Selection of gas flow meter

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Medium properties: There are many types of gas, and their compositions are complex. Gases can generally be divided into two main categories: natural gas and artificial gas. The main components of coal gas are: CO, H2, CO2, N2, O2, CH4, H2S, etc. The proportions of these components depend on the coal gas production process. Artificial gas is a gas containing combustible components, produced by processing coal as raw material. Based on the processing method, the properties of the gas, and its intended use, it can be classified into water gas, semi-water gas, and air gas (also known as producer gas), which are obtained through gasification. The gas produced by coking in the coal dry distillation process is known as coke oven gas or blast furnace gas. Gas is corrosive and toxic; the main corrosive components include hydrogen sulfide, sulfur dioxide, carbon dioxide, etc. These gases are only corrosive in the presence of water vapor. Toxic components of gas (some of which are absent in by-product gas) include hydrogen sulfide, ammonia, benzene, etc.; these components are primarily found in coke oven gas. Many gases contain dust; the amount depends on the treatment process. Some gases also contain moisture and tar. Actual operating conditions: The actual operating conditions of coal gas are quite complex, and there is no fixed pattern to them. Due to different manufacturing processes, the actual operating conditions on site vary. Some relatively common characteristics are: low hydrostatic pressure, low flow velocity, and a small allowable pressure drop. It is generally not allowed to use reduced pipe diameters to increase flow velocity. Some gas fluids contain moisture, and some of the substances being measured also have a small amount of water, which causes stratified flow at the bottom of the pipeline. In some measurement scenarios, the hydrogen content is relatively high, the fluid density is low, there is a significant difference between the minimum and maximum flow rates, the conveying pipelines have large diameters, and the lengths of straight pipe sections are relatively short. 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 properties of coal gas. Due to the presence of viscous substances such as tar, rotary volume flow meters are difficult to use, and gas turbine flow meters as well as rotameters are also not suitable. Due to the low density, low flow velocity, and wide range of flow rate variations—with the flow rate during peak gas usage being more than 20 times that during off-peak periods—the application of vortex flow meters and differential pressure flow meters is limited. Currently, the main types are differential pressure flow meters and thermal mass flow meters for gases; occasionally, gas ultrasonic flow meters are also used. Below is a description of the applications of several main types of flow meters: (1) Vortex flow meters: Due to the characteristics of coal gas—namely, low flow velocity, low pressure, a wide range of flow variations, and its corrosive nature—factors such as equipment vibration caused by pressurization devices and the random pulsating pressure of the gas can all affect the measurement accuracy of vortex flow meters. Moreover, in practical applications, it is difficult to ensure a gas flow velocity of over 5 m/s. Therefore, vortex flow meters are not suitable for measuring gas flow, and there are very few applications at present. (2) Orifice plate differential pressure flowmeter: Differential pressure flowmeters have a relatively wide range of applications and a long history. In the measurement of gas flow, they still find certain applications in specific scenarios. However, the inherent flaws and limitations of differential pressure flow meters also restrict their wider application; for instance, water droplets in the pressure guiding pipes can lead to inaccurate pressure differences. Under process conditions with large load variations, the low range ratio of differential pressure flowmeters fails to meet the requirements. The installation of flow meters with large diameters requires high standards and poses great construction difficulties; these factors impose limitations on differential pressure flow meters. Coal gas contains dust; standard orifice plates cannot be used in such an environment. To overcome this drawback, various throttling devices have been developed, such as segmental orifice plates, replaceable orifice plates, and multiple tubes connected in parallel. The effects can be either good or bad, and better results can only be achieved under specific conditions. (3) Constant-area tube differential pressure flowmeter: The constant-area tube also falls under the category of differential pressure flowmeters. In the measurement of gas flow rates at larger pipe diameters, while replaceable orifice plates are an option, their high manufacturing costs and cumbersome installation make them less attractive. Therefore, for many customers who use the measurement data solely for process monitoring and do not require high precision, differential pressure flow meters with average velocity tubes represent another viable option. The key to the success of using an average velocity tube differential pressure flow meter for measuring gas flow is to ensure that the pressure tapping tubes are not blocked by water droplets or dirty particles. Since the shut-off valves equipped in standardized constant-velocity tube products are mostly needle valve types with a small diameter, the water vapor in the fluid can condense into water droplets, and larger particle impurities in the fluid may also block the pressure Quote line. From a practical perspective, there are many issues: the failure rate is high, and maintenance requirements are substantial. Thermal gas flow meters: Thermal flow meters are flow measurement devices designed based on the principle of heat diffusion. Specifically, when a fluid flows past a heated object, the amount of heat lost by that object is proportional to the flow rate of the fluid. The sensors in this series of flow meters consist of two standard-grade RTDs; one serves as the heat source, while the other measures the fluid temperature. As the fluid flows, the temperature difference between the two sensors is linearly related to the flow rate. Through microelectronic control technology, this relationship is converted into a linear output signal representing the measured flow rate. Thermal mass flow meters have the following characteristics: they can measure extremely low flow rates, have no moving parts, and offer high reliability. The pressure loss is very small, almost negligible. Direct measurement of mass flow, no temperature or pressure compensation required. It features an extremely high range ratio of 1000:1; the price is not greatly affected by the pipe diameter, and its cost-effectiveness is even more evident for larger diameters. It is easy to install and resistant to vibration. Good repeatability and virtually maintenance-free. (5) Gas ultrasonic flowmeter: An ultrasonic gas flowmeter used to measure gas flow, which is generally a time-difference flow measurement device consisting of an ultrasonic transducer and other components, along with its own measurement tube section. Transducers are generally mounted along the pipe wall and are in direct contact with the fluid. The ultrasonic pulses emitted by one transducer are received by another transducer, and vice versa. Ultrasonic pulses travel through pipes like a ferry across a river; in the absence of fluid flow, sound waves will propagate in both directions at the same speed. If the gas flow velocity is not zero, the propagation speeds in the direction of the gas flow and against it are different; by using these two differences in propagation time, it is possible to calculate the gas flow velocity of the fluid. Gas ultrasonic flow meters feature high measurement accuracy, no pressure loss, a wide rangeability, and easy installation (no need to drill holes or weld on the pipeline). They can be used in various challenging operating conditions, such as dirty gases, gases with high moisture content, corrosive gases, and mixed gases. Additionally, they allow for bidirectional measurement. Due to differences in the technical performance of various brands, the currently high prices directly limit their applications. Overall, in terms of comprehensive comparison, thermal mass flow meters have clear advantages in measuring gas flow rates. (1) Installation: When measuring gas flow, derivative-type flow meters (including averaging pitot tubes) all require temperature and pressure compensation. The installation workload is substantial, and the installation costs are high (especially for large-diameter pipes). Due to the excessive number of installation accessories, leaks are likely to occur, **increasing the workload for daily maintenance. For thermal gas flow meters, gas flow measurement does not require temperature or pressure compensation, and the measured flow value is the mass flow rate; **this reduces the need for pressure stabilization compensation that is required in other types of flow meters when measuring gas. While reducing installation costs, it minimizes leak points and failure points, thereby ensuring the reliability of the instrument during prolonged operation. The sensor components experience almost no pressure loss, and there are no dead zones or blockages that could affect the sampling of the sensor signals. A specially designed stainless steel compression fitting interface is available; this interface is engineered to ensure that the flow meter remains undamaged even after it is removed from the pipeline multiple times. It is possible to remove the flow meter online without interruption, which brings great convenience to users’ production process management and scheduling. Additionally, for measuring gas flow in environments with corrosive media, thermal gas meters can use probes made of materials with extremely high corrosion resistance. For monitoring the flow rate of gas containing tar, two solutions can be adopted for thermal flow meters: The first solution involves using an axial probe (products featuring this design are already available internationally, as shown in the figure). To facilitate regular cleaning of accumulated dirt by customers, a thermal excavation model and drilling tools have also been developed. With appropriate measures, it is possible to ensure that in the event the probe of a thermal gas flow meter becomes clogged with contaminants, the probe can be cleaned online without shutting down the gas supply. The second approach involves giving special treatment to the probe of the thermal gas flow meter, thereby prolonging the time during which tar deposits adhere to the probe and facilitating its cleaning (as shown in the figure). 2) Measurement range: Thermal gas flow meters can monitor gas flow rates starting from a minimum flow velocity of 0.05 Nm/s, up to a maximum flow velocity of 100 Nm/s. The range ratio can reach 1:1000. The gas ultrasonic flow meter can detect flow rates in the range of 0.01 m/s to 25 m/s. These two types of flowmeters can handle large fluctuations in flow rate resulting from varying gas loads, and this is the most prominent advantage of thermal gas flowmeters over other types of flowmeters. For differential pressure flow meters, since the instrument signal (differential pressure) has a square relationship with the flow rate, their typical rangeability is only 3:1 to 4:1. With the development of modern science and technology and the application of electronic computers, the emergence of differential pressure transmitters with a wide range has led to a significant increase in the range ratio of differential pressure-type flow meters. Currently, the range ratio of differential pressure flow meters can reach 1:10; however, this range ratio is still not sufficient for many applications where the flow rate varies over a wide range. (3) Precision: Over the past forty-plus years of development, thermal mass flow meters have seen significant advancements in various aspects of their technology. Current thermal mass flow meters can achieve a comprehensive accuracy of ±1.0% FS when measuring gases of a single type (within a ratio range of 1:100), with a repeatability of ±0.25% FS, which represents a quite high level of precision for gas measurement. However, since the components of coal gas are a mixture of multiple substances (research on other mixtures is similar and can be used as a reference), any change in the composition of this mixture will lead to changes in its gas physical properties. Regarding the issue of how changes in the composition of the gas being measured affect the output signal of flow meters, most flow meters lack effective means to address this problem. Thermal mass flow meters employ a composition compensation algorithm along with a temperature compensation algorithm. The significance of this approach is that when there are changes in either the gas composition or temperature, this compensation algorithm can eliminate any errors caused by such variations. After a single calibration (usually using air as the calibration medium), it can be used under various component and temperature conditions, which saves the effort and cost associated with performing calibration again with air. It lays the technical foundation and provides a guarantee for the promotion and application of thermal mass flow meters. Given the good linearity and repeatability of thermal gas mass flow meters, for measuring gas flow rates, when it is not used for trade measurement but merely for process control purposes, thermal flow meters are undoubtedly the best choice in terms of cost-performance. Differential pressure flow meters are greatly affected by various factors; any problem at any stage—from design and manufacturing to installation—can result in relatively large errors. Especially in the measurement of low flow rates, the differential pressure generated at this time is extremely small; sometimes it’s only several tens of pascals, or even just a few pascals. With such a small differential pressure value, it is highly susceptible to interference. A tiny droplet of water, a particle of dirt, or even a bit of oil can easily nullify this differential pressure value. Therefore, when using differential pressure flowmeters to measure gas, it is impossible to achieve very high overall accuracy. Moreover, in terms of stability and ease of maintenance, differential pressure flow meters are inferior to thermal flow meters. The advantages of gas ultrasonic flow meters are undeniable; however, their purchase cost is quite high. Currently, they are mostly used in trade settlements. (4) Pressure loss: Since gas ultrasonic flowmeters are installed in a clamp-on manner (with the probe placed outside the pipeline), there is no pressure loss at all. Due to its structural design, thermal flowmeters have very low operating pressure losses; at low flow rates and large pipe diameters, these pressure losses can be considered negligible. Differential pressure flow meters, on the other hand, are different; high pressure loss is one of the main drawbacks of differential pressure flow meters. Pressure loss is a major challenge and obstacle in energy conservation. When gas is transported over long distances, higher requirements are placed on pressure loss; otherwise, it leads to a significant increase in electricity consumption as well as other additional costs. Therefore, from a long-term perspective and considering the criteria for energy conservation and emission reduction, differential pressure flowmeters are not the optimal choice for use in gas flow measurement systems. Below is a comparison of various types of flow meters: Differential pressure flow meters, Venturi flow meters, Gas ultrasonic flow meters, Thermal mass flow meters. Pressure loss: High, Low, None, Extremely low. Ability to measure two-way flow: No, No, Yes, Yes. Repeatability: 0.1%, 0.5%, 0.15%, 0.2%. Requirements for installation conditions: Strict, Moderate, Moderate, Moderate, Low. Performance at low flow rates: Poor, Poor, Good, Good. Range ratio: 10:1, 10:1, 50:1, 100:1. Service life: Short, Longer, Long, Long. Amount of maintenance required: High, High. Need for regular maintenance: Yes, Yes. Safety performance: Low, High, High, High. Price: Low, Moderate, Extremely high. Price is inversely proportional to the pipe diameter