The installation of the throttling element (standard orifice plate) The straight tube segments before and after the throttling element must be straight and must not be visible to the naked eye. Before and after the throttling piece, a sufficiently long straight pipe section is required. It should be the most reasonable that the national standard GBT2624-2006 should be used to measure the fluid flow requirement of the full pipe with a differential pressure device installed in a circular section pipe. However, in actual engineering, people often install in accordance with the first 10D after 5D. When the medium is gas, the pressure port should be positioned on the horizontal line at an angle of 45° to allow the condensation to flow into the pressure tube. For high-viscosity or corrosive media, an isolator must be installed to transmit the differential pressure signal to the transmitter via the spacer fluid. For liquid media,
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The gas collector should be installed at the highest point of the pressure guiding pipe.
2. Installation of differential pressure transmitter should ensure that there is a straight pipe section at the pressure taking point, and each side is greater than 5D (straight pipe). When taking pressure on the steam pipe, the pressure pipe should be installed on the side of the pipe. The balance tank should be installed at the highest point of the pressure pipe. The sewage pipe should be installed close to the pressure pipe connection of the transmitter. The distance between the pressure pickup point and the transmitter should be greater than 1 meter. The installation position of the transmitter should be lower than the pressure pickup point.
3. Measurement error analysis in the use of orifice flowmeter In the practical application of the orifice flowmeter, if it is used improperly, it will increase its measurement error, sometimes reaching about 10%. How to reduce its measurement error requires a reasonable selection, accurate design calculations, and manufacturing, and more attention must be paid to proper installation, maintenance, and compliance with the conditions of use to ensure that the orifice flowmeter has sufficient actual measurement accuracy. The following analysis of its measurement error:
(1) The flow calculation equation describes the uncertainty of the authenticity of the flow state, and the pulsating flow, multi-phase flow, vortex flow, asymmetrical flow, etc. do not meet the standard requirements. If the flow is still calculated based on the original meter constant, the If there is an error in the actual flow, it can be corrected or re-calculated according to the relevant calculation formula.
(2) Measurement equipment (such as primary components, secondary instruments, calculation output equipment, etc.) Under the premise of meeting the technical requirements of the standard, due to the uncertainty of each device itself and environmental conditions.
1 Incorrect plate installation. When the center of the hole plate is installed, the center of the hole is not the same as the center line of the pipe, which may cause measurement error. The occurrence of errors such as the clogging of the pressure pipe and the occurrence of protrusions such as spacers are also caused.
2 The inlet edge of the orifice is worn. Although the standard orifice plate has long been listed in the international standard ISO5167 and the Chinese national standard GB2624, it is structurally intrinsically weak and it is not durable in practical use. The useful life of an orifice plate can be ten years or longer, but the sharpness of the inlet edge begins to be destroyed only four to five months after installation and commissioning. In use, due to the abrasive action of the fluid, especially for high pressure, or high flow rate particles containing fluids and high temperature steam, etc., its inlet edge will dull faster and be ground to a circular inlet edge. The result is that at the same flow rate, the degree of contraction of the fluid after the orifice is reduced, and the differential pressure is continuously reduced, resulting in an increasingly negative flow error.
The minimum cross-sectional area of ​​the orifice at the outlet of the orifice plate has been increased after the inlet has been abraded. If the calibration can be performed under this condition, it will naturally be found that the orifice outflow coefficient has increased, but it is still used in accordance with the standard formula. The smaller coefficient, so there will be increasing negative system error.
3 Surface fouling and flow cross-sectional area change of the orifice plate. Because the orifice flowmeter is made of throttling parts, when it is used for a long time, the dirt will accumulate on the upstream of the orifice, causing the differential pressure signal to be inaccurate and directly affecting the measurement accuracy. Contamination and passivation of the orifice can cause a measurement deviation of 2 to 10% or more. If the medium to be measured is dirty (including tar), the pressure ring chamber before and after the orifice plate is blocked, the pressure at the pressure port changes or there is no pressure difference, and the flowmeter cannot work normally, resulting in a large measurement error Or no flow display.
(3) Uncertainties of the actual characteristics of the measured medium, such as component changes and sampling, analysis errors, and actual physical property changes affect the meter's normal operation, etc., which have an impact on the uncertainty of the flow value.
(4) Improper setting of transmitter zero drift and span can also cause measurement error. Because of the long time, the zero point of the transmitter will drift. If the drift is negative, the output current of the transmitter will be less than the standard 4MA, and the flow will show low. If the drift is positive, the output current of the transmitter will be greater than the standard 4MA. The display is high. If the range is set to a large value, the flow rate is displayed low, the range setting is small, and the flow rate is displayed high.
In the engineering practice of long-distance and gathering pipelines in China, orifice flowmeters, especially advanced orifice valves, have long dominated. Objectively speaking, in the use of pore analysis flowmeter to measure the flow, the need for orifice plate flowmeter device (orifice throttling device) and secondary instrumentation (differential pressure, static pressure, temperature, measured medium properties, etc.) The selection, design, installation and use must be strictly in accordance with the relevant standards. In addition, irregular maintenance and inspections are required to ensure the measurement accuracy of the flowmeter.
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