The steam flow measurement is divided into two categories in terms of measurement technology, one is superheated steam and saturated steam with high dryness (dryness x = 0.9 or more), and the other is saturated steam with low dryness. The former category can be treated as a single-phase fluid, while the latter category is a two-phase flow. Since all the current flowmeters are only suitable for single-phase fluids, the low dryness saturated steam needs to be further studied.
1. Flow measurement of superheated steam and high dryness saturated steam
The commonly used flowmeters are: throttling differential pressure flowmeter, which is still the main instrument for measuring steam flow. For example, the throttling device, the differential pressure transmitter and the three-valve group are integrated into an integrated throttling flowmeter. The throttling flowmeter solves the shortcoming of the differential pressure signal tube failure. There are also planting throttling parts, and standard nozzles are used instead of standard orifice plates. Because nozzles are compared with orifice plates, the outflow coefficient of nozzles is stable, and the outflow coefficient will not change due to the blunt edge of the sharp angle. The pressure loss is also lower than that of the orifice plate. , generally at the same flow rate and value, the pressure loss is about 30% to 50% of the orifice plate.
The vortex flowmeter measures medium temperature, that is, below 250 °C. The vortex flowmeter produced by our company is resistant to high temperature and high pressure, and the temperature resistance can reach 350 °C. It should be said that the application to steam has matured, and it is a type of flowmeter commonly used in steam measurement at present. However, it must be noted that the medium with low dryness will make the instrument coefficient deviate from the detection value and increase the measurement error.
The uniform velocity tube flowmeter and the shunt rotor flowmeter can still be used in the internal management distribution where the accuracy requirements are not too high, because the use is relatively cheap and simple, and it is suitable for the measurement of small and medium flow steam.
Target flowmeter is an electric and pneumatic target flow transmitter developed in the 1970s. It is a detection instrument for electric and pneumatic unit combination instruments. Since the force converter directly used the force balance mechanism of the differential pressure transmitter at that time, it brought many deficiencies caused by the force balance mechanism itself. For example, the measurement accuracy is low, the zero point drift, the reliability of the lever mechanism, and the poor stability.
2. Flow measurement of low dryness saturated steam
The saturated steam generated by general industrial boilers is saturated steam with high dryness (above 0.95) at the outlet, but in the process of long-distance transportation, due to many factors such as poor heat preservation or unbalanced intermittent steam use, the dryness is constantly increasing. drop, and even become a wet steam with a high water content, that is, a two-phase fluid of gas and water. The flow characteristics of two-phase fluids are fundamentally different from those of single-phase flow. Flowmeter meter coefficients or outflow coefficients measured in single-phase flow cannot be used for two-phase flow measurements. For example, the outflow coefficient in the two-phase flow test of the orifice flowmeter must be corrected for dryness. Therefore, in the flow measurement of low dryness saturated steam, the dryness parameter is a parameter that must be measured. It is a pity that there is no mature dryness meter yet. In addition, the dryness correction of the meter coefficients of other types of flowmeters has not been studied in depth. Only by solving this problem can the flow rate of low dryness saturated steam be measured.