Density correction for mixed gas flow metering

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Author: Li Baoshu, Wang Huimin, Lin Jie, Planning and Control Department, Taiyuan Iron and Steel Company. Abstract: Mixed gas is widely used in metallurgical enterprises. Measurement errors occur due to changes in the density of the mixed gas resulting from fluctuations in its composition during use. The article explores this issue. Keywords: mixed gas ； Density correction, ratio adjustment, fluctuations. 1. The importance of density correction in measuring the flow rate of mixed gas: In metallurgical enterprises, large amounts of gas are generated during coking, iron smelting, and converter steelmaking processes. After being cleaned and dust-free, this gas serves as an important secondary energy source. The capacity of the gas pipeline networks in each metallurgical enterprise is limited. Within such limited-capacity networks, if the gas mixture ratios for some users are well-balanced, those for other users will become more unbalanced, thereby causing problems with flow measurement and usage. The most commonly used gas is the mixed gas from coke ovens and blast furnaces. The main technical parameters of coke oven and blast furnace gas under standard conditions over the past few years at Taiyuan Iron and Steel Company are listed in Table 1 below: Table 1 Main Technical Parameters of Coke and Blast Furnace Gas. As can be seen from Table 1, changes in the mixing ratio of the gases pose difficulties in measuring the flow rate of the mixed gas and in controlling the combustion ratio; this in turn affects the quality of heating. Incomplete combustion may also lead to environmental pollution. Therefore, it is very important to perform density corrections for measuring the flow rate of the mixed gas. 2. How to perform density correction for the flow measurement of mixed gas using a differential pressure flow meter. Density correction in flow measurement is a different concept from pressure and temperature correction. The former refers to the change in the standard-state density of the mixed gas resulting from variations in the mixing ratio ； The latter refers to a situation where the density of the gas under standard conditions remains unchanged; it is only the changes in operating pressure and temperature that cause variations in the density of the gas under operating conditions. 2.1 Simple density correction formula for flow measurement: To distinguish it from pressure and temperature corrections, the density of the mixed gas used in the design of the throttling device is denoted as ρ_mix in the design calculations ； If the density of the mixed gas during actual operation is denoted by ρ_mix, then the simple density correction formula for flow measurement is: (1) In this formula, qVN represents the flow rate of the mixed gas under standard conditions, while qVN operating condition refers to the flow rate of the mixed gas under actual operating conditions. 2.2 Standard-state density of mixed gas: In industrial measurement, two methods are often used to determine the standard-state density of mixed gas. (1) The density of the mixed gas is calculated based on the amounts of coke oven gas and blast furnace gas available. Calculation is performed based on the flow rates of coke oven gas and blast furnace gas, as measured separately before mixing: (2) Method for determining the density of mixed gas given its calorific value. Based on the calorific value of the mixed gas determined by a calorimeter, the flow rate ratios of coke oven gas and blast furnace gas are first calculated, and then the density of the mixed gas under standard conditions is determined. In the formula, ρV in percent of coke oven gas represents the proportion of the coke oven gas flow ； Qhigh represents the calorific value of the blast furnace ； Qmix represents the calorific value of mixed gas ； Qj is the calorific value of coke oven gas in MJ/m3. By substituting the above two equations, we obtain: (3) By substituting either the result obtained from equation (2) or equation (3) into equation (1), the correction for changes in density alone can be carried out for flow measurement. 3. How to perform pressure, temperature, and density corrections for differential pressure flow meters used to measure mixed gas flow. To avoid confusion between changes in the density of the standard gas caused by variations in the mixture ratio of the mixed gas, and changes in the density under operating conditions resulting from changes in operating pressure and temperature, the correction formulas for pressure, temperature, and density in flow measurement are as follows: 4. Conclusion The properties of gas used in metallurgical enterprises are generally similar; each enterprise should carry out corrections to the readings of mixed gas flow based on the specific properties of its own gas. If converter mixed gas is used in production, the indication correction can also be carried out using the above method, as the principle is the same. (end)