Instrument users generally know, flowmeter measurement to get a more stable and accurate results, must guarantee certain straight pipe section, general requirements in the electromagnetic flow meter
upstream side should be not less than 5 d pipe diameters of straight pipe.
The downstream side should be not less than three times the pipe diameter of straight pipe section.
As far as possible in order to reduce the influence of pipe bending part for flow measurement results, the need for the meter installation process and method have a better choice.
Problem of fluid flow through the pipe, the downstream velocity distribution distortion, if two adjacent to connect different plane bend, in addition to the velocity distribution distortion will also have a whirlpool.
Most of these phenomena will influence on the downstream type flow meter (
Besides, product type, such as a few type)
Measured value, so usually required before the flow meter installed point straight pipe has a certain length, improve into the flow of the instrument.
Electromagnetic flowmeter bend influence is small, straight pipe length requirement also is far lower than the throttle differential pressure type, vortex type and other flow meter.
4, even affect the quantity is small, or because the attention of people.
Except for the straight pipe before the instrument can reduce the influence of bend, the people are also trying to reduce the impact from the installation method.
Reported that the electromagnetic flow sensor electrode axis and adjacent to the in-plane bending 45.
Oblique installation can reduce the bend effect.
Japanese industrial standard B7554 & other;
Commentary part & throughout;
List of electrode axis perpendicular to the in-plane bending, parallel, 45.
Three kinds of installation, measurement error range under different length of straight pipe, also illustrate this effect.
In 2000 Shanghai raw water company has built a pumping station equipped with DN2200 electromagnetic flowmeter, but bend downstream installation position of straight pipe length ago apparently does not comply with the provisions in 5 ~ 10 d (d
D as the pipe diameter, the same below)
Request, at this point, affecting how big is the measurement?
Due to the constraints can't at the scene with the classical measurement method (
Such as flow rate meter flow area method)
At trial to estimated bend impact additional error value.
And use must want to know before ordering additional error estimate, to determine the design scheme.
The author according to the similarity principle at a relatively small diameter electromagnetic flowmeter for tests at the scene of the laboratory simulation conditions.
Tests showed that 45.
Installed and the case is obtained under the condition of the traditional way of impact value.
Bend downstream velocity distribution affect the fluid flows through a pipe bending due to centrifugal force and diffusion effects generated by the outer wall, the wall causing shrinkage effect, the resulting transverse flow of the secondary flow, the velocity distribution distortion cause downstream, as shown in figure 1.
Figure 1, the right vertical section bend outer flow faster, horizontal section assumes the double peak velocity distribution.
Distortion will be easing.
For the most part before when electromagnetic flow sensor is a uniform magnetic field distribution in the structure design.
Inhomogeneous magnetic field theory contains electrode sounding pipe transverse plane area, each tiny liquid volume element cutting lines of magnetic force signals between the electrode and other
Role & throughout;
Each different, so is not evenly but by & other;
Role & throughout;
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 or by some.
As shown in figure 2 is attached to the Japanese industrial standard bend downstream three installation error range of different length of straight pipe.
Real flow experiments is in Shanghai guanghua & middot;
Love and its instruments co. , LTD. Is the water flow standard device for weighing method.
011, the test instrument is DN100 IFM4080K type, according to the size of the scene geometry scaling set pipe, as shown in figure 3 loaded on the flow standard device check.
The test results as shown in figure 4.
In the figure & other;
Normal installation & throughout;
As the front straight pipe length is greater than or equal to 10 d, rear straight long degree greater than or equal to 5 d installation, the test data is the ratio of refs.
Electromagnetic flowmeter full flow is 150 m.
3 m/s, made five flow point.
Under the condition of the test installation, flow sensor electrode axis in A 90 A.
2, A 0.
6, A 45 A.
Compared with the normal installation conditions that ratio, additional error in A 90 A.
2, A = = : 0.
65, A 45 A.
Experiment proves that the electrode axis 45.
Installation than traditional level (
Affected by the bend flow disturbance has a lot to improve.
Electromagnetic flow sensor in the downstream of the bend even with long enough (
The straight pipe, also should press & other;
Installation & throughout;
Bend torsion as the reduction of flow, reduce the additional error.
In this paper by embellish instrument technology co. , LTD.
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