Thermal gas mass flow meter – the best choice for measuring biogas flow

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The recovery and utilization of biogas is a beneficial systematic project for environmental protection that serves the interests of both the country and its people. Countries are investing significant human and financial resources to develop biogas recovery and utilization technologies. Its biogas power generation technology is also considered a cutting-edge technology on the international stage, with only a few countries **in possession of it. Companies are concerned with economic and social benefits, and they want to know how many kilowatt-hours of electricity can be generated from 1 cubic meter of gas Several traditional flowmeters have proven to be not very effective in biogas measurement applications. With the maturity of thermal gas mass flow meter technology and the excellent practical convenience of these devices, more and more biogas power generation companies or biogas recovery enterprises are beginning to adopt thermal gas mass flow meters. The successful use of thermal methods has resolved the long-standing problem of biogas metering that has plagued the industry. This article discusses the characteristics, principles, and applications of thermal gas mass flow meters, and compares their performance with that of other types of flow meters. Vortex flow meter (for gases) ▲ Features of vortex flow meters: no moving parts, high measurement accuracy, low pressure loss. The sensing element does not come into contact with the fluid being measured, and the output signal is independent of parameters such as the fluid’s temperature, pressure, density, composition, and viscosity. 　　▲ Principle of vortex flow meter: It uses the Karman vortex street principle from fluid mechanics to measure fluid flow rate. When a vortex generator (a non-streamlined symmetric shape such as a cylinder or triangular prism) is inserted vertically into a pipe, as fluid flows through the pipe, vortices are generated alternately on the left and right sides behind the vortex generator, forming a series of vortices. These two columns of vortices are arranged in parallel to each other, appearing alternately on the left and right, with opposite rotation directions. The frequency of the vortex, f (Hz), is related to the average flow velocity of the fluid, v (m/s), and the width of the vortex generator, d (m), by the formula: f = St * v / d. Here, St is the Strouhal number, which is related to the width of the vortex generator, d, and the Reynolds number of the fluid. ▲ Vortex street flow meters are relatively simple to use and easy to install; they come in both insert-type and pipe-type versions. During installation, care should be taken to ensure that the flow direction of the fluid is the same as the direction indicated by the flow meter. 　　▲ Disadvantages of vortex flowmeters: The range ratio is lower than that of thermal gas flowmeters (which are usually only 10:1); they are highly sensitive to vibrations; the size of the pipeline is directly proportional to its cost; temperature and pressure compensation is required for accurate measurement; in the low-range section, they are insensitive, unstable, and practically unusable for measurement. Ultrasonic flow meter (gas) ▲Features of ultrasonic flow meters: no pressure loss; simple installation; independent of the fluid’s properties such as temperature, pressure, and viscosity; no moving parts; capable of measuring dirty, corrosive gases as well as multi-component gases. 　　▲Principle of ultrasonic flow meter: It measures the propagation time of ultrasonic waves in both the forward and reverse directions within the pipe, and then calculates the flow velocity of the fluid. The flow rate of a fluid can be determined using the flow velocity of the medium, the pipe diameter, as well as a dynamic correction for the fluid based on the Reynolds number. 　　Specifically: a pair of ultrasonic sensors are arranged in a “Z” shape within the pipeline, and the distance L between the two sensors serves as the propagation path for the ultrasonic waves. The propagation times of ultrasonic waves in the forward and reverse directions between the two sensors are given by: Ts = L/(C + Vcosθ); Tn = L/(C – Vcosθ). Here, C is the speed of sound in still air, a value that varies depending on the properties of the gas, with the unit being m/s. V is the flow velocity of the gas medium, in m/s. θ is the angle between the path of the sound wave and the axis of the pipe. 　　▲Ultrasonic flowmeters are also relatively easy to use. Install it in a “Z” shape on the pipe surface; make sure to polish the area where the sensor will be installed and apply butter to it, then tighten the sensor to the pipe using fixing devices. 　　▲Disadvantages of ultrasonic flowmeters: their range ratio is slightly smaller compared to thermal gas flowmeters, and maintenance is relatively more complicated. Gases with high precision are expensive; measurement requires temperature and pressure compensation, and the equipment setup is complicated. Rotary flow meter (for gases) ▲Features of rotary flow meters: on-site display, no need for power supply; relatively low cost; There are two types of materials used in these meters – glass tube rotary flow meters and metal tube mass flow meters. It can be displayed locally or remotely, with interfaces such as HART, standard current signal, and PROFIBUS. 　　▲Principle of the rotameter: A rotameter is also a type of velocity flow meter. It consists of two parts: a conical tube that is larger at the top and smaller at the bottom, and a rotor (also known as a float) placed inside the conical tube. The weight of the rotor is balanced using the pressure difference ΔP generated as fluid passes through the gap between the rotor and the conical tube wall (throttling area). During operation, the fluid under test flows in from the lower end of the conical tube and out from the upper end. The magnitude of the upward force exerted by the fluid on the rotor, denoted as F, is always equal to the weight of the rotor, G. That is, F = ΔP*A; G = V*(ρt–ρf)g. Since F = G, it follows that ΔP*A = V*(ρt–ρf)g. Therefore, ΔP = V*(ρt–ρf)g/A (1). In this equation, V represents the volume of the rotor; A is the maximum cross-sectional area of the rotor; g is the local acceleration due to gravity; ρt and ρf are the densities of the material constituting the rotor and the fluid being measured, respectively; ΔP represents the pressure difference between the fluid above and below the rotor in the vertical direction. As can be seen from Equation (1), the pressure difference remains constant throughout the entire operation process. When the flow rate increases, the velocity of flow through the throttle area also increases. Only by increasing the throttle area and reducing the flow velocity can the pressure difference ΔP be maintained constant. Therefore, the flow rate can be determined based on the height of the rotor’s balanced position. It can be expressed by equation (2): Q=k*h(2△P/ρf)1/2 (2) Where k is the instrument constant, and h is the height to which the rotor rises. By substituting equation (1), we obtain: Q = k*h*(2gV(ρt-ρf)/(ρf*A))^1/2. ▲Rotary flow meters are relatively easy to use; all that is needed during installation is to tighten the corresponding bolts. However, it is important to ensure that the flow direction of the fluid is vertically upward. 　　▲Disadvantages of rotameters: the range ratio is only 10:1; pressure loss is relatively high; they are prone to clogging due to debris; installation and maintenance are relatively complex; they are highly affected by vibrations; temperature and pressure compensation are required; the price increases as the pipe diameter increases, with sizes generally being below DN200. Thermal gas mass flow meter ▲Features of thermal mass flow meters: a wide range ratio of up to 1000:1; high sensitivity in the low-range section; unaffected by temperature and pressure, allowing for direct measurement of gas mass; negligible pressure loss; capable of achieving high precision even for large-diameter pipes with low flow rates; price remains relatively constant regardless of pipe size; high precision of up to 1%; a wide operating temperature range of -40°C to 400°C; not sensitive to dust or particles. 　　▲Principle of the thermal mass flow meter: It is a flow meter based on the principle of thermal diffusion. Put simply, a heat source placed in a fluid loses heat as the fluid passes through it; the greater the mass flow rate of the fluid, the more heat is lost by the heat source. Under ideal conditions, the flow rate of the fluid should be equal to the amount of heat lost by the heat source. Therefore, by knowing the amount of heat, we can determine the flow rate of the fluid. 　　Specifically, in terms of implementing a thermal mass flow meter (hereinafter referred to as a thermal flowmeter), it involves the fact that such a meter has two probes: one serves as a reference point for measuring the temperature of the fluid, while the other acts as a heating source. Based on the above, we can obtain equation (3): P/△T = A + B*(Q)^m (3) Where P is the power supplied by the electronic module as a heating source; △T is the temperature difference between the heating source and the reference point; A and B are constants related to the properties of the fluid itself; m is an exponential coefficient also related to the properties of the fluid; and Q is the mass flow rate of the fluid. From Equation (3), we can see that there are two methods for performing flow measurement: fixing P corresponds to the constant power method, while fixing △T corresponds to the constant temperature difference method. We usually use the constant temperature difference method for measurement. The advantages of this method are high sensitivity in the low-range range as well as fast response speed; all products available on the market currently employ this method. 　　▲Thermal mass flow meters are easy to use and install; they are available in plug-in and pipe-type versions. Whether it is plug-in or pipeline type, maintenance and operation are extremely simple; during installation, care should be taken to ensure that the flow direction is the same as the direction indicated by the thermal flow meter. 　　▲Disadvantages of thermal mass flow meters: They are not suitable for installation in environments with high viscosity; they are also not appropriate for use in locations where the water droplet content is above 40%. Based on numerous practical application cases in various industries, the advantages of thermal gas mass flow meters in gas measurement are particularly evident, especially in situations with low pressure and complex gas compositions. Compared to other flowmeters, thermal gas mass flowmeters have advantages such as low pressure loss, a wide range, and high sensitivity in the low-range section, which makes them more conducive to energy savings. Its successful application provides equipment for accurate measurement of biogas flow in the future.