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Pressure and temperature must be considered in order to properly measure the mass flow of a steam meter
2012-4-12 12:48:20

测量方面存在不少误区,很多用户往往认为购买了高品质的流量计就可以得到准确的计量结果。 At present, there are many misunderstandings about the measurement of steam flowmeters in China. Many users often think that purchasing high-quality flowmeters can get accurate measurement results. The measurement of steam is different from other fluids such as water, air and other media. In actual measurement, there are many factors that affect its accurate measurement. Often, the flow meter itself will pass the verification, but the measurement actually feels “inaccurate”.

准确计量准确的主要问题:量程比不足;上下游直管段不足;蒸汽的密度补偿不正确;蒸汽干度的影响;蒸汽干度的影响;管道振动;差压传送误差(1)量程比不足:量程比是指一个流量计能确保给定的精度和再现性的范围内,所能测量的最大流量和最小流量之比。 The main problems that affect the accurate measurement of the steam flow meter are: insufficient range ratio; insufficient upstream and downstream straight pipe sections; incorrect compensation of steam density; effect of steam dryness; effect of steam dryness; pipeline vibration; differential pressure transmission error (1) Insufficient range ratio: The range ratio refers to the ratio of the maximum flow rate and the minimum flow rate that can be measured within a range where a flow meter can ensure a given accuracy and reproducibility. But we must be careful when it comes to the range ratio, because the range ratio is based on the actual flow rate, the maximum allowable speed of the steam system is generally 35ms, and higher flow speed will cause erosion and noise of the system. The minimum flow rate allowed by different flowmeters is different. The minimum steam flow rate that can be measured by a general vortex flowmeter is 2.8ms. For cases where the range ratio is insufficient, a large range ratio flowmeter (the lowest of GilfloILVA flowmeters) Allowed flow rate is 0.6ms, maximum range ratio can reach 100: 1) or choose multiple flowmeters in parallel. (2) Insufficient upstream and downstream straight pipe sections: For traditional vortex or orifice plate flow meters, the requirements for installing straight pipe sections in the front and rear are about 20D and 5D, respectively. If the upstream and downstream straight pipe sections are insufficient, the fluid will not develop sufficiently, and there will be distortions in the vortex and velocity profile. Distortion of velocity profile is usually caused by local obstruction of pipes (such as valves) or elbows, while vortices are generally caused by two or more spatial (stereo) elbows. Insufficient upstream and downstream straight pipe sections can be adjusted by installing flow regulators. The simplest and most effective method is to use a flow meter with lower requirements for upstream and downstream straight pipe sections. (3) Incorrect density compensation of steam: In order to measure the mass flow of steam correctly, the changes in steam pressure and temperature must be considered, that is, the steam density compensation. Different types of flowmeters are affected differently by density changes. The signal output of the vortex flowmeter is only related to the flow velocity, and has nothing to do with the density, pressure and temperature of the medium. The mass flow of the differential pressure flowmeter is related to the geometric appearance of the flowmeter, the square root of the differential pressure and the square root of the density. ① Compensation for the difference in accuracy. Temperature measurement has a great impact on the accuracy of compensation. If temperature and pressure sensors of the same accuracy class are used, the density difference caused by the temperature measurement error is greater than the pressure measurement error. ②Influencing factors of pressure measurement. In the measurement of steam pressure, a certain difference occurs between the pressure measured by the pressure transmitter and the steam pressure due to the gravity of the condensed water in the impulse pipe. If the pressure measurement error is not corrected, it will affect the calculation of steam density and cause errors in flow measurement. For the above phenomenon, zero shift can be performed in the secondary meter (in the flow computer), which is simple and accurate. ③Influencing factors of temperature measurement. From the field use of the flowmeter, in addition to the inherent error of the temperature measuring element, the temperature measurement error is also related to the non-standard installation. (4) Influence of steam dryness: At present, most of the flow meters used to measure steam flow are volume flow meters. First, the volume flow is measured, and then the mass flow is calculated from the density of the steam, that is, the steam is assumed to be completely dry. However, the steam is not completely dry. If the influence of steam dryness is not considered, the data obtained will be lower than the actual flow rate. Therefore, the secondary meter (flow computer) of the flow meter should have the function of setting the saturated steam dryness. However, it is also difficult to determine the dryness of steam in actual working conditions. If the steam quality at the inlet of the steam flow meter can be improved, the measurement accuracy of the steam flow meter can be improved. (5) Pipeline vibration: Vortex flowmeters are more sensitive to mechanical vibration, and the measurement results are susceptible to interference. Reliable support design should be made for the pipes before and after the flowmeter. If pipeline vibration is unavoidable, a differential pressure flowmeter with strong anti-interference ability should be used, such as the Spirax Sarco ILVA flowmeter. (6) Differential pressure transmission error (differential pressure flow meter): First, zero point drift. When the differential pressure transmitter is installed in the field, it is often found that the zero output is inconsistent when the zero output is factory-calibrated. This zero output deviation is called static pressure error. The adjustment method is to pass the same static pressure into the positive and negative pressure chambers, open one of the high and low pressure valves of the three valve group, and close the other, and open the balance valve. If it is suspected that the positive and negative pressure chambers are not full of the measured medium, then Exhaust air (or fluid) through the drain valve on the positive and negative pressure chambers, and then check the output of the transmitter. The second is the improper arrangement of impulse pipes. The impulse pressure pipeline should ensure a reasonable slope so that the air bubbles that may appear in the pipe rise quickly into the mother pipe, and the impurities appearing in the pipe sink quickly to the sewage valve. The impulse pipeline should be regularly inspected and maintained to ensure that there are no leaks or blockages. The inner diameter of the impulse tube is related to the nature of the fluid being measured and the total length of the impulse tube. For steam systems, the inner diameter of the impulse tube is generally about 10mm. In order to avoid the inconsistency of the medium temperature in the positive and negative pressure impulse tubes, which will lead to differences in density and transmission distortion, the positive and negative pressure impulse tubes should be arranged as close as possible. When used outdoors or in extremely cold areas, the liquid in the impulse pipe may freeze, so heat tracing is required, but the heat tracing pipe should be avoided to be directly wound on the impulse pipe, which will cause the medium to vaporize and cause false errors.

Main problems in steam measurement: Currently, flow meters are used to measure steam flow, and the measurement medium refers to single-phase superheated steam or saturated steam. For steam whose phase flow often changes, there will certainly be an inaccurate measurement problem. The solution to this problem is to maintain the superheat of the steam and reduce the water content of the steam as much as possible, for example, to strengthen the heat insulation measures of the steam pipeline and reduce the pressure loss of the steam to improve the accuracy of the measurement. However, these methods cannot completely solve the problem of inaccurate steam flow measurement. The fundamental solution to this problem is to develop a flow meter that can measure two-phase flowing media. The main problem in steam measurement: There are many types of flow meters for detecting gas flow. Speed and volume flow meters are the most common. Their common feature is that they can only continuously measure the volume flow under working conditions, and the volume flow is a state. As a function of the volumetric flow rate in the working state, the actual flow rate cannot be accurately expressed, and it is generally expressed in the standard state volume flow rate or mass flow rate in engineering. The so-called standard state volume is the volume of gas at 0 ° C and 1 standard atmosphere, or the volume at 20 ° C and 1 standard atmosphere. With mass flow as the unit of measurement, there are not many applications at present. When using a calibrated gas flow meter, the normal temperature and pressure of the gas are selected as the design conditions, and the volume flow rate in the design state is converted into a standard volume flow rate or a mass flow rate. The conversion factor includes the factor of the gas density. When the working state of the gas medium deviates In the design state, the flow indication will produce errors. In addition, changes in the composition, content, or temperature of the gas medium also have an impact on the flow measurement. Therefore, the measurement of steam flow needs to take compensation measures, and the compensation factors due to changes in the state of the steam are also more complicated.
的密度由蒸汽的温度、压力两个参数决定,而且在参数的不同范围内,密度的表达形式也不相同,无法用同一通式表示,所以不能获得统一的密度计算公式,只能个别推导求得温度、压力补偿公式。 The density of the superheated steam flowmeter is determined by the two parameters of temperature and pressure of the steam, and the expression of the density is different in different ranges of the parameters. It cannot be expressed by the same general formula, so a unified density calculation formula cannot be obtained. Individual formulas for temperature and pressure compensation can be derived. In the case of a wide range of temperature and pressure fluctuations, in addition to temperature and pressure compensation, compensation for the gas expansion coefficient ε needs to be considered. Measures to reduce measurement errors: No matter what kind of flowmeter is used to detect the saturated steam flow rate, pressure compensation measures must be taken to work under the condition of steam pressure fluctuations. This is because the flow equation contains the steam density factor and the working conditions. When the design conditions are inconsistent, errors will occur in the reading. The magnitude of the error and the working pressure are related to the deviation of the design pressure. If P >> P, a negative error will occur, otherwise a positive error will occur. The dryness condition of steam is an important condition related to whether the steam flow can be accurately measured. Currently, an online steam dryness measuring instrument is being developed. The application of the dryness meter to the steam flow measurement and compensation system will further improve the measurement accuracy. At present, the following three measures should be taken: (1) The pipelines that transport steam must have good thermal insulation measures to prevent heat loss. (2) Drain water one by one on the steam pipeline. A drain should be installed at the lowest part of the pipeline and the pipeline in front of the instrument to drain condensate in time. Large fluctuations in load reduction. Measures to reduce measurement errors: Correctly select the flow meter. For steam measurement, 5 main factors should be considered when selecting the flow meter: the characteristics of the measured fluid, the production process, installation conditions, maintenance requirements, and the characteristics of the flow meter. Vortex flowmeters are suitable for small and large factories and can meet their measurement requirements. It is best to choose manufacturers with good product quality so that the product quality can be guaranteed.