What Is an Electromagnetic Flowmeter?

Electromagnetic flow meters (EMF) are new types of flow measurement instruments developed rapidly with the development of electronic technology in the 1950s and 1960s. An electromagnetic flowmeter is an instrument that uses the principle of electromagnetic induction to measure the flow of conductive fluid based on the electromotive force induced when the conductive fluid passes through an external magnetic field.

Chinese name: Electromagnetic Flowmeter
Foreign name: Electromagnetic flowmeter
Pressure: 0.6Mpa, 1.0Mpa higher pressure
Ambient temperature: -20- + 55

Output function: 4-20ma, 0-10V, RS232, 485, etc.
Material: Carbon steel, 304, 316L stainless steel
Supply voltage: AC220V
Style: Integrated, split, plug-in, etc.
Bore diameter: DN3-2600mm

Introduction to electromagnetic flow meters

Structure of electromagnetic flowmeter

The structure of the electromagnetic flowmeter is mainly composed of a magnetic circuit system, a measurement conduit, an electrode, a housing, a lining, and a converter.

Product Flow Chart

Magnetic circuit system: Its role is to generate a uniform DC or AC magnetic field. The DC magnetic circuit is implemented by permanent magnets. Its advantages are simple structure and less interference from AC magnetic field. Surrounding, that is, the polarization phenomenon of the electrode, and causing the internal resistance between the two electrodes to increase, thus seriously affecting the normal operation of the meter. When the diameter of the pipe is large, the permanent magnet is correspondingly large, bulky and uneconomical, so the electromagnetic flowmeter generally uses an alternating magnetic field and is generated by a 50HZ power frequency power supply.

Measuring catheter: Its role is to allow the measured conductive liquid to pass through. In order for the magnetic flux to be shunted or short-circuited when the magnetic field lines pass through the measuring duct, the measuring duct must be made of a material that is non-magnetic, low-conductivity, low-thermal-conductivity and mechanical strength. Plastic, aluminum, etc.

Electrode: Its function is to lead out an induced potential signal that is proportional to the measurement. The electrodes are generally made of non-magnetic stainless steel and are required to be flush with the lining so that the fluid is not obstructed when passing through. Its installation position should be in the vertical direction of the pipeline to prevent sediment from accumulating on it and affecting the measurement accuracy.

Housing: made of ferromagnetic material, it is the outer cover of the excitation coil of the distribution system, and isolates the interference of external magnetic field.

Lining: On the inside of the measuring tube and the sealing surface of the flange, there is a complete layer of electrically insulating lining. It directly contacts the liquid to be measured, and its role is to increase the corrosion resistance of the measuring tube and prevent the induced potential from being short-circuited by the metal measuring tube wall. The lining materials are mostly polytetrafluoroethylene plastics, ceramics, etc. that are resistant to corrosion, high temperature and wear.

Converter: The induced potential signal generated by the liquid flow is very weak and is greatly affected by various interference factors. The role of the converter is to amplify and convert the induced potential signal into a unified standard signal and suppress the main interference signal. Its task is to convert the induced potential signal Ex detected by the electrode into a uniform standard DC signal.

Characteristics of electromagnetic flowmeter

1.The measurement is not affected by changes in fluid density, viscosity, temperature, pressure and conductivity;

2. There are no obstructing flow parts in the measuring tube, no pressure loss, and the requirements for straight pipe sections are low. Unique adaptability to slurry measurement;

3. Reasonable selection of sensor lining and electrode material, that is, it has good corrosion resistance and wear resistance;

4. The converter uses a novel excitation method, low power consumption, stable zero point and high accuracy. Flow range can reach 150: 1;

5. The converter can be integrated or separated with the sensor;

6. The converter uses a 16-bit high-performance microprocessor, 2x16 LCD display, convenient parameter setting and reliable programming;

7. The flow meter is a two-way measurement system, with three totalizers built in: forward total, reverse total, and difference total; it can display positive and reverse flow, and has a variety of outputs: current, pulse, digital communication , HART;

8. The converter uses surface mount technology (SMT), with self-checking and self-diagnostic functions;

9. The measurement accuracy is not affected by changes in fluid density, viscosity, temperature, pressure, and conductivity. The sensor's induced voltage signal has a linear relationship with the average flow rate, so the measurement accuracy is high.

10. There is no obstruction in the measuring pipe, so there is no additional pressure loss; there are no moving parts in the measuring pipe, so the life of the sensor is extremely long.

11. Since the induced voltage signal is formed in the entire magnetic field-filled space, which is the average value on the pipe loading surface, the straight pipe section required by the sensor is shorter and the pipe diameter is 5 times the length.

12. The converter adopts the latest and most advanced single-chip microcomputer (MCU) and surface mount technology (SMT), which has reliable performance, high accuracy, low power consumption, stable zero point, and convenient parameter setting. Click the Chinese display LCD to display the accumulated flow, instantaneous flow, flow rate, flow percentage, etc.

13. Two-way measurement system can measure forward flow and reverse flow. Special production technology and high-quality materials are used to ensure that the performance of the product remains stable for a long time.

Classification of electromagnetic flowmeter

According to the type of the applied magnetic field, the electromagnetic flowmeter mainly has two types of direct current type and inductive type. The DC electromagnetic rheometer (Figure 2) has a constant constant magnetic field B perpendicular to the tube axis, and two electrodes at C and D are used to measure the electromotive force U induced by the fluid across the magnetic field. The flow Q can be obtained by the following formula:

In the formula, A is the cross-sectional area of the pipeline; d is the diameter of the pipeline; k is the correction coefficient, which is used to modify the influence of factors not taken into account when deriving the formula (such as the fact that the flow velocity in the pipeline of the flow meter is actually not uniform). In common flowmeters, k is about 0.8, but for electromagnetic flowmeters of a specific size and used in a specific working condition, the value of k should be calibrated by the volumetric method (the volume flowing in a certain time). The magnetic field can be generated by a permanent magnet. The magnet material is generally an aluminum-nickel-cobalt alloy. When the flow rate is large, because the pipe diameter is large, an ironless field-excitation winding is used, and a nearly uniform magnetic field is generated by passing its constant DC current.

Inductive electromagnetic flowmeters can be used when the temperature of the fluid being measured is too high or the electrodes have a strong corrosive effect (Figure 2). A and B are two AC exciting windings with the same number of turns (the cross section is drawn in the figure). The windings are connected in series, but their current directions are opposite. When the fluid is stationary, the resultant magnetic flux through the induction coil C is zero, so there is no induced electromotive force in the coil. When the fluid flows, an alternating electromotive force is generated in the induction coil, and its magnitude is proportional to the flow velocity. Based on this principle, there are many modifications. For example, use an exciting winding, and place an induction coil in the opposite direction on both sides of its symmetry and connect them in series (Figure 3). When the fluid flows, the magnetic field lines move in the direction of flow, making the electromotive force in the induction coil on both sides startle Zero, which can indirectly indicate the size of the flow.

There are no other parts in the pipeline of the electromagnetic flowmeter, so in addition to measuring the flow of conductive fluids, it can also be used to measure the flow of non-conductive liquids of various viscosities (with easily ionizable substances added). Electromagnetic flow meters are often used in the nuclear energy industry.

Figure 2 DC electromagnetic flowmeter

Figure 3 Inductive electromagnetic flowmeter

The display method of electromagnetic flowmeter is divided into: split electromagnetic flowmeter and integrated electromagnetic flowmeter. The nominal diameter of the series is DN15 DN3000.

The split type electromagnetic flowmeter is an inductive instrument that measures the volumetric flow rate of the conductive medium in the tube according to the Faraday electromagnetic induction law. It uses a single-chip embedded technology to achieve digital excitation. At the same time, it uses the CAN field bus on the electromagnetic flowmeter. The technology has reached the domestic leading level.

The split electromagnetic flowmeter can also output 4-20mA current signals for recording, adjustment and control while meeting the on-site display. It has been widely used in industrial technologies such as chemical industry, environmental protection, metallurgy, medicine, papermaking, water supply and drainage, etc. Management department.

In addition to measuring the flow rate of general conductive liquids, the split electromagnetic flowmeter can also measure the volume flow of liquid-solid two-phase flow, high-viscosity liquid flow, and salts, strong acids, and strong alkaline liquids.

The integrated electromagnetic flowmeter is based on Faraday's law of electromagnetic induction and is used to measure the volume flow of conductive fluids. Due to its unique characteristics, it has been widely used in the measurement of various conductive liquids in industry. It is mainly used in chemical, paper, food, textile, metallurgy, environmental protection, water supply and drainage industries. It can be used with computers to achieve system control.

1. The electromagnetic flowmeter has no moving parts and no flow blocking parts, which will not cause pressure loss, and will not cause problems such as wear and blockage.

2. The electromagnetic flowmeter is a volume flow measuring instrument. It is not affected by the temperature, viscosity, density and conductivity (within a certain range) of the measured medium during the measurement process.

3 The range of the electromagnetic flowmeter is wide, up to 1: 100. In addition, the electromagnetic flowmeter is only proportional to the average flow velocity of the measured medium, and has nothing to do with the axially symmetrical flow state (laminar or turbulent).

4 The electromagnetic flowmeter has no mechanical inertia, is sensitive in response, can measure instantaneous pulsating flow, and has good linearity, so the measurement signal can be linearly converted into a standard signal output directly by a converter. LD-T type can indicate on the spot, LD type can transmit long distance.

Purchase of electromagnetic flowmeter

Choice of caliber and range

The diameter of the transmitter is usually the same as that of the piping system. If the portioning system is to be designed, the caliber can be selected according to the range of the mother stream and the flow rate. For electromagnetic flowmeters, a flow velocity of 2-4m / s is more appropriate. In special cases. If there is solid particles in the liquid, taking into account the wear and tear, you can choose a common flow rate 3m / s, for fluids easily attached to the pipe wall, you can choose a flow rate 2m / s.

To determine the transmitter size.

The range of the transmitter can be selected according to two principles: one is that the full scale of the meter is greater than the expected maximum flow value, and the other is that the normal flow value is greater than 50% of the full scale of the meter to ensure a certain measurement accuracy.

Lining selection

Correct selection method of lining material of electromagnetic flowmeter

The lining material should be selected according to the corrosiveness, abrasion resistance and temperature of the tested medium:

First, natural rubber (soft rubber)

1.Good elasticity, abrasion resistance and breaking force

2. Resistant to general weak acids and alkalis

3. Water measurement and sewage

Second, acid-resistant rubber (hard rubber)

Resistant to hydrochloric acid, acetic acid, oxalic acid, ammonia, phosphoric acid and 50% sulfuric acid, sodium hydroxide, potassium hydroxide at room temperature, but not resistant to strong oxidants

Test general acid, alkali, salt solution

3. Neoprene

1.Excellent elasticity, high breaking force and good wear resistance

2. Resistance to general low concentration acid and alkali, salt solution corrosion, but not resistant to oxidizing medium

<80 ;

Measurement of water, sewage, mud and ore pulp

Fourth, polyurethane rubber (Polyurethane)

1.Excellent abrasion resistance

2. Poor acid and alkali resistance

<40

Measurement of neutral and highly abrasive coal slurry, mud and ore slurry

Five, polytetrafluoroethylene (PTFE)

1.Boiling-resistant hydrochloric acid, sulfuric acid, nitric acid, aqua regia, concentrated alkali and various organic solvents

2. Good wear resistance and poor bonding performance

-80 + 180 ;

Measurement of concentration, strong alkali strong corrosive solution and sanitary media

Application site selection

1. Understand the process parameters

(1) Know the name of the liquid to be tested (provided by the user)

(2) Understand the maximum flow, commonly used flow, and minimum flow of the measured liquid (provided by the user)

(3) Understand the process pipe diameter (provided by the user)

(4) Understand the temperature of the medium (provided by the user)

(5) Understand the media pressure (provided by the user)

(6) Understand the conductivity of the measured fluid (provided by the user)

(7) Know if negative pressure exists (provided by the user)

2. Preliminary selection

(1) Determine whether to use an electromagnetic flowmeter based on the name and nature of the measured medium (determined by the salesperson)

Note: The electromagnetic flowmeter can only measure the flow of conductive liquids, but gas, oils and most organic liquids are not examples of general conductive liquids.

(2) Determine the electrode material based on the properties of the substrate

Note: The company generally provides four kinds of electrodes, such as stainless steel, Hastelloy, titanium and tantalum. Which electrode should be selected according to the nature of the relevant information manual

(2) Determine whether to use rubber or PTFE based on the medium temperature (determined by the marketer)

Note: The temperature resistance of rubber must not exceed 80C;

PTFE is resistant to 150C, and can withstand 180C instantly;

Municipal sewage can generally use rubber linings and stainless steel electrodes

(3) Select the body flange specifications (determined by the marketer) based on the media pressure learned

Note: The specification of electromagnetic flange is usually when the caliber is from DN10-250, the rated pressure of the flange is 1.6Mpa;

When the caliber is from DN250-1000, the rated pressure of the flange is 1.0Mpa;

When the actual pressure of the medium is higher than the corresponding range of the pipe diameter-pressure, it is a special order, but the maximum pressure must not exceed 6.4Mpa

(4) Determine the conductivity of the medium

Note: (1) The conductivity of the electromagnetic flowmeter must not be lower than 5uS / cm

(2) The electrical conductivity of tap water is about tens to hundreds of uS / cm. Generally, soft water (deionized water) in boilers is conductive, and pure water (highly distilled water) is not conductive

(3) The electrical conductivity of gas, oil and most organic liquids is much lower than 5uS / cm, and it is not conductive.

3 Understand user requirements

(1) Understand whether combined local display or remote remote display (provided by the user)

Note: When displaying for remote transmission, please understand the maximum distance. The maximum separation distance is 100 meters.

(2) Know if other additional functions are required (provided by the user)

Note: 1. The electromagnetic flowmeter itself has upper and lower limit flow alarms, frequency and current output functions, and no special order is required.

2. There are two types of electromagnetic flowmeter housing sealing protection: IP65 and IP68. When selecting diving type IP68, it is a special order.

3. When the electromagnetic flowmeter is to be connected to a computer, an RS-485 communication port needs to be added, which is a special order.

4 Model selection: After the above steps, the electromagnetic flowmeter model specifications can be finally determined.

How to use the electromagnetic flowmeter

The electromagnetic flowmeter has two operating states: automatic measurement state and parameter setting state.
When the meter is powered on, it automatically enters the measurement state. In the automatic measurement state, the electromagnetic flowmeter automatically completes each measurement function and displays the corresponding measurement data. In the parameter setting state, the user uses the four panel keys to complete the instrument parameter setting.
1.Key functions

1.1 The key function under the automatic measurement state.
Up key: Select the upper display content of the screen circularly;
Compound key + confirm key: enter parameter setting state;
Confirmation key: return to automatic measurement state;
In the measurement state, the contrast of the LCD display is adjusted: the small liquid crystal is pressed for a few seconds through the "composite key + up" or "composite key + down"; the large liquid crystal is achieved by adjusting the potentiometer on the back of the large liquid crystal.
1.2 Key function under the parameter setting state Down key: The number at the cursor is reduced by 1;
Up key: add 1 to the number at the cursor;
Compound key + down key: move the cursor to the left;
Compound key + Up key: move the cursor to the right;
Confirm key: enter / exit submenu;
Confirmation key: In any state, press it continuously for two seconds to return to the automatic measurement state.
Note: 1. When using the "composite key", you should first press the composite key and then simultaneously hold down the "up key" or "down key"
2. In the parameter setting state, if there is no key operation within 3 minutes, the meter will automatically return to the measurement state.
3. For the flow direction selection of flow zero correction, you can move the cursor to the leftmost "+" or "-" and use the "up key" or "down key" to switch it to the opposite of the actual flow direction.
4. Select the unit of flow rate. Move the cursor to the flow rate unit originally displayed in the "Flow range setting" menu, and then use "Up" or "Down" to switch to meet the requirements.
2. Parameter setting function key operation To set or modify the parameters of the electromagnetic flowmeter, the flowmeter must enter the parameter setting state from the measurement state. In the measurement state, press the "composite key + confirmation key" to display the state conversion password (0000). According to the security level, modify it according to the password provided by the manufacturer. After pressing the "composite key + confirmation key" again, it enters the required parameter setting state.

Installation of intelligent electromagnetic flowmeter sensor on process pipeline

1. The intelligent electromagnetic flowmeter measuring blocked pipe must be completely filled with the medium at any time, and it can not work normally under the condition that the pipe is not full or empty. When the medium is not full, you can increase the height of the outlet pipe at the rear end of the flowmeter to make the medium full, so as to avoid the dissatisfied pipe and gas from adhering to the electrode.

2. Vacuum in the pipeline will damage the lining of the flowmeter, so special attention should be paid.
3. The positive direction of flow should be consistent with the positive direction indicated by the arrow on the flow meter.
4. The intelligent electromagnetic flowmeter can be installed on straight pipes or on horizontal or inclined pipes, but the central connection of the two electrodes is required to be horizontal.
5. For liquid and solid two-phase fluids, it is best to use vertical installation so that the measured medium flows from top to bottom, which can evenly wear the lining of the flowmeter and prolong its service life.
6. Make sure there is enough space near the pipe flange for installation and maintenance.
7. If there is vibration in the measuring pipeline, there should be fixed supports on both sides of the flow meter.
8. If the measurement medium is a heavily polluted liquid, install the flow meter body in the bypass pipeline to drain and clear the flow without interrupting the process operation.
9. When installing the Teflon-lined flowmeter, the bolts connecting flanges should be tightened evenly, otherwise the Teflon-lined will be easily crushed. It is best to use a torque wrench.

Magnetic flowmeter instrument installation

Electromagnetic flowmeter method

The electromagnetic flowmeter is simply composed of a flow sensor transmitter. The installation requirements of the electromagnetic flowmeter must be installed at the lowest point of the pipeline or the vertical section of the pipeline. Use and precision requirements.

The measurement principle of the electromagnetic flowmeter does not depend on the flow characteristics. If there is a certain amount of turbulence and vortex in the pipeline in the non-measurement area (such as: elbow, tangential flow restriction or a half-open stop valve upstream), the measurement Nothing. If there is a steady state eddy current in the measurement area, it will affect the measurement stability and measurement accuracy. At this time, some measures should be taken to stabilize the velocity distribution:

Increase the length of the straight pipe section before and after

2. Use a flow stabilizer

3. Reduce the cross section of the measurement point.

1 . Requirements for the external environment

1.1 . The flow meter should be installed in a place where the temperature changes greatly or the device is exposed to high temperature radiation. If it must be installed, measures must be taken for heat insulation and ventilation.

1.2 . The flowmeter is best installed indoors. If it must be installed outdoors, it should avoid rainwater pouring, flooded water and sun exposure. Moisture and sun protection measures must be taken.

1.3 . The flow meter should be installed in an environment containing no corrosive gas. Ventilation measures must be taken when it must be installed.

1.4 . In order to facilitate the installation, maintenance and maintenance, ample installation space is required around the flowmeter.

1.5 . The installation site of the flowmeter should avoid magnetic fields and strong vibration sources. If the pipeline is vibrated, there should be fixed pipeline supports on both sides of the flowmeter.

1. Pipeline electromagnetic flowmeter

2.1 . Requirements for straight sections

In order to improve the effect of eddy current and flow field distortion, the length of the front and rear straight pipe sections of the flow meter is required to be fixed, otherwise it will affect the measurement accuracy (also can be installed rectifier, try to avoid installing near the control valve and half-open valve).

2.2 . Requirements for process tubes

The flowmeter has certain requirements on the upstream and downstream process pipes of the installation point, otherwise it will affect the measurement accuracy.

a . The inner diameter of the upstream and downstream process tubes is the same as the inner diameter of the sensor, and should satisfy: 0.98DND1.05DN (where DN : sensor inner diameter, D : process tube inner diameter)

b . Process tube and sensor must be concentric, coaxial deviation should not be greater than 0.05DN

2.3 . Requirements for the bypass

In order to facilitate the maintenance of the flowmeter, it is better to install a bypass pipe for the flowmeter. In addition, if the heavily polluted fluid and the flowmeter need to be cleaned and the fluid cannot be stopped, a bypass pipe must be installed.

a . Convenient flowmeter maintenance

b . Must be installed for heavily polluted fluids

c . Fluid cannot be stopped and the flowmeter needs to be cleaned

3 Installation requirements for plug-in electromagnetic flowmeters

3.1 . Requirements for straight pipe sections

Inlet / exit straight pipe section: the inlet should be 10 × DN; the outlet should be 5 × DN

3.2 . Requirements for grounding points

In order to make the instrument work reliably and improve the measurement accuracy, it should not be affected by the external parasitic potential. The sensor should have a good grounding and the grounding resistance is less than 10. (If the metal pipeline is well grounded, there is no need to set up a special grounding device.) 3.3 Installation Location requirements as shown

Insert the electromagnetic flowmeter according to the situation of the on-site pipeline. Flowmeters without ball valves should be installed on the pipeline without overwhelming (that is, flowmeters without ball valves can be installed without pressure installation), and holes are opened in the pipeline. Diameter 50, ready to weld the connecting welded pipe to the opening of the pipe; for the occasions requiring continuous flow loading or disallowing the medium to overflow, a ball valve must be installed, that is, a plug-in electromagnetic flowmeter with a ball valve structure is selected; a hole is opened in the pipe Diameter 50, ready to weld the connection welded pipe to the opening of the pipe.

Selection of plug-in electromagnetic flowmeter

Measuring range: Recommended use range: 0.5m / s 10m / s continuously adjustable. Maximum use range: 0.2m / s 15m / s continuously adjustable

Signal output: 1. The switching value can be set as: pulse output (up to 1000HZ); high / low flow alarm; air pipe alarm; flow direction schematic;

Fault alarm; 2. Current output: 4-20mA output

Configuration mode: 1. On-site configuration through three manual keys. 2. Field configuration via remote control. 3. On-site configuration through the Communicator.

Memory: EEPROM will not disappear, no battery is required.

Description of electromagnetic flowmeter

Electromagnetic flowmeters are widely used in sewage, fluorination, process water, tap water industry, medicine, steel, etc. Due to the principle, it can only measure conductive liquids. Although it is much better than other types of flow meters in terms of reliability and stability, customers will still have some problems in actual use. I will elaborate on the selection and installation of electromagnetic flow meters below:

First, like other flowmeters, although the electromagnetic flowmeter has a measuring range ratio of 30: 1, which is higher than that of vortex flowmeters and differential pressure flowmeters, it is also limited. When many customers set their meters, they often Compare it with a water meter, thinking that it can measure a very low flow rate. In general, it can only measure 0.1m / s. Below this flow rate, an electromagnetic flowmeter is difficult to measure correctly. Therefore, it is necessary to make clear the flow range ratio in the initial order. When ordering, you cannot order according to the original pipeline diameter. It is best to determine the instrument diameter according to your actual flow.

Second, like other flowmeters, electromagnetic flowmeters have requirements for straight pipes before and after installation, but they are less demanding than other types of flowmeters, but the most important point is to satisfy: full pipes, and then full pipes. Under the condition of unsatisfactory tube, the flow meter may jump randomly:

Third, like other flowmeters, electromagnetic flowmeters also have a protection level. Generally, the integrated protection level is IP65, and the split type is IP68 (for sensors). If the customer has requirements for the installation environment of the meter, the installation location is in an underground shade. Wells or other wet places, customers are advised to choose the split type. To avoid damage to the instrument due to wrong selection.

Fourth, the electromagnetic flowmeter can measure corrosive liquids, but at the initial stage of the order, the customer must correctly provide other properties of the measurement medium, so as to avoid the wrong selection of the electrode during the selection, which will cause the sensor to be scrapped in the later use process, which will cause inconvenience and Economic loss.

Fifth, although the electromagnetic flowmeter is more reliable, it will not be damaged under normal circumstances, but due to its principle, the sensor electrode surface has been in contact with the liquid for a long time, and the electrode surface is more likely to be contaminated. Therefore, in general, when the electromagnetic flowmeter is disassembled by the customer, it is recommended to remove it once a year to a year and a half to clean the electrode to ensure the measurement accuracy of the entire flowmeter. Any instrumentation needs to be "maintained", and electromagnetic flow meters are no exception.

6. When the main pipeline is a vertical pipeline, under normal circumstances, the water flow is required to be bottom-up, and try not to be top-down. The latter is likely to cause large fluctuations in traffic. In addition to the full pipe installation, this is also very important, followed by the distance between the straight pipe before and after.

Choose a place that is easy to maintain and easy to move. The flow meter should be installed at the rear end of the pump and must not be installed on the suction side; the valve should be installed on the downstream side of the flow.

Working principle of electromagnetic flowmeter

The electromagnetic flowmeter is a flowmeter that measures flow according to Faraday's law of electromagnetic induction. The advantage of the electromagnetic flowmeter is that the pressure loss is extremely small, and the measurable flow range is large. The ratio of the maximum flow rate to the minimum flow rate is generally above 20: 1. The applicable industrial pipe diameter range is wide, up to 3m, the output signal is linear with the measured flow rate, the accuracy is higher, and the electrical conductivity 5s / cm can be measured. Acid, alkali, salt solution, water, sewage, corrosive liquid and fluid flow of mud, pulp, pulp, etc. But it cannot measure the flow of gas, steam and pure water.

When a conductor moves in the magnetic field to cut magnetic field lines, an induced potential is generated in the conductor. The magnitude of the induced potential is proportional to the effective length of the conductor in the magnetic field and the speed of the conductor's movement in the magnetic field perpendicular to the direction of the magnetic field. Similarly, when a conductive fluid flows in a vertical direction in a magnetic field to cut magnetic induction lines of force, an induced potential is also generated on the electrodes on both sides of the pipe. The direction of the induced potential is determined by the right-hand rule, and the magnitude of the induced potential is determined by the following formula:

Ex = BDv---------------(1)

Where Ex-induced potential, V;
Bmagnetic induction strength, T
Dpipe inner diameter, m
vaverage flow velocity of liquid, m / s

However, the volume flow qv is equal to the product of the flow velocity v of the fluid and the cross-sectional area of the pipe (D²) / 4. Substituting equation (1) into this equation yields:

Qv = (D / 4B) * Ex---------(2)

It can be known from the above formula that when the pipe diameter D is fixed and the magnetic induction intensity B is kept constant, the measured volume flow has a linear relationship with the induced potential. If one electrode is inserted on each side of the pipe, the induced potential Ex can be introduced, and the magnitude of this potential can be measured to obtain the volume flow.

According to the Faraday principle of electromagnetic induction, a pair of detection electrodes are installed on the tube wall perpendicular to the measuring tube axis and magnetic field lines. When the conductive liquid moves along the measuring tube axis, the conductive liquid cuts the magnetic field lines to generate an induced potential. The detection electrode detects that the value is proportional to the flow rate, and the value is:

E = B · V · D · K

In the formula: E-induced potential;

K-coefficient related to magnetic field distribution and axial length;

B- magnetic induction intensity;

V-average flow velocity of conductive liquid;

D-electrode distance; (inner diameter of the measuring tube)

The sensor uses the induced potential E as a flow signal and transmits it to the converter. After signal processing such as amplification, conversion filtering, etc., the dot-matrix liquid crystal with backlight is used to display the instantaneous flow and accumulated flow. The converter has 4 ~ 20mA output, alarm output and frequency output, and is equipped with communication interfaces such as RS-485, and supports HART and MODBUS protocols.

Note: The parameters of different electromagnetic flowmeters are slightly different, please be sure to check the instructions when using.

According to Faraday's law of electromagnetic induction, in a uniform magnetic field with a magnetic induction intensity of B, a non-magnetically permeable pipe with an inner diameter of D is placed perpendicular to the direction of the magnetic field. When a conductive liquid flows in the pipe at a velocity of v, the conductive fluid cuts the magnetic field lines. Installing a pair of electrodes on both ends of the pipe section perpendicular to the diameter of the magnetic field can prove that as long as the flow velocity distribution in the pipe is axisymmetric, an induced electromotive force is generated between the two electrodes:

e = KBDv (3-36)

In the formula, v is the average flow velocity on the section of the pipeline, and k is the meter constant. The volume flow of the pipeline is thus:

qv = eD / 4KB (3-37)

It can be seen from the above formula that the volume flow qv has a linear relationship with the induced electromotive force e and the inner diameter D of the measuring tube, and is inversely proportional to the magnetic induction intensity B of the magnetic field, and has nothing to do with other physical parameters. This is the measurement principle of the electromagnetic flowmeter.

It should be noted that, to make formula (3-37) strictly true, the measurement conditions of the electromagnetic flowmeter must satisfy the following assumptions:

The magnetic field is a uniformly distributed constant magnetic field;

The flow velocity of the measured fluid is axisymmetrically distributed;

The liquid to be tested is non-magnetic;

The conductivity of the measured liquid is uniform and isotropic.

Technical requirements of electromagnetic flowmeter

level of accuracy

The accuracy level and the maximum allowable error of the flow meter in the specified flow range should meet the requirements of Table 1. Flowmeter error indicates the use of relative indication error.

Accuracy level and maximum allowable error

level of accuracy	0.2	(0.25)	(0.3)	0.5
Maximum allowable error	± 0.2%	(± 0.25%)	(± 0.3%)	± 0.5%
level of accuracy	1.0	1.5	2.5	/
Maximum allowable error	± 1.0%	± 1.5%	± 2.5%	/
Note: Grades without parentheses are preferred

Citation error

Reference error can also be used for the flowmeter error indication used for instantaneous flow indication. The maximum allowable error series should meet the requirements of Table 1. The accuracy grade is no longer given in the bid of the verification result, and its maximum allowable error is used. FS should also be marked after the maximum allowable error, such as ± 0.5% FS.

In a single verification of a flow meter, the error expression method of the flow meter should be given according to one of the accuracy level and the reference error; for a flow meter that uses a combination of the relative indication error and the reference error to indicate the error, a single verification One method should also be used to express their errors.

Repeatability

The repeatability of the flow meter must not exceed 1/3 of the absolute maximum allowable error specified by the corresponding accuracy class.

Measuring range of electromagnetic flowmeter

The electromagnetic flowmeter has a large measurement range, usually 20: 1 ~ 50: 1, with a wide range of selectable flow rates. The caliber range of electromagnetic flowmeters is wider than other types of flow meters, from a few millimeters to 3 meters; it can measure forward and reverse bidirectional flows. It can also measure the pulsating flow rate, as long as the pulsating frequency is much lower than the excitation frequency; the output of the instrument is linear in nature; it is easy to choose the variety of materials in contact with the fluid, and it can be applied to corrosive fluids. Because the electromagnetic flowmeter has a greater chance of measuring suspended solids or dirt than other flow meters, the probability of failure due to the adhesion layer on the inner wall is relatively high. If the conductivity of the adhesion layer is similar to the conductivity of the liquid, the meter can still output signals normally, but only change the flow area to form a hidden fault of the measurement error; if it is a high conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited; if it is an insulating adhesion layer The electrode surface is insulated to disconnect the measuring circuit. The latter two phenomena make the meter inoperable. ^[1]

The main products of electromagnetic flowmeter

The intelligent electromagnetic flowmeter uses a wide-temperature dot-matrix liquid crystal display with backlight. All displays are in Chinese. It has many functions and is practical. It is particularly convenient for users to operate, reducing unnecessary trouble and errors.

Fast delivery, low cost and easy maintenance.

How does an electromagnetic flowmeter resist interference?

When measuring large interference signals such as pulp / mineral pulp, the 8707 high signal flow tube can be used to improve the signal strength.

The intelligent electromagnetic flowmeter AFLD is designed using the principle of full intelligence, which is very different from the old or fake intelligent electromagnetic flowmeters produced by some domestic enterprises in terms of measurement accuracy, function, reliability and service life.

The service life of the electromagnetic flowmeter should be more than 10-20 years, so we fully consider the factory to this point when designing, we are very careful in every detail from the sensor to the converter, from design, material selection, process, production We are very particular about every aspect of testing, etc. We design and customize the most advanced production line of dedicated and electromagnetic flowmeters in China to ensure the long-term quality of our products.

The plug-in electromagnetic flowmeter is composed of a plug-in electromagnetic flow sensor (referred to as a sensor) and an electromagnetic flow converter (referred to as a converter). It is an instrument used to measure the volume flow of various conductive liquids in pipes. The plug-in electromagnetic flowmeter is used to measure the flow of conductive fluids in tap water, steel, petroleum, chemical, power, industry, water conservancy and other departments. It can also measure corrosive conductive liquids such as acids, alkalis and salts.

Matters needing attention for electromagnetic flowmeter

1. Accuracy level and function Select the accuracy level of the instrument according to the measurement requirements and application occasions, so as to be economical and cost-effective. For example, for the occasion of trade settlement, product transfer and energy measurement, you should choose a higher accuracy level, such as 1.0, 0.5, or higher; for process control, select different accuracy levels according to control requirements; some are only Check the process flow without precise control and measurement. You can choose a lower accuracy level, such as 1.5, 2.5, or even 4.0. At this time, you can choose a low-cost plug-in electromagnetic flowmeter.

2. When measuring the medium flow rate, meter range and caliber to measure general media, the full-scale flow of the electromagnetic flowmeter can be selected in the range of 0.5-12m / s for the measurement medium flow rate, and the range is relatively wide. The choice of instrument specifications (caliber) is not necessarily the same as the process pipeline. It should be determined according to whether the measurement flow range is within the flow rate range, that is, when the pipeline flow rate is too low to meet the requirements of the flow instrument or the measurement accuracy cannot be guaranteed at this flow rate It is necessary to reduce the caliber of the instrument to increase the flow velocity in the tube and obtain satisfactory measurement results.

3. Try to avoid ferromagnetic objects and equipment with strong electromagnetic fields, so as to avoid magnetic fields affecting the working magnetic field and flow signal of the sensor.

4. It should be installed in a dry and ventilated place as far as possible to avoid sun and rain. The ambient temperature should be -20 + 60 and the relative humidity should be less than 85%.

5. There should be ample space around the flowmeter to facilitate testing and maintenance.

Routine maintenance of electromagnetic flowmeter

You only need to perform periodic visual inspection of the instrument, check the surrounding environment of the instrument, remove dust, ensure that water and other substances are not entered, check whether the wiring is good, and check whether there are newly installed strong electromagnetic field equipment or newly installed wires across the instrument near the instrument. If the measurement medium easily stains the electrode or deposits or scales in the measuring tube wall, it should be cleaned and cleaned regularly.

Fault finding of electromagnetic flowmeter

After the flowmeter is put into operation or after a period of normal operation, it is found that the instrument is not working properly. You should first check the external conditions of the flowmeter, such as whether the power supply is good, whether the pipeline is leaking or is not full, whether there are air bubbles in the pipeline, and whether the signal cable Damage, whether the output signal of the converter (that is, the input circuit of the rear instrument) is open. Remember to repair the flowmeter blindly.

Sensor check

Test equipment: one 500M insulation resistance tester and one multimeter.

Test steps:

(1) When the pipeline is full of medium, use a multimeter to measure the resistance between terminals A, B and C. The resistance between AC and BC should be large. If the difference is more than 1 time, it may be that the electrode is leaking, and the outer wall of the measuring tube or the condensed water is adsorbed in the junction box.

(2) When the lining is dry, use an M meter to measure the insulation resistance between AC and BC (should be greater than 200M). Then use a multimeter to measure the resistance between terminals A and B and the two electrodes in the measuring tube (should be in a short-circuit connection). If the insulation resistance is very small, it means that the electrode is leaking, and the whole flowmeter should be returned to the factory for repair. If the insulation is reduced but still more than 50M and the inspection result of step (1) is normal, the outer wall of the measuring tube may be damp, and the inside of the shell can be dried with a hot air fan.

(3) Use a multimeter to measure the resistance between X and Y. If it exceeds 200, the excitation coil and its lead-out wire may be open or have poor contact. Remove the terminal board and check.

(4) Check that the insulation resistance between X, Y and C should be above 200M. If it decreases, dry the inside of the case with hot air. In actual operation, the decrease of the coil insulation will cause the measurement error to increase and the instrument output signal to be unstable.

(5) If it is determined that the sensor is faulty, please contact the manufacturer of the electromagnetic flowmeter. Generally, it cannot be resolved on site, and it needs to be repaired by the manufacturer.

Converter check

If it is determined that the converter is faulty and there is no problem after checking the external cause, please contact the manufacturer of the electromagnetic flowmeter, and the manufacturer will generally take the solution of replacing the circuit board.

Electrode maintenance

1. Before using an electromagnetic flowmeter, first calibrate the electromagnetic flowmeter with a standard pH solution. Before calibration, before operation, everyone must pay attention to cleaning the electrodes of the electromagnetic flowmeter with distilled water, and then cleaning the electrodes with the test liquid.

2. If the electromagnetic flowmeter is not used, when removing the electrode of the electromagnetic flowmeter, everyone should pay attention not to let the sensor of the electrode collide with the hard object, otherwise the use of the electrode will be affected as long as the damage occurs.

3. After using the electromagnetic flowmeter, please put the electrode of the electromagnetic flowmeter on the sleeve, and put less saturated solution in it, as long as the bulb of the electrode is moist, it is enough, but remember not to put it in distilled water. soak.

4. Usually pay attention to keep the electrode clean, do not let the two sides of its output have a short-circuit situation, otherwise it will make the measurement inaccurate and affect the use of the electromagnetic flowmeter.
In fact, there are many ways to maintain the electrodes of the electromagnetic flowmeter. Everyone should pay more attention during the use. Don't cause the electromagnetic flowmeter to fail to be used normally because of a little negligence.

Failure analysis of electromagnetic flowmeter

1. Failure during commissioning

During the commissioning period, faults generally occur during the installation and commissioning of the instrument. Once eliminated, they will not reappear under the same conditions. Common faults during commissioning are usually caused by improper installation, environmental interference and the influence of fluid characteristics.

Installation

It is usually caused by the incorrect installation position of the electromagnetic flow sensor. Commonly, if the sensor is installed at the highest point of the piping system that is prone to accumulate gas; or it is installed on a vertical pipe from the top to the bottom, it may be empty; Back pressure, the fluid is directly discharged into the atmosphere to form a non-full tube in the measuring tube.

Environmental aspect

Usually it is mainly stray current interference of pipes, strong electromagnetic wave interference in space, and magnetic field interference of large motors. Pipeline stray current interference usually achieves satisfactory results with good separate ground protection, but if a strong stray current is encountered (such as in electrolytic workshop pipes, sometimes the peak Vpp of AC potential induced on the two electrodes can be as high as 1V), Additional measures need to be taken and the flow sensor is isolated from the pipeline. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single or multiple layers of shielding.

Fluid aspect

The measured liquid contains uniformly distributed tiny air bubbles, which usually does not affect the normal operation of the electromagnetic flowmeter, but with the increase of air bubbles, the meter

The output signal of the meter will fluctuate. If the air bubble is large enough to cover the entire electrode surface, the electrode circuit will be instantly disconnected as the air bubble flows through the electrode, causing the output signal to fluctuate more.

When the low-frequency square wave excitation electromagnetic flowmeter measures the slurry with too much solid content, it will also generate slurry noise and make the output signal fluctuate.

When measuring mixed media, if the flow sensor is used for measurement before the mixing is not uniform, the output signal will also fluctuate.

Improper selection of electrode material and measured medium will also affect normal measurement due to chemical interaction or polarization phenomenon. The electrode materials should be correctly selected according to the instrument selection or relevant manuals.

Failure during operation

The fault during operation is a fault that occurs after the electromagnetic flowmeter is debugged and operated normally for a period of time. Common faults during operation are basically caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.

Sensor inner wall adhesion layer

Because electromagnetic flowmeters are often used to measure dirty fluids, after running for a period of time, adhesion layers often accumulate on the inner wall of the sensor and cause malfunctions. These failures are often caused by the conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be disconnected and the meter will not work properly. If the conductivity of the adhesion layer is significantly higher than the fluid conductivity, the electrode circuit will be short-circuited.

The meter also does not work properly. Therefore, the scale layer attached to the measuring tube of the electromagnetic flowmeter should be removed in time.

Lightning strike

Lightning strikes can easily induce high voltages and surge currents in the meter lines, causing damage to the meter. It is mainly introduced through the power line or the flow signal line between the excitation coil or the sensor and the converter, especially from the control room power line.

Changes in environmental conditions

During the debugging period, because the environmental conditions are good (for example, there is no interference source) and the flowmeter works normally, it is often easy to neglect the installation conditions (for example, the grounding is not very good). In this case, once the environmental conditions change, new interference sources appear during operation (such as electric welding on the pipeline near the flow meter, and a large transformer is installed nearby), which will interfere with the normal operation of the instrument and the output of the flow meter. The signal will fluctuate.

Common faults of electromagnetic flowmeter

Typical fault diagnosis and treatment

1. No flow output.

2. (70100)

3. (100)5s/cm

3[1]

500M

5s/cm

0.5m/s2~4m/s8m/s
500m³4500m³4125m³/h r²×0.5~8m/s=125m³/h,125m³/h0.075m~0.2975mDN80~DN3002~4m/s0.105m~0.149mDN100~DN150DN100

1. 5DN3DN3DN

2. 60-62mm

4. 3

6. L2L2

I. Influence of various media on measurement After a distance of a straight pipe section, the flow velocity partial velocity can become an axisymmetric distribution. The flow velocity is the largest at the center of the tube axis and zero at the wall of the tube. The average flow velocity is V-, as long as the flow velocity distribution is symmetrical with respect to the center axis of the measuring tube , The magnitude of the induced electromotive force generated on the electrode has nothing to do with the state of the flow velocity distribution at each point, but is directly proportional to the average flow velocity of the measured liquid. Therefore, it is one of the working conditions that the uniform magnetic field type electromagnetic flowmeter must be satisfied that the flow velocity distribution is axisymmetric. If the flow velocity distribution is asymmetric with respect to the central axis of the tube, although the total flow is the same, the induced electromotive force is large near the electrode, so the measured signal is larger than the actual flow value. On the contrary, the signal obtained when the induced electromotive force is small at 90 ° to the electrode is smaller than the actual flow value, causing measurement errors. Therefore, in order to make the flow velocity distribution axis symmetrical, it is necessary to add a straight pipe section before the flowmeter.
The influence of the magnetic field edge effect on the measurement If it is assumed that the magnetic field is always uniform along the direction of fluid flow, in fact, this means that the magnetic field in the direction of the tube axis is infinitely long and the magnetic field of the actual flow meter is finite Therefore, it is necessary to consider the influence of edge effects caused by the finite long magnetic field on the measurement. It is assumed that the tube wall is insulated, the magnetic field near the electrode is approximately uniform, and the two ends gradually weaken, forming an uneven edge, and finally falling to zero. In this way, the electric field E in the liquid is not uniform, and an eddy current will be generated. The secondary magnetic flux generated by the eddy current in turn changes the working magnetic flux at the edge of the magnetic field, which further damages the uniformity of the magnetic field. At this time, the magnitude of the induced electromotive force measured on the electrode is not the same as the magnitude of the induced electromotive force under an infinitely long magnetic field, resulting in an error. If the tube wall is conductive, due to the short-circuit effect of the conductive tube wall, the magnetic field edge effect will be more obvious. With the change of the tube wall conductivity and wall thickness, this effect will become more obvious, resulting in induced electromotive force on the electrode. Increase in losses. For electromagnetic flowmeters, it is necessary to measure the insulation of the pipe wall, so the pipe wall is usually coated with an insulating layer. If the measured medium contains a magnetically permeable substance, the magnetic field edge effect is more complicated. Due to the presence of magnetically permeable material, the magnetic field is severely distorted, resulting in a non-linear measurement. Therefore, for liquids containing liquid metals, DC excitation is generally used to reduce magnetic field edge effects.
(3) The influence of the conductivity of the measured medium, the input impedance of the electromagnetic flowmeter converter has been improved. When measuring conductive liquids, generally no error is caused by a small change in the conductivity of the medium, but for a certain converter input impedance, The conductivity of the measured medium has a lower limit, which cannot be lower than the lower limit. The conductivity of the measured medium is too large to be allowed. For example, when the electrical conductivity exceeds about 10-1 S / cm, the flow signal will be reduced and the indicated value will be changed, that is, the indicated flow value is smaller than the actual flow value. When the conductivity of the measured medium is large, the resistance of the external circuit is small. At this time, no matter how high the input impedance of the converter, the parallel result will depend on this part of the liquid external circuit to reduce the transmitter and converter. Transmission accuracy. Therefore, for an electromagnetic flowmeter, there is a certain range in which the measurement is not affected by the conductivity of the medium. The conductivity of the measured medium can neither be too large nor too small. If the conductivity of the medium is extremely high, a large eddy current will be generated in the edge region of the magnetic field, causing secondary magnetic flux, and the magnetic fields on both sides of the working magnetic field edge region will be weakened and enhanced respectively. Therefore, it is not suitable to use AC excitation for medium with high conductivity, but DC excitation. With the development of electronic technology, the increase of the input impedance of the converter will definitely reduce the lower limit of the conductivity of the measured medium.
III. Selection of flow sensor The selection of electrode material for electromagnetic flowmeter. Improper selection of electrode material and measured medium will affect the normal measurement due to chemical action or polarization phenomenon. The electrode material should be selected according to the corrosivity of the measured medium. Select the lining material of the electromagnetic flowmeter according to the corrosivity, abrasion and temperature of the measured medium. Try to choose an electromagnetic flowmeter with lightning protection.
Fourth, the flow sensor installation 1, the requirements of the installation site.
1) When measuring mixed-phase fluids, choose a place that will not cause phase separation. When measuring two-component liquids, avoid installing downstream of the mixture that is not evenly mixed. When measuring chemical reaction pipelines, install it in the fully completed reaction section.
2) Avoid negative pressure in the measuring tube as much as possible.
3) Choose a place with small vibration, especially for integrated instruments.
4) Avoid large motors and transformers nearby to avoid electromagnetic field interference.
5) A place where the sensor can be grounded separately.
6) Avoid high concentration corrosive gas in the surrounding environment as much as possible.
7). The ambient temperature is in the range of -25-60 , and the relative humidity of the environment is in the range of 10% -9O%. Avoid direct sunlight as much as possible.
8) The liquid should have the conductivity required for measurement, and the conductivity distribution should be substantially uniform. Therefore, the flow sensor should be installed to avoid the place where the conductivity is uneven. For example, if the liquid is added near the upstream, the liquid adding point is preferably located downstream of the sensor.
2. Straight pipe length requirements, electromagnetic flowmeters have lower requirements on the front and rear straight pipe sections. For 90o elbows, T-tees, concentric reducers, and fully open gate valves, it is usually only necessary to leave the electrode centerline, not the sensor inlet connection surface. For straight pipe sections> 5 times the diameter, valves with different openings require 1OD, and the downstream straight pipe sections are 3D. When measuring mixed liquids of different media, the distance between the mixing point and the flow meter must be greater than 30D.
3. The installation position and flow direction, the installation direction of the sensor can be horizontal, vertical or inclined, without restrictions. However, it is best to install solid and liquid two-phase fluids vertically and flow from bottom to top. This can avoid the disadvantages of severe local wear of the lower half of the lining during horizontal installation and solid phase precipitation at low flow rates. When installing horizontally, the electrode axis should be parallel to the horizon to prevent the insulation of the electrode surface caused by the occasional bubbles in the liquid from rubbing against the electrode surface. When installed vertically, the flow direction should be upward, so that when there is no flow or small flow, the heavy solid particles in the fluid sink, and the lighter fatty substances rise up and leave the sensor electrode area of the flowmeter.

4. The grounding sensor must be grounded separately, and the grounding resistance is below 100. In principle, the separation type should be on the sensor side, and the converter ground should be at the same ground point.