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First, the first clear that the measurement needs of the unit should have a clear number of measurement needs, mainly include: measuring medium, flow m3 / h (minimum, operating point, maximum), medium temperature °C, medium pressure MPa, installation form (pipeline or Plug-in) etc.
Second, the selection of electromagnetic flowmeter preconditions 1. The measured medium must be a conductive liquid (ie, the fluid to be measured has a minimum conductivity).
2, the measured medium should not contain more ferromagnetic medium or a large number of bubbles.
Third, should understand the measurement principle of electromagnetic flowmeter The measurement principle is based on Faraday's law of electromagnetic induction, that is, when measuring flow, the fluid flows through the magnetic field perpendicular to the direction of flow, and the flow of conductive fluid induces an average flow velocity (also called volume flow). Proportional to the voltage, the voltage signal is detected by two electrodes in direct contact with the fluid, and transmitted to the amplifier through the cable, and then converted into an output signal.
Fourth, we should understand the main features of the electromagnetic flowmeter and its main features are: (1) The measuring tube basically no pressure loss, easy to plug, has a unique adaptability to the grout type measurement; (2) low straight tube requirements; (3) low-frequency rectangular wave excitation, not It is stable and reliable with the influence of power frequency and on-site distribution interference; (4) The body of the transmitter can be made of stainless steel, and it has anti-acid, anti-alkali and anti-corrosion capabilities after the lining material is added; (5) The field display converter can use dedicated intelligence. Chips, convenient parameter setting; (6) The transmitter can be set up within the self-school system, the transmitter constants and factory calibration values ​​can be self-calibrated at any time, easy to debug and repair; (7) wide measurement range, full-scale flow rate settings can be In the range of 0.3m/s to 12m/s; (8) The plug-in type can be installed or disassembled under constant flow; (9) Wide range of applications: Can be used in chemical, metallurgy, papermaking, food, petroleum, urban water supply and other fields.
Fifth, the choice of model points 1, the first clear is to choose the pipeline electromagnetic flowmeter, or plug-in electromagnetic flowmeter.
2. Normally, the on-site non-display electromagnetic flowmeter is selected, and the 4-20 mA (or 0-10 mA) current signal output to the secondary meter of the control room can display the flow and the total amount.
3. If it is emphasized that it is convenient to observe the flow in the pipeline during on-site operation, the on-site display type electromagnetic flowmeter can be selected.
4. When the requirements of the environment or measurement accuracy are high, the safe voltage intelligent electromagnetic flowmeter can be selected.
5, in 200mm or more large diameter measuring flow or continuous flow state assembly and disassembly, can choose to insert or enhanced plug-in electromagnetic flowmeter.
6. Selection of sensor diameters 1. The caliber of the selected sensor is the same as the diameter of the connecting process pipe. The advantage is that it is easy to install (no need for different diameter pipe); the premise is that the flow rate of the fluid in the pipe must be 0.3m/s-10m/s. In the range; its applicable state is used in the early stage of the project and the flow rate of the fluid in the pipe is in a low state.
2. The caliber of the selected sensor is not the same as the diameter of the connected process pipe. Applicable status: (1) Low flow rate and stable flow rate: (2) Reduced cost performance:
7. Selection of lining materials According to the corrosivity, wearability, and temperature of the medium to be measured by the company, the choice of the ordering party may refer to the “performance and scope of application of lining materials†provided by various manufacturers.
VIII. Selection of Electrode Material According to the corrosivity of the medium to be measured by the company, the choice of the orderer may refer to the “Corrosion Resistance of Electrode Materials†provided by various manufacturers.
Nine, the choice of protection level According to the national standard GB4208-84, the enclosure protection class is: if the meter is installed below the ground and often flooded should choose submersible type; if the meter is installed on the surface, it should be selected to prevent water spray type .
Tenth, the upper limit of the flow of choice 1, the general choice of the transmitter's diameter and connecting the same pipe diameter. Because there is a strict functional relationship among the path, flow rate, and flow rate, the upper limit flow rate table of the electromagnetic flowmeter provided by the manufacturer can be consulted when selecting, and the upper limit flow value can be selected under the corresponding path.
2. When the flow rate in the process piping is too low to reach the minimum upper flow value under the corresponding path in “1â€, the path diameter of the transmitter smaller than the process piping can be selected, that is, before and after the transmitter. Reducer.
Eleventh, the choice of the installation form 1, integral type: The sensor part and the conversion part of the flowmeter will be installed in one, the advantage is that it is easy to install.
2. Separate type: The sensing part of the flow meter is to be installed on the pipeline under test, and the conversion part is installed in a room that is easy to operate. It is suitable for occasions with poor on-site environment (but its separation length should be less than 30m).
12. Selection of Grounding Ring If the pipe connecting the meter is insulative with respect to the measured medium, a grounding ring must be selected.
XIII. Selection of Ball Valves Prerequisites For process requirements, the fluid flow in the pipeline must not be stopped, and the media must not be overflowed. Ball valves need to be installed. However, when disassembling the flowmeter, it is allowed to cut off the flow (or the assembly and disassembly work does not affect the work of the pipeline), and it is not necessary to add a ball valve and directly install the flowmeter on the short pipe of the flange.
XIV. Installation Notes 1. The axis of the electrode must remain approximately horizontal.
2. Ensure that the measuring tube must be filled at all times.
3. Make sure there is enough space for bolts and nuts near the pipe flange.
4. There must be pipeline support in the pipe section where the flowmeter is installed to reduce the vibration of the pipeline.
5. Strong electromagnetic fields should be avoided near the flowmeter.
6. For long pipelines, control valves and shutoff valves should be installed downstream of the flowmeter.
7. In case of “open feed or discharge†condition, install the instrument in the low section of the pipeline.
8. Based on the electrode axis, the inlet straight pipe section should be greater than or equal to 5 times the measuring pipe diameter, and the outlet pipe should be greater than or equal to 2 times the measuring pipe diameter.
Fifteen, flowmeter grounding requirements sensor should have a good separate ground wire (copper core section ≥ 16mm2), its grounding resistance <10 ohms; if the sensor is connected to the pipe coated with insulating layer or non-metallic pipe, the sensor two The grounding ring should be set on the side (the grounding wire copper core section ≥ 16mm2).
XVI. Cable and Electrical Connections Note The separate connection cables (signal transmission and excitation) should be as short as possible from the perspective of reducing interference and saving money. Generally speaking, the conductivity of the medium, the cross-section of the excitation cable, and the type of signal cable (core, layer, and shield) must be taken into account.
Seventeen, electromagnetic flowmeter should pay attention to some matters that the liquid should have the conductivity required for the measurement, and require the conductivity distribution is generally uniform. Therefore, the flow sensor should be installed so as to avoid places where conductivity is not easily generated. For example, liquid medicine is added near the upstream of the sensor. The liquid addition point is preferably located downstream of the sensor. When used, the sensor tube must be filled with fluid (except for non-full tube types). When mixed, their distribution should be generally uniform. The liquid should be at the same potential as ground and must be grounded. If the process piping is made of plastics and other insulating materials, the transport liquid generates frictional static electricity and other causes, resulting in a potential difference between the liquid and the ground.
18. Electromagnetic flowmeter sensor installation (1) Installation location The electromagnetic flowmeter sensor housing usually has a protection class of IP65 (dust and spray water level specified in GB4208) and all the following requirements for the installation site.
1) When measuring the mixed phase fluid, select the place that will not cause phase separation; when measuring the two-component liquid, avoid installing in the downstream where the mixing is not yet uniform; when measuring the chemical reaction pipeline, install it in the downstream of the reaction fully completed section;
2) Avoid as much as possible negative pressure in the measuring tube;
3) Select a place with little vibration, especially for integrated type instruments;
4) Avoid large electric motors, large transformers, etc. nearby to avoid electromagnetic interference;
5) It is easy to realize the place where the sensor is grounded separately;
6) Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
7) The ambient temperature is in the range of -25/-10 to 50/60°C. The temperature of the integral structure is also controlled by the electronic components, and the range is narrower.
8) The relative humidity in the environment is in the range of 10% to 90%;
9) Avoid direct sunlight as much as possible;
10) Avoid rain soaking and will not be immersed in water.
If the degree of protection is IP67 (dust-proof and anti-submerged) or IP68 (dust-proof submersible), the above 8) and l0) requirements are not required.
(2) Straight pipe length requirement In order to obtain normal measurement accuracy, the electromagnetic flowmeter sensor must have a straight pipe section with a certain length upstream, but its length is lower than that of most other flow meters. After the 90° elbow, T-tube, concentric reducer, and fully open valve, it is generally considered that as long as the pipe section is 5 times the diameter (5D) of the length of the pipe from the electrode centerline (not the sensor inlet end), valves with different opening degrees Requires 10D; the downstream straight section is (2~3)D or no requirement; but prevent the butterfly valve piece from reaching into the sensor measuring tube. The lengths of upstream and downstream straight pipe sections proposed by various standards or verification regulations are also inconsistent, and are summarized in the following table, and the requirements are higher than usual. This is due to the requirement to achieve the current class 0.5 accuracy meter. ISO 6817 is the standard for the use of electromagnetic flow meters, which consider that there should be at least 10 D of straight pipe for any kind of flow disturbance, and the performance change can not guarantee more than 1%. ISO9104 and JJG198 are the documents proposed for evaluating the performance of the electromagnetic flowmeter; the length of the straight pipe section of the JJG198 listed in the table is an instrument with an accuracy of class 0.5 and above.
Spoiler name standard or strong specification number ISO6817ISO9104 technology B7554ZBN12007JJG198
Upstream elbow, T-tube, full-opening valve, diffuser 10D, or manufacturer's specification 10D5D5D10D
Reducing tube can be regarded as straight tube and other various valves 10D
Downstream Various Not Satisfied 5D Not Required 2D2D
If the valve cannot be fully opened, the valve trapping direction and the electrode shaft should be installed at an angle of 45°, so the additional error can be greatly reduced.
(3) Mounting position and flow direction The sensor can be installed horizontally, vertically or tilted, without restriction. However, it is best to measure the solid-liquid two-phase fluid vertically and flow from bottom to top. This can avoid the disadvantages of local wear in the lower half of the liner during horizontal installation, solid phase sedimentation at low flow rates, and the like.
When installing horizontally, use the axis of the electrode parallel to the horizon, not perpendicular to the horizon, because the electrode at the bottom is easily covered with sediments, and the top electrode is easily wiped by the bubbles in the liquid to cover the surface of the electrode, causing the output signal to fluctuate. . In the piping system shown in the right figure, c and d are adaptive positions; a, b, and c are unsuitable positions. The liquid may not be fully filled at point b, and the discharge port is preferably shown in the shape of f. For the solid-liquid two-phase flow c, it is also an inappropriate position.
(4) Bypass tube, easy-to-clean connection, and preset entry In order to check and adjust the zero point when the process pipe continues to flow and the sensor stops flowing, the bypass pipe should be installed. However, large diameter pipe systems are often difficult to manage because of investment and location space constraints. It is difficult to correct the measured value according to the degree of contamination of the electrode, or to determine a criterion for the degree of contamination that does not affect the measured value. In addition to the above, using a non-contact electrode or a meter with a blade cleaning device electrode to solve some problems, sometimes also need to remove the inner wall attachments, you can clean the sensor without removing the sensor as shown in the right figure.
For pipes with a pipe diameter greater than 1.5-1.6m, a manhole is preset on the pipeline near the electromagnetic flowmeter so that the inner wall of the sensor pipe is cleaned when the pipe system is stopped.
(5) Installation of negative pressure piping system The fluorine plastic lining sensor shall be used with caution in the negative pressure piping system; the positive pressure piping system shall prevent the generation of negative pressure, for example, piping whose liquid temperature is higher than room temperature, closing the upstream and downstream stop valves of the sensor and stopping After the operation, the fluid cooling shrinks to a negative pressure, and a negative pressure prevention valve should be installed near the sensor, as shown in the right figure. There are manufacturers that PTFE and PFA plastic linings used in negative pressure piping systems can be used at 20 °C, 100 °C, 130 °C when the absolute pressure must be greater than 27,40,50 kPa.
(6) The grounding sensor must be grounded separately (100n or less ground resistance). In principle, the split type should be grounded on the sensor side, and the converter grounded should be on the same ground point. If the sensor is installed on a cathodic corrosion protection pipeline, besides the sensor and the grounding ring are grounded together, use a thicker copper wire (16mm2) to bypass the connecting flanges of the sensor jumper pipe so that the cathodic protection current is between the sensor and the sensor. isolation.
Sometimes the stray current is too large. If the leakage current of the electrolytic cell along the electrolyte affects the normal measurement of the EMF, the electrical isolation between the flow sensor and its connected process can be taken. On the same cathodic protection pipeline, when the cathodic protection current affects the electromagnetic flowmeter measurement, this method can also be adopted. There is no separate installation of the electromagnetic flowmeter in the electromagnetic flowmeter; the split type converter is installed near the sensor or in the instrument room, with a large selection of places and better environmental conditions than the sensor. The degree of protection is IP65 or IP64 (dust and splash proof). The installation site requirements are the same as those in 3), 4), 6), 8), 9), and 10) of (1) in “Installation site†in the section “Flow sensor installation†in the previous section. The ambient temperature is limited by the electronic parts. The use temperature range is narrower than that listed in 7).
The distance between the converter and the sensor is limited by the conductivity of the measured medium and the signal cable type, ie, the distributed capacitance of the cable, the cross-section of the wire, and the number of shielding layers. Use the signal cable attached to the instrument (or model number) supplied by the manufacturer. Low conductivity liquids and longer transmission distances are also stipulated with three-layer shielded cables. The general instrument “operating instructions†gives the corresponding transmission distance range for different conductivity liquids. Single-layer shielded cables are usually used for industrial water or acid-base solutions and can travel over a distance of 100m.
In order to avoid interference signals, the signal cable must be put in the protective grounding pipe separately. The signal cable and the power cable cannot be installed in the same steel pipe.
IV. Operation and Maintenance (1) Zero Check and Adjustment Before the meter is put into operation, the zero point must be adjusted when the sensor is filled with liquid and static. After the start of operation, the conditions of liquid use shall be stopped at regular intervals for zero-point inspection; in particular for sedimentation and easy-to-pollution electrodes, non-cleaning fluids containing solid phases shall be inspected at the initial stage of operation to obtain experience and determine the normal inspection cycle. Exciting electromagnetic flowmeters and square wave impulse ratios are more likely to cause zero drift, so more attention should be paid to inspection and adjustment.
An example of an application error in which two deposition layers are faulty. First, for the oil drilling cementing project, the total flow of the injecting cement slurry is an important process parameter, and high-pressure electromagnetic flowmeters are often used. The meter is used intermittently, and the measuring tube is rinsed with fresh water after completion and the remaining time is an empty tube. Due to incomplete cleaning, the residual cement slurry on the inner surface of the measuring tube solidified into a thin layer, which accumulated in the past two months to form an insulating layer, covering the entire surface of the electrode, resulting in abnormal operation and eventually failing to work. The other is the electrolytic cutting process test device, using electromagnetic flow meters to control the flow of saturated salt water. After a period of use of the gap, the flow signal is gradually weakened and the signal is zero after 2 months. The reason is that iron oxide deposits the wall of the pipe during electrolytic cutting and causes a short circuit. Clear stratification immediately returns to normal.
(2) Periodically check the electrical performance of the sensor 1) Roughly measure the resistance between the electrodes. Disconnect the signal between the sensor and the transducer. The sensor is filled with liquid. Measure the resistance of the two electrodes and ground with a multimeter. Is the value range specified by the manufacturer? Inside, and the measured two values ​​are about the same. Record the resistance value measured for the first time. This value is useful for determining the cause of the sensor failure later, such as whether the deposited layer is conductive or insulating.
2) Inspect the electrode insulation resistance sensor for venting the liquid, wipe the inner wall, and measure the resistance between the two electrodes and the grounding terminal with a megohmmeter after it is completely dry.
3) Check the insulation resistance of the excitation coil Remove the wiring between the sensor excitation coil terminal and the converter and measure the insulation resistance of the coil with a megohmmeter.
Electromagnetic flowmeter selection and installation should be used
In the measurement practice of the enterprise's application of electromagnetic flowmeters, some units of measurement personnel do not pay attention to their selection and installation points, and often fail to achieve the intended purpose of measurement. Here, the author made a discussion on the selection and installation of electromagnetic flowmeters for users who would like to improve the enterprise's measurement level.