Electromagnetic flowmeter is widely used in industrial fields. However, due to the unique nature of the electromagnetic flowmeter measurement principle, its grounding problem is directed to the regular operation of the instrument and measurement accuracy.
Therefore, the correct understanding of electromagnetic flowmeter grounding reasons and familiarity with the grounding scheme of the electromagnetic flowmeter in different use occasions help the reasonable selection of electromagnetic flowmeter, installation, and commissioning fault detection.
1. The Electromagnetic Flowmeter Grounding Problem
1.1 Grounding problems arising from the root cause
From Figure 1 can be seen that the flow generates the induced electric potential E × through the electromagnetic flowmeter before and after the flange (an end to b end) between the conductor (measured medium) cutting magnetic lines of force. Therefore, the reference potential at the opposing end of E× must be consistent with the measured medium potential between the two flanges of the flowmeter.
To strictly ensure the consistency of the two. In addition to providing that the flowmeter electrode is in contact with the medium, we should also ensure reliable earth to maintain zero potential.
As the E x value is small, the full range of 2.5 to 8.0 mV. When the flow rate is shallow, only a few microvolts. Therefore, to avoid the drift of the E× value brought about by external interference (mainly electrostatic interference), we also need to transfer the weak electric potential E× measured by the sensor electrode to the converter intact and without damage through earthing.
These are the two critical points of the everyday work of electromagnetic flowmeter, but also electromagnetic flowmeter in the grounding requirements of the fundamental reason for the difference between another flowmeter.
1.2 Differences with Thermocouple Signals
When an electromagnetic flowmeter operates, the signal generated between the electrodes is at the millivolt level. As mentioned in section 1.1, this is one of the reasons why their field instruments need to be reliably earthed, but not all devices with millivolt level signals need to be earthed when they are in operation.
In order to better understand the need for grounding electromagnetic flowmeters and also to avoid us falling into cognitive misconceptions. The analogy here can be drawn with thermocouples, millivolt level signals whose field instruments do not require separate earthing measures during operation.
The thermocouple’s chavolt-level signal is generated at the hot and cold ends. It is the thermoelectric effect produced by two different materials of electrodes.
When the hot wire constitutes a positive and negative electrode and extends to the cold end through the compensation wire, the negative electrode does not need to be at the same potential as the measured medium. An electromagnetic flowmeter work electrode and media contact and maintain the same negative potential, which is the necessary condition for its work before.
2. Grounding Scheme of Electromagnetic Flowmeter
As shown in Figure 1, when the fluid cuts the magnetic lines of force to produce a flow signal, a positive potential is generated on one electrode and a negative potential on the other.
To avoid polarization reaction, the two alternate, the potential difference to the fluid itself as a zero potential. Therefore, the electromagnetic flowmeter sensor output signal grounding point should be electrically connected to the measured medium.
It is a necessary condition for the work of an electromagnetic flowmeter. The electromagnetic flowmeter can not work correctly if the situation is not met.
Therefore, the electromagnetic flowmeter converter input point must have zero potential and be conducive to the fluid to constitute a symmetrical input circuit. However, different working conditions still need to use various grounding schemes in the engineering implementation process.
2.1. Earthing solutions for installation in metallic pipelines
The medium measured by the electromagnetic flowmeter must be electrically conductive. Therefore, when the electromagnetic flowmeter is installed in an unlined metal pipe, if it is confirmed that the metal pipe has been reliably earthed, the electromagnetic flowmeter no longer needs to be separately earthed.
The reason is that the electromagnetic flowmeter through the metal gasket, bolt, and a pipe has formed a whole, the potential for 0. Here two details should be noted to avoid magnetic conductivity.
The electromagnetic flowmeter body is generally made of stainless steel or carbon steel and non-metallic lining. The aforementioned electromagnetic flowmeter does not need to be grounded separately. It is for the body of stainless steel and other non-permeable metal material.
Carbon steel plus non-metallic lining, if not without non-metallic flange, does not need to be grounded separately when using metal gaskets. However, for an electromagnetic flowmeter with non-metallic flange flanges, if the resistance value of the connecting bolt is more significant than 0.032, the metal pipe electromagnetic flowmeter grounding scheme is shown in Figure 2.
3.2 Grounding scheme for installation in non-metallic pipes
When the electromagnetic flowmeter is installed in a non-metallic pipeline, if it is a plastic pipe, insulating coating, or insulating lining of the metal pipe, the medium in the tube can not be connected to the earth through the metal pipe.
In the working condition, the two ends of the sensor should be required with a grounding ring so that the flow of the measured medium in the pipe is well grounded, with zero potential. Otherwise, the electromagnetic flowmeter can not work correctly—the non-metallic pipeline electromagnetic flowmeter grounding scheme is shown in Figure 3.
3.3. Earthing solutions installed in pipelines with cathodic protection
For buried and long-distance metal pipelines, to prevent electrochemical corrosion. Depending on the medium type, the cathodic defense must be installed either forcibly or by pushing the key.
The field instrument needs to be grounded as an independent whole for the intelligent electromagnetic flowmeter installed on the pipeline under this working condition. It cannot have any conductive circuit connection with the metal pipeline to prevent the cathodic protection of the current through the instrument directly into the earth, seriously damaging the cathodic protection effect.
At the same time, to maintain the integrity of the cathodic protection of the metal pipeline, it is also necessary to increase the conductor across the flange of the channel with the cathodic protection pipeline electromagnetic flowmeter grounding scheme, as shown in Figure 4. In Figure 4, the grounding ring and cathodic protection of the remaining metal pipe insulation, bolts, and gaskets should also be used with the metal pipe between suitable insulation measures.
4. Anti-interference measures for electromagnetic flowmeter
Intelligent electromagnetic flowmeter field instrument correctly grounded only for its regular operation under one of the necessary conditions. From the point of view of the source of interference, the external magnetic field and electric field will produce interference to the electromagnetic flowmeter. Unfortunately, people do not have better active protection measures for external magnetic field interference.
Generally, the design process requires the signal line to use twisted shielded cable through the induction balance to reduce magnetic field interference. At the same time, the choice of electromagnetic flowmeter installation site should be far away from all electromagnetic interference sources (such as high-power motors, transformers, etc.).
For the split electromagnetic flowmeter, the signal line between the converter and the sensor, the excitation line should use the manufacturer’s special cable and respectively through the pipe laying.
For the interference of the electric field, in addition to good on-site instrumentation work grounding. It should also be by the specification requirements to improve the converter to the control room cable shielding grounding—intelligent electromagnetic flowmeter electrical signal circuit connection and grounding, as shown in Figure 5.
The correct grounding of electromagnetic flowmeter, the regular commissioning, and long-term operation of electromagnetic flowmeter are crucial. But in the implementation of the project, we should recognize that “correct grounding” does not necessarily mean a separate grounding.
For example, the metal mentioned above pipe has suitable grounding measures. You can not need a separate instrument grounding. In addition, this paper focuses on the study of pipe-type electromagnetic flowmeter, for insertion type electromagnetic flowmeter, the pipe-type two flange as a whole, and insertion type electromagnetic flowmeter single connection flange corresponding to the reference to use the grounding scheme mentioned above.