How to ensure accurate measurement of steam flow rate

The measurement of steam flow rate has always been an important part of steam applications, and the main purposes of using steam flow meters include:1)Monitor the efficiency of energy use;2)Improve the control of the manufacturing process;3)Measure steam usage and conduct internal or external billing. At present, there are many misconceptions about steam measurement in China, and many users often believe that purchasing high-quality flow meters can provide accurate measurement results. The measurement of steam is different from other fluids such as water, air, and other media, and there are many factors that affect its accurate measurement in actual measurement. Therefore, it is common for the flowmeter itself to pass the calibration, but in reality, the measurement may feel "inaccurate".

The main factors that affect the accurate measurement of steam flow are as follows:1）The actual steam flow rate is lower than the minimum flow rate that can be accurately measured by the flowmeter (insufficient range ratio);2）Insufficient straight pipe sections installed upstream and downstream of the flowmeter (due to flow disturbance);3）Incorrect density compensation of steam (inaccurate temperature and pressure measurement);4）Steam contains water (without dryness compensation);5）There is vibration and interference on site (vortex flowmeter);6）Differential pressure transmission error (differential pressure flowmeter), etc.

Below is a specific discussion on the practical issues that affect steam metering: 1Insufficient range ratio Range ratio refers to the ratio of the flow rate that a flowmeter can measure to the minimum flow rate within a given range of accuracy and reproducibility. But when it comes to the range ratio, we must be careful because the range ratio is based on the actual flow rate, and the allowable speed for steam systems is generally35m/sHigher flow velocity can cause erosion and noise in the system. And different flow meters allow different flow rates. Generally, the steam flow rate that can be measured by a vortex flowmeter is2.8m/sFor situations where the range ratio is insufficient, a flow meter with a large range ratio should be used（Gilflo ILVAThe allowable flow rate of the flowmeter is0.6m/sThe range ratio can reach100：1）Or choose multiple flow meters in parallel. 2Insufficient upstream and downstream straight pipe sections For traditional vortex or orifice flow meters, the requirements for installing straight pipe sections at the front and rear are approximately20Dand5DIf the upstream and downstream straight pipe sections are insufficient, it will lead to insufficient fluid development, resulting in vortex and velocity distribution profile distortion. Flow velocity profile distortion is usually caused by local obstruction (such as valves) or bends in the pipeline, while vortices are generally caused by two or more spatial (three-dimensional) bends. Insufficient upstream and downstream straight pipe sections can be adjusted by installing flow regulators. An effective method is to use flow meters with lower requirements for upstream and downstream straight pipe sections, such as Speisak'sILVAFlow meter (upstream)6D, downstream3D）. 3Incorrect density compensation of steam (inaccurate temperature and pressure measurement) In order to accurately measure the mass flow rate of steam, it is necessary to consider the changes in steam pressure and temperature, that is, steam density compensation. Different types of flow meters are affected by density changes in different ways. The signal output of a vortex flowmeter is only related to the flow rate, and is independent of the density, pressure, and temperature of the medium. The mass flow rate of a differential pressure flowmeter is related to the geometric shape of the flowmeter, the square root of the differential pressure, and the square root of the density. Therefore, vortex flowmeters are more affected by density changes than differential pressure flowmeters.a）Compensate for differences in accuracy The accuracy of compensation obtained by using temperature compensation and pressure compensation is not only related to the accuracy of temperature sensors and pressure transmitters, but also to the type of flow meter, specific measurement conditions, and range selection of pressure transmitters. Overall, temperature measurement has a significant impact on compensation accuracy. If temperature and pressure sensors of the same accuracy level are used, the density difference caused by temperature measurement error will be greater than the pressure measurement error. pressure is7bargSaturated steam, usingPlatinum thermistor temperature measurement, with an error limit of ±0.49CAccording to the steam density table, the uncertainty of flow compensation is ±0.56%R(Differential pressure type) and ±1.11%R(Vortex style). If selected0.2The pressure transmitter is used for measurement, with an error limit of ±2kPaThe uncertainty of flow compensation is ±0.13%R(Differential pressure type) and ±0.25%R(Vortex style).b）Factors affecting pressure measurement In the measurement of steam pressure, the gravity effect of condensed water in the impulse pipe can cause a certain difference between the pressure measured by the pressure transmitter and the steam pressure. If the pressure measurement error is not corrected, it will affect the calculation of steam density and cause errors in flow measurement. Generally, for the above phenomenon,