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Reasonable selection and proper installation of electromagnetic flowmeter is very important for ensuring measurement accuracy and prolonging the service life of the instrument. The following is a brief introduction of the principles of electromagnetic flowmeter selection, installation conditions and precautions for use.
(1) Selection principle of the electromagnetic flowmeter The selection of the electromagnetic flowmeter is mainly the correct selection of the transmitter, and the converter only needs to be matched with it.
1. Diameter and Range Selection The transmitter diameter is usually the same diameter as the piping system. If the piping system is to be designed, the caliber can be selected according to the flow range and flow rate. For the electromagnetic flowmeter, the flow rate is more suitable for 2-4m/s. In special circumstances, such as liquid with solid particles, taking into account the situation of wear, the optional common flow rate ≤ 3m / s, for easily attached to the management of the fluid. Optional flow rate ≥ 2m/s.
The range of the transmitter can be selected according to two principles: First, the full-scale range of the instrument is greater than the expected maximum flow value; Second, the normal flow is greater than 50% of the full-scale range of the instrument to ensure a certain measurement accuracy.
2. The choice of temperature and pressure electromagnetic flowmeter can measure fluid pressure and temperature is a certain limit. When used, the operating pressure must be lower than the operating pressure specified by the flowmeter. At present, the working pressure specifications of electromagnetic flowmeters produced in China are:
Less than 50mm diameter, working pressure is 1.6MPa;
900mm diameter, working pressure is 1MPa;
More than 1000mm diameter, working pressure is 0.6MPa.
If there is special requirement on the pressure resistance of the transmitter, it can be specifically consulted with the manufacturer. Some manufacturers have been able to produce electromagnetic flow transmitters with a withstand voltage of 32 MPa.
The working temperature of the electromagnetic flowmeter depends on the lining material used, generally 5-70°C. If you do special treatment, you can exceed the above range, such as wear-resistant anti-corrosion electromagnetic flowmeter produced by Tianjin No.3 Automatic Meter. The transmitter allows the measured medium temperature to be -40 to 10130°C.
3. Selection of lining material and electrode material The lining material and electrode material of the transmitter must be correctly selected according to the physical and chemical properties of the medium, otherwise the instrument will be damaged quickly due to the corrosion of the lining and the electrode, and the corrosion is very strong. Once the media leaks, it can cause accidents. Therefore, the electrode and lining material must be carefully selected according to the specific measuring medium in the production process.
(b) The installation of electromagnetic flowmeters must ensure the accuracy of the electromagnetic flowmeters. The correct installation is very important.
1 transmitter should be installed in indoor dry and ventilated place. Avoid installation in places where the ambient temperature is too high. Do not subject to strong vibration. Try to avoid devices with strong magnetic fields, such as large motors, transformers, etc. Avoid installation in corrosive gases. The installation site is easy to overhaul. This is the environmental condition that guarantees the normal operation of the transmitter.
2 In order to ensure that the measuring tube of the transmitter is filled with the measured medium, the transformer is preferably installed vertically, flowing from bottom to top. Especially for liquid-solid two-phase flow, it must be installed vertically. If the site only allows horizontal installation, you must ensure that the two electrodes in the same level 3 transmitter should be installed at both ends of the valve and bypass.
4 The millivolt AC potential measured by the electrodes of the electromagnetic flow transmitter is based on the liquid potential in the transmitter. In order to make the liquid potential stable and the transmitter and the fluid maintain the same potential, in order to ensure a stable measurement, the transmitter shell and the metal tube should have good grounding at both ends, and the converter shell should also be grounded. The grounding resistance cannot be greater than 10, and cannot be shared with the grounding wire of other electrical equipment. If it is not guaranteed that the transmitter housing is in good contact with the metal pipe, use a metal wire to connect them. And then reliable grounding.
5 In order to avoid interference signals, the signal between the transmitter and the converter must be transmitted with shielded wires. It is not allowed to place the signal cable and the power cable in parallel in the same cable duct. The signal cable length must not exceed 30m.
6 The converter installation site should avoid strong AC and DC magnetic fields and vibrations. The ambient temperature is -20 to 50°C, does not contain corrosive gas, and the relative humidity is not more than 80%.
7 In order to avoid the impact of flow rate relative measurement, the flow control valve should be set downstream of the transmitter. For small diameter transmitters, since the distance from the center of the electrode to the inlet end of the flowmeter is equivalent to several times the length of the diameter D, the upstream straight pipe may not be specified. However, for large-caliber flowmeters, generally there should be more than 5D straight pipe upstream, and the downstream generally do not require straight pipe sections.
(1) Selection principle of the electromagnetic flowmeter The selection of the electromagnetic flowmeter is mainly the correct selection of the transmitter, and the converter only needs to be matched with it.
1. Diameter and Range Selection The transmitter diameter is usually the same diameter as the piping system. If the piping system is to be designed, the caliber can be selected according to the flow range and flow rate. For the electromagnetic flowmeter, the flow rate is more suitable for 2-4m/s. In special circumstances, such as liquid with solid particles, taking into account the situation of wear, the optional common flow rate ≤ 3m / s, for easily attached to the management of the fluid. Optional flow rate ≥ 2m/s.
The range of the transmitter can be selected according to two principles: First, the full-scale range of the instrument is greater than the expected maximum flow value; Second, the normal flow is greater than 50% of the full-scale range of the instrument to ensure a certain measurement accuracy.
2. The choice of temperature and pressure electromagnetic flowmeter can measure fluid pressure and temperature is a certain limit. When used, the operating pressure must be lower than the operating pressure specified by the flowmeter. At present, the working pressure specifications of electromagnetic flowmeters produced in China are:
Less than 50mm diameter, working pressure is 1.6MPa;
900mm diameter, working pressure is 1MPa;
More than 1000mm diameter, working pressure is 0.6MPa.
If there is special requirement on the pressure resistance of the transmitter, it can be specifically consulted with the manufacturer. Some manufacturers have been able to produce electromagnetic flow transmitters with a withstand voltage of 32 MPa.
The working temperature of the electromagnetic flowmeter depends on the lining material used, generally 5-70°C. If you do special treatment, you can exceed the above range, such as wear-resistant anti-corrosion electromagnetic flowmeter produced by Tianjin No.3 Automatic Meter. The transmitter allows the measured medium temperature to be -40 to 10130°C.
3. Selection of lining material and electrode material The lining material and electrode material of the transmitter must be correctly selected according to the physical and chemical properties of the medium, otherwise the instrument will be damaged quickly due to the corrosion of the lining and the electrode, and the corrosion is very strong. Once the media leaks, it can cause accidents. Therefore, the electrode and lining material must be carefully selected according to the specific measuring medium in the production process.
(b) The installation of electromagnetic flowmeters must ensure the accuracy of the electromagnetic flowmeters. The correct installation is very important.
1 transmitter should be installed in indoor dry and ventilated place. Avoid installation in places where the ambient temperature is too high. Do not subject to strong vibration. Try to avoid devices with strong magnetic fields, such as large motors, transformers, etc. Avoid installation in corrosive gases. The installation site is easy to overhaul. This is the environmental condition that guarantees the normal operation of the transmitter.
2 In order to ensure that the measuring tube of the transmitter is filled with the measured medium, the transformer is preferably installed vertically, flowing from bottom to top. Especially for liquid-solid two-phase flow, it must be installed vertically. If the site only allows horizontal installation, you must ensure that the two electrodes in the same level 3 transmitter should be installed at both ends of the valve and bypass.
4 The millivolt AC potential measured by the electrodes of the electromagnetic flow transmitter is based on the liquid potential in the transmitter. In order to make the liquid potential stable and the transmitter and the fluid maintain the same potential, in order to ensure a stable measurement, the transmitter shell and the metal tube should have good grounding at both ends, and the converter shell should also be grounded. The grounding resistance cannot be greater than 10, and cannot be shared with the grounding wire of other electrical equipment. If it is not guaranteed that the transmitter housing is in good contact with the metal pipe, use a metal wire to connect them. And then reliable grounding.
5 In order to avoid interference signals, the signal between the transmitter and the converter must be transmitted with shielded wires. It is not allowed to place the signal cable and the power cable in parallel in the same cable duct. The signal cable length must not exceed 30m.
6 The converter installation site should avoid strong AC and DC magnetic fields and vibrations. The ambient temperature is -20 to 50°C, does not contain corrosive gas, and the relative humidity is not more than 80%.
7 In order to avoid the impact of flow rate relative measurement, the flow control valve should be set downstream of the transmitter. For small diameter transmitters, since the distance from the center of the electrode to the inlet end of the flowmeter is equivalent to several times the length of the diameter D, the upstream straight pipe may not be specified. However, for large-caliber flowmeters, generally there should be more than 5D straight pipe upstream, and the downstream generally do not require straight pipe sections.
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