with no pressure loss, high precision, the advantages of the moderate price, popular with petrochemical, chemical and other industries, as an important role in flow measurement.
In practice, however, the improper operation, equipment selection, installation is unreasonable science, measurement error is hard to avoid, cause problems to users.
Therefore, the general instrument people should pay attention to various factors causing error of electromagnetic flowmeter.
In general, the main influence factors of electromagnetic flowmeter errors can be divided into three categories: improper selection, hydraulic impact and interference under test.
Improper selection 1 to test the liquid velocity of electromagnetic flowmeter can measure the velocity range is generally to 0.
5 ~ 10 m/s, economic velocity range of 1.
According to actual use for measuring the flow and the electromagnetic flowmeter is flow speed range is to determine the tube diameter measurement.
2 electrode and lining material selection electrode and lining material, come into direct contact with the liquid under test shall be according to the characteristics of the liquid under test (
Such as corrosion resistance, abrasion resistance, etc. )
Selective electrode and working temperature and lining material, such as improper selection, fast can cause adhesion, corrosion, fouling, wear, lining deformation problems, and then the measuring error comes into being.
3 excitation stability of electromagnetic flowmeter with dc field, ac sine wave excitation and excitation type double frequency rectangular wave excitation, such as dc excitation prone to electrode polarization and dc interference problems, communication sine excitation to zero, the dual-band rectangular wave excitation is both low-frequency rectangular wave excitation of excellent stability of the zero point, and the high frequency rectangular wave excitation of fluid noise inhibition ability, is an ideal way of excitation.
The actual application, should ensure the stability of the power supply voltage and frequency, as far as possible to ensure that the magnetic field intensity is constant, decrease due to the measuring error caused by the magnetic field strength changes.
4 mixed phase fluid with electromagnetic flowmeter measurement solid-liquid mixed phase fluid (
Such as silt-laden water)
Traffic, if choose by electromagnetic flowmeter calibration, single-phase liquid will produce measurement error, should choose at this time does not cause the liquid-solid phase separation of straight pipe installation of sensors.
With liquid influence 1 with liquid conductivity changes for measuring liquid conductivity is larger, can lead to show that the larger fluctuation of numerical if the problem is very serious, the control system is difficult to achieve normal operation;
And stay at low liquid conductivity measurement, the electrode is difficult to realize the normal output, if the operation scope of liquid conductivity under test in a lower value in the following, then the electromagnetic flowmeter is hard to work.
According to these situation, first of all, should be based on actual needs, combined with the relevant standards and requirements, select the type of electromagnetic flowmeter;
Second, reactor or straight pipe installation, in order to ensure the material mix, facilitate the smooth implementation of the chemical reaction;
Again and again for flow meter
2 air bubbles or a full package for under test fluid, mainly comes from the gas dissolved in the liquid for the development of the free state of inhaled air bubbles bubbles and the outside world.
Contains a large number of bubble volume of flow, influence the accuracy of measurement.
If the bubble diameter is too big, even more than the electrode diameter values, in the process of the measurement shows will appear unstable state, volatility is inevitable.
For this kind of situation, first of all, the gas collector can be installed on the electromagnetic flowmeter, at the same time according to exhaust the operating cycle;
Second, the reasonable replacement installation location;
Again, the vertical pipe installed on the electromagnetic flowmeter, ensure bottom-up direction;
Fourth, the installation of sensors, avoid too close with outfalls;
Fifth, the sensor is installed in a control valve position, in its upper position, or the downstream of the pump.
3 for measuring liquid conductivity measured liquid conductivity is too low, can increase the output impedance of the electrode, and caused by converter input impedance measurement error resulting from the load effect, if the actual conductivity is lower than the lower limit value, the instrument can not work normally, can produce value for volatility.
Countermeasures: choose other meet the requirements of low conductivity electromagnetic flowmeter, such as the capacitance type electromagnetic flow meter;
Choose other principle of flow meter, such as orifice plate, etc.
4 liquid appears asymmetric state measurement, the liquid is asymmetric under test conditions, there are mainly two kinds of flow composition: a for a single vortex flow;
Another kind is the linear flow along the pipe axis, liquid volume flow of the pipe cross section of the integral.
Aiming at shortage of straight pipe upstream, can be used to adjust the flow regulator;
Second, guarantee within a reasonable range of upstream and downstream of the pipe diameter and flow meter diameter with the same value;
Once again, to leave plenty of straight pipe upstream.
5 there is a layer of electromagnetic flowmeter measuring tube is often used to measure the cleaning fluid.
The cleaning fluid inside contain a lot of sediment and other material, make the pollution of electromagnetic flowmeter electrode surface or the pipe, measurement error phenomenon.
For this kind of situation, first of all, regular cleaning electromagnetic flowmeter;
Secondly, reasonable ascending velocity, the control in the 4 m/s status;
Again, the application of the material such as poly (tetrafluoroethylene lining.
Interference effect 1 space electromagnetic interference asked converter and sensor cable is longer, in the strong electromagnetic environment, are susceptible to interference, triggering instrument measurements nonlinear condition, it is difficult to normal display.
For this kind of situation, first of all, the introduction of shielding measures, can be carried out within the grounding steel cable introduced separately, and using standard shielded cable;
Second, the reasonable to shorten the length of the cable
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