Selection of flow meters for ultra-large pipe diameters

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By ultra-large pipe diameter, we generally refer to diameters larger than 1 meter, including round pipes/square pipes/rectangular pipes. Due to the large volume of ultra-large diameter pipes, the main consideration in design is how to reduce space occupation and save costs. Therefore, the installation length of the pipeline does not take into account the requirements for installing the flow meter, and as a result, there are no required straight pipe sections of 10 meters in front and 5 meters behind it. This poses considerable limitations for the use of most flowmeters; even if it is used reluctantly, the performance is poor. How to solve this problem? Let’s first discuss the characteristics of large-diameter pipeline flow: 1. The pipe diameter is over 1 meter, with some reaching as high as 5.6 meters. They are mostly rectangular in shape, have complex configurations, with extremely short straight sections that are often less than 1D in length. The flow velocity distribution is complex and changes continuously as the fan load varies (for large-diameter pipes, it is generally the volume of air flow that is measured). 2. Vortices exist within the pipe, and their size and position change continuously with the flow rate. 3. To reduce pressure loss, the flow velocity inside the pipe is usually below 5–10 meters per second; at room temperature, the total static pressure of air is only 15–60 Pa. 4. Gases often contain dust, and conventional differential pressure instruments are prone to clogging. Below are some instruments currently used for measuring large-diameter pipes. Classified by the number of measurement points: 1) Single-point measurement – The flow rate is determined by measuring the velocity at a specific point within the pipeline, such as the center. Examples include insert-type turbines, vortex flow meters, dual Venturi meters, and thermal mass flow meters. For these devices to function properly, the length of the straight pipe section required is 15–25D (where D is the diameter of the pipeline). Due to the unpredictable swirls present in large pipes, it is highly unlikely that there will be a constant average flow velocity at different fan loads; therefore, this type of system is rarely equipped with automatic control. It should be noted here that many flow meter manufacturers claim that their instruments have been calibrated in wind tunnels, achieving an accuracy of ±1%. It is important to remember that this refers to the accuracy of the flow velocity; in simpler terms, flow rate is the sum of the velocities across the entire flow profile. Generally speaking, single-point measurement does not yield satisfactory results when there are insufficient straight pipe sections, or when flow-blocking elements are present at either end. For applications where high linearity/repeatability and reliability are required, while accuracy only needs to be within an acceptable range, the \"On-site Grid Point Selection Calibration Method\" specifically developed and improved by SIERRA Instruments can be employed for such conditions. This method is used to select the optimal insertion point. This method employs a thermal mass flow meter; by using a single meter and selecting multiple points across the cross-section (depending on the degree of consistency between the measured data and changes in air volume), it measures variations in flow rate. Mathematical calculations are performed on the data obtained from all these points to identify the optimal measurement location, so that the flow rate measured at this location is not only linearly accurate but also reflects the average flow rate across the entire cross-section. 2) Multiple points on the measurement line: Flow rate is determined by measuring the flow velocity at multiple points along a straight line within the pipeline (the diameter for circular pipes, and the height or width for rectangular pipes); the flow meter used remains an insert-type device. Compared to fixed-point measurement, it offers improved accuracy and is easier to install; however, the straight pipe section ahead also requires at least 10 D lengths, and the flow velocity distribution within the pipe must have equipotential lines that are approximately concentric in order to achieve the necessary accuracy. For example, in a certain power plant, the area of the pipeline is less than 1 square meter. Due to the vortices present within the pipeline, their size and position keep changing as the wind volume varies. As a result, within the adjustment range of 30–50% wind volume, it was observed that as the wind volume increased, the differential pressure output by the average velocity tube actually decreased. (I’ve also encountered a situation where, when using the velocity-area method to determine the flow rate in the secondary air ducts, no matter how I adjusted the valve opening, the flow rate at that point still showed as 0!) ) Therefore, this measurement method is also difficult to use in industrial control systems. In summary, we can see that measuring the air volume in large ducts is extremely difficult. When it is not possible to change the conditions at the construction site, appropriate measures should be taken to improve those conditions in order to achieve accurate measurement of air volume. 《1》Rectifier: We already know that the key issue causing measurement problems is the lack of a straight section; as long as the flow entering the measuring section of the instrument is fully developed turbulent, things will be fine. A very good solution is to install a rectifier. What is currently recommended is the use of honeycomb rectifiers, which have been successfully applied in wind tunnel equipment for nearly a century. It is characterized by a high ratio of flow area to pipe cross-sectional area, in order to reduce pressure loss. 《2» Multi-point flow meters operate on the same principle as Pitot tubes (which are more commonly referred to as pitot tubes in China) for measuring flow velocity; they determine the flow velocity by measuring the pressure difference before and after a throttling element. The difference lies in the fact that the positions and numbers of measurement points are arranged in a matrix in accordance with relevant standards, thereby accurately reflecting the flow velocity distribution within the pipeline. The more measurement points there are, the more accurately the flow velocity distribution within the pipeline can be reflected. 《3》Automatic purging device: Since the air in the first and second stages of thermal power plants inevitably contains dust, instruments that use differential pressure measurements to determine flow rates are prone to clogging, which limits their use in such plants. To avoid this problem, a backflow device is used to purge the pipes regularly. «4» Measurement involves processing all the data within the system to generate flow values; this is a very detailed and complex process. Regarding things such as flow accumulation, it is only briefly covered here. Advantages of using multiple points: 1. High measurement accuracy. 2. Relatively easy to install, with low pressure loss. It is said (though not confirmed) that thermal power generation in the United States accounts for over 90% of the market. Our country has only adopted this technology for a few years, and it is still in the process of improvement. Of course, abroad, matrix thermal mass flow meters are gradually being used to replace traditional differential pressure flow meters. At present, this technology has already been successfully applied in some power plants in our country. High requirements are placed on brands of thermal gas mass flow meters; domestic products have a short service life, a high failure rate, and poor linearization. Currently, the main brands used in this field both domestically and internationally are SIERRA/FCI/KURZ. The main advantages of using thermal mass flow meters are: good linearity and repeatability, no pressure loss, a wide measurement range, direct measurement of mass flow without the need for temperature or pressure compensation. The pipe diameter has little impact on the cost of such meters; the larger the diameter, the greater the advantages of thermal flow meters. Installation is the simplest among all types of flow meters, resulting in lower installation costs as well as lower maintenance costs. Given these characteristics, it is increasingly becoming the mainstream method for measuring large-diameter flow rates.