The measurement of steam is different from other media. There are many factors that affect its accuracy in the actual measurement. It is often that the flowmeter verification is qualified, but the phenomenon that the actual measurement is “inaccurate” often appears. There are six factors that affect the accurate measurement of steam flow meters:
One, the insufficiency of the upstream or downstream straight pipe section. For other traditional flow meters, the requirements for installing straight pipes before and after are 20D and 5D respectively. Assuming that the upstream or downstream straight pipes are insufficient, the fluid will not fully develop, and there will be vortices and distortion of the flow velocity distribution profile. Distortion of flow velocity profile is usually caused by local obstructions (such as valves) or bent pipes, 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 a flow regulator. The simplest and most effective method is to use a flow meter with lower requirements for the upstream and downstream straight pipe sections.
Second, the turndown ratio is insufficient. Turndown ratio refers to the ratio of the maximum flow rate and the minimum flow rate that can be measured by a flowmeter that can ensure a given accuracy and reproducibility. But we must be careful when it comes to turndown, because turndown is based on the actual flow rate. Generally, the maximum allowable speed of a steam system is 35m/s. Higher flow speeds will cause erosion and noise in the system. The minimum allowable flow rate of different flowmeters is different. Generally, the minimum steam flow rate that a vortex flowmeter can measure is 2.8m/s. For the case of insufficient turndown ratio, a flowmeter with a large range ratio should be used or multiple The flowmeters are connected in parallel.
3. The density compensation of steam is incorrect. In order to correctly measure the mass flow of steam, the changes in steam pressure and temperature must be considered, that is, steam density compensation. Different types of flow meters are affected by density changes in different ways. The signal output of the vortex flowmeter is only related to the flow rate, 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 shape of the flowmeter, the square root of differential pressure and the square root of density. ① The difference in compensation accuracy. Temperature measurement has a great influence on compensation accuracy. If temperature and pressure sensors of the same accuracy level 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, due to the gravity effect of the condensed water in the pressure impulse tube, there will be 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 steam flow measurement. For the above phenomenon, zero migration can be performed in the secondary meter (in the flow computer), which is simple and accurate. ③Influencing factors of temperature measurement. Judging from the on-site use of the flowmeter, in addition to the inherent error of the temperature measuring element, the temperature measurement error is related to the non-standard installation.
Fourth, differential pressure transmission error (differential pressure flowmeter). One is zero drift. When the transmitter is installed in the field and put into operation, it is often found that the zero output of the zero output is inconsistent during factory calibration. This kind of zero output deviation is called static pressure error. The adjustment method of the steam flow meter is to pass the same static pressure into the positive and negative pressure chamber, open one of the high and low pressure valves of the three valve group, close the other, and open the balance valve. If it is suspected that the positive and negative pressure chamber is not full of the tested For medium, the gas (or liquid) can be exhausted through the drain valve on the positive and negative pressure chamber, and then the output of the transmitter can be checked. The second is the unreasonable arrangement of the impulse pipes. The impulse pipeline should ensure a reasonable slope so that the bubbles that may appear in the pipe rise quickly into the main pipe, and the impurities in the pipe sink to the sewage valve quickly. The impulse pipeline should be checked and maintained regularly to ensure that there is no leakage and no blockage. 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 temperature of the medium in the positive and negative pressure impulse pipes, resulting in differences in density and transmission distortion, the main feature of the steam flow meter is that the positive and negative impulse pipes should be arranged as close as possible. When used outdoors or in severe cold areas, the liquid in the pressure pipe may freeze, so heat tracing is required. However, it should be avoided that the heat tracing pipe is directly wound on the pressure pipe, which may cause partial vaporization of the medium and false errors.
5. The influence of steam dryness. At present, most of the flow meters used to measure the steam flow are volumetric flow meters. The volume flow is first measured, and then the mass flow is calculated by the density of the steam, that is, it is assumed that the steam is completely dry. However, the steam is not completely dry. If the effect of steam dryness is not considered, the data obtained will be lower than the actual flow rate. Therefore, the secondary instrument (flow computer) of the flowmeter should have the function of setting the dryness of the saturated steam. However, it is also difficult to determine the dryness of steam under 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.
Sixth, pipeline vibration. Steam flow meters are sensitive to mechanical vibration, and measurement results are easily disturbed. Therefore, reliable support design should be made for the pipes before and after the flowmeter.
Post time: 11-11-21