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Solutions to problems that arise during gas flow measurement! I hope everyone can add more! 1) Characteristics of gas flow rate measurement: a. Low hydrostatic pressure and low flow velocity, allowing for small pressure losses; generally, it is not feasible to increase the flow velocity by reducing the pipe diameter. b. The fluid has a high humidity; some of the substances being measured also contain a small amount of water, resulting in stratified flow at the bottom of the pipeline. c. Some measurement subjects have a high hydrogen content and low fluid density, resulting in weak signals when measured using vortex flow meters. d. The gas produced by gas generators, coke ovens, etc., generally contains viscous substances such as tar, and some also contain a certain amount of dust. e. When the measurement point is located at the compressor outlet, there is a certain degree of flow pulsation. f. The fluid is a flammable and explosive substance, so the instruments required to be explosion-proof. g. There are various pipe diameters, from small to large. h. The difference between the minimum and maximum flow rates is significant. i. Systems used for trade settlement, requiring high measurement accuracy ； As systems for general monitoring and process control, the accuracy requirements are relatively lower. (2) **Main contents of the standard – 2000** The Quality and Technical Supervision Bureau issued GB/T18215.1 \"Measurement of flow rate in main pipelines for urban artificial gas\", Part 1, which specifies the method using standard orifice throttling devices to address the technical issues related to gas flow measurement. Among these specifications: ① Requirements for the fluid: \"It should be a uniform and single-phase (or considered single-phase) fluid.\" ②Gas is washed during the purification process, so its moisture content is generally at a saturated level, with a relative humidity of 100%. ③The accuracy of measurement systems used for trade settlement should generally be better than grade 2.5. The basic error limit is expressed as a percentage of the indicated value. ④Gas flow rate is defined as the dry fraction in the wet gas. ⑤The measurement results are expressed as volumetric flow rate and converted to standard conditions. In addition to the commonly adopted values of 101.325 kPa and 20°C for defining the standard state, the traditions of the gas industry are also taken into account; other temperatures, pressures, and humidity levels agreed upon by the supply and demand parties can also be used. ⑥The throttling device adopts a multi-tube parallel configuration. ⑦In the presence of flow pulsations, the following measures have been proposed for measuring the average value. a. Adopt attenuation measures on the pipeline by installing filters (composed of containers and pipe resistances). b. Keep the instrument sensing elements as far away as possible from the pulsation source. c. Use the largest possible β and Δp to reduce the pipe diameter at the measurement point. d. Piping and instrument supports are firmly installed. e. The resistances of the two differential pressure tapping pipes are symmetrical. (2) Types and usage of instruments: There are many types of flowmeters that can be used to measure gas flow, but it is nearly impossible to find an ideal instrument for measuring coal gas flow, which is mainly due to the characteristics of coal gas. Due to the presence of viscous substances such as tar, rotary flow meters are difficult to use. Due to its low density and low flow velocity, the vortex flow meter is difficult to use. Thermal mass flow meters are also not ideal due to high moisture content and changes in gas composition. In the end, it is still the differential pressure flow meter, which has been in use for decades, that plays the leading role. ① Orifice plate differential pressure flow meter a. Replaceable orifice plate throttling device. International standard GB/T18215－2000 specifies the standard orifice plate. If the fluid at the measurement point is dirty, a throttling device with replaceable orifice plates should be used. In this way, the throttle element can be cleaned, inspected, and replaced without stopping the flow of air. The typical structure of a throttling device with replaceable orifice plates is shown in Figure 3.18. b. Annular orifice plate. The annular orifice plate is a special type of orifice plate designed specifically for measuring the flow rate of dirty fluids. Its opening is a part of a circle (an arc), and the diameter of this circle is 98% of the inner diameter of the pipe. The arc portion of the opening should be precisely positioned to be concentric with the pipe, as shown in Figure 3.19. Figure 3.18 shows the typical structure of a throttling device with interchangeable orifice plates. Figure 3.19 depicts the structure of an elliptical orifice plate. When the medium being measured is a wet gas and the pipeline is arranged horizontally, there may be a small amount of stratified liquid at the bottom of the pipeline. In such cases, using an elliptical orifice plate allows the liquid to flow smoothly through the throttling element via the elliptical portion in the lower half, whereas a standard orifice plate would prevent the liquid from passing through, leading to accumulation and affecting measurement accuracy. By the same logic, when the gas under test contains dust, and since the density of dust is much higher than that of the gas, some of the particles tend to settle near the bottom of the pipeline and be carried away by the airflow. By using an elliptical orifice plate, these particles can pass through the throttling element smoothly, without accumulating in front of it as they do with a standard orifice plate. In the metallurgical industry, there are many applications for measuring gas flow rates, and since it is common for gas to contain dust and water droplets, elliptical orifice plates are widely used. c. Parallel multi-tube configuration. The parallel configuration of multiple tubes (two, three, or four) serves three purposes; one of them is to expand the range of the measurement system. Since the gas flow velocity in pipelines is generally quite low, a orifice meter can achieve a range ratio of 3:1. By using a large-diameter orifice plate in combination with a small-diameter one, the range can be increased to 10:1. The second is to enable the disassembly and cleaning of the throttling device without shutting down the main pipeline, thereby avoiding expensive throttling devices with replaceable orifice plates. Of course, shut-off valves must be installed upstream and downstream of the throttle device. The third is to address the flow measurement issue for pipes with DN>1000. For example, 4 DN1000 throttling devices can be used in parallel to address the flow measurement for the DN2000 main pipe. The disadvantage of the parallel multi-tube configuration is that the number of devices and the investment increase exponentially. d. Gas pipe drainage and anti-freezing. In horizontally installed gas pipelines, water flow at the bottom of the pipeline is sometimes observed; therefore, a drainage device must be installed before the throttling device. A simple and reliable method is to use a water seal for automatic drainage, as shown in Figure 3.20. The relationship between the liquid level difference and pressure in the figure is as follows. Figure 3.20 Schematic diagram of water seal drainage: h = p/(gρ) (3.37), where h is the liquid level difference, in meters ； p — gas pressure, Pa ； g — gravitational acceleration, m/s2 ； ρ —— density of water, kg/m3. In cold seasons, water in drainage systems and even in gas pipes laid on the ground can freeze. To prevent frost damage, anti-freezing measures should be taken. ②Volumetric flow meter with differential pressure tube. Standard orifice differential pressure flowmeters play an extremely important role in measuring gas flow, and they have a long history of use. Since this method relies on abundant experimental data and its design and processing have been standardized, as long as it is designed, processed, installed, inspected, and used in accordance with the standards, the specified accuracy can be achieved without the need for actual flow calibration; hence, it is very convenient and has been widely adopted. However, when the pipe diameter is large, the cost of a set of replaceable orifice plate throttling devices is quite high; therefore, if the measurement data are only used for process monitoring and high precision is not required, an average velocity tube differential pressure flow meter can be chosen. The key to the successful use of the venturi differential pressure flow meter in gas flow measurement is to ensure that the pressure taps are not blocked by water droplets. Since the shut-off valves installed in standardized constant-velocity tubes are mostly needle valves with a small diameter, and the water vapor in the fluid condenses into water droplets, these droplets can easily block the flow path if the needle valves are not properly designed or if the slope of the pressure guiding tubes is not appropriate. The differential pressure signal generated by a differential pressure type venturi is generally very small. When the fluid is air at normal temperature and pressure, if the flow velocity is 10 m/s, only a differential pressure of 62.5 Pa can be achieved. In this way, a single drop of water that blocks the differential pressure transmission channel is sufficient to completely cancel out this differential pressure. Some manufacturers have replaced positive and negative pressure cut-off valves with straight-through gate valves with larger diameters, thereby creating the conditions for the reliable operation of instruments. (4) Example of calculations for orifice plate design (with the throttling element being a standard orifice plate) (5) Dust accumulation on the inside of pipes and its impact on measurements ① Dust accumulation is a common phenomenon. a. A steel plant in Shanghai used orifice plate flow meters to measure the gas flow at the outlet of its gas generators; due to the high dust content in the gas, a layer of deposits formed on the inner walls of the pipes over several years. This scale was as hard as asphalt-covered surfaces and difficult to remove, having been formed over time by coal tar and dust present in the gas. b. Another steel plant in Shanghai used differential pressure flow meters to measure gas flow. Concerned that dust accumulation on the orifice plates could affect measurement accuracy, a venturi tube was chosen as the throttling device. After more than half a year of use, it was observed that the flow readings were gradually becoming too high. Therefore, during a shutdown for maintenance, the venturi tube was disassembled and inspected; a layer of dust-containing tar had accumulated on its inner wall, even in the throat section where the flow velocity was highest. However, during the annual parking maintenance, it can be used again after being cleaned with a solution. c. Chongqing Iron and Steel Group Company used elliptical orifice plates to measure the flow rate of blast furnace gas. After several years of use, when the throttling device was removed for cleaning, it was found that 1/8 to 1/6 of the height of the elliptical part of the orifice plate was covered with deposits. d. A chemical plant in Xuzhou used an average velocity tube differential pressure flow meter to measure the flow rate in the outlet pipe of the gas generator (DN700). After using it for some time, it was observed that the flow rate reading gradually increased, being a few percent higher than the value calculated based on material balance. Upon inspection, it was found that a layer of sand sludge of varying thickness had formed on the inner wall of the pipe; the inner wall in the lower part of the horizontal pipe had a thicker layer, about 30 mm thick, while the inner wall in the upper part of the pipe had a thinner layer, about 10 mm thick. ②One of the treatment methods. Removing sediment or renewing the pipes can eliminate the sediment accumulated in the pipe sections ahead of and behind the flow meter, as well as on the surfaces of the throttling elements, without damaging the instrument; this, of course, restores the instrument’s proper measurement accuracy. However, sediments are often hard and tough, making them difficult to remove; therefore, if there is an opportunity to stop the operation, it is certainly a good idea to replace the throttle element 30D and the pipe section 15D behind it. ③Method two for handling. Correct the errors introduced by the sediment. a. Standard orifice plate differential pressure flow meter. The deposition of tar and dust in gas on the surface of standard orifice plates and the inner walls of pipes can occur in two ways. The first case involves a measurement subject in which the dust in the gas has been thoroughly washed and filtered, with only a thin layer of tar deposited on the end face of the orifice plate and the inner wall of the pipe. The second case involves measurement subjects with a high amount of dust in the gas, where a layer of \"asphalt sand\" several centimeters thick has formed on the inner wall of the pipes. In the former case, once the inner wall of the steel pipe is contaminated with tar, it exerts a certain adhesive effect on the flowing gas. There are no standard specifications regarding the extent of error introduced by this effect, making it difficult to estimate it; however, the impact is certainly minimal and can be disregarded. A layer of tar on the inner wall of the pipe may be up to 2 mm thick. However, since the diameter of gas pipelines is generally large, for example 1000 mm in nominal diameter, the impact on measurement is also minimal. In the latter case, the impact is somewhat greater; it is through an increase in the diameter ratio β that the discharge coefficient C changes, as well as C/, which in turn causes a corresponding change in the measured flow rate. For example, consider a DN1000 standard orifice plate with a β value of 0.7. At a Reynolds number of ReD equal to 2,105, the value of C is given by C = 0.5959 + 0.0312β²·1⁻⁰·¹⁸⁴₀β⁸ + 0.0029β²·⁵ = 0.5959 + 0.0312·2·1⁻⁰·¹⁸⁴₀·0·78 + 0.0029·0·7²·⁵ = 0.6040. When there is a 20mm thick layer of sediment evenly deposited on the inner wall of the pipe, β increases to 0.7365. Keeping ReD at 2,105, the value of C calculated using the same formula is 0.6009. Thus, the flow coefficient before scaling is 0.7153, while the flow coefficient after scaling is 0.6009; the change in the flow coefficient due to scaling is therefore –3.2%R. b. Venturi tube differential pressure flow meter. The deposition on the inner walls of the straight sections before and after the venturi tube can be considered to have no effect on the measurement results, as its discharge coefficient can be regarded as independent of the diameter ratio; however, the error caused by scaling in the throat is greater than that caused by a standard orifice plate. For example, in a venturi with a DN of 100 mm and a throat diameter of 700 mm, when scaling accumulates 5 mm on the inner wall of the throat, its flow cross-sectional area is reduced by approximately 2/70 compared to the original value; as a result, the flow rate is reduced by about 2.86% R. c. Orifice plate differential pressure flow meter. The effects of deposits on the inner walls of the straight sections before and after the elliptical orifice plate on flow measurement mainly consist of two aspects: one is the effect of the change in the ratio m, which is the area of the orifice opening to the cross-sectional area of the pipe, on flow measurement; the other is the effect of the reduced effective area of the elliptical orifice on flow measurement. The effect of the former is similar to that of a standard orifice plate. However, as the pipe cross-sectional area decreases, the effective area of the notched hole also decreases slightly. Therefore, m does not change much. For example, consider a DN1000 elliptical orifice plate with m equal to 0.49. When a 20mm thick layer of sediment accumulates uniformly on the inner wall of the pipe, the cross-sectional area of the pipe decreases to 0.7238 m2, while the area of the elliptical orifice decreases to approximately 0.3574 m2 (assuming that the arc of the elliptical orifice is tangent to the arc of the pipe). Therefore, β remains 0.49. The latter has a greater impact, as the open area is 0.3848 m2 in the absence of sediment, while when the sediment thickness is 20 mm, the effective open area is 0.3547 m2, which is approximately 92.18% of the value in the absence of sediment; therefore, the instrument reading is about 8.5% R higher. In actual calculations, since the radius of the notched hole is 0.98 times the pipe radius, and only 10 mm of the 20-mm-thick deposit layer blocks the notched hole, the actual impact is only half of 8.5%. d. Vortex tube differential pressure flow meter: A vortex tube flow meter measures the average flow velocity within the pipeline, multiplies it by the cross-sectional area of the flow channel, and accounts for the obstruction effects caused by the part of the vortex tube that is inserted into the pipeline. The effect of deposits on the inner walls of the straight sections before and after the venturi tube on flow measurement can, by ignoring the minor changes in the blockage coefficient, be simply regarded as the effect of a reduced flow cross-sectional area on the flow reading. For example, in a Pitot tube with an inner diameter of 1000 mm, when a deposit with a thickness of 20 mm forms uniformly on the inner wall of the tube, the flow cross-sectional area decreases from 0.7854 m2 to 0.7238 m2. With the actual flow rate remaining unchanged, the flow velocity increases; as a result, the value displayed by the instrument increases by (0.7854 – 0.7238) / 0.7238 = 8.5% R. The analyses and calculations above are idealized; the actual situation is much more complex. The thickness of the deposits on the inner wall of the pipe cannot be uniform – it is always thinner at the top and thicker at the bottom. But the method can be used. ④Predict the impact on sediments. Since it takes a long time to disassemble, inspect, and repair large-diameter flowmeters, if severe sediment accumulation is detected during the first disassembly and inspection and not removed, the impact on flow rate can be calculated based on the thickness of the sediment. If the fluid conditions remain unchanged, it is inevitable that the sediment layer will continue to thicken over time, and accordingly the impact of flow on these conditions will increase. Thus, the deposition rate can be calculated from the thickness of the sediment layer and the time taken for deposition as measured during inspections, and assuming that deposition continues at the same rate in the future, predictions can be made regarding the impact of flow in those future periods. This post was last edited by zlky2005 on 2009-3-19 16:48.]