Analysis of the reasons for inaccurate measurement of steam flow by flow meters

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This post was last edited by adsl121 on 2010-7-21 21:03. Analysis of the reasons for inaccurate measurement of steam flow by flow meters. I. Analysis of the medium being measured: 1. Superheated steam: Steam is a rather special medium; in general, what is referred to as steam is superheated steam. Superheated steam is a common power source, often used to drive turbines, which in turn drive generators or centrifugal compressors. Superheated steam is obtained by heating saturated steam. It contains absolutely no liquid droplets or mist, and is a real gas. The temperature and pressure parameters of superheated steam are two independent parameters, and its density should be determined by these two parameters. 　　After being transported over long distances, superheated steam, as operating conditions such as temperature and pressure change – especially when the degree of superheating is not high – loses heat and its temperature drops, causing it to transition from a superheated state to a saturated or supersaturated state, thereby becoming saturated steam or supersaturated steam containing water droplets. When saturated steam is suddenly subjected to a significant pressure drop and the liquid undergoes adiabatic expansion, it also turns into superheated steam, thus forming a two-phase flow of vapor and liquid. It causes difficulties in measurement, leading to inaccurate results. 　　2. Saturated steam: Steam that has not undergone heat treatment is called saturated steam. It is a colorless, tasteless gas that is non-flammable and non-corrosive. Saturated steam has the following characteristics. 　　(1) There is a one-to-one correspondence between the temperature and pressure of saturated steam; only one independent variable exists between them. 　　(2) Saturated steam tends to condense; if heat is lost during transmission, liquid droplets or mist form within the steam, resulting in a decrease in temperature and pressure. Steam containing droplets or mist is called wet steam. Strictly speaking, saturated steam is a two-phase fluid that contains droplets or mist to some extent; therefore, the same gas equation of state cannot be used to describe it in different states. The content of liquid droplets or mist in saturated steam reflects the quality of the steam, which is generally expressed by the parameter known as dryness. The dryness degree of steam refers to the percentage of dry steam in a unit volume of saturated steam, denoted by “x”. 　　(3) It is difficult to accurately measure the flow rate of saturated steam, as it is hard to maintain a consistent degree of dryness in the saturated steam. Generally, flow meters are unable to accurately detect the flow rate of two-phase fluids; fluctuations in steam pressure cause changes in steam density, resulting in additional errors in the readings given by the flow meters. Therefore, in steam metering, it is necessary to ensure that the dryness of the steam at the measurement point meets the required standards; compensation measures should also be taken when needed to achieve accurate measurements. II. Analysis of measuring instruments At present, flow meters are used to measure steam flow, and the medium being measured is single-phase superheated steam or saturated steam. For steam with frequently changing phase flow, there is certainly a problem of inaccurate measurement. The solution to this problem is to maintain the superheat of the steam and minimize its moisture content; for example, by improving the insulation of the steam pipes and reducing pressure losses in the steam, in order to enhance the accuracy of the measurements. However, these methods cannot completely resolve the issue of inaccurate steam flow measurement; the fundamental solution to this problem is to develop a flow meter capable of measuring two-phase flowing media. 　　There are many types of flow meters used to measure gas flow rates, among which velocity-type and volumetric flow meters are the most commonly used. Their common feature is that they can only measure the volumetric flow rate under operating conditions; since volumetric flow rate is a function of the state, it cannot accurately represent the actual flow rate. In engineering practice, the volumetric flow rate or mass flow rate under standard conditions is generally used for representation. The so-called standard state volume is the volume of a gas at 0°C and 1 standard atmosphere, or the volume at 20°C and 1 standard atmosphere. The use of mass flow rate as a unit of measurement is not common at present. When using a calibrated gas flow meter, the normal temperature and pressure of the gas are taken as the design conditions; the volumetric flow rate under these design conditions is converted into a standard volumetric flow rate or mass flow rate. The conversion factor includes an element related to the gas density, and when the operating conditions of the gas medium deviate from the design conditions, errors will occur in the flow rate readings. Furthermore, changes in the composition, concentration, or temperature of the gas medium also affect flow measurement; therefore, compensatory measures are necessary when measuring steam flow, and the compensation factors become more complex due to the changes in the state of the steam. 　　The density of superheated steam is determined by two parameters: temperature and pressure. Moreover, within different ranges of these parameters, the expression for density varies, and it cannot be represented by a single formula; therefore, there is no unified formula for calculating density. Instead, compensation formulas for temperature and pressure must be derived individually. In situations where temperature and pressure fluctuate significantly, in addition to performing temperature and pressure compensation, it is also necessary to consider compensation for the gas expansion coefficient ε. 　　Regardless of the type of flow meter used to measure the flow rate of saturated steam, pressure compensation measures must be taken when operating under conditions of fluctuating steam pressure. This is because the flow equation includes a factor related to the density of the steam; when the operating conditions differ from the design conditions, errors occur in the readings. The magnitude of these errors is related to the difference between the actual operating pressure and the design pressure – a value of P_actual > P_design results in negative errors, while otherwise positive errors occur. The dryness condition of steam* is an important factor for accurately measuring steam flow rates; currently, the following three measures should be taken: (1) The pipelines used to transport steam must have adequate insulation to prevent heat loss. 　　(2) Drainage should be provided for each section of the steam pipeline; traps should be installed at the lowest points of the pipeline as well as on the pipes in front of the instruments, to remove condensate water promptly. 　　(3) During boiler operation, it is necessary to avoid excessive bubble levels and minimize large fluctuations in load.