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The accuracy of the flow meter and the installation of the straight pipe section

In the past decade or two, in the context of globalization of trade, many substances in the economic field have been accounted for by the measurement of flow meters. Buyers and sellers are not willing to bear the economic losses caused by the error of flow measurement. For example, if an enterprise uses 10,000 tons of water per day and uses a flow meter error of ±2%, it will probably pay an additional 150,000 yuan per year. If it is expensive, such as natural gas or oil, the loss will be more prominent. Therefore, there is an urgent need to improve the accuracy of the flow meter. For industrial control systems, in the past, flow meters were considered to have good repeatability and high accuracy. Now, it seems that this view can no longer meet the new requirements. For example, in thermal power plants, because the air volume cannot be accurately measured, in order to ensure that the boiler furnace is not incompletely burned due to lack of oxygen, the air is polluted by the environmental protection department. The gas supply causes the combustion efficiency to drop. Every 1% of the peroxygen supply causes 5% of the excess air to be heated, resulting in waste of fuel and fan power. From the perspective of energy saving and consumption reduction, the flow meter is also urgently needed. Accuracy. 1 Installation of straight pipe length is an important factor that is easily overlooked. The manufacturer of the flow meter has indicated the accuracy in the technical documents. It should be determined after the laboratory has been rigorously calibrated, but under the site conditions, Most flow meters are difficult to achieve this accuracy. 1.1 The effect of the length of the straight pipe on the accuracy For most flow meters (except for volumetric and scientific, but the diameter does not exceed 0.25m), the flow velocity distribution has a significant impact on accuracy. International and domestic industry standards clearly stipulate that there should be 30D in front of the flow meter and 5D straight pipe length (D is the inner diameter of the pipe). Only in this way, the flow velocity distribution in the pipe can reach full development turbulence and have a certain distribution. Regularity, using the flow coefficient calibrated in the laboratory, can be guaranteed to achieve the accuracy stated by the manufacturer [1]. However, the actual installed site, from the process point of view, there will be a variety of various resistance parts (elbows, valves, reducers, manifolds?), its outlet flow velocity distribution is also unpredictable and very complicated. From the point of saving the site, it is also impossible to arrange the length of such a long straight pipe required for the flow meter. Therefore, the accuracy achieved by flow meters in field applications will be much lower than the data provided by the manufacturer. 1.2 The length of the straight pipe section is insufficient. The problem of insufficient length of the straight pipe section has long been neglected. The reason is that even if the length of the straight pipe section in the field does not reach the length required by the regulations, there will still be output, and the instrumentation of the first line will come. It is said that the instrument works as if it is still "normal", but it is far from the necessary accuracy. Just like a gun without a crosshair, you can still shoot, but the bullet can't hit the bull's eye. This problem is especially prominent for instruments that use the measured point velocity to calculate flow (such as double venturi, plug-in vortex, turbine, and hot type). The national standard ISO07145 [2] of this type of instrument stipulates that the front straight pipe section should have (30). ～50) The length of D is likely to have an accuracy of ±3%. Obviously, it is very difficult to meet at the scene. 2 flow regulators (flowconditioners) 2.1 flow regulators can improve the flow rate distribution to solve the problem of insufficient length of the straight pipe on the site, but also to maintain the flow meter with high accuracy, the International Organization for Standardization Closed Pipe Flow Professional Committee (ISOTC30) has been recommended for many years The various flow regulators [3] shown in Figure 1 are used, such as tube bundles (AGA/ASME, ISO, AGA, ASME??), plate holes (Mitsubishi), and combined (Zanker, Sprenkle). Among them, the combined effect is good, the pressure loss is several times higher than the first two; and the plate hole structure is simple, easy to process, install, low pressure loss, and great development potential. The above three categories have their own strengths, and the selection needs to be adapted to local conditions. The installation of the flow regulator can maintain the flow meter with high accuracy when the length of the straight pipe section is not up to the specification. But why has it not been promoted and applied for a long time? 2.2 Advantages and disadvantages of using the flow regulator The flow regulator not only increases the cost and maintenance workload, but also requires some straight pipe lengths on site. As shown in Figure 1, the bundle structure itself requires (2 ~ 4) D length (D tube inner diameter), and Before and after installation, it is required to have the front (4 ~ 5) D, the rear (between the adjuster and the flow meter) (3 ~ 4) D, a total of about 10D, which is unrealistic and difficult to achieve for the length of the straight pipe section that has been stretched. Secondly, there are the following shortcomings, such as: 1 increased cost, the processing cost of a composite flow regulator is no less than a throttling device; 2 increases the installation and maintenance workload, if the fluid contains dust, or solid Phases and condensates, which are not cleaned in time for long-term use, will be deposited below the level adjuster. The result will be an asymmetrical consequence of the velocity distribution, which will not add to the snake's foot. This will cause additional pressure loss. In view of the above various reasons, although it has been recommended by ISOTC for many years, it has not been widely recognized by the engineering community. However, the rectification effect it has obtained has inspired the people with lofty ideals in the industry and provided a basis for the development of new flow meters in the future. Class (a) is used to eliminate vortices, (b) is used to eliminate vortices and to improve moderate velocity distribution distortion, and (c) is used to eliminate vortices and improve severe velocity distribution distortion. The use of a flow regulator increases the resistance, and the pressure loss varies with the distortion of the speed profile, with (a) being the smallest and (c) being the largest. 3 Porous Balanced Throttle Device 3.1 Basic Principles The advantages and disadvantages of using a flow regulator have been described earlier in this paper. Because it requires a certain length of straight pipe section, it restricts its popularization and application. In the long-term practice, people found that reducing the distance between the flow regulator and the throttle, has little effect on the rectification effect. Finally, the rectifier and the throttle are combined into one, that is, the rectifier is directly used as a section. The new throttling device of the flow piece not only saves costs, but also solves the length of the straight pipe that has been very cramped. Inspired by this concept, in 2002, Rosemount Company of the United States first introduced a four-hole orifice plate called Condi2tioningOrifice (Fig. 2a) according to a four-tube rectifier similar to AGA/ASME. It is said that the straight pipe length can be shortened to 2D, still maintains high accuracy. 3.2 Porous Balanced Flow Device Based on this, in 2004, A+Flowtek Company of the United States introduced a 17-hole porous orifice plate (Fig. 2b) [4]. The 17 holes are divided into three groups, and one hole in the center has the largest pore diameter. The remaining 16 wells are divided into two groups, each of which has 8 holes, the center of which is divided into 2 different diameters, the middle 8 holes are second, and the outer diameter is the smallest. A+Flowtek calls it BalancedFlowmeter, translated as balanced flowmeter. According to its manual, the relevant technical parameters are as follows: accuracy ± 0.15%; straight pipe requirements: first 0.5D, after 0.5D; range ratio: 10:1; pressure Loss: Δω is 30%. 3.3 Description of the length of the straight pipe before the above Whether these technical parameters are as perfect as the manufacturer's propaganda is worthy of discussion. In particular, the expression of the length of the straight pipe required is generally determined in the industry as the distance from the front end face of the throttle member (ie, the porous orifice plate) to the outlet of the upstream resistance member is the length of the front straight pipe section, and the porous balance flowmeter is made of a porous orifice plate. Combined with the short tube to supply the whole. It refers to the upstream straight pipe section 0.5D refers to the distance between the inlet flange of the balance meter and the resistance member. The length of the short pipe of the balance flowmeter is (2~10D), which is to occupy the straight pipe section on the spot. Intentionally or unintentionally, the effect is to mislead the user, not to seek truth from facts. 4 Porous rectifying throttling device Based on the development of technology and market demand, Tianjin Runtai Automation Instrument Co., Ltd. has developed a porous rectifying throttling device and carried out the outflow coefficient, the length of the front straight pipe section, etc. in a third-party laboratory. Test and test data show that the porous rectifying throttling device developed by Tianjin Runtai Automation Instrument Co., Ltd. has better rectification effect in the case of short straight pipe sections, which is worthy of promotion. Some data and related explanations are now divided. It is as follows: (1) Name: The throttling device of the device is a porous orifice plate arranged according to a certain regularity. Its function has a rectifying function in addition to throttling, so it is determined as a porous rectifying throttling device according to its main function. . (2) Experimental equipment: For the water flow laboratory of aerospace department, the experimental equipment flow uncertainty is 0.05%, the differential pressure transmitter accuracy is 0.075%, the test medium is water, and the test tube diameter is DN100. (3) Selection of β value: It is suggested that the β value is between 0.50 and 0.65, the β value is too large and the intensity is weakened, and the β value is too large. (4) Comparison of three structural properties Under the condition that the length of the front straight pipe section is about 30D, the repeatability, uncertainty and linearity of the outflow coefficient of the three types of porous throttling devices are compared. The data shows that A The technical indicators of the type (Fig. 3a) are better than the B type (Fig. 2b) and C type (Fig. 3b). Therefore, it is important to test the length of the different front straight tubes for the A-type porous rectifying throttle device. The relevant data is shown in Table 1. (5) Length of front straight pipe section: The A-type rectifying throttle device is tested for three straight pipe lengths (30D, 5D, 2D), and the resistance component is 90° elbow. The test data is shown in Table 2. From the data in Table 2, the relative error between the outflow coefficient and the outflow coefficient of the reference (30D) can be controlled within ±1% when the A-type throttle is only 2D in the front straight pipe section; while the current straight pipe length reaches 5D. The relative error between the reference and the outflow coefficient can be less than ±0.3%, indicating that the rectification effect is better, and it is suitable for the field where the straight pipe section is not long and requires high accuracy. (6) Permanent pressure loss: due to the fact that the fluid does not form a large vortex after passing through the porous throttling device, it causes a large permanent pressure loss. Under the same β value, it should be smaller than the orifice plate, and larger than the venturi, this prediction is still To be confirmed by experiments. 5 Summary (1) New type of throttling device that has emerged in the past 10 years Since the trade settlement, industrial metering and control, there is an urgent need for a flow meter that can not maintain the high accuracy of the straight pipe length. The inner cone flowmeter has been in the limelight for a while, but the disorderly rush to promote, regardless of the irrational selection of occasions, has caused serious accidents, should learn lessons, but should not be completely negated, the current domestic and international introduction of the porous throttling device, The structural strength is superior to the inner cone. The test of the data also shows that the superior performance is worthy of being gradually applied. (2) Agency and production imitation and innovation It should be acknowledged that there is still a gap between China and the industry in the field of industrial automation. Since the reform and opening up, many domestic products have introduced foreign advanced products and advanced technologies to China, and indeed have played a role in promoting technological progress. However, foreign products are too expensive in the case of monopoly. In order to break this situation, it is not satisfied with the product but should be based on self-production, and production can not be limited to imitation (the inner cone flowmeter should be a lesson). Seriously studying the advanced technology of foreign countries, and through the problems in the application, we will find that many foreign products are not so perfect, they are constantly improving and perfecting. Why are we still satisfied with counterfeiting products that others have long since fallen behind? On the basis of earnest study, we can start to improve and create new products with independent intellectual property rights in China.

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