The principle of electromagnetic induction makes electromagnetic flowmeter In the case of ensuring the measurement accuracy, the cost of production and operation is reduced as much as possible, and the stability and service life are also guaranteed. However, the electromagnetic flowmeter will also have faults. The small editor has carried out tracking and Research on the electromagnetic flowmeter fault, and combined with years of maintenance experience, summed up the following three categories of failures:
1、 The main faults caused by fluid are as follows:
1. The liquid contains gas,
The dissolved gas in the liquid will not affect the flow measurement, while the bubble will affect the measurement and may cause failure. The bubbles in the liquid come from: ① the air in the pipe is not exhausted completely; ② it is inhaled from outside the pipe system; ③ the dissolved gas is transformed. The former two are described in the previous section, and the latter is caused by the change of liquid temperature and pressure in the pipe. In the process industry, the pressure / temperature of pipeline liquid often changes. When the liquid pressure decreases or the temperature rises, the dissolved gas will be transformed into free bubbles. For example, if the liquid is lower than room temperature and remains in the pipeline for a period of time (such as parking), the dissolved air may be converted into bubbles; when the liquid is higher than room temperature, it will gradually cool and contract in the closed pipe at both ends, forming a local vacuum Dissolved air or vaporized vapor to form bubbles. In the initial stage of process restart, the bubble often appears the phenomenon of output sloshing, which tends to be normal after a period of operation. When the opening of the control valve is very small, the easily gasified liquid will sometimes vaporize and form bubbles.
2. The liquid contains solids,
The liquid contains powder, particle, or fiber and other solid, which may cause faults: ① slurry noise; ② electrode surface contamination; ③ conductive deposition layer or insulation deposit layer covering electrode or lining; ④ lining is worn or deposited, changing flow area. Due to the gradual deposition of conductive materials, this kind of fault usually does not appear in the commissioning period, but will show after a period of operation. The possibility of similar faults should be analyzed if the normal operation starts and the flow indication becomes smaller and smaller as time goes on.
3. The conductivity is not uniform,
In the proportioning process, the "dosing" liquid is often injected into the liquid, and the injection liquid is usually added by reciprocating pump. If the conductivity of the injected liquid is different from that of the main liquid, and the mixture has not been mixed evenly, if the electromagnetic flow sensor is installed downstream, the output of the instrument will shake because of the sharp change of the conductivity. Although the liquid conductivity is larger than the threshold value and changes slowly, it will not affect the normal measurement of the electromagnetic flowmeter. This phenomenon often occurs in the process of adding coagulant to raw water of water treatment project.
4. Improper material matching between liquid and liquid contact,
The main parts of mismatching fault with liquid contact material are electrode and grounding ring. In addition to corrosion resistance, mismatching is mainly due to electrode surface effect. The surface effects are as follows: ① chemical reaction (passivation film on the surface), ② electrochemical and polarization phenomena (generating electric potential), ③ catalytic action (formation of gas mist on the electrode surface). Grounding rings also have these effects, but to a lesser extent.
5. Flow noise.
2、 The main faults caused by environment are as follows:
1. Strong magnetic field,
The practical experience of the influence of strong magnetic field is not much, because the installation should be far away from the strong magnetic field.
2. Strong electromagnetic waves,
electromagnetic flowmeter It shall meet the requirements of electromagnetic compatibility and work normally under the specified radiation electromagnetic field environment, which will not cause performance degradation or abnormal operation of electromagnetic flowmeter in this environment.
3. Stray current in pipeline,
After proper grounding, the influence of most stray current in pipeline can be avoided. Sometimes, when the flow sensor is bridged and grounded by thick wire, it will be affected by stray current, and other measures need to be taken.
4. Ground potential change,
The change of ground potential will affect the flow measurement, for example, the change of ground potential is caused by the voltage drop on the grounding wire due to other equipment, and it will also affect the measurement if there is a large common mode interference.
5. Moisture immersion.
When applied in water supply and drainage industry, the flow sensor is often installed in the instrument well below the horizon, which is often immersed in the rain water which is not discharged in time, or even immersed in the water for a long time. Even if the protection level of the enclosure is IP67 or IP68, such accidents often occur because the sealing gasket of the terminal box cover or the sealing ring leading in the cable is not compressed and sealed, the ferrule is not installed, or the ferrule is not matched with the outer diameter of the cable. If the sealing gasket such as the terminal box cover of the flow sensor installed on the ground is not well sealed, the moisture will be absorbed and condensed into water by the breathing effect of temperature change. The cable lead-in device of terminal box is not installed with sealing ring or is not tightly pressed, and condensate water on the cable surface can easily enter into the terminal box. Such cases are also common. In the process of construction, the cable is cut off intentionally or unintentionally and then connected again, and sealed with tape. This hidden danger will not form a fault in the early stage of operation, but the sealing is aging for a long time, the moisture is inhaled at the joint, and the cable insulation is reduced. Water and moisture invade the terminal box, which reduces the insulation strength and insulation resistance. There will be no flow signal output in the flow signal circuit, and the excitation coil circuit will form zero offset or instability. If necessary, the sealing joint can be watered with silica gel. The non hermetic structure of the excitation coil protection shell absorbs moisture due to respiration. If the liquid temperature is lower than the room temperature, it is easy to condense on the outer wall of the measuring tube. If the liquid temperature is lower than 0 ℃, it will frosting, which will make the flow signal circuit short circuit and failure.
3、 The main fault sources from pipeline system and related equipment are as follows:
1. Gas is stored in the pipeline;
One of the reasons for the gas storage in the pipeline is that the air in the pipe can not be completely discharged before the pipeline is put into operation, and the remaining gas accumulates at the high point of the pipeline, which is entrained by the liquid and flows out in the form of bubbles; the other reason is that the small bubbles entrained in the liquid gradually accumulate and are stored in the high point of the pipeline. The fault shows that the measured value of flow is inconsistent with the actual value and the output is shaking.
2. Pulsating flow generated by reciprocating pump or control valve oscillation;
Reciprocating pump or control valve oscillates to produce pulsating flow. Reciprocating pump pumps pump liquid flow and the measuring point is not far away from the pump. The pulsating flow will make the output shake and sometimes produce measurement error. In order to reduce the influence of pulsation on electromagnetic flowmeter, the excitation frequency or damping can be increased, and damping devices such as air chamber can be installed in the pipeline. The mismatching of pipeline flow control makes the opening and closing oscillation of control valve also form pulsating flow.
3. The flow state changes during use.
In the process of using, the change of flow state usually leads to failure after debugging and normal operation for a period of time. Although the probability of this kind of failure is not high, this factor should not be forgotten when analyzing the cause of failure. The falling off of pipe lining and other components will lead to this phenomenon.
4. Improper installation;
Sometimes, the output value is not consistent with the actual value.
(1) The inner diameter between the flow sensor and the connecting pipe is mismatched and the difference is too large;
(2) The flow passage between the flow sensor and the flow channel;
(3) The flow deflector near the front of the flow sensor produces serious velocity distribution distortion or rotating flow, and the straight pipe section is insufficient.
5. The tube is not full;
Due to insufficient back pressure or poor installation position of flow sensor, the liquid in the measuring tube can not be filled, and the fault phenomenon has different performance due to the degree of underfilling and flow condition. If a small amount of gas is stratified flow or wavy flow in the horizontal pipeline, the fault phenomenon shows that the error increases, that is, the flow measurement value is inconsistent with the actual value; if the flow is bubble flow or plug flow, the fault phenomenon will not only be inconsistent with the measured value, but also output shaking due to the gas phase covering the electrode surface instantaneously. If the gas part of the flow area increases in the horizontal pipe stratified flow, that is, the liquid is not full, the output sloshing will also occur. If the liquid is not full and the surface of the liquid is below the electrode, the phenomenon of output overfilling will occur.