The electromagnetic flowmeter features an integrated LCD display for cumulative flow, instantaneous flow, and velocity. With no moving parts inside the pipe, it operates without clogging and boasts strong anti-interference capabilities. Based on Faraday’s law of electromagnetic induction, the electromagnetic flowmeter offers high measurement accuracy, multiple functions, and outputs analog signals to drive downstream instruments. The multi-functional instrument includes bidirectional flow measurement, range switching, upper and lower flow limit settings, empty pipe and power cut-off, small signal cut-off, flow display and total flow calculation, automatic verification and fault self-diagnosis, communication with a host computer, and motion configuration. Electromagnetic flowmeters are widely used in chemical, wastewater, hydropower, textile, papermaking, food, industrial machinery, heating, environmental protection, energy, and other industrial fields. Below, the editors at Xianji.com will introduce the installation requirements and wiring methods for electromagnetic flowmeters, methods for handling anti-interference issues, causes and solutions for abnormal fluctuations in measurement values, common fault causes and solutions, and factors affecting the accuracy of electromagnetic flowmeters. Let’s take a look!
Electromagnetic flowmeter installation requirements, wiring methods, and anti-interference solutions
Electromagnetic flowmeter installation requirements and wiring methods
I. Installation Requirements:
1. If the measuring pipeline vibrates, fixed supports should be installed on both sides of the flowmeter.
2. The flow direction of the fluid should be consistent with the direction indicated by the arrow on the flowmeter housing.
3. Control valves and shut-off valves should be installed downstream of the sensor, not upstream of the electromagnetic flowmeter.
4. The electromagnetic flowmeter must never be installed at the pump inlet or outlet; it should be installed at the pump outlet.
II. Wiring Method:
1. Equipotential bonding should be established between the flowmeter housing, the measured fluid, and the pipeline connection flange, and the connection should be grounded.
2. When installed on a vertical pipeline, the flow direction of the measured fluid should be from bottom to top. When installed on a horizontal pipeline, the two measuring electrodes should not be directly above or below the pipeline.
3. The length of the upstream straight pipe section and the installation support method should meet the design document requirements. 4. Flange Connection: This connection method is relatively traditional. Both ends of the flow meter have flanges for connection. When connecting to the pipeline, simply bolt the flanges at both ends to the flanges on the pipeline. This connection allows for unidirectional installation. The sensor using this connection is relatively small and is only suitable for use in small pipelines.
Electromagnetic flowmeter anti-interference problem handling methods
Electromagnetic flowmeters are widely used and are familiar to most people. A significant issue with electromagnetic flowmeters is interference. Electromagnetic interference (EMC) refers to various electromagnetic effects that degrade the performance of control system equipment, transmission channels, or the entire system.
The entire process of EMC interference formation involves the interference signal emitted by the interference source traveling through a coupling channel to the affected equipment. The three elements of interference are called the three components of an interference system: the interference source, the interference propagation path, and the affected equipment.
Classification of Electromagnetic Interference: In industrial measurement and control systems, EMC is a critical issue affecting normal operation. It can originate internally (due to its own inherent interference) or externally (due to external interference). When analyzing EMC, the system refers to the entirety of electrical or electronic equipment designed, managed, and controlled.
1. Internal Interference of Electromagnetic Flowmeter
Interference sources within the system can be categorized as follows:
(1) Power supply interference. Power supply interference mainly originates from the power supply and its leads.
(2) Ground interference. Ground interference is caused by the shared ground wire within the system. When current flows through the common ground wire in all circuits, a voltage drop occurs on the ground wire, creating mutual noise.
(3) Coupling interference of signal channels. When a long transmission line is required, the signal is easily interfered with during transmission, leading to distortion or loss of signal quality. The interference mainly includes electromagnetic induction interference from the surrounding electromagnetic field on the transmission line; and crosstalk between transmission lines caused by the distributed capacitance and mutual inductance between two or more signal lines with different signal strengths when they are close together.
2. External Interference of Electromagnetic Flowmeter
External interference sources of the system can be divided into:
(1) Natural interference. Natural interference includes lightning and changes in the electric field of the atmosphere. Lightning can generate high-amplitude high-frequency surge voltages on transmission lines, causing interference to the system.
(2) Power interference. With more and more electronic devices connected to the power grid, some potential interferences will appear in the system. These interferences include power line interference, electrical fast transients, surges, voltage changes, lightning transients, and power line harmonics.
(3) Power frequency interference. Power supply equipment and output lines generate power frequency interference. If a section of the signal transmission line is parallel to the power supply line, this low-frequency interference will couple to the signal line and become interference.
(4) Radio frequency interference. Communication equipment, radio broadcasting, television, radar, etc., will emit strong radio waves through antennas.
(5) Electrostatic discharge. With the adoption of modern chip technology, components have become very dense in a very small geometric size. These high-speed, millions of transistors are highly sensitive and easily damaged by external electrostatic discharge.
(6) Automotive noise. Automobiles generate noise in the very high frequency (VHF) to ultra-high frequency (UHF) bands during operation.
(7) Discharge interference. Partial discharge can be divided into three types: positive corona discharge, negative corona discharge, and spark discharge.
(8) Glow discharge. Glow discharge is gas discharge.
(9) Arc discharge. Arc discharge is metal mist discharge; a typical example of arc discharge is in metal arc welding.
Propagation Paths of Electromagnetic Interference
Electromagnetic interference in electromagnetic flowmeters can be divided into two main categories based on its transmission path: conducted interference, mainly caused by interference signals generated by electronic equipment interfering with each other through conductive media or common power lines; and radiated interference, which refers to interference signals generated by electronic equipment being transmitted through space to another electrical network or electronic equipment. From the perspective of the affected sensor, interference coupling can be divided into two main categories: conducted coupling and radiated coupling.
Methods for Suppressing Electromagnetic Interference
Based on the three elements of electromagnetic interference, the following three methods for solving electromagnetic interference problems are proposed:
1. Suppressing Electromagnetic Interference Generated by Interference Sources
① Shielding
Shielding involves using a shielding structure to isolate two spatial regions, controlling the induction and radiation of electric fields, magnetic fields, and electromagnetic waves from one region to another.
② Filtering
Filtering refers to classifying various signals according to their frequency characteristics and controlling their direction. It is a technique that provides transmission poles for signals within certain frequency ranges and transmission zeros for signals within other frequency ranges.
③ Grounding A ground wire is the implementation of grounding, that is, connecting certain ground potentials in a circuit, or connecting a part of electronic or electrical equipment to the earth, according to certain requirements using necessary metal conductors or wires.
④ Wiring Different types of signals are transmitted by different cables. Signal lines and power cables should be avoided from being laid close together parallel to each other to reduce electromagnetic interference.
2. Cut off the propagation path of interference.
3. Improve the electromagnetic interference resistance of sensitive equipment (reduce sensitivity to interference).
Causes and Handling Methods of Abnormal Fluctuations in Electromagnetic Flowmeter Measurement Values
I. In general, abnormal fluctuations in current meter measurements may be caused by:
1. Installation at the high end of the pipe;
2. Surrounding interference, such as the noise from frequency converters, large motors, etc.;
3. The measured liquid is not completely filled;
4. Non-metallic pipes are not grounded;
5. Damage to the flowmeter itself.
II. Handling Methods
1. If abnormal fluctuations occur in the measurement of an electromagnetic flowmeter, check and troubleshoot according to the relevant circumstances. 1. If the flow meter is installed at the high end, a drain valve can be installed at the inlet to release the gas, thus resolving fluctuations in the measurement results. If the flow meter is not installed at the high end, it may be caused by electromagnetic interference. The power supply should be disconnected and replaced. If it is a non-metallic pipe, a grounding ring should be installed to ensure sufficient contact between the medium and the pipe.
2. Bubble formation in liquids occurs through two pathways: absorption from the outside and the transformation of dissolved gas (air) into free bubbles. If the liquid contains large bubbles, they can cover the entire electrode when passing through it, causing a momentary open circuit in the flow signal input circuit, resulting in fluctuations in the output signal.
3. A simple method of identification is to cut off the excitation circuit current of the magnetic field when fluctuations occur. If the instrument still displays an unstable reading, it is mostly due to the influence of bubbles. If the electrode resistance is measured with a pointer-type multimeter at this time, the circuit resistance of the electrode will be higher than normal. However, this test requires extensive experience and data accumulated by the tester over a long period. Common Fault Causes and Solutions for Electromagnetic Flowmeters
I. Output Even Without Liquid Flow:
1. Open circuit in the signal transmission cable connection to the converter;
2. Open circuit in the signal cable connection to the electrode;
3. Contamination or insulation layer deposits on the electrode surface;
4. Poor grounding or open circuit.
Solutions:
1. Reconnect the cable properly;
2. Open the sensor and reconnect it;
3. Clean the electrode surface;
4. Connect the ground wire properly.
II. Output Signal Exceeds Full-Scale Range
1. Incorrect signal cable wiring or broken cable connection;
2. Incorrect converter parameter settings;
3. Incompatible converter and sensor models.
Solutions:
1. Check if the signal loop connection is normal. If the signal loop is broken, the output signal will exceed the full-scale value. In this case, the signal cable needs to be reconnected correctly. At the same time, check if the cable insulation performance is intact. If it no longer meets the requirements, replace it with a new cable.
2. Carefully check if the converter’s parameter settings and zero point and full-scale values meet the requirements. 3. If the converter and sensor models are found to be incompatible, an exchange with the manufacturer is required.
III. Excessive Error:
1. Zero point too high;
2. Incomplete liquid filling;
3. Excessive power supply distortion;
4. Poor grounding.
Solutions:
1. Readjust the zero point;
2. Improve pipeline conditions to ensure the sensor is always filled with liquid;
3. Improve power supply conditions to meet normal operating conditions;
4. Ensure proper grounding.
Factors Affecting the Accuracy of Electromagnetic Flowmeters
I. Electromagnetic Flowmeter Connection Cable Issues
Electromagnetic flowmeters consist of a specific cable connecting the sensor and converter into a system. Cable length, insulation condition, number of shielding layers, distributed capacitance, and conductor cross-sectional area all affect the measurement results, and in severe cases, may even cause the flowmeter to malfunction. Solutions:
1. The shorter the cable, the better. Its length should be within the allowable range. The maximum length is determined by the conductivity of the liquid being measured, the number of shielding layers, distributed capacitance, and conductor cross-sectional area.
2. Avoid intermediate joints. The ends should be properly treated and connected.
3. Use cables of the specified type whenever possible.
II. Electrode and Lining Material Selection for Electromagnetic Flowmeters
Since the electrodes and lining materials are in direct contact with the liquid being measured, the selection should be based on the characteristics of the liquid (e.g., corrosiveness, abrasiveness) and the operating temperature. Inappropriate selection can lead to problems such as rapid adhesion, corrosion, scaling, wear, and lining deformation, resulting in measurement errors. Therefore, this should be given high priority during equipment selection. III. Excitation Stability Issues of Electromagnetic Flowmeters Electromagnetic flowmeters employ excitation methods such as DC excitation, AC sinusoidal excitation, and dual-frequency rectangular wave excitation. DC excitation is prone to electrode polarization and DC interference, while AC sinusoidal excitation is susceptible to zero-point fluctuations. Dual-frequency rectangular wave excitation, however, combines the excellent zero-point stability of low-frequency rectangular wave excitation with the strong noise suppression capabilities of high-frequency rectangular wave excitation, making it a more ideal excitation method. In practical applications, it is crucial to ensure the stability of the power supply voltage and frequency to guarantee a constant magnetic field strength and minimize measurement errors caused by variations in magnetic field strength.