Major issues affecting steam flow metering

(1) Insufficient upstream and downstream straight sections. For conventional vortex or orifice flowmeters, the front and rear installations of straight sections are required to be approximately 20D and 5D, respectively. If the upstream and downstream straight sections are insufficient, the fluid may not fully develop, and the vortex and flow velocity profile distortion may occur. Distortion of the velocity profile is usually caused by local obstructions (eg, valves) or bends in the pipe, which are generally caused by two or more spatial (stereo) bends. The lack of upstream and downstream straight pipe sections can be adjusted by installing flow regulators. The simplest and most effective method is to use flow meters with lower requirements for upstream and downstream straight pipe sections.

(2) Insufficient range ratio. The turndown ratio refers to the ratio of the maximum flow to the minimum flow that can be measured within a given range of accuracy and reproducibility of a flowmeter. However, we must be careful when it comes to turndown ratios, because the turndown ratio is based on the actual flow rate. The maximum permissible speed of a steam system is 35m/s. Higher flow speeds can cause system erosion and noise. While the minimum flow rate allowed for different flowmeters is different, the minimum steam flow rate that can be measured by a typical vortex flowmeter is 2.8m/s. For a lack of range ratio, a large number of flowmeters or multiple flowmeters should be selected. Flowmeters are connected in parallel.

(3) The steam density compensation is incorrect. In order to correctly measure the mass flow of steam, changes in steam pressure and temperature must be considered, ie steam density compensation. Different types of flowmeters have different ways of affecting density changes. 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 differential pressure flowmeter's mass flow is related to the geometric shape of the flowmeter, the square root of the differential pressure and the square root of the density. 1 Difference in compensation accuracy. Temperature measurement has a great influence on compensation accuracy. If temperature and pressure sensors of the same accuracy class are used, the difference in density due to temperature errors is greater than the error in pressure measurement. 2 pressure measurement influencing factors. In the measurement of steam pressure, a certain difference between the pressure measured by the pressure transmitter and the steam pressure occurs due to the gravitational effect of the condensed water in the pressure-inducing pipe. If the pressure measurement error is not corrected, it will affect the calculation of the steam density and cause the error of steam flow measurement. For the above phenomenon, zero point migration can be performed on the secondary table (flow computer), which is simple and accurate. 3 temperature measurement influencing factors. From the point of view of the flow meter's field use, the temperature measurement error is related to the non-standard installation except for the inherent error of the temperature measurement element.

(4) Differential pressure transmission error (differential pressure flowmeter). One is zero drift. When the 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 steam flow meter is adjusted by passing the same static pressure to the positive and negative pressure chambers. One of the high and low pressure valves of the three valve group is opened and the other is closed, and the balance valve is opened. If it is suspected that the positive and negative pressure chambers are not yet full of measured The medium can be drained (or effusion) by the drain valve on the positive and negative pressure chambers, and then the output of the transmitter can be checked. The second is the unreasonable layout of the pressure tube. The pressure-increasing pipeline shall ensure a reasonable slope, so that bubbles that may appear in the pipe rise faster into the main pipe, and impurities occurring in the pipe sink underground to the sewage valve faster. The pressure piping should be checked and maintained regularly to ensure no leakage or clogging. The inner diameter of the impulse pipe is related to the nature of the fluid to be measured and the total length of the impulse pipe. For the steam system, the inner diameter of the impulse pipe is generally about 10mm. In order to avoid inconsistency of the medium temperature in the positive and negative pressure induction pipes, resulting in a difference in density, causing transmission distortion, the main characteristics of the steam flow meter positive and negative pressure piping should be arranged as close as possible. When used in outdoor or severe cold areas, the liquid in the pressure tube may freeze, so heat insulation is needed. However, the heat tracing tube should not be directly wound around the pressure tube, causing vaporization of the medium and false error.

(5) The effect of steam quality. At present, most of the flowmeters used to measure the steam flow are volumetric flowmeters. First, the volume flow rate 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, steam is not completely dry. If the effect of steam quality is not considered, the data obtained will be lower than the actual flow rate. Therefore, the flowmeter's secondary instrument (flow computer) should have the function of setting the saturated steam dryness. However, it is also difficult to determine the dryness of steam under actual 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.

(6) Pipe vibration. Steam flowmeters are relatively sensitive to mechanical vibrations, and the measurement results are susceptible to interference. The flowmeters must be designed for reliable support before and after the flowmeter.