The factors influencing electromagnetic flowmeter measurement and selection considerations?
1, all kinds of media's influence on the measurement
(1) the influence of the velocity distribution
Know by fluid mechanics, the liquid flow in the pipe, the pipe of each point on the cross section flow rate is not equal, but both laminar and turbulent flow, after a certain distance of straight pipe, the velocity differential can be axially symmetric distribution, flow velocity in the center of the tube axis place for the biggest, in the pipe wall is zero, the average flow velocity V -, as long as the relative velocity distribution measuring tube central axis of symmetry, the induced electromotive force on the electrode size and velocity distribution state of each point has nothing to do, but just is proportional to the measured average flow velocity of liquid. As a result, the velocity distribution of axial symmetry is uniform magnetic field model of the electromagnetic flowmeter must meet one of the working conditions. If the relative velocity distribution of pipe center axis of asymmetric, although the total flow rate is the same, but near the electrode induction electromotive force is big, so the measured signal is greater than the actual flow rate value. In contrast, in 90 ° with electrodes into induction electromotive force small signal is smaller than the actual flow rate value, cause measurement error. Therefore, in order to make the axisymmetric flow velocity distribution, flow meter, straight pipe is necessary.
(2) the edge effect of magnetic field effect on the measurement
If assumed along the direction of flow of fluid, the magnetic field is uniform, in fact, this means that the magnetic field along the tube axis direction for infinite magnetic field and the actual flow meter is limited long so I must consider finite magnetic field produces the edge effect of the impact on the measurement. Near the pipe wall is supposed to be insulated, electrode is roughly uniform magnetic field, both ends is waning, form uneven edge, the last drop to zero. So, make the internal electric field E liquid is uneven, will produce eddy current. By eddy current secondary magnetic flux, in turn, change the magnetic field generated by the edges of the magnetic field homogeneity of the magnetic flux to make further work would be damaged. At this moment, on the electrode measured induced electromotive force and infinite magnetic field induction electromotive force is not the same size, produced the error. If the wall is conductive, due to the effect of short circuit of the conducting tube wall, the edge effect of magnetic field will be more apparent, as the change of wall thickness and wall conductivity, this influence will be more obvious, which leads to the electrode on the loss of the induced electromotive force increases. For electromagnetic flowmeter, measurement of wall insulation is necessary, so the pipe wall is usually coated with insulation layer. If measured medium containing guide magnetic materials, magnetic field edge effect is more complex. As a result of the existence of permeable material, serious distortions in the magnetic field, caused by the nonlinear measurement. So for the measured liquid contains liquid metal, generally USES the dc excitation in order to reduce edge effect of magnetic field.
(3) the influence of the conductivity of measured medium
At present, the electromagnetic flowmeter converter input impedance has improved measuring liquid conductivity, generally will not caused by a slight change of conductivity of medium error, but for certain converter input impedance, the electrical conductivity of measured medium has a lower limit value, cannot be lower than the lower limit value. The electrical conductivity of measured medium is too big is not allowed. For example when the conductivity of more than 10-1 s/cm, can reduce traffic signals, change the indicated value, the instructions flow value is less than the actual flow. When the electrical conductivity of measured medium large, open circuit of resistance is small, then no matter how high is the input impedance of the converter in parallel will depend on the result of the open circuit to reduce this part of the liquid between the transmitter and transducer transmission precision. So, for an electromagnetic flowmeter measurement is not affected by medium conductivity is a range, conductivity measured medium neither too big nor too small. If the electrical conductivity of medium is extremely high, will produce very big eddy current magnetic field edge area, cause secondary magnetic flux, make work magnetic field on either side of the edge area is weakened and enhancement, respectively. So high conductivity measurement medium unfavorable use ac excitation, and the application of dc excitation. With the development of electronic technology, the improvement of the converter input impedance, will can reduce the lower limit of the conductivity measured medium.
2, the common faults occurring in the use of electromagnetic flowmeter
(1) failure caused by installation
Instrument installation common failure usually by improper installation, caused environmental interference and fluid characteristics. On how to install, usually caused by electromagnetic flow sensor installation position is not correct. Common such as sensor installed on the easy accumulation of gas pipeline, or installed on the top-down vertical tube, may appear after emptying or sensors without back pressure, fluid directly discharged into the atmosphere, and form the measuring tube is not full package.
(2) the operating period of failure
Run-time failures are electromagnetic flowmeter by the commissioning and normal operation after a period of fault, common run-time failures generally by the flow sensor wall adhesion layer, lightning strike, and the changing environmental conditions and other factors.
❶ running environment Generally is the main pipeline stray current interference space strong electromagnetic interference, large motor magnetic field interference, etc. Pipeline stray current interference usually take good separate grounding protection can obtain satisfactory results, but if strong stray current, such as electrolytic workshop pipeline sometimes induction in two electrode potential ac peak Vpp can still need to take other measures up to 1 v and flow sensor and pipe insulation, etc. Generally by the signal cable into space electromagnetic interference, usually adopts single layer or multi-layer shielding protected.
❷ sensor wall adhesion layer
Because the electromagnetic flow meter used to measure the dirt fluid, run after a period of time, often on the inner wall of the sensor failure resulting from the accumulation of adhesion layer. These failures are often caused by adhesion layer conductivity is too big or too small. If attachments for insulating layer, the electrode circuits will be cut-off, instrument can't work normally. If adhesion layer conductivity is significantly higher than fluid conductivity, the electrode will appear short circuit loop, instrument can't work normally. So, should keep clear of the electromagnetic flowmeter measurement tube adherent fouling layer.
❸ lightning strike
Lightning easily induced in the instrument line, high voltage and surge current makes the instrument damage. It mainly through the power cord or excitation coil or the flow line between the sensor and converter approaches introduced, particularly from the control room power supply cord into account for the vast majority of.
❹ fluid aspects
Liquid containing tiny bubbles usually does not affect the normal measurement, uniform distribution of bubble increase will make the output signal wave, if the air bubble to flow through the electrode covering the entire surface of electrode, the electrode signal circuit transient circuit, the output signal will cause a big wave. Low-frequency, 16 Hz to 50, 50 6 Hz rectangular wave excitation electromagnetic flowmeter measuring liquid containing solid exceeds a certain content will produce slurry noise, the output signal will also have a certain degree of volatility. Two or more than two liquid pipeline mixed process, if the two kinds of liquid conductivity or their potential difference between electrodes, before not mixing the access to the flow sensor for the flow measurement, the output signal will fluctuate. Electrode material and improper matching measured medium, produce the chemical action, such as oxidation passivation or electrode surface insulation film, and electrochemical polarization and so on, all can interfere with the normal measurement. Measurement of mixed media, if before not mixing into the flow sensor measurements, will also make the output signal wave.
3, the selection of flow sensor
Electromagnetic flowmeter selection of electrode materials, electrode materials and improper matching measured medium, will affect the normal due to the chemistry or polarization measurement, should choose according to the measured medium corrosive electrode materials. According to the corrosion resistance, wear resistance and temperature of the measured medium choose electromagnetic flowmeter lining material. As far as possible choose to have lightning protection function of electromagnetic flowmeter.
4, flow sensor installation
(1) to the requirements of the installation site.
Ⅰ. Mixed phase flow measurement, the choice does not cause phase separation, measurement of two-component liquid, avoid in has not yet been mixing downstream, measuring chemical reaction line, put them in response to fully complete.
Ⅱ. As far as possible to avoid the measuring tube into a negative pressure.
Ⅲ. Select the little vibration, especially for a size instrument.
Ⅳ. Avoid near a big motor, transformer, etc., lest cause electromagnetic interference.
The place of separate grounding Ⅴ. Easy to implement sensor.
Ⅵ. As far as possible to avoid the environment with high concentration of corrosive gas. Within 25 to 60 ℃ environment temperature, environment relative humidity within the scope of the 10% - 9 o %, as far as possible, avoid direct sunlight.
Ⅶ. Liquid should be measuring the electrical conductivity, and require substantially uniform conductivity distribution. Therefore flow sensor installation to avoid uneven prone to conductivity of places, such as adding liquid near the upstream, charging point best located at the downstream of the sensor.
(2) the straight pipe length requirement
electromagnetic flowmeter to straight pipe before and after the request is lower, about 90 o bent pipe, tee tee, concentric reducer, usually as long as from the center line of the electrodes, after full open valve inlet side not sensor junction surface > length of 5 times the diameter of straight pipe, the different opening of valve is 1 od, downstream straight pipe for the 3 d. Measuring the mixed liquid of different medium, the point and the distance between the flow meter at least greater than 30 d..
(3) the installation position and direction of flow
sensors installed direction of horizontal, vertical or inclined, is not restricted. But the measurement of solid, liquid two phase fluid is best installed vertically, from bottom to top. When mounted horizontally lining that can avoid lower partial wear serious, shortcomings and so on low velocity when the solid phase precipitation. Level to make the electrode axis parallel to the horizon, when installation to prevent due to occasionally save air bubbles in the liquid caused insulation grazed the electrode surface also can prevent the bottom electrode is covered by sediments. Vertical installation, flow direction should be brought up, so I can make no flow or small flow rate, fluid inclusions in the heavier solid particles sinking, and lighter body fat class material rising leave flowmeter sensor electrode area.
(4) grounding
the sensor must be separate grounding, grounding resistance under 100 Ω. Separation type grounding in principle should be at the side of the sensor, converter grounding should be in the same place.
5, summary
Electromagnetic flowmeter is widely used in industrial measurement of conductive liquid, it has simple structure, wide range than, in using a small amount of maintenance, the above is all sorts of factors, which affect the electromagnetic flowmeter simple introduction and understanding, hope to provide help for the general user.