1. How it works
The working principle of electromagnetic flowmeters is based on Faraday's law of electromagnetic induction. When a conductor moves in a magnetic field, there will be induced electromotive force at both ends of the conductor perpendicular to the direction of the magnetic field and the direction of movement. The magnitude of the electromotive force is proportional to the speed of the conductor movement and the magnitude of the magnetic induction.
Figure 2-9 Working principle of electromagnetic flowmeter
In Figure 2-9, when a conductive fluid flows at an average flow rate υ(m/s) through an insulated tube with an inner diameter of D(m) equipped with a pair of measuring electrodes, and the tube is in a uniform magnetic field with a magnetic induction intensity of B(T), then an electromotive force E(V) perpendicular to the direction of the magnetic field and the direction of flow is induced on the pair of electrodes. E=BDυ (2-2) from the law of electromagnetic induction
In general, the volumetric flow rate qv (m³/s) can be written
It can be obtained from Eq. (2-2) and Eq. (2-3).
Hence the electromotive force can be expressed as
When B is a constant, Eq. (2-4) is in order
Eq. (2-4) is rewritten as:
qv=kE (2-6)
It can be seen that the flow rate qv is proportional to the electromotive force E.
On the one hand, the electromagnetic flow converter provides a stable excitation current to the excitation coil of the electromagnetic flow sensor to achieve the B constant in Figure 2-9, and at the same time, the electromotive force induced by the sensor is amplified and converted into a standard current signal or frequency signal, which is convenient for the display, control and adjustment of the flow rate. Figure 2-10 shows the converter circuit structure.
Figure 2-10 Converter circuit structure
The electromagnetic flow converter is used to measure the volumetric flow rate of the conductive fluid in a closed pipeline. It is widely used in petrochemical, iron and steel metallurgy, water supply and drainage, water conservancy and irrigation, water treatment, environmental protection sewage total control, papermaking, medicine, food and other industrial and agricultural departments of the production process flow measurement and control, and is suitable for the total amount measurement of conductive liquid.
2. Selection of electromagnetic flowmeter
According to the relevant data, 2/3 of the failures of the flowmeter in practical application are caused by the wrong selection and wrong installation, so special attention needs to be paid to the selection.
The following are the selection steps of electromagnetic flowmeter:
Step 1: Collect data.
1) Name of the fluid to be measured;
2) Maximum and minimum flow rates;
3) Maximum working pressure;
4) Maximum temperature and minimum temperature.
The maximum and minimum flow rates must conform to Table 2-1.
Table 2-1 Maximum and minimum flow rates of a flow meter
The flow rate is calculated using the following formula:
Q＝Vπ(D/2)² （2-7）
During the ceremony,
Q is the liquid flow rate (m³/h);
V is the liquid flow rate (m/s) (the range is generally 0.1~15 m/s);
D is the inner diameter of the pipe (mm).
The following points must be taken into account in the collection of data:
1) The actual maximum working pressure must be less than the rated working pressure of the flowmeter.
2) The maximum working temperature and the minimum working temperature must meet the temperature requirements specified by the flow meter.
3) Determine if there is a negative pressure situation.
4) The user can choose the corresponding electromagnetic flowmeter according to the flow rate in the flow range table, if the inner diameter of the selected electromagnetic flowmeter is inconsistent with the inner diameter of the current process pipeline, the pipe should be reduced or expanded. If the pipeline is being reduced, it should be considered whether the pressure loss caused by the reduction will affect the process.
5) Considering the product price, you can choose a smaller-diameter electromagnetic flowmeter, which is relatively less investment.
6) When measuring clean water, the economic flow rate is 1.5~3m/s; when measuring the solution that is easy to crystallize, the flow rate should be increased appropriately, 3~4m/s is appropriate, which plays the role of self-cleaning, preventing adhesion and deposition, etc.; when measuring abrasive fluids such as slurry, the flow rate should be appropriately reduced, 1.0~2m/s is appropriate, so as to reduce the wear of the lining and electrode.
Step 2: Selection of electrode material.
The material of the electrode should be selected according to the corrosivity of the fluid to be measured (see Table 2-2), and the relevant corrosion manual can also be consulted, and experiments should be done for special fluids.
Table 2-2 Electrode materials and their corrosion resistance
Step 3: Lining material selection.
The lining material should be selected according to the corrosiveness, abrasion, and temperature of the medium to be measured (see Table 2-3).
Table 2-3 Selection of lining materials
Step 4: Selection of protection level.
There are generally three types of protection levels for electromagnetic flowmeters:
IP65: anti-spray type, allow the faucet to spray water on the sensor from any direction, the water spray pressure is 30kPa, the water output is 12.5L/s, the distance is 3m, the water inlet into the shell will not reach the harmful level, and the meter will not be affected.
IP67: Anti-short-term immersion type, the sensor is immersed in water, the water ingress of the shell will not reach the harmful level in a short time, and the instrument will not be affected.
IP68: Submersible type, the sensor is immersed in water for 1m, after continuous diving, the water ingress into the shell will not reach the harmful level, and the instrument will not be affected.
3. Mechanical and electrical installation of electromagnetic flowmeter
The correct selection of the installation point and the correct installation of the electromagnetic flowmeter are very important links, if the installation link is wrong, the light will affect the measurement accuracy, the heavy will affect the service life of the electromagnetic flowmeter, and even damage the electromagnetic flowmeter.
(1) Mechanical installation Special attention should be paid to the following when choosing the installation location:
(1) The axis of the measuring electrode must be approximately horizontal, the measuring pipe must be completely filled with liquid, and there must be at least a straight pipe section of 5D (D is the inner diameter of the flowmeter) in front of the flowmeter and a straight pipe section of at least 3D (D is the inner diameter of the flowmeter) in the rear (see Figure 2-11).
Figure 2-11 Mechanical installation of an electromagnetic flowmeter
(2) The flow direction of the fluid is consistent with the direction of the arrow of the flowmeter; if there is a vacuum in the pipeline, the lining of the flowmeter will be damaged, special attention should be paid; there should be no strong electromagnetic field near the flowmeter; there should be ample space near the flowmeter for installation and maintenance; the flowmeter with PTFE lining should be installed and the bolts connecting the two flanges should be tightened evenly, otherwise it is easy to crush the PTFE lining, and it is best to use a torque wrench.
(3) It should be installed at the lower part of the horizontal pipeline and vertically upwards (see Figure 2-12), avoid installing at the highest point of the pipeline and vertically down (see Figure 2-13), and should be installed at the rising part of the pipeline (see Figure 2-14)
Figure 2-12 is installed at the lower part of the horizontal pipeline
Figure 2-13 Avoid installing the pipe at the highest point and vertically downward
Figure 2-14 Installed at the rise of the pipeline
When installing an open discharge pipe, it should be installed at the lower part of the pipe.
(4) If the pipe drop is more than 5m, install an exhaust valve downstream of the sensor (see Figure 2-15), install a control valve and a shut-off valve downstream of the sensor, but not upstream of the sensor (see Figure 2-16), and the sensor should never be installed at the inlet of the pump, but at the outlet of the pump (see Figure 2-17).
Figure 2-15: Install an exhaust valve downstream of the sensor
Figure 2-16: Install a control valve and a shut-off valve downstream of the sensor
Figure 2-17 Mounting position of the sensor and pump
(5) Figure 2-18 shows the method of installing a flowmeter in the measuring well.
(2) Electrical grounding As shown in Figure 2-19, in order to prevent external interference, the electromagnetic flowmeter requires that the measured medium, the user pipeline and the instrument shell must be in the same potential that is, "ground", the instrument must be connected to an independent grounding point, other electrical equipment is not allowed to be connected to the same grounding line, and the grounding resistance should be less than 10Ω. There are two kinds of grounding methods: external grounding and internal grounding, the so-called external grounding is the use of grounding rings, and the so-called internal grounding is the use of grounding electrodes in multiple electrodes.
Figure 2-18 Measuring flowmeter in the well
1—inlet 2—overflow pipe 3—inlet gate 4—cleaning hole 5—flowmeter 6—short pipe 7—outlet 8—discharge valve 9—blowdown valve
Figure 2-19 Grounding of flowmeter 1—Measuring grounding 2—Grounding wire (16mm² copper wire)
(1) When the instrument is installed on a metal pipe with no paint or lining, the grounding wire can be connected to the two pipe flanges, so as to form a reliable contact between the pipeline and the liquid.
(2) When the instrument is installed on a plastic pipe or a pipe with an inner wall insulation, a grounding ring should be installed at the outlet and inlet of the sensor, or an internal grounding electrode should be used to connect the measurement grounding with the liquid.
(3) The instrument is installed on the cathodic protection pipeline, and the pipe with electroerosion protection is usually insulated inside and out to make the liquid groundless to the ground. Attention must be paid to the following points during installation: the two ends of the sensor should be equipped with suitable grounding rings, which rely on the seal to insulate the sensor flange of the pipe flange; the grounding ring must be connected to the sensor and the measuring grounding wire by a copper wire with a cross-sectional area of 16mm² (see Figure 2-20); the two pipe flanges connected with the instrument must be connected with a copper wire with a cross-sectional area of 16mm² (see Figure 2-21); the shaft sleeve and washer of insulating material are used to insulate the flange connecting bolts from the flange.
Figure 2-20 Grounding ring 1—Measuring grounding 2—Grounding wire (16mm² copper wire) 3—Grounding ring
Figure 2-21 Connecting wires 1—Measuring grounding 2—Grounding wires (16mm² copper wires) 3—Grounding rings 4—Connecting wires (16mm² copper wires)
(3) Electrical wiring If the split installation is adopted, the connected signal cable adopts customized special cable, the shorter the cable, the better; the excitation cable can choose YZ medium-sized rubber sheathed cable, its length is the same as the signal cable; the signal cable and the power supply cable must be strictly separated, can not be laid in the same pipe, can not be laid in parallel, can not be stranded together, should be worn separately in the steel pipe; the signal cable and the excitation cable are as short as possible, the excess cable can not be rolled together, the excess cable should be cut off, and the joint should be re-welded, when the cable enters the electrical interface of the sensor, make a U-shape at the port, so as to prevent rainwater from penetrating into the sensor。
Transferred from China Special Equipment Safety and Energy Conservation Promotion Association