Density correction for mixed gas flow measurement

1. Importance of density correction for mixed gas flow measurement

In metallurgical enterprises, a large amount of gas is generated along with coking, ironmaking, and converter steelmaking. These gases are important secondary energy sources after cleaning and dust removal.

The capacity of the gas pipeline network of each metallurgical enterprise is limited. In the limited capacity pipeline network, if the gas mixing ratio of some users is well balanced, the gas mixing ratio of other users will be more unbalanced, which will cause problems for flow measurement and use. The

most used gas is the mixed gas of coke oven and blast furnace. The main technical parameters of the standard state of coke oven and blast furnace gas of TISCO in the past few years 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 gas mixing ratio will bring difficulties to the mixed gas flow metering, fuel ratio control, etc., which will also affect the heating quality and may cause environmental pollution due to incomplete combustion. Therefore, the density correction of the mixed gas flow meter is very important.

2. How to perform density correction for mixed gas flow metering with a differential pressure flowmeter

Density correction for flow metering is different from pressure and temperature correction. The former refers to the change in the standard state density of the mixed gas itself due to changes in the mixing ratio; the latter refers to the fact that the standard state density of the gas has not changed, but the working state density of the gas has changed due to changes in working pressure and temperature.

2.1 Simple density correction formula for flow metering

In order to distinguish it from pressure and temperature correction, the density of the mixed gas when designing the throttling device is represented by ρ mixed standard design; the density of the mixed gas during actual operation is represented by ρ mixed standard working, then the simple density correction formula for flow metering is:

(1)

Where qVN is the mixed gas flow rate under the standard state, and qVN working condition is the mixed gas flow rate under the actual working condition.

2.2 Standard state density of mixed gas

In industrial measurement, two methods are often used to calculate the standard state density of mixed gas

(1) The density of mixed gas is calculated based on the known coke oven gas and blast furnace gas volumes. Calculate based on the flow rates of coke oven gas and blast furnace gas detected before gas mixing:

(2) Method for calculating the density of mixed gas when the calorific value of mixed gas is known. According to the calorific value of mixed gas detected by the calorific value meter, first calculate the flow ratio of coke oven gas and blast furnace gas respectively, and then calculate the density of the mixed gas in standard state.

In the formula, ρVcoke% is the proportion of coke oven gas flow; Qhigh is the calorific value of blast furnace; Qmixed is the calorific value of mixed gas; Qcoke is the calorific value of coke oven gas in MJ/m3.

Substituting the above two equations into:

(3)

Substituting the result of either (2) or (3) into (1) will complete the correction of the simple density change in flow measurement.

3. How to perform pressure, temperature, and density corrections for mixed gas flowmeters of differential pressure type

In order to avoid confusion between the density change of standard gas caused by the change of mixed gas ratio and the density change of working state caused by the change of working pressure and temperature, the correction formula for pressure, temperature, and density used in flow measurement is:

4. Conclusion

The properties of coal gas in metallurgical enterprises are similar. Each enterprise should make corrections to the indication of mixed gas flow rate according to its own coal gas properties. If converter mixed coal gas is used in production, the indication correction can also be made according to the above method, because the principle is the same. (end)