference working conditions. Users should pay attention to the actual application working conditions. There is a difference. According to the JB/T "Electromagnetic Flowmeter" industry standard, the reference working conditions are: ambient temperature: 20±2; relative humidity: 60% ~ 70%; power supply: rated voltage ±1%; installation conditions: upstream straight pipe section Length>10D; Downstream straight pipe length>5D; Preheating time:>15min.

3.1 Error curve The error curve given by the manufacturer represents the linearity change trend of the flow meter within its measurement range, which corresponds to the given degree index. Taking the electromagnetic flowmeter of Kaifeng Instrument Factory as an example: the degree is: ±0.3% of the indication value (flow rate ≥1m/s); or ±3 mm/s (flow rate <1m/s). Therefore, at 0.5 m/s, the flow meter allows an error of ±0.5%, and at 0.3 m/s, the flow meter allows an error of ±1.0%.

3.2 Working condition flowUnder the working conditions of the meter, due to the influence of various factors, the measurement accuracy may be different from the real flow calibration accuracy given by the manufacturer under reference conditions. For example, according to industry standards, for every 10% change in temperature, the change in measurement accuracy should not be greater than 1/2 of the basic error limit of the instrument. When the temperature changes by 20%, the measurement accuracy of ±0.3% may become ±0.6%. During the operation of the flow meter, the user should consider the impact of factors such as noise interference, restrictions on installation conditions, ambient temperature changes, humidity changes (in a long-term humid environment, the insulation strength of the flow meter decreases.) on the measurement accuracy of the flow meter. Usually a 0.5-level flow meter actually reaches level 1, and it should be considered that the user's operation and maintenance of the flow meter is better.

4 Measurement range and system zero point 4.1 Measurement range The flow range of electromagnetic flowmeter is relatively wide, and the corresponding flow rate range is 0~10m/s. Some manufacturers’ flowmeters are 0~15m/s. Theoretically, changing the range will not affect the measurement accuracy and linearity of the flow meter, but choosing an appropriate range will help improve the unit resolution of the 4-20mA analog output signal. If the 100m3/h range can meet user requirements, do not choose the 200m3/h range, otherwise the unit resolution of the analog output signal will be reduced by half. 4.2 Under normal operation of the system zero point, the system zero point of the electromagnetic flowmeter will change with the long-term operation of the system due to factors such as aging of components, reduction of the insulation strength of the excitation wire package, polarization and contamination of the measuring electrode, and increase in the system ground resistance (potential). It will cause changes and drifts in the system zero point. Users should regularly check the system zero point of the flow meter and perform adjustments and maintenance. Corresponding to the flow rate of 1m/s, if the system has a zero point of ±5mm/s, it will cause an additional error of ±0.5%. Generally speaking, when the flow rate is small, the additional error caused by the system zero point is larger. Since the system zero point always exists more or less, the manufacturer will adjust it to a minimum according to the regulations when the flow meter is calibrated for real flow out of the factory. The user also needs to make adjustments at the application site. Therefore, the error curve given by the manufacturer objectively reflects the existence of the zero point of the flow meter system, thus causing the nonlinearity of the instrument. If only the measurement accuracy of the electromagnetic flowmeter is given without giving the error curve, it actually cannot clearly reflect the measurement accuracy of the flowmeter in the entire measurement range.

5 Signal Reference and DC Noise 5.1 Signal Reference Electromagnetic flowmeter is based on Faraday’s law of electromagnetic induction (when the conductive fluid traverses the magnetic field, an electromagnetic flow meter is induced in the conductive fluid that is proportional to the fluid flow rate) For the basic principle of flow meter. In order to effectively pick up the mV-level flow signal induced on the two measuring electrodes and suppress the interference signal, the flow signal is differentially transmitted from the sensor to the differential amplifier signal input end of the converter, and the "zero resistance" fluid medium " "Ground" serves as the signal ground terminal of the differential amplifier. The differential amplifier amplifies the difference in flow signals and overlapsThe common mode interference signal added to the flow signal is suppressed and attenuated. When there is common mode interference such as ground loop ground current, electrode polarization voltage, electrostatic coupling voltage between the excitation loop and the electrode measurement loop, as long as the working parameters of the differential amplifier are symmetrical, the common mode interference will not affect the amplifier's response to the flow signal. Amplified unless the common-mode interference voltage exceeds the common-mode rejection range of the amplifier. During the installation and maintenance of electromagnetic flowmeters, users must fully realize the importance of grounding the flowmeter system. In order to explain the difference between groundings more clearly, some manufacturers call the ground used as the reference for flow signals called liquid ground ( Wetted parts: grounding electrode or grounding ring, etc.), the physical ground is called the earth. Liquid ground not only requires the grounding resistance to be as small as possible, but also has good stability and reliability. The purpose of the earth is mainly to reduce the interference and protection of the flow meter system from the external environment, such as electromagnetic field radiation, loop current between the fluid medium ground and the earth, lightning strikes, etc.

5.2 DC noise When the flow rate is relatively stable, if the flow meter output fluctuates greatly, the main influencing factor is the DC noise superimposed on the measurement signal. If the fluid medium ground is in good contact with the earth, the cause of DC noise is the polarization voltage present on the electrodes. Between the electrode and the electrolyte fluid medium, a certain electric field is generated due to the directional movement of positive and negative ions in the liquid, thus forming a polarization voltage that drifts between the electrode and the liquid ground. The polarization voltage is superimposed on the flow signal in the form of common mode interference, blocking the differential amplifier so that the flow signal cannot be amplified. The flow meter cannot complete normal signal sampling, or its drift changes cause fluctuations in the flow signal. In the water supply industry, polarization voltage generally does not occur. However, in the chemical industry, due to the corrosion of the fluid medium and the difference between the electrode and the liquid-contacting material, the polarization voltage is easily generated. Therefore, the liquid-contacting material is required to be consistent with the measuring electrode material. Even if a polarization voltage is generated, the potential that generates the polarization voltage is the same because the materials are the same, so the common mode voltage between the electrode and the measurement reference is small.

6. Online calibration of ultrasonic flowmeters. Electromagnetic flowmeter manufacturers use standard container method, weighing method or standard table method to calibrate electromagnetic flowmeters with real flow. At present, there is no online calibration equipment for electromagnetic flowmeters recognized by all parties. ABB provides users with a device called "CalMaster", which can be regarded as an expert evaluation system for electromagnetic flowmeters. The flowmeter is evaluated online. Based on the comparison between the historical parameter record when the product leaves the factory and the online product operating status parameters, the possible error in the measurement accuracy of the electromagnetic flowmeter compared with the original real-flow calibration when it leaves the factory is given. If the user feels that there is a problem with the measurement accuracy during the use of the flowmeter, he should send it to the relevant department for verification. Using the method of online comparison of ultrasonic flowmeters, we can qualitatively evaluate whether the electromagnetic flowmeter is in good operating condition and eliminate the possibility of large measurement errors in the electromagnetic flowmeter. The measurement accuracy of ultrasonic flowmeter is lower than that of electromagnetic flowmeter because electromagnetic flowmeterThe meter measures the surface average flow velocity on a circular area, and the nominal diameter and out-of-roundness error of the electromagnetic conduit have been corrected by the meter coefficient during the real flow calibration at the manufacturer. However, the ultrasonic flowmeter measures the linear average flow velocity of the axial section of the pipeline. The influence of flow velocity distribution distortion should be considered, and it is difficult for users to give quantitative values ​​for on-site pipeline diameter and out-of-roundness errors. The length of the straight pipe section of the ultrasonic flowmeter requires 10D upstream and 5D downstream. The actual length of the single-channel ultrasonic flowmeter should be longer, which is often not achieved under site conditions. Factors such as pipe wall corrosion, coating protection and thickness also affect the measurement accuracy of ultrasonic flow meters.

7. The pump flow rate cannot determine whether the electromagnetic flowmeter is accurate. In the process of using the electromagnetic flowmeter, users often judge whether the electromagnetic flowmeter is accurate based on the pump flow rate. However, pump flow cannot determine whether the electromagnetic flowmeter is accurate. The flow value and lift value specified on the pump nameplate should be nominal values ​​representing the pump capacity. Under actual working conditions, the pump flow rate changes due to changes in lift, efficiency, power and pipe network load. Class C pump flow may achieve a tolerance of ±8%. Multiple pumps operate in parallel, and the flow rate flowing into the main pipe is not necessarily equal to the sum of the flow rates of each pump.

8 Measuring electrode pollution protection It is difficult to quantify the extent to which the pollution of the electromagnetic flowmeter affects the measurement accuracy of the electromagnetic flowmeter. When the fluid medium may cause sediments and contaminate the electrodes, appropriate flow rates (selected pipe diameters) and installation methods should be selected in terms of design and installation to avoid excessive sediments. Electrode contamination physically manifests as an increase or decrease in the signal input impedance of the electrode measurement loop. As long as the impedance does not exceed a certain limit, there will be no side effects on the electromagnetic flowmeter measurement. High input impedance (1011Ω) differential amplifier design parameters can avoid the influence of electrode contamination (increased impedance). All electromagnetic flowmeters can achieve this technical index. Therefore, in the water supply industry, electrode contamination is not a problem that should be paid attention to in the application of electromagnetic flowmeters.

9 There are two main forms of lightning protection in the system: one is the discharge between different charged cloud layers, and the other is the discharge from the cloud layer to the ground. The latter is the main source of lightning damage. Direct lightning strikes, Lightning electrostatic induction and electromagnetic induction are the main factors causing instrument damage. The purpose of instrument lightning protection is not to avoid lightning strikes, but to protect the instrument from being damaged by lightning waves. When designing electromagnetic flowmeter products, manufacturers have considered certain protective measures for the lightning protection and anti-interference of the flowmeter system. For example: the power supply part is equipped with a transient suppression diode or discharge tube; the power supply and signal input/output are electrically isolated; the digital communication interface uses anti-lightning devices; the signal connection cable between the sensor and the converter is triple shielded; the flow meter is required The system is well grounded, etc. However, in areas with frequent and intense lightning, users should take further measures: install a 1:1 transformer and lightning arrester at the power input end to avoid power supply breakdown. The signal analog output is equipped with a lightning arrester. The flowmeter system must be well grounded, and the liquid ground must be well connected to the earth to avoid the flow of lightning current conducted by pipelines and fluid media.Instrument body.