The measurement of coal gas, natural gas, and steam (hereinafter referred to as the three gases) has been a thorny issue for many years. Due to inadequate measurement methods and management problems, not only do measurement disputes frequently occur, but the state also needs to allocate huge sums of money annually for "legal" subsidies to cover the supply and sales gaps in coal gas and natural gas. With the development of environmental protection and the improvement of people's living standards, the amount of gas used by industrial and urban residents is increasing daily. Therefore, scientifically solving the measurement problems of coal gas, natural gas, and heating steam is becoming increasingly important. I. Current Status of the Three Energy Gases The coal gas used in China's urban areas is mainly coke oven gas, followed by oil-based coal gas. In the metallurgical industry, in addition to coke oven gas, there is blast furnace gas; in the machinery industry, there is producer gas; and in the chemical industry, there is water gas. The main components of coke oven gas and oil-based coal gas are hydrogen, methane, and carbon monoxide. Coal gas contains naphthalene, ammonium hydrates, and tar. These components easily separate from the gas and condense and accumulate on the inner wall of the pipe and on the surface of other components, rendering many existing flow measurement instruments inoperable. It can be said that the problem of gas production, transportation, and industrial gas metering has remained unsolved. Previously, the only barely usable metering method was the orifice plate, but due to severe "fouling" problems, frequent decontamination was necessary, making quantitative assessment of metering accuracy difficult. Currently, many gas production and consumption units simply do not meter coke oven gas. Natural gas includes gas well gas and associated natural gas from oil and gas, serving as both a high-quality fuel and a chemical raw material. Natural gas metering involves two scenarios: metering of unprocessed (impure) natural gas and metering of processed (clean) natural gas. Unprocessed natural gas often contains liquids, solid particles, and other precipitates that may contaminate measuring instruments, causing significant metering difficulties. Therefore, a flow meter with satisfactory metering performance has remained elusive; orifice plates and vortex flow meters are currently the main metering methods in use. Metering clean natural gas should not be particularly difficult, but due to inadequate processing technology and unclean pipelines, the natural gas delivered to users during the gathering and transportation process is sometimes unclean. Currently, orifice plates are the primary metering method for clean natural gas in my country. Vortex flow meters and vortex shedding flow meters are only used for smaller pipe diameters. Turbine and ultrasonic flow meters are still in the initial trial stage. Steam, including saturated steam and superheated steam, is widely used. The difficulty in metering saturated steam lies in two aspects: firstly, the correction for steam dryness and the potential measurement problems of two-phase steam-water flow are difficult to solve; secondly, saturated steam users are mostly small and medium-sized enterprises with large fluctuations in steam consumption, making it difficult for metering instruments to adapt. These two reasons, coupled with the inherent limitations of the instruments, result in excessive measurement errors. Metering superheated steam is less troublesome than metering saturated steam. Current metering methods for steam, in addition to traditional orifice plates and nozzles, include vortex flow meters and anuba flow meters. For steam with higher temperatures and larger pipe diameters, orifice plates and nozzles are still the main traditional metering methods. II. Drawbacks of Orifice Plate Metering From the current status of energy gas metering in China, orifice plates are still widely used as a metering method for coal gas, natural gas, and steam pipelines, as well as industrial gas (steam), especially in applications with diameters of DN200mm and above. The drawbacks of orifice plate metering are a long-standing consensus within the industry. To draw wider attention and promote the use of new measurement technologies to improve the current metering situation for these three gases, it is necessary to revisit this long-standing issue. 1. Orifice plates inevitably suffer from "sharp edge erosion" and "fouling." The measurement principle dictates that from the day of installation, the actual outflow coefficient of the orifice plate increases daily (the instrument reading becomes increasingly lower). In applications with dirty fluids and high flow velocities, this change is often quite significant. A special investigation revealed that a newly manufactured orifice plate meeting standard requirements, after being disassembled and inspected after a period of use, had its sharp orifice edges dulled, contrary to the standard requirement. The orifice plate end area, which has high requirements for flatness and surface roughness in the standard, accumulated many contaminants. When used to measure high-humidity gases and dirty fluids, liquid and solid substances accumulated in the dead water zone near the orifice plate. Since the orifice plate discharge coefficient is greatly affected by the sharpness of the inlet edge and "contamination," the discharge coefficient, which was originally ±0.6% at installation, suddenly increased by several percent. According to reports in foreign professional journals, in situations with dirty fluids and high flow velocities, it is not uncommon for the discharge coefficient to increase by more than ten percent due to "edge erosion" and "contamination." 2. The orifice plate range ratio is too small, only 3-4 in the mid-Reynolds number region, while the actual fluctuation range of the user's gas (steam) consumption is often much larger. The metering characteristics of an orifice plate dictate that its actual discharge coefficient varies with the Reynolds number (Re) of the measured fluid. This is especially true at a Reynolds number of 5 × 10³. III. Discussion on Improving Three-Gas Measurement Methods Given the serious drawbacks of orifice plate metering, with the development of flow measurement technology, several new or improved flow measurement instruments have entered the three-gas measurement field in the past decade or so. However, due to the special nature of three-gas measurement conditions and the limitations of these instruments in measurement and performance, although they have achieved good measurement results in some situations, the situation of "orifice plates playing the leading role" in the three-gas measurement field has not changed. Last year, Dalian Sonica Company launched an internal Venturi tube flow meter. Its measurement principle and application practice show that this new generation of differential pressure flow meter completely overcomes the various drawbacks of orifice plate metering and has good adaptability to the three-gas measurement conditions. The following is a brief analysis of several measurement methods that may replace orifice plates for three-gas measurement. 1. Thermal (Wire) Gas Mass Flow Meter: Thermostatic gas mass flow meters can, in principle, be used for the flow measurement of various gases, including coal gas and natural gas. However, in practice, they are only suitable for measuring clean gases with stable and dry operating conditions, and are particularly unsuitable for measuring dirty and high-humidity gases. First, the measurement principle dictates that differences in the thermal properties of the measured medium have a significant impact on the measurement results. Therefore, the instrument must be calibrated using the actual measured medium. In practical use, additional errors caused by variations in the composition of the measured medium are unavoidable and difficult to quantify. Second, it is unsuitable for measuring dirty gases; the water content in the gas cannot be too high, and liquids must absolutely not be present in the medium. If a pasty substance, especially water droplets or other condensates, is present in the measured airflow, it could clog the probe's sensitive element. The sudden change in the cooling effect of the medium on the sensitive element will cause incalculable measurement errors, and may even render the instrument malfunctioning. Third, the sensor directly measures the gas velocity at a specific point or points within the pipe. Converting this to the flow rate of the fluid within the pipe depends on the gas velocity distribution within the pipe—which depends on the Reynolds number and the pipe wall roughness. If a fixed calculation factor is used, errors arising from this are also unavoidable. 2. Anuba Flow Meter: The Anuba (also known as an averaging pitot tube or other names) flow meter operates on the same principle as the Pitot tube, achieving simultaneous multi-point measurement. It features a simple structure, convenient installation and maintenance, and low pressure loss. It is relatively inexpensive for large-diameter pipe measurements and is used not only for water but also for gases and steam. However, the Anuba's measurement principle and flow calculation method inherently limit its accuracy (flow coefficient uncertainty is 1%). Under significant variations in operating conditions, the actual measurement error can far exceed the design calibration value. Wear on the edge of the pressure tap by the fluid can gradually alter the flow coefficient, similar to the abrasion of sharp edges on orifice plates, leading to performance instability. When measuring dirty gases, the pressure tap is prone to clogging, which not only changes the discharge coefficient but can also sometimes prevent normal operation. While recent improvements in Anuba technology have enhanced wear and clogging resistance, they still cannot fundamentally solve the problem. It is absolutely unsuitable for measuring dirty gases such as coke oven gas and unclean natural gas. Anuba is only suitable for applications where the measured medium is relatively clean, the operating conditions are relatively stable, and high-accuracy measurement is not required. 3. Vortex Flow Meter: Vortex flow meters have a simple structure, are easy to install and maintain, are durable, and can measure high-humidity gases and other relatively dirty gases. However, their measurement accuracy is relatively low, around 1.5% without temperature and pressure correction. Due to the spiral blades forming the vortex initiator, the pressure loss caused by the gas flowing through the vortex initiator is too large, making it unsuitable for applications with low operating pressure and strict pressure loss requirements. Secondly, the measured gas velocity cannot be too low; otherwise, the vortex initiator cannot start, creating a measurement blind zone (currently, vortex flow meters used for natural gas metering have this measurement defect of not measuring small flow rates). When used to measure dirty gases such as coal gas, the accumulation of dirt in the vortex initiator and clogging of the vibration frequency detector are also significant problems. 4. Vortex flow meter. Vortex flow meter has a simple structure, is easy to install and maintain, and has a high measurement accuracy (excluding temperature and pressure corrections). The accuracy is 0.5-1% for liquids and 1-1.5% for gases and steam. The range is relatively wide, generally 20:1. In recent years, vortex flow meters have been used to measure steam, and their development has been rapid in the DN200mm range. They are also used for natural gas metering. The main limitations of vortex flow meters are as follows: (1) Due to the limitations of the detection principle, the lower limit of the flow Reynolds number to be measured is required to be relatively high. The gas flow velocity should generally be greater than 4m/s. If the flow velocity is low, the measurement accuracy will be reduced or even small flow rates will not be measured. In addition, the reliability of operation is also problematic in environments close to strong vibration fields. (2) Due to the limitations of the detection principle, the measuring pipe diameter cannot be too large (large pipe diameters will cause the frequency signal to be lost due to the low frequency of the vibration vortex at low flow velocities. Currently, the mature technology products of manufacturers are all below DN300mm). Due to the limitations of the temperature resistance of the signal detector, the temperature of the measured medium cannot be too high, preferably below 350℃. (3) It is not suitable for measuring dirty fluids such as coal gas and non-clean natural gas, which may cause serious pollution to the frequency signal detector. (4) It has high requirements for the length of the straight section of the inlet pipe (slightly higher than the requirements for orifice plates). If the installation conditions are not met, a large additional measurement error will occur. 5. Precision gas turbine flow meter has a measurement accuracy of 0.2-0.5% and can be used for the measurement of various clean gases, including clean natural gas. It is also used abroad for natural gas trade settlement measurement. Since the measuring element of the turbine flow meter is a rotating impeller and uses bearings, the fluid being measured must be clean, and it should be calibrated frequently during use. Obviously, it is not feasible to use turbine flow meters for the measurement of non-clean natural gas and coal gas. When used for the measurement of clean natural gas, the gas must be clean. When used for newly opened natural gas pipelines, the turbine flow meter often cannot maintain normal operation because the pipeline is not clean. 6. Ultrasonic gas flow meters have seen rapid development abroad in recent years and have begun to be used in the trade settlement measurement of medium and large diameter natural gas. my country has also conducted a small number of trials on imported ultrasonic gas flow meters with diameters of DN200mm-400mm. Multi-channel ultrasonic gas flow meters with medium and large diameters can achieve an accuracy better than 0.5%, but they are too expensive and their reliability needs to be tested in practice. Single-channel ultrasonic gas flow meters with smaller diameters have lower measurement accuracy and are not cheap. In addition to the price issue, ultrasonic gas flow meters also have limitations in terms of usage conditions, mainly: (1) The gas being measured must be clean to ensure that it does not contaminate the probe, so it is not suitable for measuring coal gas and non-clean natural gas. (2) The density of the gas being measured and the operating pressure inside the pipe cannot be too low. This is especially true for larger pipe diameters, so it is not suitable for measuring low-pressure large-pipe gas. (3) In situations with high-frequency vibration noise, ultrasonic flow meters may sometimes fail to work properly. In summary, considering both performance and price, the gradual application of multi-channel ultrasonic gas flow meters in the trade settlement and metering of clean natural gas in large-diameter and relatively large-diameter pipes in my country has a certain degree of practicality. However, its widespread application in natural gas gathering and transportation and industrial gas metering is not feasible. 7. Elbow Flow Meter: Also known as a bend flow meter, the elbow flow meter measures fluid flow by utilizing the quantitative relationship between the static pressure difference (differential pressure) formed between the outer and inner radii of the bend and the average flow velocity when fluid passes through the curved channel of the elbow. Therefore, it is also a type of differential pressure flow meter, mostly used for gas measurement. It has seen some development in my country in recent years. Elbow flow meters have advantages such as simple structure, stable performance, good repeatability, and low price. Their main disadvantages are low measurement accuracy, with a flow coefficient uncertainty generally around 4%, and unsuitability for measuring low-pressure, low-velocity fluids. The low measurement accuracy of elbow flowmeters is determined by their measurement principle: the velocity distribution of the fluid at the elbow is exceptionally complex and prone to aging; the flow coefficient is highly sensitive to changes in the Reynolds number and pipe wall roughness; and the non-axisymmetric velocity distribution of the fluid before entering the bend has a much greater impact on the flow coefficient than that of a standard orifice plate. Furthermore, when used for gas measurement, how to correct for density changes at the pressure tapping section is a variable that is difficult to control precisely. Actual flow calibration of the flow coefficient using computer software correction techniques can improve the measurement accuracy of elbow flowmeters; however, apart from the Reynolds number, which is easily corrected dynamically, the other variables mentioned above are difficult to address using software correction methods. Theoretically, if actual flow calibration is performed using the actual medium within the actual operating conditions, and then real-time correction is performed according to the empirical formula derived from the calibration results, satisfactory measurement accuracy can be obtained. However, for general industrial metrology, this is often difficult to achieve. Therefore, elbow flowmeters are only suitable for applications where the operating conditions do not vary significantly, the requirement for high measurement accuracy is not high, and good measurement repeatability is sufficient. 8. Internal Venturi Flow Meter The internal Venturi flow meter is a patented technology product independently developed by Dalian Sonica Electronics Co., Ltd. It is a new generation of differential pressure flow measurement instrument that integrates the advantages of classic Venturi tube, annular orifice plate and wear-resistant orifice plate. The technical characteristics of internal Venturi tube are: (1) High measurement accuracy and good stability. Due to the absence of sharp edge erosion and dirt accumulation problems of orifice plate, the discharge coefficient can remain constant for a long time during the measurement process. (2) Strong adaptability to the measured medium. It can measure various liquids, gas volumes and vapors, including fluids containing solid particles and high moisture content gases. (3) Wide measurement range. The range ratio is greater than 10:1, and even the range ratio reaches 15:1, 25:1, and the linearity of the discharge coefficient is still less than 0.4%. (4) Low requirement for the length of the straight pipe section. The shortest straight pipe section length only needs to be about 1/8 of the orifice plate, generally 1-3D, thus effectively avoiding or reducing the additional measurement uncertainty of the measurement system. (5) The pressure loss is small, about 1/3 to 1/5 of that of an orifice plate. The excellent metering characteristics of the internal venturi tube make it an ideal replacement for orifice plates, suitable for widespread use in the metering of various coal gases, natural gas, and steam. The disadvantage of the internal venturi tube is that a high-performance differential pressure transmitter must be configured to achieve high measurement accuracy, but this is not difficult to achieve today. • Conclusion The current state of energy gas metering in my country urgently needs to be improved. A one-size-fits-all approach is neither possible nor appropriate to adopt improved measurement methods. Decision-makers should conduct a comprehensive and objective analysis and comparison of the various available metering methods based on their own measurement conditions and the need to improve metering effectiveness; this will likely lead to the correct choice.