Take the orifice plate as an example, there is a circular hole in the middle of the orifice plate, and its diameter is smaller than the internal classic of the pipeline. When the fluid passes through the orifice plate, the velocity of the fluid at the orifice plate will increase (according to the continuity equation of the fluid). At the same time, according to Bernoulli equation, the pressure of the fluid in the pipeline will decrease after the fluid velocity increases, so a pressure difference is formed before and after the orifice plate, which is proportional to the fluid velocity in the pipeline. Two different pressures are taken out through the pressure guide pipe and converted into electrical signals by the differential pressure transmitter.
This principle is also true for venturi tubes, but it is somewhat different in structure. The inner diameter of the venturi tube is the same as that of the pipeline, and the inner diameter in the middle is smaller than that at both ends. It is also because the diameter of the fluid decreases when it flows through the middle of the venturi tube, which speeds up the flow rate and reduces the static pressure. Therefore, the differential pressure of Venturi tube is taken out at the entrance and middle of Venturi, and the electrical signal is also converted by differential pressure transmitter.
For Pitot tube and A Niu rod, these detecting elements are somewhat different from orifice plate and venturi tube. They insert two back-to-back pipes into the center of the pipeline of the measured fluid, with the positive pressure side leading to the direction of the fluid and the negative pressure side facing away from the direction of the fluid, because when the fluid meets an obstacle on the flow channel, a vortex area with low pressure will be formed on the back of the obstacle, and the flow of the fluid can be measured through these two different pressure differences.
For the differential pressure flowmeter, the most important thing is that all the connecting pipes must not leak, because the pressure difference generated by the components detecting the fluid is very small, and any leakage will produce great detection errors. At the same time, the extraction position of the pressure guide tube is also a very important link for different detection fluid objects. For gas fluid, the pressure guide tube should be taken out at the upper part of the pipeline at 45, mainly to prevent the liquid possibly contained in the fluid from entering the pressure guide tube and causing detection errors; For liquid fluid, the pressure guide pipe should be taken out at the lower part 45 of the pipeline, which is also to prevent the gas in the fluid from entering the pressure guide pipe.
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