Selection of steam flowmeter
There are two main types of instruments for measuring steam: vortex flowmeters and differential pressure flowmeters . Compared with the vortex flowmeter , the differential pressure flowmeter has a large pressure loss, low measurement accuracy, and differential pressure transmission errors. Based on the performance of all aspects, a vortex flowmeter is selected.
主要原理： 在流体中设置旋涡发生体（阻流体），从旋涡发生体两侧交替地产生有规则的旋涡，这种旋涡称为卡曼涡街，如图1所示。 The main principle of the vortex flowmeter is to set a vortex generator (fluid resistance) in the fluid, and alternately generate regular vortices from both sides of the vortex generator. This vortex is called Kaman vortex street, as shown in Figure 1 . The vortex rows are arranged asymmetrically downstream of the vortex generator.
The pulse frequency signal output by the vortex flowmeter is not affected by the fluid physical properties and composition changes, that is, the meter coefficient is only related to the shape and size of the vortex generator and the pipeline within a certain Reynolds number range, so the measured volume flow is more accurate . However, as the flow meter needs to detect the mass flow in the material balance and energy measurement, the output signal of the flow meter should monitor the volume flow and fluid density at the same time. The fluid physical properties and components have a direct impact on the mass flow of the flow meter .
，被测介质来流的平均速度为U ，旋涡发生体迎面宽度为d ，表体通径为D ，根据涡街原理，有如下关系式 Suppose the frequency of vortex occurrence is f , the average velocity of the incoming flow of the measured medium is U , the width of the oncoming body of the vortex generator is d , and the diameter of the surface body is D. According to the principle of vortex street, there is the following relationship:
（ 1 ） f = SrU1 / d = SrU / md ( 1 )
旋涡发生体两侧平均流速， m/s ； Where U1-- average flow velocity on both sides of the vortex generator, m / s ;
斯特劳哈尔数； Sr--Strauhal number;
旋涡发生体两侧弓形面积与管道横截面面积之比 m-- the ratio of the arcuate area on both sides of the vortex generator to the cross-sectional area of the pipe
为 The volume flow qv in the pipeline is
(2) qv = πD2U / 4 = πD2mdf / 4Sr (2)
为 The mass flow M in the pipeline is
×ρ M = qv × ρ
Simple and firm structure, convenient installation and maintenance (compared with differential pressure flowmeter).
There are many types of fluids, such as liquid, gas, vapor, and some miscible fluids.
～ ±2% ） R 。 The accuracy is taught high (compared with differential pressure type and float type flowmeter), and it is generally the measured value ( ± 1% ~ ± 2% ) R
： 1或20 ： 1 。 Range width up to 10 : 1 or 20 : 1 .
～ 1/2 ）。 Low pressure loss (approximately 1/4 to 1/2 of orifice plate flowmeter).
Output pulse signal proportional to flow rate, suitable for total measurement, no zero drift;