The structure of the electromagnetic flowmeter is mainly composed of a magnetic circuit system, a measuring conduit, an electrode, a casing, a lining, and a converter.
Magnetic circuit system: its role is to produce a uniform DC or AC magnetic field. The DC magnetic circuit is realized by a permanent magnet, which has the advantages of simple structure and less interference by the alternating magnetic field, but it is easy to polarize the electrolyte liquid in the measuring duct, so that the positive electrode is surrounded by negative ions, and the negative electrode is positive ion Surrounding, that is, the polarization phenomenon of the electrode, and causing an increase in internal resistance between the two electrodes, thus seriously affecting the normal operation of the meter. When the diameter of the pipe is large, the permanent magnets are correspondingly large, bulky and uneconomical, so the electromagnetic flowmeter generally adopts an alternating magnetic field and is generated by the excitation of a 50HZ power frequency power source.
Measuring catheter: its function is to let the conductive liquid to be tested pass. In order to make the magnetic flux diverted or short-circuited when the magnetic flux passes through the measuring catheter, the measuring catheter must be made of non-magnetic, low electrical conductivity, low thermal conductivity and mechanical strength. Non-magnetic stainless steel, FRP, high strength can be used. Plastic, aluminum, etc.
Vortex flowmeter working principle
The working principle of the vortex flowmeter is to arrange a vortex generator in the fluid, so that the vortex is alternately generated on both sides of the body, and the vortex column is asymmetrically arranged downstream of the vortex generator to generate a certain frequency, by the formula f= St*v/(1-1.27d/D)*d, (St is the Strauhal number, which is a dimensionless number, related to the vortex generator and Reynolds number; v is the flow velocity; d is the incident head width; D is the nominal diameter) to get the flow rate.
In general, the vortex flowmeter output signal (frequency) is not affected by changes in fluid properties and composition, which means that the meter factor is only related to the shape and size of the vortex generator and the Reynolds number. Its advantages are: simple and firm structure, convenient installation and maintenance; suitable for a variety of fluids, liquid, gas, steam and some mixed phases are applicable; high precision, generally up to ± 1% R; flow range is wide, up to 10 : 1 or 20:1 or more; low head loss; no zero drift; relatively cheap price; disadvantage: not suitable for low Reynolds number Re <20000, limited use of high viscosity, low flow rate, small diameter The requirements for the environment are high, and places with vibration should be eliminated as much as possible, and the upstream side needs to have a long straight pipe section; the meter factor is lower, and the larger the diameter, the lower the diameter. The signal resolution is reduced, so the aperture should not be too large, generally used in DN15~DN300mm.
Several problems of steam metering and their solutions
Steam is one of the important energy sources for enterprise production. It is the main heat energy for urban central heating and an important indicator for economic accounting. Therefore, the accuracy of steam metering is particularly important.
In order to solve the problem of heating for employees, the company has updated the original boiler equipment, adding 4 new boilers (2 sets of 25t/h, and another 2 sets of 35t/h). The DCS realizes the automatic control of the boiler system, and the product is saturated steam.
Analysis of problems and influencing factors
Starting from the boiler ignition operation in November 2010, the steam flow often shows the maximum value, and the differential pressure signal measured by the differential pressure transmitter exceeds 20 mA. The same is true when the load is low and the actual flow is small. In this case, steam flow measurement does not provide a safety reference for boiler operation, and it cannot be used for cost accounting.
In response to this phenomenon, the design of the project uses a V-cone flowmeter, equipped with an intelligent differential pressure transmitter, and the measurement signal is sent to the DCS for calculation and display.