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Application and Prospect of temperature and pressure compensated vortex flowmeter in energy measurement

Abstract: This paper analyzes the principle and structure of vortex flowmeter with temperature and pressure compensation; This paper expounds the application and Prospect of this new product in energy measurement in our factory

key words: four in one flowmeter; Density compensation; Performance index

vortex flowmeter is a new type of measuring instrument that has been widely used at home and abroad in the past one or two decades. It has a great trend to gradually replace the traditional flow measuring instrument composed of throttling device + differential pressure transmitter. Its popularity is determined by its high cost performance, accurate measurement, wide range ratio and simple maintenance

however, ordinary vortex flowmeter, like other types of flowmeter, has an obvious disadvantage, that is, in the measurement of steam or gas flow, if the process condition changes too much, its density value will not be constant, and it will deviate too much from the design value, which will bring great errors. In our actual energy measurement work, we encounter many such situations. Due to various reasons, the steam or gas in some pipelines cannot be the same as the design value, which is constant at a constant temperature and pressure value, and there are always great fluctuations. Therefore, we have to install pressure sensors and temperature sensors before and after the flowmeter, and then send the signal to the secondary display instrument for temperature and pressure compensation to correct the density and reduce the measurement error. However, this brings many problems such as installation, maintenance and cost. Our experimental machine is mainly used to test the bearing capacity of material mechanics

therefore, we hope to have a highly integrated vortex flowmeter to solve this problem. This year, we tried three vortex street flowmeters of Yiwu instrument company, which integrate high-precision temperature, pressure, flow sensors and intelligent flow software calculation. Since its application, the application effect is very good. Next, I briefly introduce this four in one vortex flowmeter:

1. Principle and structure

1. Measurement principle: the temperature sensor, pressure sensor and flow sensor send the signals detected respectively to the header circuit board, and the circuit part processes these three groups of signals separately, and automatically uses the measured temperature and pressure values through the software from the schematic diagram of vortex flowmeter with temperature and pressure compensation in Figure 1, Convert or find out the density value under working conditions, then correct it to the flow value, and finally output it to the secondary display instrument in the form of current or frequency. In some places where it is inconvenient to provide power, the battery powered local indicating type can also be selected. At this time, the cost is cheaper, because the secondary display instrument can be omitted

2. Structure and composition

this is the four in one vortex flowmeter. Behind the triangular baffle is the flow sensor, which can measure the vortex frequency of the vortex street. It can be piezoelectric or capacitive (capacitive

has better performance); A temperature sensor is installed downstream to measure the medium temperature in the pipeline; The pressure sensor is installed in front of the triangle block to measure the medium pressure in the pipeline. The output signals of the three sensors are connected to the circuit part of the meter and processed by them

to sum up, this vortex flowmeter has realized the unified integration of three sensors with high-performance thermoelectric and power-saving materials and technical circuit software, that is, the four are integrated

second, density compensation

first, let's analyze why we need to compensate the density of measured gas or steam. As we all know, the volume flow directly measured by vortex flowmeter is the volume flow under working conditions. We generally settle the gas, steam, etc. based on the mass flow, which involves the density problem, that is, we implement the following mathematical model:

W = q· ρ


w: mass flow

q: volume flow under working condition

ρ: Density under working conditions

the following is an example to illustrate the importance of density. In the measurement of superheated steam flow, it is assumed that the pressure under the design working condition is 10kgf/cm2 (a), the temperature is 300 ℃, and the flow at this time is 30t/h (W set). Check the density table ρ= 3.800kg/m3。 However, under the actual working condition, the pressure fluctuates in the range of 10.0 ~ 1110kgf/cm2 (a) and the temperature fluctuates in the range of 300 ~ 320 ℃, that is, the maximum density under the working condition can reach 4.191kg/m3( ρ 1) , up to 3.660kg/m3 at least( ρ 2) At this time, the indicated value of flow is:

w1 = ρ 1/ρ · W set = 4.191 ÷ 3.800 × 30=33.09t

W2 = ρ 2/ρ The change of resistance is converted into the change of voltage or current through Wheatstone bridge · w = 3.660 ÷ 3.800 × 30 = 28.89t

that is, the actual steam flow should change within the range of 28.89 ~ 33.09t/h. At this time, compared with the design value, the error:

σ Average = σ 1+ σ 2 = (w1-w set)/W Set + (w2-w set)/W set

= (28.89-30)/30 + (33.09-30)/30 = 6.6%

at this time, the error size is obvious, indicating that if the density is not accurate, it will affect the final mass flow indication and bring large deviation. Due to the fluctuation of the actual process conditions, we must add temperature and pressure compensation to make density correction at any time

the software of the vortex flowmeter has three density correction methods for the model:

1. Saturated steam: check the density table at temperature

2. superheated steam: check the density table for temperature and pressure

3. general gas mathematical model:

ρ = ρ Subject × (P+0.1013) × 106/(t+273.15)

the first two models are used for measuring steam. It directly implements the steam density table into the software. As long as the flowmeter measures the temperature and pressure of the medium, it can intelligently query the density, and automatically calculate it in combination with the measured volume flow, so as to indicate the mass flow. The third model is mainly for general gases. It can be seen from the formula that after measuring the temperature and pressure of the medium, it intelligently converts the density value under the working condition into the density value under the working condition according to the density value under the standard condition entered in advance, and then automatically calculates it in combination with the measured volume flow, so as to indicate the mass flow

III. performance index

this four in one vortex flowmeter has the following excellent performance:

1. High cost performance: the price of this vortex flowmeter is less than 1/3 higher than the old flowmeter, but its performance is far higher than the latter. 2. High measurement accuracy: on the one hand, compared with ordinary vortex flowmeter, when measuring steam or gas, due to the use of temperature and pressure compensation and the correction of density, its measurement accuracy is greatly improved, which can be clearly witnessed in the example; On the other hand, due to the function of the software, it can be calibrated in the vortex flowmeter

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