Since the beginning of the 21st century, China has introduced ultrasonic flowmeter in the field of natural gas flow measurement, and with the development of China's natural gas industry, hundreds of ultrasonic flowmeters have been or will be used for high-pressure, high-volume natural gas trade measurement. Ultrasonic flowmeter in high pressure, high flow of natural gas flow measurement, although with other flowmeter unparalleled advantages, but because the actual conditions of use tend to differ greatly with the laboratory conditions.
Therefore, in order to ensure its metrological performance, still need to pay attention to several aspects of the problem, such as: due to noise, dirt and other effects caused by ultrasonic flowmeter measurement accuracy is reduced, or even completely unable to work at the same time, because the temperature and pressure measurement instrument is also Ultrasonic Flow Meter part of the system, so the temperature and pressure measurement accuracy will directly affect the accuracy of the metering system.
In the piping system, there is always a certain amount of noise due to the presence of valves, rectifiers, and various types of choke pipes. However, the changing operating conditions (such as flow, pressure and temperature) and various types of "noise (Flow regulators, pressure regulating valves, etc.) make it difficult to determine the size of the noise.
In general, the main sources of noise are: ① flow through the pipeline high-speed air; ② into the probe; ③ rectifier; ④ pressure or flow control valve. According to the working principle of the ultrasonic flowmeter, it can be known that if the frequency of the noise is consistent with the operating frequency range of the ultrasonic flowmeter, it will interfere with the detection of the ultrasonic pulse, which will affect the accurate measurement of the transmission time and eventually lead to the measurement of the volumetric flow The
Ultrasonic flowmeter is particularly sensitive to noise generated by buck components, and even some low noise valves have a greater impact on ultrasonic flowmeters than pressure regulators because valves with low noise technology are primarily visible to human ears Of the noise range for noise reduction, and this range and affect the ultrasonic signal to noise ratio of the sound spectrum is not exactly the same.
Water, iron sulfide powder or other dirty natural gas flow through the ultrasonic flow meter, the dirt gradually accumulated in the flow meter surface tube and ultrasonic probe, may affect the accuracy of ultrasonic flowmeter, the main impact of the following three aspect:
① reduce the flow meter body of the effective diameter, flow meter readings high.
② dirt on the surface of the ultrasonic probe to shorten the transmission time, flow meter readings are high; but if the flowmeter wall has a more obvious corrosion, after cleaning, the flowmeter diameter will increase. Since the flow is proportional to the square of the inner diameter. Therefore, the flowmeter measured flow will be less than the actual flow through the flow meter, resulting in low flow meter reading.
(3) The change of the surface roughness of the inner wall of the watch body and the upstream pipe section causes the change of the flow velocity distribution, which affects the accuracy and stability of the flowmeter. Differences in the channel arrangement of the ultrasonic flowmeter of different manufacturers lead to different degrees of sensitivity to the impact of dirt accumulation. Experiments have shown that dirty deposits have a greater impact on the small ultrasonic flowmeter with smaller caliber.
Ultrasonic Flow Meter system is generally composed of flowmeter, pressure transmitter, temperature transmitter (or temperature platinum resistance), flow computer and online component analyzer, by input components and pressure, temperature, flow computer can be directly The flow rate of the flowmeter is converted to the volume used under standard reference conditions for trade metering. Pressure and temperature measurements directly affect the accuracy of the metering system.
In the Ultrasonic Flow Meter system, the temperature and pressure measuring instruments used in the field are qualified by laboratory tests, but there will still be cases where the temperature or pressure of the flow computer is not accurate. In the process of natural gas solidification of the Ultrasonic Flow Meter system, it was found that the error caused by volumetric volume under standard reference conditions could be as high as ± 0.5% due to the inaccuracy of pressure or temperature measurement.