The Necessity of Energy Monitoring Over the past decade, China's GDP growth rate has consistently exceeded 10% annually, attracting global attention. However, this high growth has come at the cost of high energy consumption. China's energy consumption per unit of output is 2.5 times the international average, and 2.5, 4.9, and 8.7 times that of the United States, the European Union, and Japan, respectively, consuming 1.42 billion tons of coal annually. If GDP were to double during the 11th Five-Year Plan period, at the current energy consumption level, annual coal production would need to reach nearly 3 billion tons. This is unsustainable and unbearable from the perspectives of resources, production, safety, transportation, and environmental protection. During the 10th Five-Year Plan period, the central government issued energy conservation targets, requiring a 15-17% reduction in energy consumption per unit of output while ensuring GDP growth. However, the actual implementation resulted in a 7% increase. Without vigorous efforts in energy conservation, high energy consumption will inevitably become a bottleneck for further economic growth in China. Therefore, during the 11th Five-Year Plan period, the central government prioritized energy conservation and emission reduction as a key aspect of economic development, requiring a 20% reduction in energy consumption per unit of output while ensuring GDP doubling. To ensure implementation, the international standard GB 17167 was formulated, requiring all units and equipment consuming a certain level of energy to install monitoring instruments for evaluation, using scientific methods and numerical data to assess the effectiveness of energy conservation and consumption reduction. The Role of Flow Meters in Energy Conservation Monitoring Facts demonstrate that mere publicity about energy conservation and consumption reduction is ineffective; legislation is necessary to supervise with concrete figures and indicators, and to ensure implementation at the level of local governments and enterprise leaders. Therefore, the national standard GB 17167-2006 was formulated, with five mandatory clauses (4.3.2, 4.3.3, 4.3.4, 4.3.5, 4.3.8). These clauses not only stipulate that units and equipment consuming a certain level of energy must install monitoring instruments, but also clearly define the amount of energy consumed, the types of monitoring instruments, and the accuracy levels required. In energy monitoring instruments, besides electricity meters and weighing instruments for solid coal, all other gaseous and liquid energy sources (such as crude oil, refined oil, heavy oil, residual oil, natural gas, liquefied gas, coal gas, etc.), as well as energy-carrying media (steam), must use flow meters. Even for solid coal, in continuous operation in process industries, impeller flow meters or gas-solid two-phase microwave flow meters can be used. Therefore, flow meters play a crucial and irreplaceable role in energy conservation and consumption reduction monitoring. Characteristics of Energy Monitoring Flow Meters There are more than ten principles and nearly 200 types of flow meters. Choosing the right one is not a simple task. Due to space limitations, this article omits general selection rules and focuses on two issues to consider when selecting flow meters in energy monitoring: accuracy. In energy monitoring, the accuracy of flow meters should be given high priority. GB17167 has made reasonable and clear provisions for this. Since flow meters are required to accurately quantify the effects of energy conservation and consumption reduction, they should possess the necessary accuracy. Otherwise, with limited knowledge, how can we assess the energy-saving effect and "prescribe the right medicine" to further implement energy-saving measures? Of course, higher accuracy isn't always better, as high-accuracy instruments are very expensive, and the company's affordability must be considered. Therefore, GB17167 realistically sets different requirements based on the monitored object: for example, for measuring fuel oil flow, the accuracy should be 0.5–1 grade; for measuring natural gas and coal gas flow, it can be 2 grade; and for measuring water and steam flow, it can be as low as 2–2.5 grade. It's important to emphasize accuracy here, not just the dispersion of the flow coefficient, but also the flow uncertainty of the calibration device. Therefore, some instruments, such as those that calculate flow by measuring point velocity (e.g., dual Venturi, Pitot tubes, etc.), are unsuitable for energy monitoring, especially in cases with large pipe diameters, insufficient straight pipe length, and undesirable flow velocity distribution within the pipe. Permanent Pressure Loss Flow meters used for monitoring energy reduction not only lack energy efficiency themselves but also consume energy in the form of pressure loss. The amount of energy consumed depends on the instrument's structure. When fluid flows through resistance components within an instrument (such as orifice plates, nozzles, internal cones, vortex generators, detection rods, targets, etc.), vortices will be generated behind the resistance components. Similar to friction in mechanical motion, this is an irreversible isentropic process that consumes the fluid's energy, making it impossible for the fluid pressure to return to its original value. Flow meters such as nozzles and internal cones all experience this loss. While Tubes have an expansion section that converts the fluid's kinetic energy into potential energy, pressure loss due to friction between the fluid and the tube wall also exists, although it is much smaller than the pressure loss caused by vortices. The formula for calculating permanent pressure loss is omitted here; please refer to the R, W, and Miller Flow Measurement Engineering Handbook. The fluid energy loss caused by internal resistance components will slow the flow and reduce the flow rate. To maintain normal process operation, it is necessary to increase the power of the pump (or fan); this increased power is due to the pressure loss of the flow meter. For some instruments, this is a significant amount. The annual operating costs of three commonly used instruments (orifice plates, internal cones, and averaging pitot tubes) at different diameters are calculated and listed in Table 1. The fluid used in the above calculations is air, with flow rate calculated based on an average flow velocity of 25 m/s in the pipe, a temperature of 20℃, and a pressure of 102 kPa. The fan efficiency is 0.85, the orifice plate efficiency is 0.62, and the inner cone efficiency is 0.7. The annual operating hours are 8760 hours (365 × 24), and the electricity cost is 0.8 RMB per kilowatt-hour. If the parameters change, the results will differ. Therefore, the above calculations can only serve as a qualitative assessment of the energy consumption of different instruments. The data listed in Table 1 show that when the orifice plate diameter is greater than 0.3 meters and the inner cone diameter is greater than 0.5 meters, the annual operating cost is excessively high and unacceptable. However, it is still applicable when the pipe diameter is small. Averaging pitot tubes and similar insertion-type instruments have low pressure loss and are particularly suitable for large-diameter pipes. Selection of Several Types of Flow Meters Throttling Devices These instruments can withstand harsh operating conditions and account for more than 50% of the flow meter market share. They have a long history, and new types of instruments continue to emerge based on application problems. ■ Orifice Plates: Orifice plates have accumulated decades of usage experience and established a database of tens of thousands of data points. They also meet both international and domestic standards and currently have the highest installation rate. However, the recent release of the new ISO 5167 standard, which further requires longer straight pipe sections, presents significant challenges to their application. From an energy-saving perspective, they are the type of flow meter with the highest pressure loss. When the pipe diameter is large, the annual operating costs caused by pressure loss are unacceptable, and accuracy cannot be guaranteed due to the inability to meet the required straight pipe length. Therefore, their market share has been declining both domestically and internationally. ■ Internal Cone Flow Meters: Introduced by McCrometer in 1986, internal cone flow meters are suitable for multiphase dirty fluids (such as coke oven gas) due to fluid scouring. Furthermore, because the cone has a rectifying effect, the required straight pipe length is only a few times the pipe diameter. After the release of the new ISO 5167 standard, they received considerable attention in the industry. However, some claims, such as low pressure loss and high accuracy, are exaggerated. Tianjin Kete Company is a professional manufacturer and seller of internal cone pipes. They have conducted systematic research and testing on the accuracy and pressure loss of internal cone pipes, with the following results: Accuracy: Calibration was performed on internal cone pipes with diameters ranging from 100 to 800 mm, and the results are listed in Table 2. The data shows that when the pipe diameter is large, the dispersion of the outflow coefficient (which also reflects accuracy) caused by processing and installation can reach 5%. Pressure Loss: The formula for calculating the permanent pressure loss εe of the internal cone is: εe = (1.3 - 1.25b)εe, where εe is the output differential pressure. Table 3 shows the ratio of permanent pressure loss to output differential pressure for orifice plates and internal cone pipes based on different b values. The data shows that the pressure loss of the internal cone, especially when the b value is small (0.4–0.45), is very close to that of the orifice plate; however, if a larger b value is selected, it means that the annular channel is enlarged, the rectification effect will decrease, and the accuracy will be affected. Therefore, the b value is usually selected between 0.55 and 0.65. Therefore, the internal cone flow meter is not an energy-saving instrument, and the reason for promoting the internal cone flow meter based on the requirement of "energy saving and consumption reduction" is insufficient. The Lo-loss tube has an inlet cone angle of approximately 40°, and after throttling, there is an expansion section with a cone angle of approximately 5-7°. The fluid can smoothly convert kinetic energy into potential energy without generating vortices, resulting in low pressure loss. It is an energy-saving instrument, but if higher accuracy is required, a longer straight pipe section is still necessary. ■ The shuttle-type, a patented product published in 2005, combines the advantages of the internal cone annular channel requiring a shorter straight pipe while avoiding its disadvantage of higher pressure loss; it also takes advantage of the Lo-loss tube's lack of vortices and orderly recovery of kinetic energy into potential energy. A tail cone is added to the internal cone to form a shuttle body, thus resulting in lower pressure loss. Furthermore, the patent incorporates measures to improve accuracy to adapt to shorter straight pipe sections in the field. ■ The insertion type is an instrument that calculates flow rate by measuring the velocity at one (or several) points within the pipe. The characteristics of flow meters are simple structure, low cost, easy installation and disassembly, and low pressure loss; however, their accuracy is often only ±2-5%. For flow meters that only measure the velocity at a single point, the accuracy may be less than ±3% when the straight pipe section length is insufficient, limiting their selection as energy monitoring instruments. Common types include: ■ Double Venturi tubes: Their outstanding advantage is simple structure and large output differential pressure, but according to ISO 7145 assessment, their accuracy can only reach ±3% in the best case, which basically cannot meet the accuracy requirements of energy monitoring instruments. ■ Thermal flow meters: They have good low-speed performance but are limited to measuring clean, dry gases. Not only the straight pipe section length but also the gas composition has a significant impact on accuracy, resulting in lower accuracy and making them unsuitable for energy monitoring. The irregular-shaped Pitot tube (also known as a measuring tube) has been widely adopted in the domestic metallurgical industry in the past year or two. It is an insertion flow meter used to measure waste gas (converter gas, coke oven gas, etc.). Its structure is similar to the Japanese irregular-shaped Pitot tube (see Japanese Industrial Standard 28808). Its advantages lie in the cut perpendicular to the pipe axis, which facilitates the discharge of solid dust and liquid droplets in the waste gas, unlike ordinary averaging pitot tubes which are prone to clogging. In addition, it is calibrated individually, eliminating the problem that the differential pressure measured by an averaging pitot tube may not be the average flow velocity differential pressure in the pipe. Due to its special structure, its velocity coefficient is approximately 0.85, and calibration is necessary before use. From the perspective of energy monitoring, the low pressure loss and applicability to measuring dirty waste gas are outstanding advantages. However, whether the accuracy can reach the ±2-3% mentioned in the relevant article is questionable. Its only three measuring points are located above the pipe, and the position and number of measuring points do not meet the requirements of ISO 3966, failing to fully reflect the velocity distribution in the pipe. The article "ignores" the interference coefficient introduced by the imperfect velocity distribution in its error analysis. When the straight pipe section is insufficient, it will be the biggest source of error. ■ Averaging Pipe: The averaging pitot tube calculates flow rate by measuring the velocity at multiple points within the pipe. Its accuracy is generally ±1.5% to 2%, making it suitable for energy monitoring applications with large pipe diameters where accuracy requirements are not too high (such as water measurement). If the straight pipe section is insufficient, two averaging pitot tubes can be inserted simultaneously at close cross-sections in the pipe, at 90° to each other. The number and location of measuring points can generally meet the requirements of ISO 3966. Regarding the issue of easy clogging, a backflushing method can be used, or the advantages of the measuring tube can be incorporated to design a new type of instrument that combines the ease of installation, low cost, and resistance to clogging of the averaging pitot tube. ■ Flow Meters with No Resistance Components: This is the fastest-growing type of flow meter in the past five years. According to authoritative websites, the annual growth rates of the following three types of instruments in the global market are 2%, 6.9%, and 10.4%, respectively. Their characteristics include no resistance components within the pipe, low pressure loss, simple mechanical structure, increasingly sophisticated functions, and high accuracy. ■ Electromagnetic: Electromagnetic flow meters are currently the best-selling product in the flow meter market, but they cannot measure the flow rate of oil or gaseous energy. ■ Although Coriolis pipes have no resistance components, the fluid being measured must be forced to rotate 180° to generate Coriolis force, resulting in higher pressure loss but accuracy as high as ±0.2-0.5%. The diameter is generally less than 0.2 meters, there are no straight pipe section requirements, and the price is relatively high. European and American countries mostly use it to measure energy and valuable fluids to reduce disputes in trade accounting. It is not yet widely used in China. ■ Ultrasonic flow meters, because they can measure various fluids including gas and liquid, not only have low pressure loss but also accuracy as high as ±0.5%, and a wide range, are currently the fastest-growing and most suitable flow meters for energy monitoring. The technology of ultrasonic liquid flow meters is relatively mature. However, the propagation resistance of sound waves in gases is much greater than that in liquids. According to GE in the United States, the received reflected information is only one ten-thousandth of the transmitted information, with a time difference of only 10⁻⁹-10⁻¹² seconds. In the past seven or eight years, the problem of weak information transmission has been largely solved abroad, and products have been launched, with corresponding standards established. They have become the main metering instrument for domestic and international natural gas trade settlement, but the price is still relatively high. Domestically, products from Shanghai and Beijing have entered the market, but they still suffer from drawbacks such as insufficient accuracy and low resistance to noise interference. Although some are used for measuring blast furnace and coke oven gas, they have not yet fully entered the trade metering field. In the past two years, clamp-on ultrasonic gas flow meters have been introduced abroad, with some manufacturers even claiming an accuracy of ±0.5%. However, according to senior foreign experts, due to the influence of many factors such as pipe material, coating, thickness, and installation location, objectively speaking, an accuracy of ±2% is already quite difficult. Currently, ultrasonic gas flow meters, with their simple structure, complete functions, high accuracy, easy installation, and applicability to various pipelines, are the most promising flow instruments for energy monitoring. Discussion This article emphasizes two key characteristics of flow instruments used for energy monitoring: necessary accuracy and low pressure loss. Currently, there is no perfect instrument. Each instrument can only function under specific conditions. For example, the internal cone flow meter is superior to the classic throttling device when the straight pipe section is short, while still maintaining high accuracy. However, the pressure loss is not as small as advertised. Similarly, insertion flow meters have low pressure loss and are inexpensive, but their accuracy often fails to meet monitoring requirements. Ultrasonic flow meters have low pressure loss and high accuracy, but they are too expensive and can only be used for important trade measurements, not for general users. Therefore, repeated comparisons are necessary when selecting a flow meter, and one should not rely solely on one-sided claims. In the instrument manuals, manufacturers often describe the instrument's performance as excellent, such as its accuracy. While some of this may be fabricated or misleading, many responsible manufacturers have indeed obtained these technical indicators through calibration in relatively ideal laboratory conditions. The problem is that industrial sites generally cannot provide ideal laboratory conditions. This factor should not be ignored when selecting an instrument. Over the past century, some flow meters, such as orifice plates and nozzles, have been continuously improved through application, accumulating tens of thousands of experimental data points and establishing standards. However, some drawbacks, such as high pressure loss, excessively long straight pipe sections that cannot be met on-site, and poor long-term stability, are difficult to overcome, especially after the publication of the new ISO 5167 standard, which further restricts its application. Against this backdrop, a number of new instruments have emerged, such as internal cone, insertion, and ultrasonic instruments. These instruments, when first introduced, have some shortcomings that require continuous improvement and refinement in practice. Faced with this issue, what attitude should industry experts take—unconditional criticism or blind support? Neither is advisable. Only by affirming the advantages, pointing out the shortcomings, and proposing improvement suggestions can new products be continuously improved and gradually replace older products that cannot adapt to the ever-evolving engineering technology. "A thousand sails pass by the sunken ship"—this is an objective law that is independent of human will.