Design and Application of Insertion Type Gas Flow Meters

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This post was last edited by liaifeng on 2020-6-4 at 18:49. Summary: To address the issues associated with throttle flowmeters in measuring gas flow, an insert-type gas flowmeter was designed, which consists of a pressure tapping head, a pressure tapping and conducting device, and a sealing and fixing mechanism. After the use of plug-in gas flow meters for measuring the flow of coke oven gas, blast furnace gas, mixed gas, and other types of gas at the Laiwu branch of Shansteel Co., Ltd., the gas measurement data has become more accurate, production has become more stable, and production costs have been reduced. 1 Preface The devices currently used for measuring gas flow rates are mainly throttle flow meters, such as orifice plates, internal components, and V-cones. These flow measurement devices have several drawbacks in use, including the need to stop production for installation, high pressure losses, tendency to clog, and difficulty in clearing blockages. Taking coke oven gas as an example, insufficient gas purification capacity, high levels of impurities, and the formation of naphthalene at low temperatures can cause blockages in the throttling devices, which severely affects the measurement of gas flow and may even lead to blockages in the pipes at those throttling locations. When a throttle device becomes clogged, it can only be cleared during a shutdown of the process; thus, there are few opportunities for maintenance, and the task is difficult. This is especially true for large-diameter pipes, as removing and reinstalling them requires the use of heavy lifting equipment, which makes the process time-consuming and labor-intensive. To this end, it is necessary to develop an insert-type gas flow meter that is resistant to clogging, has low pressure loss, and can be installed and maintained online. 2 Working Principle of Insertion Flow Meters Insertion flow meters are differential pressure type flow meters that utilize the Pitot tube principle to measure the flow velocity at the center of a pipeline, and then convert this value into volumetric and mass flow rates. They are used to determine the flow velocity at the center of a fluid-carrying pipeline. Such meters consist of a pressure sensing tube and a pressure probe; the pressure sensing tube has full-pressure channels and static-pressure channels. The side of the pressure probe facing the flow carries full-pressure holes that are connected to the full-pressure channels, while the side facing away from the flow carries static-pressure holes that are connected to the static-pressure channels. During operation, the pressure tapping head of the insert-type flow meter is inserted into the center of the pipeline; the total pressure port is aligned with the direction in which the fluid flows, while the static pressure port is aligned with the direction opposite to that of fluid flow. The difference between total pressure and static pressure represents the actual differential pressure at the center of the pipeline. This value is then used, together with the wind tunnel calibration curve of that pressure tapping head, to determine the standard differential pressure at that point. The flow rate of the fluid is calculated based on this standard differential pressure ； At the same time, a pressure transmitter is required to measure the fluid pressure, and a thermistor is used to measure the fluid temperature. The standard differential pressure signal, pressure signal, and temperature signal are all fed into the flow integrator or the DCS system; this allows for the calculation of the flow equation using the pressure sensors, as well as for pressure and temperature compensation of the fluid, in order to ensure measurement accuracy, just like in the case of magnetic float level gauges. 3 Design and Installation of Insertion Flow Meters 3.1 Pressure Transducers Pressure transducers are an important component of insertion flow meters, used to obtain the differential pressure signal generated by the fluid. The pressure tap features a curved surface design; when it is inserted into the center of the pipe, the front side wall and the total pressure port are aligned with the direction of fluid flow, while the back side wall and the static pressure port are aligned with the opposite direction of flow. The blocking element is located on the curved surface adjacent to the total pressure port, and this element causes the fluid to flow away from the back side wall, thereby reducing the amount of fluid that flows toward the back side wall – and consequently, reducing the amount of fluid flowing toward the static pressure port inside the pipe. This increases the differential pressure, enhancing the stability and accuracy of the measurements. Since the total pressure port is aligned with the direction of the incoming flow, a high-pressure area is formed within the port, with a pressure slightly higher than that inside the pipeline. This prevents impurities in the gas from entering the total pressure port, thus avoiding blockage of the pressure probe. 3.2 Pressure tapping and conduction device: The operator of the electromagnetic flowmeter explains that the pressure tapping and conduction device is a mechanism that transfers the differential pressure generated by the pressure tap to the differential pressure transmitter, thereby enabling the conversion of physical signals. The pressure tap is connected to the three-valve assembly above via two pressure conduits, and the three-valve assembly is in turn connected to the differential pressure transmitter, allowing for the measurement of the standard differential pressure on both sides of the pressure tap. The three-valve assembly uses model 3051, and can be connected directly to the vast majority of differential pressure transmitters available on the market. The pressure guide tube features a small cross-section, with an outer diameter of only 30 mm; this design hardly hinders the flow of gas within the pipeline, resulting in minimal pressure loss. 3.3 Sealing and Fixing Devices The sealing base is primarily used for fixing and sealing plug-in flow meters. Since the gas inside the pipeline may be toxic, a gas leak could lead to serious accidents; therefore, it is very important to ensure proper sealing of the flow meters. The sealing and fixing device is mainly composed of a pipeline welding base, a ball valve, an elastic sealing ring, and a sealing compression device, and its structure is shown in Figure 1. In this case, the pipeline welding base is directly welded to the pipeline, thereby enabling the connection between the flow meter and the pipeline ； Ball valves are used to seal the gas in the pipeline, ensuring gas isolation in the pipeline before and after the flow meter is inserted or removed ； The elastic sealing ring is made of corrosion-resistant and high-temperature resistant elastic graphite; its inner diameter matches that of the outer diameter of the pressure guide tube. After the flow meter is inserted, a sealing compression device applies pressure to the elastic sealing ring, which deforms under this force and fits tightly against the pressure guide tube, thereby achieving gas sealing. http://www.jssanchang.com/d/file/jishu/d8de62bdf1791e4cca5c868cafdd4ed6.jpg 3.4 Calculation Method: Before leaving the factory, insert-type flow meters need to be calibrated using a wind tunnel; the flow multiplier K of such meters is determined by calculating based on the standard flow rate and standard differential pressure. In actual use, compensation is required based on the fluid’s density, pressure, and temperature. The differential pressure model is used for flow rate calculation, and its formula is as follows: Where M represents the mass flow rate, in kg ； K is the flow rate multiplier ； ΔP is the differential pressure value, in kPa ； ρ is the medium density, kg/m3 ； ρ denotes the density of the medium under standard conditions, in kg/m3. 3.5 Installation Method: The insert-type flow meter can be installed with the flow stopped, or it can be installed online using pressure-compatible hole-opening tools. During installation, simply make a circular hole with a diameter of 30 mm in the pipe, weld a sealing fixing device above the hole, and insert the flow meter. Thanks to its support for online plug-and-play functionality, which prevents production disruptions caused by gas supply interruptions, it can be installed at certain measurement points where a shutdown of the gas supply is not feasible due to manufacturing process requirements. Before inserting the pipe, the insertion dimensions should be calculated, and the orientation of the total pressure and static pressure ports should be checked to ensure that after installation, the ends of these two ports are located on the center line of the pipe, that is, at the center of the pipe. Additionally, the total pressure port should be aligned with the direction of flow of the fluid, while the static pressure port should be aligned with the opposite direction of flow. The overall installation method is shown in Figure 2. http://www.jssanchang.com/d/file/jishu/f61276622ee67925cacc6689bdd7cc0b.jpg 4 Conclusion: Insertion-type gas flow meters have been used at the Laiwu branch of Shansteel for measuring the flow rate of coke oven gas, blast furnace gas, and mixed gas. They completely eliminate the drawbacks associated with throttle elements such as orifice plates in terms of installation, measurement, and maintenance. These meters offer accurate measurement results, resistance to clogging, low pressure loss, and the ability to be installed and maintained online, making them an ideal alternative to traditional flow meters. By using insert-type flow meters for gas measurement, the gas measurement data becomes more accurate, production becomes more stable, and costs are reduced ； It provides accurate and reliable monitoring and measurement data for the enterprise’s production and operations, which helps the company to manage gas usage efficiently, improve cost accounting, reduce energy consumption, and enhance efficiency.