Accuracy analysis of steam vortex flowmeter measurement
Abstract: This paper briefly introduces the method of steam flow measurement and analyzes the causes of the large measurement error.
According to the actual situation of steam flow, the methods and measures to improve the accuracy of flow measurement are put forward, and the matters that should be paid attention to when selecting the correct flow meter for steam flow are available for the flow measurement work.
1 Uncertainty in superheated steam flow measurement  Â
The superheated steam is obtained by heating the saturated steam. The temperature and pressure of superheated steam are two independent parameters, and other state parameters are determined by these two parameters. After the superheated steam is transported, it changes with the working conditions (such as temperature and pressure), especially in the case of low superheat, because the heat loss temperature decreases and it changes from superheated state to saturated or supersaturated state. Become saturated steam or supersaturated steam with water droplets. The saturated steam is suddenly decompressed abruptly, and a part of the droplets will also be converted into superheated steam during adiabatic expansion, thus forming a vapor-liquid two-phase flow medium. Generally, the flow meter cannot accurately detect the flow rate of the two-phase fluid, thereby generating a flow measurement error. .
2 Uncertainty in measurement of saturated steam flow   Â
There is a one-to-one correspondence between the temperature of the saturated steam and the pressure, and there is only one independent parameter between the two. The saturated steam is easily condensed, and if there is heat loss during the transfer, droplets or liquid mist are formed in the steam, which causes a decrease in temperature and pressure. Steam containing droplets or liquid mist is called wet steam. Strictly speaking, saturated steam contains more or less droplets or liquid mist, so it cannot be described by the same gas state equation in different states. The content of droplets or liquid mist in saturated steam reflects the quality of the steam and is generally expressed by the dryness parameter. The dryness of steam refers to the percentage of dry steam in a unit of volume of saturated steam. It is difficult to accurately measure the saturated steam flow. Generally, the flowmeter cannot accurately detect the flow of the two-phase fluid. The steam pressure fluctuation will cause the steam to change in volume, and the flow measurement will produce additional error. Therefore, in the measurement of saturated steam, it is necessary to try to keep the steam break at the measuring point stable, and if necessary, take compensation measures.
3 measurement error analysis
Flow meters are currently used to measure steam flow, and the measurement medium refers to superheated steam or saturated steam in a single-phase flow. For steam with a constantly changing phase flow, there is a problem of inaccurate measurement. In this regard, it is necessary to maintain the superheat of the steam, to minimize the water content of the steam, such as strengthening the heat preservation measures of the steam pipe, reducing the pressure loss of the steam, etc., in order to improve the accuracy of the measurement. However, these methods do not completely solve the problem of accurate steam flow measurement. The fundamental solution to this problem is to develop a flow meter that can measure two-phase flow media. There are many types of flowmeters for detecting gas flow. They are common in speed and volumetric flowmeters. Their common feature is that the specific volume flow of the gas can be measured, and the specific volume flow Gv is a function of the state. The specific volume flow of the gas does not accurately reflect the actual flow. In this regard, engineering is generally expressed in terms of standard state versus volume flow or mass flow. When using a graduated gas flowmeter, the normal gas temperature and pressure are selected as design conditions, and the specific volume flow rate in the design state is converted into a standard specific volume flow rate or a mass flow rate, and the conversion coefficient contains a gas specific volume factor when the gas works. The state deviates from the set state and an error will be produced from the volume flow measurement. In addition, variations in the zen, content, or temperature of the gas have an effect on flow measurement. Therefore, the measurement of the steam flow requires more compensation measures, and the compensation factor due to the state change of the steam is also complicated. The specific volume of superheated steam is determined by two parameters of temperature and pressure of steam, and the expression of specific volume is different in different parameter ranges, which cannot be expressed by the same formula, so a unified specific volume calculation formula cannot be obtained. Only the temperature and pressure compensation of the public body can be derived. In the case where the temperature and pressure fluctuation range are large, in addition to temperature and pressure compensation, compensation for the superheated steam volume expansion coefficient ε needs to be considered.
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