With the advent of the energy shortage era and the continuous improvement of people’s environmental awareness and high attention to safe production, the central heating business has developed rapidly. Heat energy becomes a commodity, and steam supply and charging are naturally the basis of heat companies. Steam metering is increasingly important. Guangzhou Hengyun Thermal Power Co., Ltd. is a central heating company for Guangzhou Economic and Technological Development Zone, High-tech Zone, Export Processing Zone and Bonded Zone. The heat source is the secondary steam of Guangzhou Hengyun Group's thermoelectric (A, B, C, D) plants. The three steam boilers built by the Thermal Power Company have a steam supply capacity of 300t/h, and the total length of the steam pipe network is about 50km. Steam users currently use more than 50 factories in the region. The characteristics of industrial users are as follows: (1) The steam consumption of each user varies greatly, the small one is less than 1t/h, the largest one is as high as 50t/h, and the corresponding steam flowmeter size is from DN10 to DN300mm; (2) continuous production is continuous The use of steam, especially some large users, will cause major economic losses if accidental stoppage of steam occurs; (3) The volume of flow changes will vary depending on the production period. Therefore, the basic requirements for the selection of steam metering instruments are: stability, reliability, wide range ratio, high accuracy, and low maintenance workload. In recent years, we have adopted a steam flow measurement system based on differential switched capacitor vortex flowmeters to achieve significant results and economic benefits. 1 differential switching capacitance vortex flowmeter 1.1 Basic working principle Vortex flowmeter is based on the "Carmen vortex street" principle to achieve flow measurement instrumentation. Non-streamlined columnar resistance components are installed in the fluid pipeline. Within a range of Reynolds number Re, vortex separation occurs on both sides of the Re, and two rows of alternate vortex columns are formed and flow downstream one after another, continuously. It is like a street made up of vortices. It is vividly called "Vortex Street." When the ratio of the vortex column width h to the distance L between two adjacent vortexes in the same column satisfies h/L=0.281, the vortex array geometry is stable and is called Karman vortex street. The vortex-separating resistance components are called vortex generators (see Figure 1). When the vortex generating body is a triangular cylinder, the relationship between the vortex separation frequency f (unit: Hz) and the average flow velocity v in the pipeline and the width d of the flow surface of the vortex generator is: Where: St is the Strouhal number (dimensionless), in a certain Re range, St is a constant; v1 is the average flow velocity of the fluid on both sides of the body, m / s; D is the pipe diameter, m; d / D is the ratio of the column width, that is, the ratio of the width of the vortex body to the diameter of the tube; v is the average flow rate before the vortex body, m/s. Due to the relationship between the average instantaneous flow qv[m3/h] in pipe, the pipe diameter D and the average flow velocity v: So f=qv·4St/[(1-1.25d/D)·(d/D)··D3]=K·qv (3) Where: K is the meter factor, L-1. K=4St/[(1-1.25d/D)·(d/D)··D3] (4) Experiments show that when the Reynolds number Re≥20000, the Strouhal number St is basically a constant value, such as the triangular column body St=0.16, on the other hand, for a given form of the specific instrument, its geometric dimensions D, d and The correlations have been determined so that the K value of the meter coefficient is also determined, regardless of fluid properties and composition. Therefore, the operating state volume flow qv is linearly proportional to the vortex frequency f: Qv=f/K (5) Therefore, as long as the vortex frequency is accurately detected, the volumetric flow rate can be known. This is the detection basis and basic working principle of the vortex flowmeter. How to accurately detect the vortex frequency has become the key to determine the overall performance of vortex flowmeters. The obvious difference between various vortex flowmeters is also the different detection technology (components) used, such as the thermal eddy from the early thermistor elements. Street flow meters, later market more stress-type piezoelectric crystal element vortex flowmeter, to the recent development of differential switched capacitor vortex flowmeter and so on. 1.2 Structure Theory Differential Switched Capacitor Vortex Flowmeter (hereinafter referred to as capacitive vortex street) uses a differential switched capacitor sensor (hereinafter referred to as DSC sensor) to detect the vortex frequency signal generated by Karmen vortex. The entire signal detection process is shown in Figure 2. The DSC sensor is an external cantilever type structure (Fig. 3), which is independent of the instrument vortex generator but is radially inserted into the center hole of the generator and communicates with the pressure guiding holes on both sides to detect the change of Karman vortex pressure. It is essentially a capacitive mechanical-electric converter. A vibrating tongue with a cylindrical shape and a sealed lower end and a thin tube wall serves as a common electrode of the capacitor. Inside, there are two arc-shaped electrodes attached to the lower end of the same support but insulated from each other. A tiny gap filled with air remains between the two electrodes and the inner wall of the common electrode vibration tongue, constituting two capacitors. The dielectric is air, and the capacitance thereof is inversely proportional to the distance between the electrode and the inner wall of the vibration tongue. When the vibration tongue is deformed or displaced, the two capacitors lose their balance and produce differential capacitance. When the vortex is generated, a slight pressure difference is formed on both sides, and the vortex pressure causes left and right flexible vibration of the vibration tongue. Since the two internal electrodes and their bearings are still fixed, a capacitance gap increases (the capacitance becomes smaller), At the same time, the other capacitance gap is reduced (the capacitance becomes larger) and vice versa, that is, the size of the two capacitances changes periodically due to the relative change in distance, and a differential signal is generated. This is also the only signal detected by the differential switched capacitor detection circuit. The differential signal is in the form of a sine wave and is processed by a pre-amplifier and is output in the form of a PFM (pulse frequency modulation) pulse signal. 2.3 Performance Features When the pipe is vibrated, the inertial force of the vibration acts on the tongue and the electrode at the same time, causing them to deform in the same direction. Because the vibration tongue matches the vibration characteristics and geometry of the two electrodes and the support, they are for external vibration. The response is synchronous, that is, the deformation variables are consistent, and there is no relative change in the two capacitance gaps, so no differential signal is generated. Therefore, the influence of strong mechanical vibration in any direction, frequency ≤ 500 Hz, and acceleration ≤ 1g can be eliminated. The use of modern engineering materials and methods extends its operating temperature range to -200°C to +400°C without any problems such as cracking or aging, and can withstand thermal shocks up to 100°C/s. The wall of the vibration tongue is very thin, so the sensitivity is very high, that is, the lower limit of the flow rate can be very low (the flow rate is 0.2m/s when the medium is water); because the vibration tongue is cylindrical stainless steel, the mechanical structure is strong and can withstand high The pressure of the flow vortex acts so that the upper limit of the flow rate is high (when the medium is water, the flow rate is 9m/s, and the medium is steam and gas, the flow rate is 75m/s). Due to the balance between the two extremes of flow rate, the range ratio is very wide (up to 40 : 1). 2 Temperature Pressure Compensation and Measurement System Vortex flowmeter is a speed type flowmeter. Its direct detection result is to give the volume flow under the operating state. Within its measurement range, this volume flow is not affected by the nature of the fluid temperature, pressure, density, viscosity, composition, etc. The change is therefore accurate and does not require any temperature or pressure compensation. However, when measuring steam flow in engineering, it is customary to use mass flow qm, and the unit is expressed in kg/h or t/h. Therefore, the above-mentioned volume flow qv needs to be introduced to convert the steam density Ï(p,T) parameters: Qm(V,p,T)=qv(V)·Ï(p,T) (6) The steam density Ï is directly affected by the steam working conditions such as temperature T, pressure p, etc. For example, the steam temperature is 190°C. When the pressure rises from 1.1MPa to 1.2MPa, the density increases by about 10%. A certain amount of additional error occurs when steam properties change or operating conditions fluctuate. Therefore, the temperature and pressure must be detected at the same time as the volume flow detection, that is, the real-time compensation of Ï by T, p to eliminate the additional error of the system, and the final required test result is the accuracy of the mass flow, and these compensation operations Work is done automatically in the flow totalizer. Therefore, when using a vortex flowmeter to measure steam flow, temperature and pressure compensation is necessary. In this measurement system, the volumetric flow qv, pressure p, and temperature T of the steam are converted to PFM pulses, 4 to 20 mADC, and resistance values ​​by the detection of a capacitive vortex flowmeter, pressure transmitter, and thermal resistance (Pt100), respectively. The signal and the input of the intelligent flow integrator at the scene are used for calculation and display. The reading is the compensated steam mass flow qm. In addition, because the steam users are dispersed and the distance is far away, it brings a lot of inconvenience to management. Therefore, we use the communication function (RS485) of the flow totalizer and the wireless transmission technology of the private network to form a network with the host computer (smart terminal). The measurement data of each measurement point is transmitted in real time to the heat company's host for monitoring and storage. The flow, pressure, temperature and other parameters of any user point can be inquired at any time in the host computer and the power-down situation can be achieved, thereby achieving remote monitoring and management. , Also become part of the enterprise information construction. In summary, the system for each measurement point is shown in Figure 4. 3 Application Examples Among the more than 50 measurement points, the following are several typical application examples. 3.1 Large flow, long pipelines The Tingyi and Topjin companies under the Tingyi Group use capacitive vortex street DN200 and DN250 respectively. The steam flow rate is between 20-30t/h. The steam pipeline is about 5km long. It is located at the end of the heat source point, sometimes the steam is overheated and saturated. Critical state, but the operation has been normal. Another user's malt company has a pipeline length of about 3km. In the design stage, due to lack of estimation of the steam consumption, only DN200 capacitive vortex street is used. However, when it is actually used, the steam consumption is much more than the original design consumption, which is about 30t/h. Sometimes it even reaches 40t/h or more. Excessive flow rate results in extremely high velocity of steam in the pipeline (the site can often hear the loud whine in the pipeline). This imposes higher requirements on the performance of the capacitive vortex street, but it has been operating well since its use. 3.2 Large pipes, small flow Lianzhong Stainless Steel Co., Ltd. builds DN250 steam pipes and uses DN200 capacitive vortex street according to the long-term steam planning. Although the initial steam consumption is only 5 to 10 t/h, the flow rate is relatively small compared to DN200 capacitive vortex street. It still falls within its measurement range, so it can theoretically measure accurately. To facilitate the understanding of the vortex street operation situation, the Thermal Power Company added a stress vortex flowmeter (using a piezoelectric crystal sensor) to the steam supply end of the same pipeline as a comparison, but due to the loss of small flow rate, poor stability, etc., The reading error is 5% to 30%. Obviously, this kind of data has lost its reference value. Later it was changed into a capacitive vortex street. As a result, the measurement data of the two capacitive vortex streets remained basically the same. 3.3 Large changes in traffic Most of steam users with small steam pipes have large variations in steam consumption, because these users are mainly concentrated in heating and stirring, baking and hotels and living areas, such as Huida Chemicals, Bingxin Paper, and Jabil. Districts, Xingang Port and other units, steam consumption due to different seasons (such as more in winter and less in summer) and different periods (such as cooking, bathing, and less in other time) are very different, if you install a traditional orifice flowmeter Requires 2 to 3 sets of switches, and 1 set of vortex can meet the requirements. Taking Bingxin Paper Company as an example, the steam consumption can reach 3t/h at full load, and the steam consumption is less than 0.1t/h when it is at the low peak of steam, so the flowmeter is required to be very wide. The range ratio, otherwise it will cause the loss of small flow, but from the field installed DN80 capacitive vortex street operation situation, there is almost no problem of the loss of small flow, thus avoiding both sides of the steam supply for measurement due to inaccurate Caused disputes. 3.4 Mechanical Vibration of Pipes There are always some vibration phenomena on the spot steam pipe. The mechanical vibration of the pipe caused by the operation of machinery and equipment or the driving of the vehicle is even more difficult to avoid. The most typical ones are the two steam users of Lijia Company and Green Arrow Xiangjiao Co., Ltd., and their factories are tight. By road, steam branch pipelines are inevitably going through major traffic routes. The former originally used the DN25 stress vortex street. As a result, it is often possible to accurately predict large-scale vehicles to pass through sudden changes in the detection signal of the site (reflected as instantaneous flow readings increase sharply), which indicates that it is easily disturbed by vibration; the latter installs DN80 capacitors. The vortex flowmeter, which had the same value as the large-scale vehicle, showed no change in its display value, which proved its good anti-vibration performance. 3.5 Interstitial Steam Honda Motor Co. installed a DN80 capacitive vortex flowmeter with a steam flow rate of about 5 t/h. Because of intermittent use, frequent shutdowns and use of steam are required within one day, which will obviously cause frequent impacts on vortex streets, and the resulting disturbances will increase. Therefore, it is required that the vortex street used has a high resistance to flow. Impact, thermal shock, and resistance to vibration. From the field observation and measurement data analysis, the capacitive vortex street can fully adapt to this working condition, and the instrument operation has been normal. 4 Precautions Vortex application effect is a comprehensive reflection of product performance, selection and installation. An analysis of several issues that are apt to be neglected in the application is made to draw attention and attention. (1) In the design and selection phase, the size of the vortex orifice must be selected according to the actual process parameters of the steam, and the pipe size is only used as a reference. Experience has shown that there are often large margins in the design of pipelines, and the size of steam pipelines is generally larger. If the caliber of vortex street is selected as the original pipeline, it will often result in a decrease in accuracy (loss of small flows) and even inability to measure the consequences; The vortex caliber selected by the parameters often has a minor file. At this time, installing a vortex street requires a concentric shrinking tube. (2) The installation must be strictly based on the construction specifications. For example, the length of the straight pipe before and after the pipe must meet the technical requirements of the vortex flowmeter. The center line of the vortex instrument body is coaxial with the center line of the straight pipe. The gasket cannot extend into the pipe. This can prevent partial flow area from being covered. Distortion of the flow velocity distribution, flow field interference caused by the measurement error; steam pipe measurement section to ensure that there is no condensate; for the newly installed process pipeline, in the purge must first remove the vortex, cleaned and then can be loaded back. (3) The electrical wiring must be standardized, and the site's waterproof and anti-high temperature measures must be in place. For example, the optimal height of the insulation layer at the meter body should not be more than half that of the head bracket, so that the holder has a heat-insulating part (lower half) to prevent heat radiation, and no heat-insulating part (upper half) can prevent Heat is transferred to the meter head; when the pipe is horizontal, the meter head should be placed as horizontally as possible to minimize the effect of hot air on the preamplifier electronics. 5 Concluding remarks The core of this measurement system is the differential switched capacitor vortex flowmeter, which plays a decisive role in the success of the measurement. Several years of practical application have shown that this type of capacitive vortex street is stable and reliable, and has strong anti-vibration interference. There is no sensitivity of the stress vortex flowmeter to vibration, and its measurement data is credible and fair. There is also no such problem as stress vortex damage due to lightning strikes; the structure is strong, there are no moving parts, there are no wear and blockage problems, no maintenance, low cost of use; high sensitivity, wide range ratio, reduced loss of small flow The instrument has high precision (up to 0.5%), and it will hardly change in long-term operation, and the improvement of precision has obvious economic value for steam metering. Its successful application provides a very effective way to solve the problem of accurate measurement of high-temperature steam in heat companies. Large Flat Washers, Fender Washers, Metric Flat Washers, Aluminum Flat Washers, Stainless Steel Flat Washers Ningbo Brightfast Machinery Industry Trade Co.,Ltd , https://www.brightfastener.com
f=St·v1/d=St·v/[(1-1.25d/D)·d] (1) 
