The influence of electromagnetic flowmeter installation condition
Electromagnetic flowmeter is a kind of speed meter, its biggest advantage is measured from the fluid density, the influence of the parameters such as temperature, pressure and viscosity. Is an ideal electromagnetic flowmeter measurement signal only related to the volume of fluid in pipe flow and has nothing to do with the velocity distribution of physical quantities. The pipe flow field distribution in the signal has a certain degree of influence on the measurement. In cannot guarantee under the condition of nothing to do with the velocity distribution, according to the classical analysis, if the flow velocity in the center of axisymmetric flow, induced potential and only is proportional to the pipe cross section of average flow velocity.
In practice, for process requirements, in the upstream of electromagnetic flowmeter are often required to install some choke pieces such as elbow, reducer, divergent pipe, valve, etc., due to the existence of choke, causes the flow of distortion, secondary flow and vortex flow. Destroyed the original full development of the velocity distribution, to restore its symmetry fluid velocity distribution, need a certain length of straight pipe.
"Electromagnetic flowmeter flow measurement method" regulation for the specific requirements, and provides the front spoiler a single bend, planar double bend, three-dimensional bending curve, the influence of the different butterfly valve opening. Specification of the electromagnetic flowmeter to the disturbed flow has no specific provisions, only the principle of working fluid in the flow sensor is fully developed and no rotating flow, regardless of whether the cone Angle is less than 15 °, the divergent tube should be connected.
Domestic development of gauge diameter greater than 2 m, 2 m in diameter of the mount divergent tube flow standard device, directly connected to the 2.2 m above the instrument, to be extended to 2.6 m instrument, appear the phenomenon of poor repeatability, efficacy for analysis reasons of the poor repeatability of large diameter electromagnetic flowmeter, the divergent tube directly connect the inlet in the electromagnetic flow sensor, found the considerable influence on flow measurement value. Finally found the cone Angle is close to 15 ° of divergent inlet side pipe connected directly to the electromagnetic flow sensor, produces about 10% of the influence of the measured value, and the repeatability is poor. Between divergent tube and sensor added a length L = 1 dn after short tube, affect the quantity can be reduced to about 5%, increase the length of the influence quantity will continue to decrease. In addition, also found that reducing pipe connected directly to the electromagnetic flow sensor, little effect on the flow measurement value.
Practice shows that when the electromagnetic flowmeter is installed on the outside, a prerequisite for the gate valve downstream straight pipe length is insufficient, on the basis of function distribution, two electrode electromagnetic flowmeter can be used when installation electrode axis and bend, gate valve, such as axis 45 ° inclined installation, to avoid electromagnetic velocity sensitive area near the installation way, to reduce the additional error caused by the asymmetry of velocity.