在现场实际应用时，它的测量误差往往会增大，有时可达到10%--20%，特别是在采用孔板流量计作为工艺生产过程的物料(水、蒸汽、煤气及原料)的计量，进行经济核算和物料平衡时，减少测量误差尤为重要。 When the orifice plate flowmeter is applied in the field, its measurement error will often increase, sometimes it can reach 10% -20%, especially when the orifice plate flowmeter is used as the process production material (water, steam, gas and gas). Raw material) measurement, economic calculation and material balance, it is particularly important to reduce measurement errors. However, it must be noted that not only reasonable selection, accurate design calculation and processing, but also proper installation, maintenance and compliance with the conditions of use are required to ensure that the orifice plate flowmeter has sufficient actual measurement accuracy. Here are some reasons for measuring errors:
1. The working state of the measured fluid changes. If the working state of the measured fluid (such as temperature and pressure) and the corresponding fluid density, viscosity, and pipe roughness are not consistent with the design, Some meter constants are used to estimate the flow rate, and there will be errors with the actual flow rate, which can be corrected or redesigned according to relevant calculation formulas. 2. The orifice plate was installed incorrectly. When the commissioning was just started, the gas flow meter showed too small. It was inspected that the orifice plate was installed in reverse, but the sharp side of the orifice plate should face the fluid flow as the inlet end. One side is the exit end, pay attention to the direction. In addition, when the center of the orifice of the orifice plate is not concentric with the centerline of the pipe during installation, it will also cause measurement errors. Blockage of the impulse tube and the appearance of protrusions such as gaskets also cause the error. 3. The entrance edge of the orifice plate is worn. Because the orifice plate is used for a long time, especially in the case that the measured medium is mixed with solid particles and other debris, or chemically corroded, the geometry and size of the orifice plate will change. The sharpness of the entrance edge of the orifice plate is dulled by the impact or corrosion of the medium, so that the pressure difference ΔP generated when an equal amount of fluid passes through decreases, thereby causing the displayed value to be low. 。 In severe cases, the orifice flowmeter needs to be replaced. 4. The scaling of the orifice plate surface and the change of the flow cross-sectional area. In the field use, the surface of the orifice plate may be contaminated with a layer of dirt, or due to the long-term deposition of impurities in the front and rear corners of the orifice plate, or due to strong corrosion The effect will make the cross-sectional area of the pipeline gradually change, and the leakage and blockage of the impulse pipe will cause measurement errors. Due to the dirty gas used in our company (including tar), the pressure ring chambers at the front and back of the orifice plate are blocked, there is no pressure difference between the pressure taps, and no flow indicator is displayed on the flowmeter. After removal, clean the two ring chambers and replace the pressure guiding tube. , The flow meter display is normal. 5. The transmitter zero point drift and improper range setting will also cause measurement errors. Due to the long time, the transmitter zero point will drift. If the drift is negative, the transmitter output current will be less than the standard 4MA, and the flow rate will be low. If the drift is positive, the output current of the transmitter will be greater than the standard 4MA, and the flow rate will be higher. If the range setting is larger, the flow rate will be lower, and if the range setting is smaller, the flow rate will be higher.
安装的7个条件： 7 conditions for orifice plate flow meter installation:
1. In order to ensure that the flow of fluid forms a fully developed turbulent velocity distribution in front of the orifice flowmeter, and make this distribution a uniform axisymmetric shape, so: 1) The straight pipe section must be round, and In the front 2D range of the flow part, the roundness requirements are very strict, and there is a certain roundness index. Specific measurement method: (A) OD, D / 2, D, 2D 4 vertical pipe sections in front of the throttle, measure at least 4 individual diameters of the inner diameter of the pipe at a maximum equal angular distance, and take the average value D. The difference between the single inner diameter measurement value and the average value must not exceed ± 0.3% (B) After the throttle, 8 inner diameter single measurement values are measured at the OD and 2D positions by the above method. Comparing any single measurement value with D, the The maximum deviation must not exceed ± 2%. 2) A sufficiently long straight pipe section is required before and after the throttle. This long straight pipe section is related to the form of the local resistance member in front of the throttle and the diameter ratio β. 3) The length of the straight pipe section between the first resistance member and the second resistance member on the upstream side of the throttling member can be 1/2 of the listed value according to the form of the second resistance member and β = 0.7 (regardless of the actual β value) 2. The straight pipe section used to install the orifice plate flowmeter should be smooth. If it is not smooth, the flow coefficient should be multiplied by the roughness to correct the sparseness. 3. The straight pipe section before and after the orifice plate flowmeter must be straight, and there must be no visible bend. 4. When the upstream side of the throttle is an open space or a large container with a diameter ≥ 2D, the length of the straight pipe between the open space or the large container and the throttle shall not be less than 30D (15D). If there are other local resistance parts between the throttle and the open space or the large container, in addition to the minimum straight pipe length 1 that meets the requirements between the throttle and the local resistance parts, from the open space to the The total length of the straight pipe sections between the flow pieces must not be less than 30D (15D). 5. Open the stainless steel three-valve balance valve, slowly open the valve at the high and low pressure end of the orifice plate, and close the stainless steel three-valve balance valve after the fluid passes the flow meter. 6. Open the inlet and outlet valves, and the opening of the inlet and outlet valves must be the same. 7. Connect the signal and power cables.