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The use of electromagnetic flowmeter and common faulty instructions

Electromagnetic flowmeters are already very mature products. Due to their high measurement accuracy, they have become the products of larger market demand. Electromagnetic flowmeters also have different classification examples of submersible electromagnetic flowmeters, plug-in electromagnetic flowmeters according to different demand environments. Battery powered electromagnetic flowmeters, etc. However, the electromagnetic flowmeter also has many demanding requirements in the installation and use. Kaifeng Liyuan Flowmeter Co., Ltd. (formerly Kaifeng Flowmeter Factory) will give you the following precautions for the installation of the electromagnetic flowmeter.

1 Installation and use precautions

1.1 General Things to Note When Using

The liquid should have the conductivity required for the measurement and require a substantially uniform conductivity distribution. Therefore, the flow sensor installation should avoid the place where the conductivity is not uniform, for example, the liquid is added near the upstream, and the liquid addition point is preferably located downstream of the sensor. The sensor tube must be filled with liquid when in use (except for non-full tube type). When the liquid is mixed, its distribution should be substantially uniform. The liquid should be at the same potential as the ground. Must be grounded. For example, when the process pipe is made of an insulating material such as plastic, the liquid is transported to cause rubbing static electricity and the like, causing a potential difference between the liquid and the ground.

1.2 Flow sensor installation

1.2.1 Installation site

Generally, the electromagnetic flow sensor housing protection is extremely IP65 (dust-proof and dust-proof grade specified in GB4208), and all the following requirements are imposed on the installation site.

(1) When measuring the mixed phase fluid, select the place that will not cause phase separation; when measuring the two-component liquid, avoid the downstream of the mixture that has not been uniformly mixed; when measuring the chemical reaction pipeline, it should be installed in the fully completed reaction section;

(2) Avoid changing the inside of the measuring tube into a negative pressure as much as possible;

(3) Select a place with small vibration, especially for integrated instruments;

(4) Avoid large motors and large transformers nearby to avoid electromagnetic field interference;

(5) It is easy to realize the place where the sensor is grounded separately;

(6) Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;

(7) The ambient temperature is in the range of -25/0 to 50/600 °C, and the integral structure temperature is also subject to electronic components, and the range is narrow;

(8) The relative humidity of the environment is in the range of 10% to 90%;

(9) Avoid direct sunlight as much as possible;

(1O) Avoid rainwater immersion and will not be immersed in water.

1.2.2 Straight pipe length requirements

In order to obtain normal measurement accuracy, the electromagnetic flow sensor I also has a length of straight pipe, but its length is lower than most other flow meters. 9O. After bending, T-shaped tube, concentric reducer, and full-opening valve, it is generally considered that as long as the center line of the electrode (not the connection end of the sensor) is 5 times the diameter (5D) length of the straight pipe section, the valve of different opening degree needs 1OD The downstream straight pipe section is 2 to 3D or no requirement. However, it is necessary to prevent the butterfly valve disc from extending into the sensor measuring tube. The lengths of the upstream and downstream straight pipe sections proposed by various standards or verification procedures are also inconsistent. As shown in Table 1, the requirements are higher than the usual requirements. This is because the requirements for the current 0.5-level precision instrument are guaranteed.

The use of electromagnetic flowmeters and common faults

1.2.3 Installation position and flow direction

The sensor can be mounted horizontally, vertically or tilted without restriction. However, it is best to measure the solid-liquid two-phase fluid vertically and flow from bottom to top. This can avoid the disadvantages of severe local wear of the lower half of the lining during horizontal installation and solid phase precipitation at low flow rates.

When installing horizontally, make the electrode axis parallel to the horizon, not perpendicular to the horizon, because the electrode at the bottom is easily covered by sediment, and the top electrode is easily covered by the occasional bubbles in the liquid to cover the surface of the electrode, causing the output signal to fluctuate. . In the piping system shown in Figure 1, c and d are suitable positions; a, b, and e are unsuitable positions, and liquids at b may not be filled with gas at a and e, and the pipe at the end of e may be short or may not be filled. The discharge port is preferably shown in the shape of a crucible. It is also unsuitable for the solid-liquid two-phase flow C.

1.2.4 Grounding

The sensor must be grounded separately (grounding resistance below 100Ω). In principle, the grounding should be on the sensor side and the converter grounding should be at the same grounding point. If the sensor is mounted on a cathodic corrosion protection pipe, in addition to the grounding of the sensor and the grounding ring, a thicker copper wire (16mm2) is used to bypass the sensor connecting the two connecting flanges of the pipe, so that the cathodic protection current is between the sensors. isolation.

The use of electromagnetic flowmeters and common faults

1.3 Converter installation and connection cable

The integrated EMF does not have a separate converter; the split converter is installed near the sensor or the instrument room. The choice of the place is large, the environmental conditions are better than the sensor, and the distance between the converter and the sensor is subject to the conductivity of the measured medium and the signal cable type. , that is, the distributed capacitance of the cable, the cross section of the wire, and the number of shield layers. Use the signal cable attached to the instrument (or the specified model) from the manufacturer. When the conductivity is low and the transmission distance is long, a three-layer shielded cable is also specified. The general instrument "Instruction Manual" gives the corresponding transmission distance range for different conductivity liquids. Single-layer shielded cables are used for industrial water or acid-base fluids with a distance of 100m.

In order to avoid interference signals, the signal cable must be worn separately in the grounding protection steel tube. The signal cable and power cable cannot be installed in the same steel pipe.

3 failure

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

(1) Adhesion layer on the inner wall of the sensor. Since electromagnetic flowmeters are commonly used to measure dirty fluids, after a period of operation, the adhesion layer is often accumulated on the inner wall of the sensor to cause a malfunction. These failures are often caused by the electrical conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be broken, and the meter will not work normally; if the conductivity of the adhesion layer is significantly higher than the fluid conductivity, the electrode circuit will be short-circuited and the meter will not work properly. Therefore, the fouling layer attached to the measuring tube of the electromagnetic flowmeter should be removed in time.

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

(3) Changes in environmental conditions. During commissioning, because the environmental conditions are still good (for example, there is no source of interference), the flowmeter works normally, and it is often easy to neglect the installation conditions (for example, the grounding is not very good). In this case, once the environmental conditions change, a new source of interference occurs during the operation (such as welding on the pipeline near the flowmeter, installing a large transformer nearby, etc.), which will interfere with the normal operation of the meter, the output of the flowmeter. The signal will fluctuate.

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