The orifice flowmeter is a high-range differential pressure flow device that combines a standard orifice plate with a multi-parameter differential pressure transmitter (or differential pressure transmitter, temperature transmitter and pressure transmitter) to measure gas, The flow of steam, liquid and natural gas is widely used in process control and measurement in the fields of petroleum, chemical, metallurgy, electric power, heating, water supply and so on.
1 Influence of the measured fluid characteristics
Due to the nature of the natural gas itself, complex changes occur with changes in the ambient temperature, which affects the measurement accuracy of the flow meter. For the measurement of natural gas, the working temperature and pressure of the natural gas must be determined. Because the change of the external temperature will change the pressure and temperature of the natural gas itself, it may cause excessive density change and compression coefficient change. Low-density gases present difficulties with certain measurement methods, in which case the selected measurement method is changed, or temperature and/or pressure correction is performed to ensure accurate measurement accuracy.
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2 Instrument performance impact
The error caused by the orifice flowmeter itself mainly includes: the right angle sharpness of the inlet of the orifice plate; the size of the pipe diameter is not in accordance with the calculation; the thickness of the orifice plate is mistaken; the step of the throttle fitting is stepped and eccentric; the flatness of the upstream end face of the orifice plate; The size produces steps, eccentricity; pressure-taking position; welding, weld seam protrusion; pressure hole processing is not standardized or blocked; throttle parts have different axial degrees and so on. These factors all have the potential to affect the repeatability of the orifice. The repeatability is determined by the principle and manufacturing quality of the instrument itself. It is an important indicator in process control applications. The accuracy is dependent on the repeatability and is related to the calibration system. In practical applications, the repeatability of the meter is disturbed by many factors including fluid viscosity, density, etc., which will affect the measurement accuracy. This effect is more pronounced if the meter output characteristics are non-linear.
3 Impact of flowmeter installation
The installation error caused by the deviation of the pipeline arrangement is universal, and the main reason for this is that the length of the straight pipe section cannot be met.
4 Environmental conditions
Although the flowmeter installation can be used normally, due to changes in the environmental conditions and expected conditions, some performance parameters and hardware aspects of the instrument have also changed, which will change the flowmeter measurement results.
Ways to measure the problem of orifice flowmeter
1 Strengthen management. Improve the quality of personnel
The reason why the orifice flowmeter is easy to deviate from the standard is the working principle and structural characteristics of the instrument itself. The error of the instrument itself is generated during manufacture. The installation and usage errors are due to corrosion, abrasion and contamination of the fluid medium during installation or long-term use. Caused by. Therefore, the flow metering system should be installed in strict accordance with the technical requirements to eliminate installation errors.
2 Correct selection of measuring instruments
The measuring instrument should be equipped with a standard throttling device. When the flow variation range is large, wide-range intelligent differential change should be considered. Other types of measuring instruments should be carefully selected in the corresponding standard for measuring natural gas. The measurement model of the secondary instrument should be consistent. The requirements of SY/T6143-1996 calculate Fz input components and calculate C values in real time.
3 Strengthen technical training and learn from foreign experience
Introduce, digest, and absorb new natural gas metering technologies developed in recent years. From the perspective of foreign countries, in the international trade of natural gas, energy metering has almost completely replaced the traditional volumetric measurement, and volume measurement in China's natural gas measurement field still dominates. In order to be in line with international standards as soon as possible, the users can work together with various research institutes to carry out the test of natural gas energy measurement, and complete the standardization on this basis. In order to speed up the test progress, it is recommended to introduce foreign online gas chromatography gas chromatography system and flow computer, and develop a natural gas energy metering system suitable for China's national conditions on the basis of digestion and introduction technology.
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