| Accompanied by construction of many transnational or transregional gas pipeline, the natural gas market is expanding and the trade is growing year by year. Only accurate measurement can provide a scientific basis for the rational use and energy saving of natural gas. It also can maximize economic benefits.Actual flow test of natural gas flow meter is a developmental trend. It is currently the only way to test ultrasonic flow meter and other high-precision flow meter.Improving the test accuracy of the flowmeter, need more accurate flow standard device, as well as eliminating errors in actual flow tests. Wuhan flow measurement Test Station use natural gas in East Gas Transmission pipeline as test medium. Before the gas using, it will be throttled and pressure regulated. Due to the large pressure drop, throttle will bring in great decreasign of gas temperature and test accuracy will be affected.Based on the study of hydrodynamic theory and the theory of elastic-plastic mechanics, combined with the gas source and test process in the Wuhan station, the thesis analized impact of temperature on the test results. First, the fluctuations of the temperature and pressure will lead to changes in the gas density within the pipe section between two flow meters and affect the value flow meters showing. Secondly, gas temperature change will bring in heat distortion of the test tube section and change pipeline volume. It also affects natural gas volume and test accuracy. It was found that in the test, the gas temperature drop 1℃,0.0025% error will be brought in. For the test line is very long, temperature drop easily occurs. For high precision flow meters, this error is not negligible. As a result, the natural gas re-heat program has been designed to heat natural gas after throttling and reduce temperature fluctuations during test.For the flow of natural gas, the flow pattern is also an important influencing factor. When natural gas flows through some pipe fittings, local flow rate may change dramatically. The existence of Asymmetric distribution and eddy will also affect the test of flowmeters. In order to observe the influence of flow pattern on gas test accuracy, we use FLUENT to simulate gas flow in the test pipe. In actual use, there are a number of combinations of standard flow meters and tested flow meters. In order to obtain the effect law of flow pattern, we chosed some typical combinations to simulate the influences of combination types, the number of standard meters (ie the number of inlet pipes), flow rate and pressure. According to the simulation results, based on comprehensive analysis and comparison, the appropriate pipe volume can be determined according to the volumetric flow rate of each cross section of the outlet tube and its error. |
