Sixteen rules for selection and installation of electromagnetic flowmeter
/div> & nbsp;& nbsp; In the practice of applying electromagnetic flowmeter in enterprises, the metrologists of some units do not pay attention to the selection and installation points, and often fail to achieve the expected purpose of measurement. In this paper, the author discusses the selection and installation points of electromagnetic flowmeter, for the netizens who are interested in improving the level of enterprise measurement.
& nbsp;& nbsp; 1、 The first thing is to define the measurement demand of the unit
& nbsp;& nbsp; There are several measurement requirements that should be specified, mainly including: measurement medium, flow rate m3 / h (minimum, working point, maximum), medium temperature ℃, medium pressure MPa, installation form (pipeline type or plug-in type), etc.
& nbsp;& nbsp; 2、 The precondition of choosing electromagnetic flowmeter
& nbsp;& nbsp; 1. The medium to be measured must be a conductive liquid (i.e. the fluid to be measured must have a minimum conductivity)& nbsp;& nbsp;< BR> & nbsp;& nbsp;& nbsp; 2. The measured medium should not contain more ferromagnetic medium or a large number of bubbles.
& nbsp;& nbsp; 3、 The principle of electromagnetic flowmeter should be understood
& nbsp;& nbsp; The measurement principle is based on Faraday's law of electromagnetic induction, that is, when measuring the flow rate, the fluid flows through a magnetic field perpendicular to the flow direction, and the flow of conductive fluid induces a voltage proportional to the average flow rate (also known as volume flow rate). The voltage signal is detected by two electrodes in direct contact with the fluid and transmitted to the amplifier through a cable, Then it is converted into an output signal.
& nbsp;& nbsp; 4、 The main characteristics of electromagnetic flowmeter should be understood
& nbsp;& nbsp; Its main features are as follows
& nbsp;& nbsp; (1) there is no pressure loss in the measuring pipe and it is not easy to block, so it has a unique adaptability for slurry measurement< BR> & nbsp;& nbsp; (2) the requirement of straight pipe section is low< BR> & nbsp;& nbsp; (3) low frequency rectangular wave excitation, not affected by power frequency and field distribution interference, stable and reliable operation< BR> & nbsp;& nbsp; (4) the transmitter body can be made of all stainless steel, which has the ability of anti acid, anti alkali and anti-corrosion after adding lining material< BR> & nbsp;& nbsp; (5) the field display converter can use special intelligent chip, which is convenient for parameter setting< BR> & nbsp;& nbsp; (6) a self calibration system can be set inside the transmitter to self calibrate the transmitter constant and factory calibration value at any time, which is convenient for debugging and maintenance< BR> & nbsp;& nbsp; (7) wide measurement range, full scale flow rate can be set in the range of 0.3m/s-12m/s< BR> & nbsp;& nbsp; (8) the plug-in type can be installed or disassembled under the condition of continuous flow< BR> & nbsp;& nbsp; (9) wide range of application: it can be used in chemical industry, metallurgy, paper making, food, petroleum, urban water supply and other fields.
& nbsp;& nbsp; 5、 Key points of model selection
& nbsp;& nbsp; 1. The most important thing is to choose pipeline geomagnetic flowmeter or plug-in electromagnetic flowmeter< BR> & nbsp;& nbsp; 2. Generally, the electromagnetic flowmeter without field display is selected, and its output 4-20mA (or 0-10mA) current signal is sent to the secondary instrument in the control room, and the flow and total amount can be displayed< BR> & nbsp;& nbsp; 3. If it is convenient to observe the flow in the pipeline during field operation, the electromagnetic flowmeter with field display can be selected< BR> & nbsp;& nbsp; 4. When the requirements of environment or measurement accuracy are high, the safe voltage intelligent electromagnetic flowmeter can be selected< BR> & nbsp;& nbsp; 5. The plug-in electromagnetic flowmeter or enhanced plug-in electromagnetic flowmeter can be selected preferentially when the flow is measured in the large diameter pipe above 200 mm or in the continuous flow state.
& nbsp;& nbsp; 6、 The key points of choosing sensor caliber
& nbsp;& nbsp; 1. The diameter of the selected sensor is the same as that of the connected process pipe, which has the advantage of convenient installation (no reducer is required); The premise is that the velocity of fluid in the pipe must be within 03 M / S-10 M / s; It is suitable for the early use of the project and the fluid velocity in the pipe is in a low state< BR> & nbsp;& nbsp; 2. The diameter of the selected sensor is different from that of the connected process pipe, and its applicable conditions are as follows: (1) low flow rate and stable flow rate; (2) low cost performance
& nbsp;& nbsp; 7、 Selection of lining materials
& nbsp;& nbsp; According to the corrosivity, abrasiveness and temperature of the tested medium, it is selected by the ordering party. Please refer to the "table of lining material performance and scope of application" provided by various manufacturers.
& nbsp;& nbsp; 8、 Selection of electrode materials
& nbsp;& nbsp; According to the corrosivity of the tested medium, it is selected by the ordering party. Please refer to the "corrosion resistance table of electrode materials" provided by various manufacturers.
& nbsp;& nbsp; 9、 Selection of protection level
& nbsp;& nbsp; According to the national standard gb4208-84, the protection grade of the enclosure includes: if the instrument is installed below the ground and is often flooded, the submersible type should be selected; If the instrument is installed on the ground, it is better to choose the water proof type.
& nbsp;& nbsp; 10、 Selection of upper limit flow
& nbsp;& nbsp; 1. Generally, the diameter of the transmitter is the same as that of the connecting pipe. Because there is a strict functional relationship among diameter, flow rate and flow rate, you can refer to the upper limit flow meter of electromagnetic flowmeter provided by the manufacturer to select the upper limit flow value under the corresponding diameter< BR> & nbsp;& nbsp; 2. When the flow rate in the process piping is too low to reach the minimum upper limit flow value under the corresponding diameter in "1", the diameter of the transmitter can be selected to be smaller than that of the process piping, that is, a reducer can be added before and after the transmitter.
& nbsp;& nbsp; 11、 Selection of installation form
& nbsp;& nbsp; 1. Integral type: the sensing part and the conversion part of the flowmeter are installed in one, and the advantage is that it is easy to install< BR> & nbsp;& nbsp; 2. Separate type: the sensing part of the flowmeter is installed on the measured pipe, and the conversion part is installed in the room which is convenient for operation. It is suitable for poor site environment (but the separation length should be less than 30m).
& nbsp;& nbsp; 12、 Selection of grounding ring
& nbsp;& nbsp; If the pipeline connecting the instrument is insulated relative to the measured medium, the grounding ring shall be selected.
& nbsp;& nbsp; 13、 The selection premise of ball valve
& nbsp;& nbsp; For the situation that the flow of fluid in the pipeline should not be stopped and the medium should not overflow, the ball valve should be installed. When the flow meter is allowed to be cut off (or the installation and removal operation does not affect the pipeline work), it is not necessary to install the ball valve, but directly install the flowmeter on the short flange pipe.
& nbsp;& nbsp; 14、 Key points for installation
& nbsp;& nbsp; 1. The axis of the electrode must be kept approximately horizontal< BR> & nbsp;& nbsp; 2. Ensure that the measuring tube must be filled at all times< BR> & nbsp;& nbsp; 3. Make sure there is enough installation space for bolts and nuts near the pipe flange< BR> & nbsp;& nbsp; 4. The pipe section with flowmeter should be supported by pipeline to reduce the vibration of pipeline< BR> & nbsp;& nbsp; 5. Strong electromagnetic field should be avoided near the flowmeter< BR> & nbsp;& nbsp; 6. For long pipeline, control valve and shut-off valve shall be installed downstream of flowmeter< BR> & nbsp;& nbsp; 7. In case of "open feed or discharge" status, the instrument shall be installed in the low section of the pipeline< BR> & nbsp;& nbsp; 8. Based on the electrode axis, the inlet straight pipe section should be greater than or equal to 5 times the diameter of the measuring pipe, and the outlet pipe should be greater than or equal to 2 times the diameter of the measuring pipe.
& nbsp;& nbsp; 15、 Grounding requirements of flowmeter
& nbsp;& nbsp; The sensor shall have a good single grounding wire (copper core section ≥ 16mm2), and its grounding resistance shall be less than 10 ohm; If the pipe connecting the sensor is coated with insulating layer or non-metallic pipe, the grounding ring shall be set on both sides of the sensor (the cross section of copper core of grounding wire is ≥ 16mm2).
& nbsp;& nbsp; 16、 Key points for cable and electrical connection
& nbsp;& nbsp; The separated connecting cable (signal transmission and excitation) is as short as possible in terms of reducing interference and saving money. Generally, consideration should be given to the conductivity of the medium, the cross-section of the excitation cable and the type of the signal cable (core, layer and shielding).
& nbsp;& nbsp; 1、 The first thing is to define the measurement demand of the unit
& nbsp;& nbsp; There are several measurement requirements that should be specified, mainly including: measurement medium, flow rate m3 / h (minimum, working point, maximum), medium temperature ℃, medium pressure MPa, installation form (pipeline type or plug-in type), etc.
& nbsp;& nbsp; 2、 The precondition of choosing electromagnetic flowmeter
& nbsp;& nbsp; 1. The medium to be measured must be a conductive liquid (i.e. the fluid to be measured must have a minimum conductivity)& nbsp;& nbsp;< BR> & nbsp;& nbsp;& nbsp; 2. The measured medium should not contain more ferromagnetic medium or a large number of bubbles.
& nbsp;& nbsp; 3、 The principle of electromagnetic flowmeter should be understood
& nbsp;& nbsp; The measurement principle is based on Faraday's law of electromagnetic induction, that is, when measuring the flow rate, the fluid flows through a magnetic field perpendicular to the flow direction, and the flow of conductive fluid induces a voltage proportional to the average flow rate (also known as volume flow rate). The voltage signal is detected by two electrodes in direct contact with the fluid and transmitted to the amplifier through a cable, Then it is converted into an output signal.
& nbsp;& nbsp; 4、 The main characteristics of electromagnetic flowmeter should be understood
& nbsp;& nbsp; Its main features are as follows
& nbsp;& nbsp; (1) there is no pressure loss in the measuring pipe and it is not easy to block, so it has a unique adaptability for slurry measurement< BR> & nbsp;& nbsp; (2) the requirement of straight pipe section is low< BR> & nbsp;& nbsp; (3) low frequency rectangular wave excitation, not affected by power frequency and field distribution interference, stable and reliable operation< BR> & nbsp;& nbsp; (4) the transmitter body can be made of all stainless steel, which has the ability of anti acid, anti alkali and anti-corrosion after adding lining material< BR> & nbsp;& nbsp; (5) the field display converter can use special intelligent chip, which is convenient for parameter setting< BR> & nbsp;& nbsp; (6) a self calibration system can be set inside the transmitter to self calibrate the transmitter constant and factory calibration value at any time, which is convenient for debugging and maintenance< BR> & nbsp;& nbsp; (7) wide measurement range, full scale flow rate can be set in the range of 0.3m/s-12m/s< BR> & nbsp;& nbsp; (8) the plug-in type can be installed or disassembled under the condition of continuous flow< BR> & nbsp;& nbsp; (9) wide range of application: it can be used in chemical industry, metallurgy, paper making, food, petroleum, urban water supply and other fields.
& nbsp;& nbsp; 5、 Key points of model selection
& nbsp;& nbsp; 1. The most important thing is to choose pipeline geomagnetic flowmeter or plug-in electromagnetic flowmeter< BR> & nbsp;& nbsp; 2. Generally, the electromagnetic flowmeter without field display is selected, and its output 4-20mA (or 0-10mA) current signal is sent to the secondary instrument in the control room, and the flow and total amount can be displayed< BR> & nbsp;& nbsp; 3. If it is convenient to observe the flow in the pipeline during field operation, the electromagnetic flowmeter with field display can be selected< BR> & nbsp;& nbsp; 4. When the requirements of environment or measurement accuracy are high, the safe voltage intelligent electromagnetic flowmeter can be selected< BR> & nbsp;& nbsp; 5. The plug-in electromagnetic flowmeter or enhanced plug-in electromagnetic flowmeter can be selected preferentially when the flow is measured in the large diameter pipe above 200 mm or in the continuous flow state.
& nbsp;& nbsp; 6、 The key points of choosing sensor caliber
& nbsp;& nbsp; 1. The diameter of the selected sensor is the same as that of the connected process pipe, which has the advantage of convenient installation (no reducer is required); The premise is that the velocity of fluid in the pipe must be within 03 M / S-10 M / s; It is suitable for the early use of the project and the fluid velocity in the pipe is in a low state< BR> & nbsp;& nbsp; 2. The diameter of the selected sensor is different from that of the connected process pipe, and its applicable conditions are as follows: (1) low flow rate and stable flow rate; (2) low cost performance
& nbsp;& nbsp; 7、 Selection of lining materials
& nbsp;& nbsp; According to the corrosivity, abrasiveness and temperature of the tested medium, it is selected by the ordering party. Please refer to the "table of lining material performance and scope of application" provided by various manufacturers.
& nbsp;& nbsp; 8、 Selection of electrode materials
& nbsp;& nbsp; According to the corrosivity of the tested medium, it is selected by the ordering party. Please refer to the "corrosion resistance table of electrode materials" provided by various manufacturers.
& nbsp;& nbsp; 9、 Selection of protection level
& nbsp;& nbsp; According to the national standard gb4208-84, the protection grade of the enclosure includes: if the instrument is installed below the ground and is often flooded, the submersible type should be selected; If the instrument is installed on the ground, it is better to choose the water proof type.
& nbsp;& nbsp; 10、 Selection of upper limit flow
& nbsp;& nbsp; 1. Generally, the diameter of the transmitter is the same as that of the connecting pipe. Because there is a strict functional relationship among diameter, flow rate and flow rate, you can refer to the upper limit flow meter of electromagnetic flowmeter provided by the manufacturer to select the upper limit flow value under the corresponding diameter< BR> & nbsp;& nbsp; 2. When the flow rate in the process piping is too low to reach the minimum upper limit flow value under the corresponding diameter in "1", the diameter of the transmitter can be selected to be smaller than that of the process piping, that is, a reducer can be added before and after the transmitter.
& nbsp;& nbsp; 11、 Selection of installation form
& nbsp;& nbsp; 1. Integral type: the sensing part and the conversion part of the flowmeter are installed in one, and the advantage is that it is easy to install< BR> & nbsp;& nbsp; 2. Separate type: the sensing part of the flowmeter is installed on the measured pipe, and the conversion part is installed in the room which is convenient for operation. It is suitable for poor site environment (but the separation length should be less than 30m).
& nbsp;& nbsp; 12、 Selection of grounding ring
& nbsp;& nbsp; If the pipeline connecting the instrument is insulated relative to the measured medium, the grounding ring shall be selected.
& nbsp;& nbsp; 13、 The selection premise of ball valve
& nbsp;& nbsp; For the situation that the flow of fluid in the pipeline should not be stopped and the medium should not overflow, the ball valve should be installed. When the flow meter is allowed to be cut off (or the installation and removal operation does not affect the pipeline work), it is not necessary to install the ball valve, but directly install the flowmeter on the short flange pipe.
& nbsp;& nbsp; 14、 Key points for installation
& nbsp;& nbsp; 1. The axis of the electrode must be kept approximately horizontal< BR> & nbsp;& nbsp; 2. Ensure that the measuring tube must be filled at all times< BR> & nbsp;& nbsp; 3. Make sure there is enough installation space for bolts and nuts near the pipe flange< BR> & nbsp;& nbsp; 4. The pipe section with flowmeter should be supported by pipeline to reduce the vibration of pipeline< BR> & nbsp;& nbsp; 5. Strong electromagnetic field should be avoided near the flowmeter< BR> & nbsp;& nbsp; 6. For long pipeline, control valve and shut-off valve shall be installed downstream of flowmeter< BR> & nbsp;& nbsp; 7. In case of "open feed or discharge" status, the instrument shall be installed in the low section of the pipeline< BR> & nbsp;& nbsp; 8. Based on the electrode axis, the inlet straight pipe section should be greater than or equal to 5 times the diameter of the measuring pipe, and the outlet pipe should be greater than or equal to 2 times the diameter of the measuring pipe.
& nbsp;& nbsp; 15、 Grounding requirements of flowmeter
& nbsp;& nbsp; The sensor shall have a good single grounding wire (copper core section ≥ 16mm2), and its grounding resistance shall be less than 10 ohm; If the pipe connecting the sensor is coated with insulating layer or non-metallic pipe, the grounding ring shall be set on both sides of the sensor (the cross section of copper core of grounding wire is ≥ 16mm2).
& nbsp;& nbsp; 16、 Key points for cable and electrical connection
& nbsp;& nbsp; The separated connecting cable (signal transmission and excitation) is as short as possible in terms of reducing interference and saving money. Generally, consideration should be given to the conductivity of the medium, the cross-section of the excitation cable and the type of the signal cable (core, layer and shielding).
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