Keywords: electromagnetic flowmeter, design and selection, measurement accuracy, zero point

　　Subject: Discussion on application experience of electromagnetic flowmeter in water supply industry

　　As a good linearity, wide range ratio, high reliability and high accuracy flowmeter, electromagnetic flowmeters have been widely used in the water supply industry, especially in the measurement of source water and factory water in water inlets and outlets of water plants. Status. Although the design and process technology of electromagnetic flowmeters are constantly improving, in the process of actual application at the user's site, many factors will directly affect the performance of the instrument, and cannot meet the user's expectations. During the entire process of meter design, selection, installation, commissioning, and maintenance, the user needs to ensure the basic conditions for normal operation of the meter as far as possible, so that the meter runs in a good working state.

　　1 Design selection

　　1.1 Necessary conditions to ensure the measurement accuracy of electromagnetic flowmeters

　　⑴ The fluid medium under test must be conductive;

　　⑵ The fluid medium to be tested must fill the pipeline;

　　⑶ The flowmeter measurement system must be well grounded;

　　⑷ The flow meter should meet the requirements of the length of the straight pipe before and after;

　　避免 Avoid strong electromagnetic field interference near the flowmeter.

　　1.2 General Selection Principles

　　确定 Determination of caliber

　　Electromagnetic flowmeter can continuously measure the flow in a wide flow range. Within the specified flow (flow rate) range (0.5 ～ 10m / s), the measurement range can be adjusted arbitrarily. In general, the diameter of the flowmeter is equal to the diameter of the process pipeline, which can meet the requirements of the working conditions, and is easy to install without pressure loss.

　　关系 The relationship between flow, velocity and caliber is as follows:

　　Q = 0.0028274 * D2 * v

　　V = Q /(0.0028274 * D2)

　　Where: Q = flow rate (m3 / h)

　　D = nominal diameter of flowmeter (mm);

　　V = fluid flow rate (m / s).

　　⑵ Recommended flow rate

　　A. From the perspective of accuracy, economy and durability, the recommended flow rate range is between 1 ~ 5m / s. In this range, the flowmeter has high measurement accuracy, good linearity, small power loss, and less wear of the fluid medium on the lining and electrodes of the flowmeter.

　　B. For fluid media containing solid particles, the recommended flow rate range is between 1 and 3 m / s. This choice helps avoid excessive wear on the flowmeter lining and electrodes caused by suspended solids.

　　C. For fluid media that may cause sediment in the pipeline, the recommended flow rate range is between 2 and 5 m / s. If the requirements are not met, if the pressure loss allows, a flowmeter with a diameter smaller than the pipe diameter can be selected, and a reducer can be installed. Higher flow rates tend to eliminate excessive deposits. The flow meter is installed vertically or installed at the V-shaped elbow, which is easy to eliminate excessive deposits.

　　⑶ Corrosion resistance

　　The selection of flowmeter electrode material and lining material should be selected according to the corrosivity of the fluid medium to be measured. For fluid media with complex components such as mixed acids, a hanging test should be performed.

　　⑷ The caliber of the instrument is different from the caliber of the process pipe

　　a. In order to make the instrument work in a proper flow rate range, when the flow rate in the process pipeline is relatively low, the process flow is relatively stable, and a certain pressure loss is allowed, the diameter of the instrument can be selected smaller than the diameter of the process pipeline, and reducers are added before and after the instrument. Increase the local flow rate in the meter.

　　b. For large-caliber process pipelines, when the flow velocity in the pipeline is relatively low and the process flow is relatively stable, instruments with smaller calibers can be selected. Adding reducers before and after the instrument can reduce the cost of the instrument on the one hand, and can also The meter operates in a linear flow range.

　　C. In order to ensure the accuracy of the instrument measurement, the central cone angle of the reducer should not be greater than 15 °, and the upstream side of the reducer joint should have at least 5 times the straight pipe section of the process pipe diameter.

　　2 Installation

　　2.1 Installation site requirements (the installation method shown in Figure 1 is recommended)

　　The installation place and position of the electromagnetic flowmeter can be installed horizontally, vertically and obliquely 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:

　　⑴ The measuring tube of the flowmeter must be filled with fluid medium (that is, no empty tube or full tube is allowed).

　　轴线 The electrode axis of the flowmeter should be approximately horizontal.

　　⑶ The straight upstream pipe section of the flowmeter (measured from the electrode axis) is at least 5D long, and the downstream straight pipe section of the outlet is 2D long.

　　⑷ The direction of flow of the measured fluid should be consistent with the direction indicated by the flow direction mark of the flowmeter.

　　⑸ For the convenience of installation, maintenance, and maintenance, sufficient space for operation and maintenance should be ensured near the pipe flange.

　　⑹ When the diameter of the pipeline is not consistent with the diameter of the flowmeter, you can install a gradually expanding tube or a reducing tube at both ends of the flowmeter, and the cone angle should be less than 15 °

　　安装 The installation place of electromagnetic flowmeter should avoid strong magnetic field and strong vibration source. There should be fixed supports on the pipes on both sides of the flowmeter.

　　⑻ The flowmeter converter installed separately should be installed in a ventilated and dry place, and it should avoid rainwater pouring and flooding. In order to prevent the electrical components of the meter from getting wet, the insulation performance is reduced and damaged.

　　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 be well grounded separately. If the pipe connected to the flowmeter is coated with insulation or non-metallic pipe, the flowmeter should be equipped with a ground (liquid) ring.

　　3 Measurement accuracy and error curve

　　测量 The measurement accuracy and error curves given by the electromagnetic flowmeter manufacturer refer to the technical indicators under reference working conditions. Users should note that there are differences from actual application conditions.

　　According to JB / T 9248-1999 "Electromagnetic Flowmeter" industry standard, the reference working conditions are:

　　Ambient temperature: 20 ℃ ± 2 ℃;

　　Relative humidity: 60% ～ 70%;

　　Power supply: rated voltage ± 1%;

　　Installation conditions: length of upstream straight pipe section> 10D;

　　The length of the downstream straight pipe section is> 5D;

　　Warm-up time:> 15min.

　　3.1 Measurement accuracy

　　Now most manufacturers use the percentage of the indicated value to indicate the measurement accuracy (or basic error limit) of the meter. For example, the level 0.3 flowmeter has a measurement accuracy of ± 0.3%. More regular manufacturers have their own controlled product flow verification procedures. The Kaifeng Instrument Factory regulations stipulate that the flowmeters are calibrated under the reference working conditions. The inspection measurement accuracy is controlled within ± 0.28%, which is better than the industry standard.

　　3.2 Error curve

　　的 The error curve given by the manufacturer indicates the trend of linearity change of the flow meter in its measurement range, which corresponds to the accuracy index given (the error curve is shown in Figure 2). Take the electromagnetic flowmeter of Kaifeng Instrument Factory as an example:

　　Accuracy: ± 0.3% of displayed value (flow velocity ≥ 1m / s); or ± 3 mm / s (flow velocity <1m / s).

　　Therefore, at 0.5m / s, the flowmeter allows an error of ± 0.5%, and at 0.3m / s, the flowmeter allows an error of ± 1.0%.

　　3.3 Working conditions

　　Under working conditions, due to various factors, the measurement accuracy may be different from the actual flow calibration accuracy given by the manufacturer under reference conditions. For example, according to industry standards, for every 10 ° C change in temperature, the measurement accuracy should not change more than 1/2 of the basic error limit of the meter.

　　When the temperature changes by 20 ° C, the measurement accuracy of ± 0.3% may become ± 0.6%.

　　During the operation of the flowmeter, the user should consider the influence of noise interference, restrictions on installation conditions, changes in ambient temperature, and humidity (in a long-term humid environment, the insulation strength of the flowmeter decreases). Generally, the 0.5 level flowmeter actually reaches level 1, and it should be considered that the user has better operation and maintenance of the flowmeter.

　　4 Measuring range and system zero

　　4.1 Measuring range

　　The flow range of the electromagnetic flowmeter is relatively wide, and the corresponding flow rate range is 0 ~ 10m / s. Some manufacturers' flowmeters are 0 ~ 15m / s. In theory, changing the measuring range will not affect the measurement accuracy and linearity of the flow meter, but choosing a suitable measuring range will help improve the unit resolution of the 4-20 mA analog output signal. If the 100m3 / h range can meet the user's requirements, do not select the 200m3 / h range, otherwise the unit resolution of the analog output signal will be reduced by half.

　　4.2 System zero

　　Under normal operating conditions, the system zero point of the electromagnetic flow meter will cause the system due to long-term operation of the system, due to factors such as aging of components, reduced insulation strength of the excitation wire package, polarization and pollution of the measuring electrode, and increased system ground resistance (potential) Zero point changes and drift, the user should regularly check the system zero point of the flow meter for adjustment and maintenance. Corresponding to a flow of 1m / s, if the system has a zero of ± 5mm / s, it will cause an additional error of ± 0.5%. In general,

　　较小 When the flow is small, the additional error caused by the zero point of the system is greater.

　　Because the zero point of the system always exists more or less, the manufacturer will adjust it to the minimum according to the regulations when the flow meter is calibrated at the factory, and the user needs to adjust it at the application site. Therefore, the error curve given by the manufacturer objectively reflects the existence of the zero point of the flowmeter system, thus causing the nonlinearity of the instrument. If only the measurement accuracy of the electromagnetic flowmeter is given without 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 a flowmeter based on Faraday's law of electromagnetic induction (when a conductive fluid traverses a magnetic field, it is induced in the conductive fluid proportional to the flow velocity of the fluid) as the basic principle. 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 transmitted in a differential manner from the sensor to the differential amplifier signal input of the converter. "Ground" is used as the signal ground terminal of the differential amplifier. The differential amplifier amplifies the difference of the flow signal, and suppresses and attenuates the common mode interference signal superimposed on the flow signal. When there is common mode interference such as ground loop current, electrode polarization voltage, electrostatic coupling voltage between excitation circuit and electrode measurement circuit, as long as the working parameters of the differential amplifier are symmetrical, common mode interference will not affect the amplifier's effect on the flow signal. Amplify unless the common-mode interference voltage exceeds the common-mode rejection range of the amplifier.

　　During the installation and maintenance of the electromagnetic flowmeter, the user must fully realize the importance of grounding the flowmeter system. In order to explain the difference in grounding more clearly, some manufacturers call the ground referenced by the flow signal as the liquid-contact ground ( Wetted parts: ground electrode or ground ring, etc.), called the earth physically. Wetted ground not only requires the ground resistance to be as small as possible, but also has good stability and reliability.

　　The earth is mainly used to reduce the interference and protection of the external environment to the flowmeter system, such as electromagnetic field radiation, the loop current of the fluid medium and the earth, and lightning strikes.

　　5.2 DC noise

　　When the flow rate is relatively stable, if the flowmeter output fluctuates greatly, the main influencing factor is the DC noise superimposed on the measurement signal. If the fluid medium ground is good, the cause of the DC noise is the polarization voltage present on the electrodes. Between the electrode and the electrolyte fluid medium, due to the directional movement of positive and negative ions in the liquid, a certain electric field is generated, so a polarized voltage that drifts between the electrode and the wetted ground is formed. The polarization voltage is superimposed on the flow signal in the form of common mode interference, blocking the differential amplifier, making the flow signal unable to be amplified, the flow meter cannot complete normal signal sampling, or the flow signal fluctuates due to its drift. In the water supply industry, polarization voltage is generally not generated. However, in the chemical industry, due to the corrosion of the fluid medium and the difference between the electrode and the wetted material, it is easy to generate a polarized voltage. Even if a polarization voltage is generated, the potential for the polarization voltage is the same due to the same material, so that the common mode voltage between the electrode and the measurement reference is minimized.

　　6 Ultrasonic flowmeter online calibration electromagnetic flowmeter

　　The manufacturer adopts standard container method, weighing method or standard meter method to transmit the value, and the actual flow is used to calibrate the electromagnetic flowmeter. At present, no online calibration equipment for electromagnetic flowmeters has been seen. ABB provides users with a device called "CalMaster", which can be considered as an expert evaluation system for electromagnetic flowmeters. The flowmeter is evaluated online. According to the comparison of the historical parameter records when the product leaves the factory and the product's operating state parameters when it is online, it shows how much possible errors the electromagnetic flowmeter has compared with the measurement accuracy of the actual flow calibration when it was originally shipped. In the process of using the flow meter, if the user feels that there is a problem with the measurement accuracy, he should send it to the relevant measurement department for verification. The ultrasonic flowmeter online comparison method can be used to qualitatively evaluate whether the electromagnetic flowmeter is in good working condition, and eliminate the possibility of large measurement errors in the electromagnetic flowmeter.

　　的 The measurement accuracy of the ultrasonic flowmeter is lower than that of the electromagnetic flowmeter, because the electromagnetic flowmeter measures the average surface flow velocity on a circular area, and the nominal diameter and out-of-roundness error of the electromagnetic catheter have been shipped from the factory. The calibration time is corrected by the meter coefficient. The ultrasonic flowmeter measures the average line velocity of the axial section of the pipeline. The influence of the distortion of the flow velocity distribution should be considered, and the on-site pipeline diameter and out-of-roundness error are difficult for the user to give a quantitative value.

　　直 The length of the straight pipe section of the ultrasonic flowmeter requires 10D upstream and 5D downstream. The mono ultrasonic flowmeter should actually be longer, and the field conditions often cannot be achieved.

　　(3) Factors such as tube wall corrosion, coating protection and thickness also affect the measurement accuracy of the ultrasonic flowmeter.

　　7 The pump flow cannot judge whether the electromagnetic flowmeter is accurate

　　In the process of using the electromagnetic flowmeter, the user often evaluates the accuracy of the electromagnetic flowmeter based on the pump flow rate. However, the pump flow cannot determine whether the electromagnetic flowmeter is accurate.

　　的 The flow value and head value specified on the pump nameplate should be the nominal values ​​that represent the pump capacity. Under actual operating conditions, the pump flow rate changes due to changes in head, efficiency, power, and network load.

　　级 Class C pump flow may reach a tolerance of ± 8%.

　　⑶ Multiple pumps are operated in parallel, and the flow rate entering the main pipe may not be equal to the sum of the flow rates of the pumps.

　　8 Measuring electrode pollution protection

　　The degree to which the contamination of the thallium electrode affects the measurement accuracy of the electromagnetic flowmeter is difficult to quantify.

　　⑴ When the fluid medium may cause deposits to pollute the electrode, you should choose a suitable flow rate (selected pipe diameter) and installation method in terms of design and installation to avoid causing excessive deposits.

　　污染 Electrode pollution physically manifests as the signal input impedance of the electrode measurement circuit increases or decreases. As long as the impedance does not exceed a certain limit, there will be no side effects on the measurement of the electromagnetic flowmeter. High input impedance (1011Ω