What is an electromagnetic flowmeter
What is an electromagnetic flowmeter?
Electromagnetic flowmeter is an instrument for measuring the volume flow of conductive liquid based on Faraday’s law of electromagnetic induction. Its conversion principle is the famous Faraday law of electromagnetic induction, that is, the conductor cuts the electromagnetic line through the magnetic field to produce electromotive force. The magnetic field of the flow sensor is realized by excitation, which is divided into DC excitation, AC excitation and low-frequency square wave excitation. Electromagnetic flowmeter is simply composed of flow sensor and transmitter.
In the electromagnetic flowmeter, the conductive medium in the measuring tube is equivalent to the conductive metal rod in Faraday test, and the two electromagnetic coils at the upper and lower ends produce a constant magnetic field. When a conductive medium flows through, an induced voltage will be generated, which is directly proportional to the flow rate of the fluid, and then the flow value Q is obtained.
An electromagnetic flowmeter is a transducer that measures fluid flow induced by a voltage flowing through a liquid through a magnetic field. A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow rate perpendicular to the flux line.
The physical principle of electromagnetic flowmeter is electromagnetic induction. Electromagnetic flow meters require conductive fluids (e.g., water containing ions) and electrically insulated tube surfaces (e.g., rubber lined steel tubes). If the direction of the magnetic field is constant, the electrochemical and other effects on the electrode will make it difficult to distinguish the potential difference from the potential difference caused by fluid flow. In order to alleviate this situation in modern electromagnetic flowmeter, the magnetic field is constantly reversed, so as to offset the electrochemical potential difference, which will not change direction with the magnetic field. However, this prevents the use of permanent magnets in electromagnetic flow meters.
How is electromagnetic flowmeter calculated
The electromagnetic flowmeter applies the principle of Faraday phenol water induction law, that is, when the conductive object moves to cut the magnetic wire in the magnetic field,
Induced electromotive force is generated in the conductor. The induced electromotive force E is:
Flow Q: Q = 3600 × V × S
In the formula:
- K – instrument coefficient
- B – magnetic induction intensity (T)
- D – electrode spacing (m)
- V – average velocity of fluid (M/s)
- S – inner sectional area of conduit (M2)
When measuring the flow rate, the conductive liquid flows through the magnetic field perpendicular to the flow direction at the flow rate v. the flow of the conductive liquid induces a voltage proportional to the average flow rate, and its induced voltage signal is detected through the electrode in direct contact with the liquid. For the same flowmeter, s, B and D are constant, so the flow is directly proportional to the induced electromotive force E (or flow rate V).
Difference between electromagnetic flowmeter and ordinary flowmeter
Electromagnetic flowmeter is used to measure conductive liquid, which is often used in environmental protection industry. Metal tube rotameter can measure gas and liquid. For glass rotameter, it is difficult to measure opaque liquid because of intuitive observation of flowmeter.
Measuring range of electromagnetic flowmeter
Generally, the measured medium flow rate of industrial electromagnetic flowmeter should be 2 ~ 4m/s. under special circumstances, the minimum flow rate should not be less than 0.2m/s and the maximum should not be greater than 8m/s. If the medium contains solid particles, the common flow rate shall be less than 3m/s to prevent excessive friction between the lining and the electrode; For viscous fluid, the flow rate can be greater than 2m/s. a larger flow rate is helpful to automatically eliminate the effect of viscous substances attached to the electrode and improve the measurement accuracy.
When the measuring range Q has been determined, the diameter D of the flowmeter can be determined according to the above flow rate V range, and its value is calculated by the following formula: Q=πD2V/4
In the formula:
- Q: Flow (m2/h)
- D: Pipe inner diameter
- V: Flow rate (M/h)
The range Q of the electromagnetic flowmeter shall be greater than the expected maximum flow value, and the normal flow value shall be slightly greater than 50 of the full scale of the flowmeter.
Characteristics of electromagnetic flowmeter
The measurement accuracy is not affected by the changes of fluid density, viscosity, temperature, pressure and conductivity. The sensor induced voltage signal has a linear relationship with the average flow rate, so the measurement accuracy is high.
There is no choke in the measuring pipeline, so there is no additional pressure loss; There are no movable parts in the measuring pipe, so the service life of the sensor is very long.
Because the induced voltage signal is formed in the whole space filled with magnetic field and is the average value on the pipe load surface, the straight pipe section required by the sensor is short and the length is 5 times the pipe diameter.
Only the lining and electrode of the sensor are in contact with the measured liquid. As long as the electrode and lining materials are reasonably selected, they can be resistant to corrosion and wear.
The LDE converter adopts the latest and most advanced single chip microcomputer (MCU) and surface mount technology (SMT), with reliable performance, high precision, low power consumption, stable zero point and convenient parameter setting. Click the Chinese LCD to display the cumulative flow, instantaneous flow, flow rate, flow percentage, etc.
Two way measurement system can measure forward flow and reverse flow. Special production technology and high-quality materials are adopted to ensure that the performance of the product remains stable for a long time.
Advantages and disadvantages of electromagnetic flowmeter
Advantages of electromagnetic flowmeter
- 1. The measuring channel of electromagnetic flowmeter is a smooth straight pipe without flow blocking detection parts. Because it is not easy to obstruct, it is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, mineral slurry, slurry and sewage.
- 2. The electromagnetic flowmeter does not produce the pressure loss caused by the detection flow, and the resistance of the instrument is only the resistance along the pipeline with a unified length. The energy-saving effect is obvious. It is most suitable for the large-diameter water supply pipeline requiring low resistance loss.
- 3. The volume flow measured by electromagnetic flowmeter is not significantly affected by the changes of fluid density, viscosity, temperature, pressure and conductivity (as long as it is above a certain threshold).
- 4. Compared with most other flow instruments, the requirements of front straight pipe section are lower.
- 5. The measurement range is large, usually 20:1-50:1, and the optional flow range is wide. The fullness value and liquid flow rate can be selected within 0.5-10m/s.
- 6. The caliber range of electromagnetic flowmeter is wider than that of other types of flow instruments, from a few millimeters to 3m. It can measure positive and negative two-way flow and pulsating flow, as long as the pulsating frequency is much lower than the excitation frequency. The instrument output is linear in nature.
- 7. It is easy to select the type of material in contact with the fluid and can be applied to aggressive fluids.
Disadvantages of electromagnetic flowmeter
- 1. Electromagnetic flowmeter can not measure liquids with low conductivity, such as petroleum products and organic solvents. Gas, steam and liquid containing more and larger bubbles cannot be measured.
- 2. Because of the limitation of lining material and electrical insulation material, the universal electromagnetic flowmeter can not be used for liquids with high temperature above 200 ℃; At the same time, it shall not be used for the medium with too low temperature, and the insulation shall be damaged due to condensation (or frost) outside the measuring tube.
Classification of electromagnetic flowmeter
At present, electromagnetic flowmeter has developed into many types, and there are many classification methods:
1. Classification by excitation mode
- 1) The number of DC excitation electromagnetic flowmeter is very small, which is only used to measure the flow of liquid metal, such as mercury at room temperature and liquid sodium and potassium at high temperature.
- 2) The early electromagnetic flowmeter of AC power frequency excitation type uses 50Hz power frequency mains excitation. Because it is vulnerable to electromagnetic interference and zero drift, it has been gradually replaced by low-frequency rectangular excitation. However, when measuring the liquid-solid two-phase flow such as mud and slurry, the low-frequency rectangular wave excitation method can not overcome the peak noise generated by the solid passing over the electrode surface, while the power frequency AC excitation instrument does not have this disadvantage. Therefore, some electromagnetic flowmeters at home and abroad still use the AC excitation method.
- 3) The low-frequency rectangular wave excitation type is used for low-frequency rectangular wave excitation with low power consumption and no disorder of zero point. It is the main excitation mode of electromagnetic flowmeter at present. There are two kinds of waveforms: “positive negative” and “positive zero negative zero”. The excitation frequency of some electromagnetic flow meters can be set by the user. Generally, small-diameter instruments use higher frequency and large-diameter instruments use lower frequency.
- 4) The waveform of dual frequency excitation current is to superimpose high-frequency rectangular wave on low-frequency rectangular wave, which is mainly to overcome the slurry noise and activity noise existing in binary rectangular wave excitation and improve the stability and response characteristics of the instrument.
2. Classification according to the composition of sensors and converters
- 1) Separate type this is the main type of electromagnetic flowmeter. The sensor is installed on the smooth pipeline, and the converter is installed in the instrument room or around the sensor easy to install and operate, with an interval of tens to hundreds of meters. The advantage is that the converter can be far away from the harsh environment on site, and the inspection, adjustment and measurement parameters of electronic devices are more convenient.
- 2) The all-in-one sensor and converter are assembled together and installed on the process pipeline to directly output the current (or frequency) scale signal reflecting the flow. The advantage is that the connection length of the flow signal line and the excitation line between the sensor and the converter is shortened, and there is no such external wiring, so the electrical wiring is simple and the price is relatively cheap. However, it is easily limited by pipeline layout. If it is installed in a place that is not easy for people to access, it is very inconvenient to maintain; In addition, the electronic devices in the converter are installed on the pipeline and are vulnerable to liquid temperature and pipeline vibration.
3. Classification by connection mode
According to the connection method between flow sensor and pipeline, there are flange connection, flange clamping connection, sanitary connection and threaded connection.
4. Classification by purpose
- 1) General purpose electromagnetic flowmeter is the main type of electromagnetic flowmeter, which is used in the fields of metallurgy, petrochemical, papermaking, light textile, water supply and drainage, sewage treatment, medicine, food, biology and fine chemical industry. There is a range requirement for the conductivity of the measured medium, which generally cannot exceed its upper and lower limits.
- 2) Explosion proof type is used in places with explosive atmosphere. Because the excitation current energy is large, it is still mainly flameproof. In recent years, intrinsically safe electromagnetic flowmeter, i.e. safe spark electromagnetic flowmeter, has emerged abroad. The excitation power is greatly reduced and can be made into one, all of which can be installed in dangerous areas.
- 3) The sanitary electromagnetic flowmeter used in medicine, food and biochemistry industries shall meet the relevant hygienic requirements in terms of regular bacteria arrival, easy disassembly and cleaning, etc.
- 4) The anti immersion type is used for sensors installed under the ground, which can withstand short-term water immersion.
- 5) The submersible type is used to measure the flow on the free surface of open channel or non full pipe closed channel. The sensor is located at the lower part of the open channel closure baffle and works under water for a long time. It is different from the general electromagnetic flowmeter in structure and use.
- 6) Plug in type electromagnetic flow sensor for large pipe diameter. The sensor is inserted radially from the pipe opening to calculate the flow by measuring the local flow velocity. The accuracy is low, but the price is cheap. It is suitable for the control system.
Application of electromagnetic flowmeter
- 1. The general-purpose electromagnetic flowmeter is an important example of electromagnetic flowmeter, which is used in metallurgy, petrochemical, papermaking, light textile, water supply and drainage, sewage treatment, medicine, food, biology and fine chemical industry. There is a category requirement for the conductivity of the measured medium, and individual can not exceed its high and low limits.
- 2. Explosion proof type is used in occasions with explosive atmosphere. Because the excitation current energy is large, it is still mainly flameproof. In recent years, foreign countries have shown a substantial insurance type, that is, the insurance spark electromagnetic flowmeter. The excitation power has decreased greatly. It can be made into a whole and installed in the injured area.
- 3. Sanitary electromagnetic flowmeter used in medicine, food and biochemistry industries shall meet the requirements of relevant hygiene in terms of timely bacteria arrival, easy assembly and cleaning.
- 4. The anti immersion type is used for sensors installed in the air, which can withstand short-time water immersion.
- 5. The submersible type is used to measure the flow of open channel or non full pipe closed channel when flowing freely on the water surface. The sensor is located at the lower part of the open channel closure baffle and temporarily immersed in water. It is different from individual electromagnetic flowmeter in structure and application.
- 6. Pull out type is used for electromagnetic flow sensor with large pipe diameter. The sensor is pulled out radially from the pipe opening, and the flow is calculated by measuring part of the flow velocity. The accuracy is low, but the price is cheap, which is practical for the control system.
Application of electromagnetic flowmeter in chilled water
Application in the measurement of chilled water cooling capacity: electromagnetic flowmeter has obvious advantages in the flow measurement of chilled water. For example: no pressure loss, high precision and good stability. However, due to the characteristics of chilled water, some problems should be paid attention to in the use of electromagnetic flowmeter.
The first is the inner village of the electromagnetic flowmeter. Pay attention to the negative pressure of the inner village. When the measuring medium has certain corrosivity, we should use PTFE memory material. However, the electromagnetic flowmeter made of tetrafluoroethylene can not bear negative pressure.
There are two ways to solve the problem:
- First, the use of skeleton memory has the disadvantage of increasing the investment cost.
- Second, change the original vent valve into a one-way valve (check valve). When there is a small negative pressure in the pipeline, the check valve will automatically open to suck air from the atmosphere, so as to ensure safety.
The above are the matters needing attention of electromagnetic flowmeter in chilled water measurement. Another flowmeter suitable for chilled water measurement is TDOA ultrasonic flowmeter, which should also pay attention to some problems in use. For example, since the sensor and cable plug may be wrapped in the insulation layer, in order to prevent damage to the sensor caused by condensation, the protection grade of cable plug shall be IP68 submersible protection grade.
Can electromagnetic flowmeter measure liquid mixture
Electromagnetic flowmeter is used to measure conductive liquid. What is the measurement result of mixed liquid electromagnetic flowmeter?
Liquid containing bubbles
The medium mixed with tiny bubbles can be measured normally, but when the bubble volume is too large, the electrode may be covered by gas, the circuit will be cut off instantly, the output will shake and can not function normally.
Liquid in solid-liquid two-phase flow
Solid-liquid two-phase fluids containing non ferromagnetic particles or fibers can also measure the volume flow of the two phases. Fluids with high solid content, such as drilling mud, drilling cementing cement slurry, pulp, etc. As solids flow together in the carrier liquid, there is sliding between them and there is a difference in speed. The instrument for single-phase liquid inspection will produce additional error when used in solid-liquid two-phase flow.
When the large particles in the slurry rub the electrode surface, the sharp slurry noise occurs in the rectangular excitation electromagnetic flowmeter with low frequency, and the flow signal becomes unstable, select the high-frequency instrument and the instrument with strong ability to suppress the slurry noise.
Liquid containing ferromagnetic substance
For the fluid containing strong magnetic substances, the permeability in the measuring tube changes according to the content of strong magnetic substances, resulting in measurement error. However, by placing the electromagnetic flowmeter EMF compensated on the outside of the magnetic flux detection line in the magnetic circuit, the influence of mixing of strong magnetic substances can be reduced. The test report shows that the water contains the slurry of iron concentrate with liquid-solid weight ratio of about 4:1 and particle size ≤ 0.15mm. The comparison flow test of clean water and slurry is carried out with an 80mm diameter instrument. The general instrument indication changes by 7% – 10%. For the instrument equipped with magnetic flux detection coil, the indication error is within ± 2% FS.
When measuring the slurry containing ore particles, attention should be paid to the wear degree of the sensor lining. The expansion of the inner diameter of the measuring tube will produce additional errors. In this case, it is proposed to install the sensor on the vertical piping while selecting the ceramic lining or polyurethane rubber lining with good wear resistance, so as to make the wear of the piping uniform and eliminate the serious local wear of the lower half of the horizontal installation.
The above brief introduction shows that the intelligent electromagnetic flowmeter can be used to measure the mixture, avoid relevant problems, and also obtain good measurement results.
Lightning protection method for electromagnetic flowmeter converter
Thunderstorm season, thunder has become the biggest threat to electromagnetic flowmeter. I remember that a few years before we started using the electromagnetic flowmeter, it was a headache for us to encounter thunderstorm weather. More than half of the electromagnetic flowmeter converters were damaged by lightning every year due to thunderstorm weather. So how can we prevent the converter of electromagnetic flowmeter from being lightning protected?
Only with good grounding protection can the electromagnetic flowmeter be measured accurately. Maybe in the environment without such interference, non grounding will not affect the accurate measurement of the flowmeter, but once the environment changes, the flowmeter will not work accurately and stably. Therefore, we’d better do a good job in sensor grounding protection during installation to create a good working environment for the electromagnetic flowmeter.
Lightning protection of electromagnetic flowmeter:
- 1. The length of grounding wire of lightning arrester shall not exceed 1m, and the cross-sectional area shall be more than 6mm Square multi strand copper core insulated conductor. If there is no grounding grid nearby, a simple grounding grid shall be made near the lightning arrester. The method is as follows: use three flat steels or angle steels with a length of 1.5m to drive them into the ground according to the regular triangle, weld the three flat steels with flat steel strips, weld bolts at one corner, and then connect the grounding wire of the lightning arrester to install the panel on the grounding plane of 220VAC lightning protection grounding wire.
- 2. The length of the signal cable between the converter and the sensor shall not be greater than 50m, and the signal cable must be sleeved with galvanized pipe. If the galvanized pipe is in the air, the galvanized pipe shall be reliably grounded with conductor, and the grounding resistance shall be less than 10.
- 3. The power supply is 220V one-way AC. an air switch is installed in the instrument box. The lightning arrester and the air switch are connected in parallel. The conductor comes out of the air switch and is connected to a three-point socket.
Structure of electromagnetic flowmeter
The structure of electromagnetic flowmeter is mainly composed of magnetic circuit system, measuring conduit, electrode, shell, lining and converter.
- 1. Measuring catheter: its function is to let the measured conductive liquid pass through. In order to make the magnetic flux shunted or short circuited when the magnetic force line passes through the measuring conduit, the measuring conduit must be made of non-magnetic, low conductivity, low thermal conductivity and materials with certain mechanical strength. Non-magnetic stainless steel, FRP, high-strength plastic, aluminum, etc. can be selected.
- 2. Magnetic circuit system: its function is to produce uniform DC or AC magnetic field. The DC magnetic circuit is realized by permanent magnet, which has the advantages of simple structure and less interference by AC magnetic field, but it is easy to polarize the electrolyte liquid in the measuring catheter, so that the positive electrode is surrounded by negative ions, and the negative electrode is surrounded by positive ions, that is, the polarization phenomenon of the electrode, which leads to the increase of locust internal resistance of the two electrodes, which seriously affects the normal operation of the instrument. When the pipe diameter is large, the permanent magnet is also large, bulky and uneconomical, so the electromagnetic flowmeter generally adopts alternating magnetic field and is excited by 50Hz power frequency power supply.
- 3. Housing: made of ferromagnetic material, it is the housing of distribution system excitation coil and isolates the interference of external magnetic field.
- 4. Electrode: its function is to lead out the induced potential signal proportional to the measured. Electrodes are generally made of non-magnetic stainless steel and are required to be flush with the lining so that the passage of fluid is not obstructed. It should be installed in the vertical direction of the pipeline to prevent sediment from accumulating on it and affecting the measurement accuracy.
- 5. Converter: the induced potential signal generated by liquid flow is very weak and greatly affected by various interference factors. The function 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 amplify and convert the induced potential signal ex detected by the electrode into a unified standard DC signal.
- 6. Lining: there is a complete layer of electrical insulation lining on the inner side of the measuring conduit and the flange sealing surface. It directly contacts the measured liquid, and its function is to increase the corrosion resistance of the measuring conduit and prevent the induced potential from being short circuited by the pipe wall of the metal measuring conduit. The lining materials are mostly corrosion-resistant, high temperature resistant and wear-resistant polytetrafluoroethylene plastics, ceramics, etc.
How to correctly select the lining material of electromagnetic flowmeter
Electromagnetic flowmeter is widely used in various industries, such as petroleum and chemical industry. In fact, the most important thing for the selection of electromagnetic flowmeter is the selection of its lining. How to choose the lining, we should first understand it to help us choose the lining.
1. Ordinary rubber, natural rubber, soft rubber, hard rubber.
The operating temperature is 60 ℃, which is characterized by elasticity and good wear resistance. Generally used in urban water supply and drainage and other fields, the corrosion resistance is relatively poor.
2. Polytetrafluoroethylene, also known as PTFE, also known as F4.
It is one of the commonly used lining materials. Because of its stable chemical properties, it is generally used for sanitary liquid or strongly corrosive liquid, such as concentrated acid, concentrated alkali, etc.
3. Polyperfluoroethylene propylene, also known as F46.
This material is similar to PTFE, but its wear resistance is stronger than PTFE. Similarly, the medium temperature can reach 100 ℃.
4. Polyurethane rubber, also known as polyurethane.
It has excellent wear resistance, but it is insufficient for corrosivity, and the temperature shall not exceed 80 ℃. It is generally used for the measurement of industrial and mining environment with high wear resistance requirements, such as mineral slurry, coal slurry and other media.
5. Polyfluoroethylene, also known as FS.
It has similar characteristics to F4 material, but the bearing temperature is slightly worse. Generally, the medium temperature does not exceed 80 ℃, which has high cost performance and low cost compared with F4 material.
6. Neoprene, also known as Cr, also known as neoprene.
It is characterized by good wear resistance and excellent elasticity. It is generally used in water supply and drainage, sewage treatment and other fields. Poor corrosion resistance and no oxidation resistance are its disadvantages.
7. Ceramic material
Ceramics is undoubtedly the best and absolute high-end product of all materials. The only disadvantage is that the price is not grounded, the manufacturing process is complex, the process requirements are very high, and the price is very high.
Power supply mode of electromagnetic flowmeter
- 1) 220V power electromagnetic flowmeter is the most commonly used method at present. There is 220V power supply in general industrial site. The flowmeter only needs to be installed according to the operation requirements and can be put into use after power on. The 220V power supply electromagnetic flowmeter can be made into one or element type;
- 2) 24V power supply electromagnetic flowmeter is mainly used in industrial sites where AC power supply does not work. Considering the allowable or low-voltage DC current according to the safety factor, or use the flow accumulator to provide electromagnetic flowmeter from the accumulator. Only 24V can be selected.
- 3) At present, battery powered electromagnetic flowmeter has not been widely used, which is mainly used when there is no power supply on site, but its price is much higher than that of the same product.
Price determinants of electromagnetic flowmeter
With the emphasis on environmental protection, electromagnetic flowmeter has attracted more and more attention and developed faster and faster. Electromagnetic flowmeter plays a very important role in large diameter pipeline. Electromagnetic flowmeter has different caliber and different price, and the price varies greatly among different manufacturers. So what factors determine the price of electromagnetic flowmeter.
The price of electromagnetic flowmeter is mainly related to the following six factors:
- 1. Structure method: the price of different electromagnetic flow meters is very different. Qingtian instrument electromagnetic flow meter is divided into integrated electromagnetic flow meter, split electromagnetic flow meter, etc;
- 2. Raw materials and lining materials: different raw materials will bring different prices to the electromagnetic flowmeter, such as carbon steel, 304 and 316 materials, and lining materials such as rubber, PTFE, PFA, polyurethane, etc;
- 3. Electrode materials: the electrode materials of electromagnetic flowmeter mainly include stainless steel, titanium, tantalum, platinum electrode, etc;
- 4. Connection method: the connection method of electromagnetic flowmeter is pipe type/plug-in type. The pipe type is divided into flange connection, clamp connection and flange clamping type;
- 5. Output signal method: the output signal method will also affect the price of electromagnetic flowmeter, such as pulse signal and current signal;
- 6. Communication method: 485 communication method, 232 communication method, HART communication method and modebas communication method are selected for electromagnetic flowmeter.
The price of electromagnetic flowmeter will also be affected by other factors, such as brand, after-sales and so on. When users choose electromagnetic flowmeter at the beginning, they must select the correct type, provide the correct use environment and measurement medium, and inform the supplier, and we will provide a reasonable quotation.
Selection of electromagnetic flowmeter
As a kind of flowmeter, electromagnetic flowmeter is widely used because of its simple structure and no pressure loss when fluid flows through the flowmeter; Electromagnetic flowmeter can measure domestic wastewater, industrial wastewater and so on, and even measure the flow of liquid-solid two-phase.
The main characteristics of electromagnetic flowmeter are: the measurement process is not easily affected by liquid density, temperature and viscosity; The speed measurement scale is relatively wide. Of course, the measurement of electromagnetic flowmeter is limited, because electromagnetic flowmeter can only measure conductive liquid, not gas and steam. So what should be considered in the selection of electromagnetic flowmeter?
The medium measured by electromagnetic flowmeter needs the requirement of conductivity. The liquid conductivity needs to be greater than 5 μs/cm。
2. Accuracy class
The accuracy of electromagnetic flowmeter is high. Because the measurement range of liquid flowmeter is large, the accuracy must be small. The larger the range is, the greater the error is. Therefore, the accuracy needs to be strictly controlled. The accuracy of precision flowmeter is 0.5-1, while the accuracy of conventional glass rotation and gold rotation is 2.5. The difference is still a little big. In terms of metering and billing, high-precision electromagnetic flowmeter should be selected.
3. Selection of flow rate and diameter
The flow rate of electromagnetic flowmeter is required to be between 0.5-15m/s, and the reasonable flow rate is generally between 1.5-3m/s. If the flow rate is too fast, the PTFE lining may be scratched if it contains particles. Flow rate and diameter need a reasonable selection.
4. Selection of grounding ring and electrode raw materials
The mismatch between the measured liquid and the electrode or grounding ring causes disadvantages. In addition to corrosion, this mismatch also has the appearance effect of the electrode, mainly including chemical reaction, polarization phenomenon and catalyst effect. The grounding ring also has these problems, but the impact level is smaller.
5. The solid phase is contained in the body
Assuming that the liquid contains particles, the possible disadvantages are: the accumulation layer is covered on the surface of the electrode and fabric, the fabric is worn, and the cross-sectional area of the pipe becomes smaller. For some relatively simple crystalline liquids, when the fluid flows through the flow meter, a layer of solid is hung on the inner wall. Because there are the same problems with other types of flowmeter, the flowmeter with very short measured pipeline and simple disassembly can be selected to facilitate disassembly after crystallization to stop protection.
Others are the requirements of some communication modes, such as 4-20mA, Hart, 485 communication, etc., which can be selected.
Selection of electrode materials for electromagnetic flowmeter
In the front flow meter market, environmental protection treatment and charging treatment are adopted for urban pipeline treatment and industrial wastewater discharge. For measuring the discharge flow, the flow instrument is needed, and the electromagnetic flowmeter is the most suitable instrument. Of course, electromagnetic flowmeter can not only measure sewage, but also conductive liquid. However, there are many kinds of components in different liquids, and the corrosive strength is also different. Therefore, when selecting electromagnetic flowmeter, we must distinguish the chemical properties and select the corresponding electrode.
- Stainless steel: it has corrosion resistance in weak oxidation environment. It is usually used for water, organic acid (acetic acid, lactic acid), weak base (ammonia), etc.
- Iron: it has corrosion resistance in oxidizing environment, especially in the environment where grasping ions exist. It is used in seawater and various grasping chemical solutions (grasping Chemical Press, grasping potassium), etc.
- Tantalum: it has corrosion resistance in strong oxidizing and reducing environment, but it can not be used for alkali, hydrofluoric acid and fuming sulfuric acid. It is used for concentrated hydrochloric acid, sulfuric acid, nitric acid, aqua regia, etc.
- Titanium: it has corrosion resistance to alkali and acid without ions. It is used for concentrated alkali, sulfuric acid, nitric acid, etc.
- Corrosion resistant and heat resistant nickel base alloy B: it has corrosion resistance in non oxidizing environment, especially for hydrochloric acid and fluoric acid. It is suitable for hydrochloric acid, fluoric acid, etc.
- Corrosion resistant and heat resistant nickel base alloy C: it is used in medium oxidation and reduction environment. It is suitable for all kinds of organic acids, inorganic acids and bases.
- Platinum iridium alloy: it has corrosion resistance to all acids and bases except aqua regia, but the price is expensive. It is used for phosphoric acid, fluoric acid, hydrochloric acid, sulfuric acid, nitric acid and bases.
- Stainless steel coated tungsten carbide: due to its high hardness, tungsten carbide has good wear resistance. It is used for fluid and solid-liquid two-phase slurry containing more sediment.
How to clean the electrode of electromagnetic flowmeter
The electrode of the intelligent electromagnetic flowmeter plays a very important role in the measurement of instrument flow. Moreover, because the fluid measured by the intelligent electromagnetic flowmeter is chemical liquid, it is inevitable that the electrode will be polluted by the fluid during long-term use. Therefore, in order to ensure the good condition of the flowmeter at all times, It is also essential to learn the cleaning method of electrode.
How to clean the electrode of intelligent electromagnetic flowmeter:
- 1. When the electrode is polluted by oil medium, the electrode can be cleaned with cleaning agent or pure water. After cleaning, soak the electrode in potassium solution for six hours;
- 2. When the electrode is polluted by platinum medium, because a layer of oxide film will be formed in the platinum medium, the electrode surface can be cleaned with toothpaste. After cleaning, the oxide film on the surface can be cleaned with pure water;
- 3. Another common method is to use the electric breakdown method. The principle is to add high voltage between the electrode and the fluid, because the adhesion of electric shock is relatively strong. At this time, the attachments on the electrode surface will be broken down due to high voltage and then washed away by current;
- 4. Ultrasonic cleaning method. The main purpose is to concentrate the ultrasonic energy on the contact surface between the electrode and the medium and break the dirt to achieve the purpose of cleaning.
Summary: when the user determines that the measured fluid will produce sediment due to long-time flow, it is necessary to clean the electrode regularly when measuring the flow with the intelligent electromagnetic flowmeter, which can also effectively keep the electromagnetic flowmeter in good working condition at all times.
When using and installing the electromagnetic flow meter, the measuring pipeline of the electromagnetic flowmeter must be filled with liquid, and the electrode axis should be in a horizontal position. The electromagnetic flowmeter shall be kept away from the power supply, and shall be separately and reliably grounded. There shall be no strong vibration around, and attention must be paid to daily maintenance.
Installation of electromagnetic flowmeter
How to choose the installation location of electromagnetic flowmeter
Electromagnetic flowmeter is a professional flow instrument for measuring conductive liquid. At present, the environmental protection industry is necessary for electromagnetic flowmeter in the treatment of sewage and wastewater. Because the electromagnetic flowmeter has long service life, simple measurement and accurate accuracy. The manufacturer supports the selection of electromagnetic flowmeter, but the customer may need to choose carefully in installation, and the manufacturer can recommend the installation location. So how to choose the installation location of electromagnetic flowmeter?
In order to ensure reliable and stable operation of transmission line, the following requirements shall be paid attention to when selecting the installation site:
- 1. Places without vibration and strong electromagnetic field (such as near large motors and frequency converters);
- 2. The vertical installation shall be carried out as far as possible, and the horizontal installation shall be carried out at a low place, and the pipe apex shall not be installed, so as to ensure that the pipe is full and prevent bubbles;
- 3. For the requirements of straight pipe section, it is best to ensure the first 10 and the last 5 times of pipe diameter;
- 4. When installing the welding type, pay attention not to connect the instrument for welding to prevent damaging the flowmeter;
- 5. The shell shall be grounded as much as possible, and the shielding wire shall be grounded at one end, and the connection point shall not be grounded together with strong current;
- 6. If the installation position needs to be selected to meet the maintenance requirements for simple maintenance. At the same time, the side pipe can be installed, and the fluid flows through the side pipe in case of failure;
- 7. Avoid places with direct sunlight and high temperature, which will demagnetize;
- 8. For other requirements that should be considered, the signal routing shall not be together with strong current lines.
Installation environment requirements of electromagnetic flowmeter
Electromagnetic flowmeter is paid more and more attention in the flow instrument industry. The higher the requirements of sewage and wastewater treatment in the environmental protection industry, the greater the demand for electromagnetic flowmeter. Then the electromagnetic flowmeter can be divided into two types. According to the assembly mode, it can be divided into integrated and split electromagnetic flowmeter, and according to the connection mode, it can be divided into flange type, clamping type, clamp type, plug-in type, etc. The electromagnetic flowmeter is very easy to use, but the installation environment should also be selected correctly.
The enclosure protection grade of electromagnetic flow (unit: cubic meter per second) sensor is generally IP65 (International gb4208 enclosure protection grade – dust and water collision level). The main requirements for installation environment are:
- (1) The sensor shall be installed in a dry and ventilated place to avoid damp, ponding and flooding, and direct sunlight and rain shall be avoided as far as possible.
- (2) It shall be avoided to install in the place where the ambient temperature is too high as far as possible. The electromagnetic flowmeter with integrated structure is also subject to the ambient temperature of electronic components, which should be lower.
- (3) There shall be no strong leakage current on the pipeline where the sensor is installed, and it shall be far away from the equipment with strong current characteristics: wave particle radiation, such as large electromechanical and large transformer, so as to avoid electromagnetic interference;
- (4) There shall be no strong vibration on the pipeline or ground where the sensor is installed, especially the integrated instrument;
- (5) The location where the sensor is installed shall consider the space for on-site maintenance by the staff. The function of electromagnetic flowmeter is to generate uniform DC or AC magnetic field. The DC magnetic circuit is realized by permanent magnet, which has the advantages of simple structure and less interference by AC magnetic field, but it is easy to polarize the electrolyte liquid in the measuring catheter, so that the positive electrode is surrounded by negative ions, and the negative electrode is surrounded by positive ions, that is, the polarization phenomenon of the electrode, which leads to the increase of internal resistance between the two electrodes, which seriously affects the normal operation of the instrument.
After the installation location and environment of electromagnetic flowmeter are determined, what installation requirements should we pay attention to?
For the installation of electromagnetic flowmeter, first of all, pay attention to that the sensor itself cannot be used as the load support point, it cannot support the adjacent working pipeline, and the pipeline holding it shall bear the load. In order to obtain normal measurement accuracy, there must also be a certain length of straight pipe upstream of the electromagnetic flow sensor, but its length is lower than that of most other flow instruments. After elbow, T-shaped pipe, concentric reducer and fully open gate valve, a straight pipe section with a length of 5 times the diameter (5d) from the flange connecting surface at the inlet end of the sensor shall be required, and 10d shall be required for valves with different opening degrees; The downstream straight pipe section is (2-3) d; However, prevent the butterfly valve disc from extending into the sensor measuring pipe. The length of upstream and downstream straight pipe sections proposed in various standards or verification regulations is also inconsistent, and the requirements of some electromagnetic flow meters are higher than the usual requirements. This is because in order to meet the requirements of the current 0.5 precision instrument.
The sensor can be installed horizontally, vertically or obliquely (the fluid must flow horizontally or obliquely upward) without restriction. However, ensure that the measuring pipe is coaxial with the process pipe. The deviation of its axis shall not exceed 2mm. The measurement of solid-liquid two-phase fluid is best installed vertically and flows from bottom to top.
This can avoid serious local wear of the lower half of the lining during horizontal installation and solid precipitation at low flow rate.
When welding or flame cutting the pipeline adjacent to the sensor, isolation measures shall be taken to prevent the lining from heating, and it must be confirmed that the signal line of the instrument converter is not connected to prevent damage to the converter. During horizontal installation, the electrode axis shall be parallel to the horizon, not perpendicular to the horizon, because the electrode at the ground is easy to be covered by sediments, The top electrode is easy to be wiped by occasional bubbles in the liquid to cover the electrode surface and make the output signal fluctuate.
For your correct measurement, you should pay attention to the following requirements when selecting the position on the pipeline:
- (1) the sensor can be installed on either straight pipe or horizontal or inclined pipe, but the central connection of the two electrodes is required to be horizontal.
- (2) the medium shall flow through the full pipe at the installation position to avoid the full pipe and gas adhering to the electrode.
- (3) for liquid-solid two-phase fluid, it is better to adopt vertical installation to make the lining of the sensor wear evenly and prolong the service life
- (4) when the medium in the installation position of the flowmeter is not enough, the method of raising the half rear pipe of the flow can be adopted to make it full. It is strictly prohibited to install the flowmeter at the highest point and water outlet of the pipe.
- (5) modify the installation method of pipeline: when the medium flow rate fails to meet the requirements, the flowmeter with smaller diameter shall be selected. At this time, the reducing cone pipe or some pipelines shall be modified to make it the same diameter as the sensor, but the front and rear straight pipe sections must at least meet the following requirements: the front straight pipe section ≥ 10dn and the rear straight pipe ≥ 5dn (DN is the pipe diameter).
Only by selecting the position suitable for the installation of the electromagnetic flowmeter and correctly installing the electromagnetic flowmeter according to the instructions in the manual can the stability of the flowmeter in normal use be guaranteed as much as possible.
In order to make the electromagnetic flowmeter play its maximum function normally, we suggest the following:
- 1. The electromagnetic flowmeter transmitter shall be installed in a dry and ventilated place indoors. Avoid installation where the ambient temperature is too high, avoid strong vibration, and try to avoid equipment with strong magnetic field. Such as large motor, transformer, etc. Avoid installation in places with corrosive gases. The installation site shall be convenient for maintenance, which is the environmental condition to ensure the normal operation of the transmitter.
- 2. Valves and bypasses shall be installed at both ends of the electromagnetic flow transmitter.
- 3. In order to ensure that the measuring tube of the electromagnetic flow transmitter is filled with the measured medium, the transformer should be installed vertically and flow from bottom to top. Especially for liquid-solid two-phase flow, it must be installed vertically. If only horizontal installation is allowed on site, it must be ensured that the two electrodes are in the same horizontal plane.
- 4. In order to avoid interfering with the signal, the signal between the transmitter and the converter must be transmitted by shielded wires. It is not allowed to place the signal cable and power line in the same cable steel pipe in parallel. Generally, the length of signal cable shall not exceed 30m.
- 5. The AC potential of several millivolts measured by the electrode of the electromagnetic flow transmitter is based on the liquid potential in the transmitter. In order to stabilize the liquid potential and maintain equipotential between the transmitter and the fluid to ensure stable measurement, the transmitter shell and both ends of the metal pipe shall be well grounded, and the converter shell shall also be grounded. The grounding resistance shall not be greater than 10 and shall not be shared with the grounding wires of other electrical equipment. If it is impossible to ensure good contact between the transmitter shell and metal pipes, they shall be connected with metal wires and then reliably grounded.
- 6. In order to avoid the influence of flow velocity on relative measurement, the flow regulating valve shall be set downstream of the transmitter. For small-diameter transmitters, the distance from the electrode center to the inlet end of the flowmeter is equivalent to several times the length of diameter D? Therefore, no regulations can be made for the upstream straight pipe. However, for the flowmeter with large diameter, the upstream shall have a straight pipe section of more than 5D, and the downstream shall not have a straight pipe section.
- 7. The installation site of the converter shall be free from strong AC and DC magnetic fields and vibration. The ambient temperature is – 20-50 ℃, there is no corrosive gas, and the relative humidity is no more than 80%.
Installation method of electromagnetic flowmeter
1. Horizontal and vertical installation
The sensor can be installed horizontally and vertically, but it should be ensured to avoid the influence of sediment and bubbles on the measuring electrode, and it is better to keep the electrode horizontal axially. When installed vertically, the fluid shall flow from bottom to top. The sensor cannot be installed at the highest position of the pipe, which is easy to accumulate bubbles.
2. Ensure full pipe installation
Ensure that when the flow sensor is measuring, the pipeline is filled with the measured fluid, and the non full pipe state cannot occur. If the pipeline is not full or the outlet is vented, the sensor shall be installed on a siphon.
3. Installation between elbow, valve and pump
In order to ensure the stability of measurement, a straight pipe section shall be set at the front and rear of the sensor, and its length is given in the figure below. If this is not possible, a current stabilizer shall be used or the cross-sectional area of the measuring point shall be reduced.
4. The sensor cannot be installed at the water inlet of the pump
In order to avoid negative pressure, the sensor shall not be installed at the water inlet of the pump, but at the water outlet of the pump.
5. Inlet straight pipe section and outlet straight pipe section of sensor
The ideal installation location should be enough straight pipe sections before and after the measuring point. The inlet straight pipe section shall be ≥ 5D, and the outlet straight pipe section shall be ≥ 3D (D is the nominal diameter of the sensor). The straight pipe section at the inlet of the plug-in type shall be ≥ 20d, and the straight pipe section at the outlet shall be ≥ 7d (D is the nominal diameter of the sensor).
6. Installation when the pipeline outlet is vent
When the outlet is in the vent state, the sensor shall not be installed at the vent position of the pipeline, but at the lower position. When the sensor is installed below the pipeline, it shall be ensured that the sensor is filled with liquid and there shall be no empty pipe.
7. Series and parallel installation
If several sensors need to be connected in series on the same pipe in sequence, the distance between each sensor shall be at least the length of 2 sensors. If more than two sensors are installed in parallel with each other, the distance between the sensors must be greater than 1m.
The flow signal generated by the sensor is very small and only a few millivolts at full scale, so the sensor should be well grounded. The grounding requirements of electromagnetic flowmeter include two aspects:
1. From the working principle of electromagnetic flowmeter and the loop of flow induced signal current, the grounding terminal of sensor and converter must have the same potential as the measured medium.
2. Grounding. Take the earth as zero potential to reduce external interference. Generally, the process pipes are metal pipes, which are grounded, which is easy to meet. However, in case of large external electromagnetic field interference, the electromagnetic flowmeter shall be provided with a separate grounding device. The grounding wire shall be multi strand copper wire with a section greater than 5mm2. The grounding wire of the sensor must not be connected to the common ground wire of the motor or other equipment to avoid the influence of leakage current. The grounding resistance shall be less than 10 Ω.
- a. The sensor is installed on the metal pipe (there is no insulating coating on the inner wall of the metal pipe).
- b. The sensor shall be installed on the plastic pipe or the pipe with insulating lining. The two ends of the sensor shall be installed with grounding ring, grounding flange or short pipe with grounding electrode.
8. Make the measured medium flowing in the pipe short circuit with the earth and have zero potential, otherwise, the electromagnetic flowmeter cannot work normally.
Electromagnetic flowmeter is simply composed of flow sensor and transmitter. The installation requirement of electromagnetic flowmeter is that it must be installed at the lowest point of the pipeline or the vertical section of the pipeline, but it must be in the case of full pipe. The requirement for the straight pipe section is 3D after 5D, so as to ensure the use of electromagnetic flowmeter and the requirements for accuracy. The measurement principle of electromagnetic flowmeter does not depend on the characteristics of flow. If there is certain turbulence and vortex in the pipeline in the non measurement area (such as elbow, tangential flow limiting or semi open stop valve in the upstream), it has nothing to do with the measurement. If there is steady-state eddy current in the measurement area, it will affect the stability and accuracy of measurement. At this time, some measures should be taken to stabilize the velocity distribution: increase the length of front and rear straight pipe sections, and use a flow stabilizer to reduce the section of measurement points.
Requirements for external environment
The flowmeter shall not be installed in places with great temperature change or exposed to high-temperature radiation of equipment. If it must be installed, heat insulation and ventilation measures must be taken.
It is better to install the flowmeter indoors. If it must be installed outdoors, rain pouring, ponding flooding and sun exposure shall be avoided, and moisture-proof and sunscreen measures shall be taken. 1.3. The flowmeter shall not be installed in the environment containing corrosive gas. When it must be installed, ventilation measures must be taken.
In order to facilitate installation, maintenance and repair, sufficient installation space shall be provided around the flowmeter. 1.5. The installation site of the flowmeter shall avoid magnetic field and strong vibration source. If the pipeline vibration is large, there shall be supports for fixing the pipeline on both sides of the flowmeter.
Pipeline electromagnetic flowmeter
Requirements for straight section in order to improve the influence of eddy current and flow field distortion, there are certain requirements for the length of front and rear straight pipe sections of flowmeter installation, otherwise the measurement accuracy will be affected (rectifier can also be installed, and it is avoided to install after being close to regulating valve and half open valve as far as possible).
Requirements for process pipes the flowmeter has certain requirements for the upstream and downstream process pipes at the installation point, otherwise the measurement accuracy will be affected.
- a. The inner diameter of the upstream and downstream process pipes is the same as that of the sensor, and shall meet: 0.98dn ≤ D ≤ 1.05dn (where DN: inner diameter of the sensor, D: inner diameter of the process pipe);
- b. The process pipe and sensor must be concentric, and the coaxial deviation shall not be greater than 0.05dn2.
Requirements for bypass pipe in order to facilitate the maintenance of flowmeter, it is best to install bypass pipe for flowmeter. In addition, for heavily polluted fluid and flowmeter that needs cleaning and cannot be stopped, bypass pipe must be installed.
- a. Facilitate the maintenance of flowmeter;
- b. Heavy polluted fluid must be installed;
- c. The fluid cannot be stopped and the flowmeter needs to be cleaned.
Installation requirements for plug-in electromagnetic flowmeter
Requirements for straight pipe section inlet/outlet straight pipe section: inlet shall be ≥ 10 × DN； The outlet shall be ≥ 5 × DN.
Docking point requirements: in order to make the instrument work reliably and improve the measurement accuracy, it shall not be disturbed by external parasitic potential/the sensor shall be well grounded, and the grounding resistance shall be less than 10. (if the metal pipeline is well grounded, there is no need to set a special grounding device).
As for the installation position, insert the electromagnetic flowmeter as shown in the figure. According to the different pipeline conditions on site, the flowmeter without ball valve shall be installed on the pipeline without pressure (that is, the flowmeter without ball valve can be selected for installation without pressure). The diameter of the opening on the pipeline is 50, and the connecting welded pipe is ready to be welded on the opening of the pipeline; For occasions requiring continuous flow loading and unloading or not allowing medium overflow, a ball valve must be installed, that is, a plug-in electromagnetic flowmeter with ball valve structure shall be selected; The diameter of the opening on the pipe is 50, and the connecting welded pipe is ready to be welded on the opening of the pipe. Type selection and measurement range of plug-in electromagnetic flowmeter:
- Recommended application range: 0.5m/s-10m/s continuously adjustable.
- Maximum application 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; ATC alarm; Flow direction diagram; Fault alarm.
2. Current output: 4-20mA output configuration mode:
- Field configuration through three manual keys.
- Field configuration through remote control.
- Field configuration through manual operator.
- Memory: EEPROM memory that will not disappear without battery storage.
What errors should be avoided when using electromagnetic flowmeter
Electromagnetic flowmeter is used in various fields, such as chemical industry, environmental protection, sewage treatment, metallurgy and so on. This is because electromagnetic flowmeter has the characteristics of high precision, long service life and convenient maintenance. However, not only good products can be used well. When using electromagnetic flowmeter well, we must pay attention to avoid some wrong use methods. In the process of use, due to the influence of various factors, it is easy to cause the inaccurate measurement results of electromagnetic flowmeter. So what are the reasons for the error of electromagnetic flowmeter?
1. The liquid in the pipe is not full
Due to insufficient back pressure or poor installation position of the flow sensor, the liquid in the measuring pipe is not filled, and the fault phenomenon has different manifestations due to the degree of non filling and flow conditions.
If the flow is bubble flow or plug flow, the fault phenomenon is not only the measured value is inconsistent with the actual value, but also the output shaking due to the instantaneous covering of the electrode surface by the gas phase;
If a small amount of gas flows in stratified or wavy flow in the water pipe, the fault phenomenon is that the error increases, that is, the measured flow value is inconsistent with the actual value;
If the gas-phase part of the flow cross-sectional area increases in the stratified flow of horizontal pipeline, that is, the degree of liquid incompleteness increases, and the output sloshing will also occur;
If the liquid is not full enough, so that the liquid level is below the electrode, the output overfilling will occur.
2. The liquid contains solid phase
The liquid contains solids such as powder, particle or fiber, and the possible faults are:
- ① Slurry noise;
- ② Contamination of electrode surface;
- ③ A conductive deposition layer or an insulating deposition layer covering the electrode or lining;
- ④ The lining is worn or covered with sediment, and the flow cross-sectional area is reduced.
3. Electromagnetic flowmeter shall be used with caution for liquid that may crystallize.
Some easily crystallized chemical materials can be measured normally when the temperature is normal. Because the pipes conveying fluid have good heat tracing and insulation, they will not crystallize during insulation work, but the measuring pipe of electromagnetic flow sensor is difficult to implement heat tracing and insulation. Therefore, when the fluid flows through the measuring pipe, it is easy to cause a layer of solid on the inner wall due to cooling.
Since there is also a crystallization problem when using a flowmeter with other principles for measurement, a “ring” electromagnetic flow sensor with a very short measuring pipe length can be selected without other better methods, and the heat tracing and insulation of the upstream pipeline of the flowmeter can be strengthened. In terms of pipe connection method, it is considered that the flow sensor can be easily disassembled and maintained once crystallized.
4. Problems caused by improper selection of electrode and grounding ring materials
For the electromagnetic flowmeter that fails due to the mismatch between the material and the measured medium, the parts in contact with the medium include electrode and grounding ring. In addition to corrosion resistance, as long as it is the electrode surface effect.
Connection mode between electromagnetic flowmeter and pipeline
Before using the electromagnetic flowmeter, it should be well connected with the pipeline. This is required for all pipeline flowmeters, which is well known. As its application scope is more and more extensive, various pipelines have many requirements for its connection. Therefore, there are many connection methods between it and the pipeline, mainly including flange connection, clean connection, clamp connection and threaded connection, which will be introduced respectively below.
- 1. Flange connection: this connection method is commonly used. Both ends of the electromagnetic flowmeter have flanges that can be connected. When connecting with the pipeline, it is only necessary to fix the flanges at both ends and the flanges on the pipeline with bolts. This connection can be a one-way device. This connected sensor has a small volume and is only suitable for use in small pipelines.
- 2. Clamping connection: this method is relatively advanced. Some electromagnetic flowmeters without flanges choose this connection method, which can be clamped between two flanges of the pipeline with bolts, and the connection method is relatively simple. It is necessary to know the installation method of electromagnetic flowmeter, so that there will be no error during clamping connection.
- 3. Clean connection: it can be used for the connection of some small-diameter pipes. The clean connection mode is also called clamp connection mode. Through this connection, the electromagnetic flowmeter can be quickly installed and disassembled, and its daily cleaning and protection are also very convenient.
- 4. Threaded connection: usually, electromagnetic flow meters in medical and food industries will use this connection method or be used in some small-diameter electromagnetic flow meters. In addition, this connection also has certain applications in petroleum and exploration.
Selecting the connection mode of flowmeter suitable for site requirements is the first step of flowmeter selection and the most basic parameter requirements, which will facilitate installation, connection and maintenance in the future.
What causes inaccurate detection of electromagnetic flowmeter
The same is true for the installation of electromagnetic flowmeter, which needs to be installed carefully. The quality of installation will directly affect the later operation process and work efficiency. Correctly installed according to the principle, the electromagnetic flowmeter has high stability. Generally, some faults rarely occur, but it is characterized by the need to remind: changes in environmental conditions and limited installation working conditions will lead to some problems. We need to learn to prevent them, mainly in the following cases, What we need to avoid.
- (1) The electromagnetic flowmeter has requirements for the pipeline, and the requirements for the straight pipe section of the pipeline are high, which can be met as much as possible in the design of new water plants, but it becomes difficult to meet the above conditions for the transformation and installation of old water plants, It will affect the stability of the equipment. The solution is to install a rectifier to improve the flow characteristics.
- (2) If the flow rate of the measured water is too low, it will also affect the measurement accuracy. In particular, the flow rate of the flow meter installed at the boundary of some districts and cities is too low because it is at the end of the pipe network of the regional company and the pressure difference is small. The solution can be improved by opening the valves on both sides of the boundary as much as possible.
- (3) When there is gas in the pipeline, it will cause measurement fluctuation and instability. If we want to compete, we must find the source of gas and solve it. If necessary, we can install an exhaust valve in the pipeline.
- (4) The sensor electrode surface of the flowmeter installed in the raw water pipeline is polluted by sludge deposition, and the shape and sound measurement. The solution is to select scraper electrode and regularly maintain and clean the well.
- (5) Foreign matters accumulate in the inlet pipe of electromagnetic flowmeter. It will cause high error to the measurement accuracy. The solution is to remove the foreign matters accumulated in the pipeline in time.
- (6) The converter of electromagnetic flowmeter is damaged by lightning, so that it cannot be measured. The solution is to install corresponding lightning arresters at the signal input end, excitation current input end and working power supply end of the converter respectively.
How to verify electromagnetic flowmeter
Since the electromagnetic flowmeter must be used online continuously, it is almost impossible to dismantle it and transport it to the national measurement and Testing Center for verification. Therefore, it is necessary to verify the accuracy of large-diameter electromagnetic flowmeter used in the field. The accuracy verification of electromagnetic flowmeter is also very useful for the management of electromagnetic flowmeter, ensuring its accuracy and reliability, accumulating original comparison data, and future verification and verification. The accuracy verification of electromagnetic flowmeter can use the volume of clean water tank and the calibration equipment of electromagnetic flowmeter. Comprehensively verify the accuracy of the electromagnetic flowmeter to determine the accuracy of the electromagnetic flowmeter in the application process of the water plant, ensure the authenticity of the measurement data, or whether to replace the electromagnetic flowmeter.
1. Visual inspection and instrument method shall be adopted
Use gs8 to check whether the resistance of excitation coil, insulation resistance between signal lines, grounding resistance and other items of the sensor meet the pre factory standards, and whether the zero point and output current of electromagnetic flowmeter converter meet the accuracy requirements. The specific detection methods are as follows:
- (1) Measure the resistance value of the excitation coil to judge whether the excitation coil has inter turn short circuit (the resistance value between measuring line No. “7” and “8”), and the resistance value should be between 30 Ω and 170 Ω. If the resistance is the same as the factory record, it is considered that the coil is good, and then indirectly evaluate that the magnetic field strength of the electromagnetic flowmeter sensor has not changed.
- (2) Measure the insulation resistance of the excitation coil to the ground (measuring line No. “1” and “7” or “8”) to judge whether the sensor is damp. The resistance value shall be greater than 20 megohm.
- (3) Measure the contact resistance between electrode and liquid (measuring line numbers “1” and “2” and “1” and “3”), and indirectly evaluate the general condition of electrode and lining layer surface. For example, whether the electrode surface and the lining layer are attached with the deposition layer, and whether the deposition layer is conductive or insulating. The resistance value between them shall be between 1K Ω and 1 megohm, and the resistance values of wire numbers “1” and “2” and “1” and “3” shall be roughly symmetrical.
- (4) Close the valve on the pipeline and check the zero point of the whole machine when the electromagnetic flowmeter is full of liquid and the liquid does not flow. Make appropriate adjustments as appropriate.
- (5) Check the insulation resistance of each core wire of signal cable and excitation cable, and check whether the shielding layer is intact.
- (6) Use GS8 to calibrate the instrument and test the output current of the converter. When zero flow is given, the output current shall be: 4.00ma; When 100% flow is given, the output current shall be 20.00ma. The error of output current value shall be better than 1.5%.
2. Verification of clean water tank volume method:
The clear water tank volume method is often used by water supply enterprises to verify the measurement accuracy of electromagnetic flowmeter for factory water in water plants.
Under the condition of accurate measurement of the geometric dimensions of the clean water tank and reducing the operation errors, a higher comparison reference function can be obtained. The principle of clean water tank volume method is: use high-precision steel ruler to measure the actual spatial plane size of clean water tank and suction well, and accurately calculate the actual plane area of clean water tank and suction well. First, adjust the water level of the clean water tank to a higher level and close all outlet valves.
After the water level of the clean water tank is stable, use the level transmitter of the clean water tank and use a high-precision steel ruler to manually and accurately measure the water level of the clean water tank and suction well. In order to correct the error caused by the leakage of valves such as clean water tank, measure the water level of clean water tank and suction well again after a certain interval, and calculate the water leakage per unit time, so as to correct the effluent measurement and reduce the error. The purpose of recording the cumulative flow of the electromagnetic flowmeter to be verified and manually measuring the liquid level of the clean water tank and suction well is to verify the accuracy of the liquid level transmitter. Then start the water pump, open the water outlet valve, and after a certain time, close the water outlet valve to stop the water supply pump.
When the water level of the clean water tank is stable, the level transmitter of the clean water tank and the water suction well shall be used again, and the water level of the clean water tank and the water suction well shall be measured manually and accurately with a high-precision steel ruler. The water level of the clean water tank and the water suction well shall be recorded again, and the cumulative flow of the electromagnetic flowmeter to be verified shall be recorded. Finally, the water level height difference ⊿ h between the clean water tank and the suction well is calculated, so as to calculate the actual water volume of the clean water tank and the suction well. The actual water volume is equal to the height difference ⊿ h multiplied by the plane area and the corrected water volume.
Then calculate the water volume of the electromagnetic flowmeter to be verified, subtract the cumulative volume of the electromagnetic flowmeter from the actual water volume of the clean water tank, and obtain the error between them, so as to verify the accuracy of the metering system of the electromagnetic flowmeter for the outgoing water. Using the clean water tank volume method to verify the measurement accuracy of the electromagnetic flowmeter of the factory water, it needs to be carried out under the condition that the clean water tank is completely static, so the data obtained is more accurate. The calculation formula is as follows:
- E = (Q meter-Q instrument)/Q meter × 100%
In the formula:
- E is the error between the two;
- Q meter is the volume calculated by the falling height difference of the clean water tank;
- Q instrument is the flow value accumulated by the flowmeter during verification.
Maintenance of electromagnetic flowmeter
- 1. Before using the electromagnetic flowmeter, calibrate the electromagnetic flowmeter with standard pH value solution. After calibration and before operation, we must pay attention to cleaning the electrode of the electromagnetic flowmeter with distilled water, and then clean the electrode with measuring solution again.
- 2. If the electromagnetic flowmeter is not used, when removing the electrode of the electromagnetic flowmeter, we should pay attention not to let the tactile sensor of the electrode collide with hard objects, otherwise any damage will affect the use of the electrode.
- 3. After using the electromagnetic flowmeter, you should sleeve the electrode of the electromagnetic flowmeter and put less saturated solution in it. Just ensure that the bulb of the electrode is wet, but remember not to soak it in distilled water.
- 4. Pay attention to keep the electrode clean and do not let the outputs on both sides of it short circuit, otherwise the measurement will be inaccurate and the use of electromagnetic flowmeter will be affected. In fact, there are many ways to maintain the electrode of the electromagnetic flowmeter. You should pay more attention in the process of use. Don’t make the electromagnetic flowmeter unable to be used normally in the future because of your little negligence.
After the flowmeter is put into operation or normally put into operation for a period of time, if it is found that the instrument is not working normally, first check the external conditions of the flowmeter, such as whether the power supply is good, whether the pipeline is leaking or not in a non full pipe state, whether there are bubbles in the pipeline, whether the signal cable is damaged, and whether the output signal of the converter (i.e. the input circuit of the rear instrument) is open. Remember to dismantle and repair the flowmeter blindly.
Test equipment: one 500m Ω insulation resistance tester and one multimeter.
- (1) When the pipeline is full of medium, use a multimeter to measure the resistance between terminals a, B and C. the resistance between a-c and B-C should be equal. If the difference is more than 1 time, it may be the leakage of the electrode, the adsorption of condensate on the outer wall of the measuring tube or in the junction box.
- (2) When the lining is dry, measure the insulation resistance between a-c and B-C with m Ω meter (it shall be greater than 200m Ω). Then use a multimeter to measure the resistance between terminals a and B and the two electrodes in the measuring tube (it should be in short circuit connection state). If the insulation resistance is very small, it indicates that the electrode leaks, and the whole set of flowmeter shall be returned to the factory for maintenance. If the insulation drops but there is still more than 50m Ω and the inspection result in step (1) is normal, the outer wall of the measuring tube may be damp.
- The inside of the shell can be dried with a hot air blower.
- (3) Use a multimeter to measure the resistance between X and Y. if it exceeds 200 Ω, the excitation coil and its outgoing line may be open circuit or poor contact. Remove the terminal board for inspection.
- (4) Check the insulation resistance between X, y and C, which shall be more than 200m Ω. If it decreases, dry the inside of the shell with hot air. During actual operation, the decrease of coil insulation will lead to the increase of measurement error and the instability of instrument output signal.
- (5) If the sensor is judged to be faulty, please contact the manufacturer of electromagnetic flowmeter. Generally, it cannot be solved on site and needs to be repaired by the manufacturer.
If it is determined that the converter is faulty and there is no problem due to external reasons, please contact the manufacturer of electromagnetic flowmeter. The manufacturer will generally solve it by replacing the circuit board.
Fault analysis 1. Faults in commissioning period. Faults in commissioning period generally occur in instrument installation and commissioning stage. Once eliminated, they will not appear again under the same conditions in the future. Common faults during commissioning are usually caused by improper installation, environmental interference and fluid characteristics. In terms of installation, it is usually the fault caused by the incorrect installation position of the electromagnetic flow sensor, such as installing the sensor at the highest point of the pipe system easy to accumulate gas; Or installed on the vertical pipe from top to bottom, emptying may occur; Or there is no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere to form a non full pipe in the measuring pipe. In terms of environment, it is usually mainly pipeline stray current interference, space strong electromagnetic wave interference, large motor magnetic field interference, etc. Generally, satisfactory results can be obtained by taking good separate grounding protection for pipeline stray current interference. However, in case of strong stray current (such as the pipeline in the electrolytic workshop, sometimes the AC potential peak VPP induced on the two electrodes can be as high as 1V), additional measures and insulation between the flow sensor and the pipeline need to be taken. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single-layer or multi-layer shielding. In terms of fluid, the measured liquid contains uniformly distributed micro bubbles, which usually does not affect the normal operation of the electromagnetic flowmeter, but with the increase of bubbles, the output signal of the instrument will fluctuate. If the bubbles are large enough to cover the whole electrode surface, the electric circuit will be opened instantaneously as the bubbles flow through the electrode, resulting in greater fluctuation of the output signal. When the electromagnetic flowmeter excited by low-frequency square wave measures the slurry with excessive solid content, it will also produce slurry noise and make the output signal fluctuate. When measuring the mixed medium, if it enters the flow sensor 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 the normal measurement due to chemical action or polarization. Electrode materials shall be selected correctly according to instrument selection or relevant manuals. Operation period fault operation period fault is the fault that occurs after the electromagnetic flowmeter has been debugged and operated normally for a period of time. The common operation period fault is basically caused by the adhesion layer on the inner wall of the flow sensor, lightning strike and changes in environmental conditions. Because the electromagnetic flowmeter is often used to measure dirty fluid, the adhesion layer on the inner wall of the sensor often accumulates after running for a period of time. These faults are often caused by too large or too small conductivity of the adhesion layer. If the attachment is an insulating layer, the electrode circuit will be open and the instrument cannot work normally; If the conductivity of the attachment layer is significantly higher than that of the fluid, the electrode circuit will be short circuited and the instrument will not work normally. Therefore, the attached scaling layer in the measuring pipe of electromagnetic flowmeter should be removed in time. Lightning strike is easy to induce high voltage and surge current in the instrument line, resulting in instrument damage. It is mainly introduced through the power line or excitation coil or the flow signal line between the sensor and the converter, especially from the power line in the control room. Changes in environmental conditions during commissioning, due to good environmental conditions (e.g. no interference source), the flowmeter works normally. At this time, it is often easy to ignore the installation conditions (e.g. grounding is not very good). In this case, once the environmental conditions change and new interference sources appear during operation (such as electric welding on the pipeline near the flowmeter, installation of large transformer nearby, etc.), the normal operation of the instrument will be disturbed and the output signal of the flowmeter will fluctuate.
Common faults and Solutions
(1). If the measured liquid contains bubbles, it will lead to measurement deviation or small-scale fluctuation.
Fault discrimination method: when there is shaking in the measurement, the power supply shall be cut off immediately. At this time, the instrument still has display and the display is extremely unstable, which indicates that it is caused by too many bubbles in the liquid. If you use a multimeter to measure the electrode resistance, you will find that it is much higher than normal.
- 1. The electromagnetic flowmeter shall be installed in the liquid bottom-up pipeline or directly installed in U-shaped pipe at the lowest point of the pipeline.
- 2. An air collecting bag and an exhaust valve are installed upstream of the electromagnetic flowmeter.
(2). Non full pipe phenomenon. When the pipe is not full, you can see that the liquid contains bubbles. The fault discrimination method is the same as the above (1) method of liquid with bubbles. At this time, the electrode resistance will be significantly high. If conditions permit, you can go to the back end to see the liquid discharge port, If the discharged liquid is obviously not full, it should be judged that the installation of the electromagnetic flowmeter is not full. Troubleshooting: the installation pipeline of electromagnetic flowmeter can be installed in U-shape.
(3). Electrode corrosion. The electrodes of ordinary electromagnetic flowmeter are normally acid and alkali resistant and are easy to be corroded. Fault discrimination method: when bubbles and signal interference are eliminated, the measured display value will shake, and the data will disappear when it is transmitted to the sensor.
Troubleshooting: replace the electrode in case of this phenomenon.
How to choose the right flow meter
Selection of caliber and range
The transmitter diameter is usually the same as that of the pipeline system. If the bypass system needs to be designed, the diameter can be selected according to the range of flow mother and flow rate. For the electromagnetic flowmeter, the flow rate is 2-4m/s. In special cases, if there are solid particles in the liquid, considering the wear, the common flow rate can be ≤ 3m/s. for the fluid easy to attach to the pipe wall, the flow rate can be ≤ 2m/s.
The range of the transmitter can be selected according to two principles: one is that the full range of the instrument is greater than the expected maximum flow value, and the other is that the normal flow value is greater than 50% of the full range of the instrument to ensure a certain measurement accuracy.
Correct selection of lining material for electromagnetic flowmeter.
The lining material shall be selected according to the corrosivity, abrasiveness and temperature of the measured medium:
Natural rubber (soft rubber)
- 1. Good elasticity, wear resistance and breaking force;
- 2. Resistant to general weak acid and weak alkali corrosion;
- 3. Water and sewage measurement.
Acid resistant rubber (hard rubber)
1. Understand process parameters
- (1) Know the name of the measured liquid.
- (2) Understand the maximum flow, common flow and minimum flow of the measured liquid.
- (3) Understand the process pipe diameter.
- (4) Understand the medium temperature.
- (5) Understand the medium pressure.
- (6) Understand the conductivity of the measured fluid.
- (7) Know whether there is negative pressure.
It can resist the corrosion of hydrochloric acid, acetic acid, oxalic acid, ammonia, phosphoric acid and 50% sulfuric acid, sodium hydroxide and potassium hydroxide at room temperature, but it is not resistant to the corrosion of strong oxidants.
2. Preliminary selection
(1) Determine whether to use electromagnetic flowmeter according to the known name and nature of the measured medium (determined by the salesman).
Note: electromagnetic flowmeter can only measure the flow of conductive liquid, while gas, oil and most organic liquids are not examples of general conductive liquid.
(2) Determine the electrode material according to the known dielectric properties.
The company generally provides four kinds of electrodes: stainless steel, Hastelloy, titanium and tantalum. Which electrode should be selected should refer to the relevant data manual according to the dielectric properties
(2) Determine whether to use rubber or PTFE lining according to the known medium temperature (determined by the marketer)
- The temperature resistance of rubber shall not exceed 80C;
- Teflon temperature resistance 150C, instant resistance 180C;
- Generally, rubber lining and stainless steel electrode can be used for urban sewage.
(3) According to the known medium pressure, select the flange specification of the table body (determined by the salesman)
- The specification of electromagnetic flange is usually that when the diameter is dn10-250, the rated pressure of flange is ≤ 1.6Mpa;
- When the diameter is dn250-1000, the rated pressure of flange is ≤ 1.0MPa;
- When the actual pressure of the medium is higher than the above pipe diameter pressure corresponding range, it is a special order, but the maximum pressure shall not exceed 6.4Mpa.
(4) Determine the conductivity of the medium
- (1) the conductivity of electromagnetic flowmeter shall not be lower than 5us/cm;
- (2) The conductivity of tap water is about tens to hundreds of us/cm. Generally, the soft water (deionized water) of boiler is conductive, and the pure water (highly distilled water) is not conductive;
- (3) The conductivity of gas, oil and most organic liquids is much lower than 5us/cm and is not conductive.
3. Understand user requirements
(1) Know whether it is combined local display or split remote display.
Note: when it is split remote transmission display, please understand the maximum distance. The maximum separation distance is 100m.
(2) Find out if additional functions are required.
- 1. The electromagnetic flowmeter itself has the functions of upper and lower limit flow alarm, frequency and current output, and there is no need for other special orders.
- 2. The enclosure sealing protection grade of electromagnetic flowmeter includes IP65 and IP68. When submersible IP68 is selected, it is a special order.
- 3. When the electromagnetic flowmeter needs to be connected with the computer, the RS-485 communication port needs to be added, which belongs to a special order.
- 4. Type selection: after the above steps, the model and specification of electromagnetic flowmeter can be finally determined.
A suitable instrument can bring you great benefits in production, on the contrary, it will bring some unnecessary trouble. Therefore, you must be careful in model selection, take several aspects into account and buy appropriate products.
Source: Network Arrangement – China Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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