Electromagnetic flowmeter, as a kind of flowmeter, is widely used, because of its simple structure, the flow of fluid through the flowmeter will not occur pressure loss;Electromagnetic flowmeter can measure the domestic waste water, industrial waste water and so on, and can even measure the flow of liquid-solid two-phase.The main characteristics of electromagnetic flowmeter are: the measurement process is not easy to be affected by the density, temperature, viscosity of the liquid;The speed measurement scale is relatively wide.Of course, there are limits to the measurement of electromagnetic flowmeters, because electromagnetic flowmeters can only measure conductive liquids, not gases and steam.So what should be considered in the selection of electromagnetic flowmeter?
2. Accuracy level
The precision of electromagnetic flowmeter is high, because the liquid flowmeter measurement range is large, the measurement range is large so the accuracy must be small, the larger the range error is also larger, so the need for strict control of precision.The accuracy of precision flowmeter is 0.5-1 level, while the accuracy of conventional glass and gold rotation is 2.5 level.The difference is still a little big.In the measurement and billing, or to choose a high precision electromagnetic flow meter.
3. Selection of velocity and diameter
The velocity requirement of electromagnetic flowmeter is 0.5-15m/s, and the reasonable velocity is generally 1.5-3m/s.The flow rate is too fast, if contains particulate matter may scrape the lining teflon.Flow rate and diameter, need to have a reasonable selection.
4. Selection of grounding ring and electrode raw material
Because the measured liquid and the electrode or grounding ring do not match, resulting in defects, this mismatch in addition to corrosion problems, but also the appearance of the electrode effect, the first includes: chemical reaction, polarization phenomenon, catalytic effect.Grounding rings also have these problems, but at a lower level.
5. Solid phase in the body
If particles are assumed to be present in the liquid, the possible disadvantages are: covering up the accumulation layer on the electrode and the surface of the fabric, wearing of the fabric, and reducing the cross-sectional area of the pipe.Some relatively simple crystallization of the liquid, when the fluid flow rate timing caused hanging on the inner wall of a layer of solid, because the use of other types of flow meters also have the same problem, so you can choose to be measured pipeline is very short, and simply disassembled the flow meter, after crystallization can be easily disassembled to stop the protection.
1. In high-temperature environment, near heat sources such as heating, direct sunlight, excessive humidity, rain or water.
2. Vibration.After compressor, constant displacement pump and oscillating faucet, vortex street, vortex inlet and coriolis flow meter cannot be installed.
3. Erosion.Wet, is in the humidity is too good environment, such as can not avoid closed should choose corrosion resistant material as the instrument shell.
4. Explosion-proof and flammable.The explosion - proof safety accident of the instrument should be increased.
5. Dust.Instrument transmitters should be separated or shielded.
6. Electromagnetic interference: the instrument should be equipped with a protective cover around the strong electromagnetic interference to shield electromagnetic interference.Be in bad working environment, the detection instrument of flowmeter is evasive hard, and secondary table (transmitter) otherwise, should not use integrative instrument at this time.
How to reduce the impact of electromagnetic flowmeter bend installation method
As is generally known to instrument users, in order to obtain relatively stable and accurate results of flow meter measurement, a certain straight pipe segment must be guaranteed.
General requirements in the electromagnetic flow meter upstream side should be no less than 5D pipe diameter of the straight section.The downstream side shall have no less than 3 times the pipe diameter of the straight section.In order to minimize the impact of the elbow on the flow measurement results, it is necessary to have a better choice on the process and method of the flowmeter installation.
1. Raise questions
After the fluid flows through the bend, the flow velocity distribution distortion is generated downstream. If two adjacent curved pipes are connected in different planes, there will be vortex in addition to the flow velocity distribution distortion.These phenomena will affect the measurement values of most types of flow meters installed downstream (except for a few types such as volumetric type), so it is usually required to have a certain length of straight pipe segment before the installation point of flow meters to improve the flow into the meter.Electromagnetic flowmeter is less affected by bending pipe, straight pipe length requirements are far lower than throttle differential pressure type, vortex street type and other flow meters.However, the accuracy of the current electromagnetic flowmeter is 0.5 percent, even 0.2% to 0.4. People pay attention to it even though the influence is small.
In addition to the straight pipe section in front of the instrument can reduce the impact of bending, people also try to reduce the impact of installation methods.
Tilting installation can reduce the impact of pipe bending.Japanese industrial standard B7554 part "explanation" list of electrode axis perpendicular to the in-plane bending, parallel, 45 ° three installation, measurement error range under different length of straight pipe, also shows the effect a..In 2000, a company in Shanghai planned to install DN2200 electromagnetic flowmeter in a pump station that had been built. However, the length of the front straight pipe at the downstream installation location of the elbow pipe obviously did not meet the requirements of 5D ~ 10D(D is the pipe diameter, the same below) in the regulations. At this time, how big is the measurement error?Due to the limitation of conditions, it is not possible to use the classical measurement method (such as the tachometer tachometer area method) in the field to compare the test, so as to estimate the additional error caused by the bend.And the user must know the additional error estimate before ordering to determine the design solution.According to the principle of similitude, the experiment of electromagnetic flowmeter with smaller diameter is carried out in laboratory.Tests showed that 45 ° bend effect can be reduced with installation.The influence value of the traditional installation method under the installation condition is obtained.
2. Influence of downstream velocity distribution of elbow
Due to the centrifugal force, the fluid flowing through the elbow will have diffusion effect along the outer wall and contraction effect on the inner wall, which will lead to secondary flow of transverse flow and distortion of velocity distribution in the downstream.
The velocity at the outer edge of the curved pipe in the right vertical section is faster than that at the horizontal section.The distance from the bend increases with the flow.The distortion tends to moderate.
Most electromagnetic flow sensors are designed with non-uniform magnetic field distribution.Inhomogeneous magnetic field theory contains electrode sounding pipe transverse plane area, each tiny liquid volume element cutting lines of magnetic force between the electrode signal "role", therefore is not evenly but according to the "role" non-uniform design, the magnetic field intensity at various points in the ideal velocity distribution under the condition of distortion will not affect the flow measurement value.However, the actual instrument is still affected by some.