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Natural gas flowmeter categories and impact measurement accuracy analysis

based on the current development of natural gas measurement instruments, from the perspective of measurement principle, natural gas flowmeter can be divided into five categories, including ultrasonic flowmeter, turbine flowmeter, waist wheel flowmeter, orifice flowmeter and skin film meter []. The specific working principles and characteristics of various flow meters are shown in Table 1

2 analysis of factors affecting measurement accuracy

2.1 pressure and temperature

the state of natural gas is very sensitive to the changes of pressure and temperature of a full range of product brands, including Zytel, Zytel HTN, crastin, Rynite, Delin, Hytrel, Vamac. Under the measurement standard state, the standard range of natural gas is reasonably adjusted according to the temperature and pressure of medium materials and the actual natural gas operation, It can effectively reduce the measurement deviation. In the north, the temperature difference between winter and summer is large, and the error range of natural gas flow measurement is 3% - 8%. If the medium pressure and temperature measurement specifications are not formulated, the gas company will have a certain loss []

2.2 the meter measures the ambient temperature at the same time

the measurement accuracy of natural gas is also changed by the change of ambient temperature. If the number of bits of the AD converter, that is, the resolution is low, then the measurement result will not change accurately, and the measurement accuracy will be reduced. Being in the unstable state of temperature for a long time will lead to problems in the instrument. One of the instruments in the metering device is a flow sensor, which is made of a kind of thermal expansion material. The flow sensor is very sensitive to the change of working environment temperature. When the temperature of the measuring environment is very low, the natural gas measurement will be slow and the measurement error will be large, which is generally 2.6-3.9 times of the normal measurement value, indicating that the change of temperature difference in the working environment has a great impact on the measurement accuracy of the measuring instrument [5]

2.3 skills and training

the improvement of instrument operators' understanding of measurement specifications and skills is conducive to reducing artificial measurement deviation. For the personnel engaged in flow measurement, it is necessary to strengthen skill training and learning, and improve the overall comprehensive level of the measurement team. First, it is necessary to understand the performance of the measurement instruments, detect the instrument flow under different pressures and temperature differences, and select the meter with appropriate model and performance according to the working environment; Secondly, carry out installation training for operators, especially understand the working principle of the meter, and avoid incorrect installation of measuring instruments due to improper human operation, which will cause large measurement deviation. In view of the above problems to meet their own needs and even the needs of the world, it is necessary to strengthen the skill training of staff, popularize the knowledge of natural gas flow measurement errors, and master the working principle of measurement devices, so as to effectively ensure the safety of natural gas transmission and operation of gas companies

3 analysis of influencing factors on the accuracy of different types of flow meters

3.1 measurement accuracy analysis of speed flow meters

ultrasonic flow meters are the most widely used in speed flow meters, and speed flow meters also include turbine flow meters, vortex street flow meters, and precession vortex flow meters. The main factors that have a great impact on the measurement accuracy of speed flowmeter are:

(1) fluid density and viscosity. The greater the density and viscosity, the greater the metering resistance, and the lower the metering accuracy. The metering accuracy can be improved only when the fluid flow rate and flow state are relatively stable

(2) installation requirements for turbine flowmeter. The installation deviation of measuring instruments will also cause measurement errors

(3) mechanical parts. The size of the instrument components will also affect the measurement results of the turbine meter. If the fluid contains impurities or the flowmeter operates for a long time, it will wear the bearing pressure and reduce the measurement accuracy

3.2 measurement accuracy analysis of positive displacement flowmeter

the most typical positive displacement flowmeter is the waist wheel flowmeter. The calculation formula of flow is as follows:

q =? N · V

where: q is the volume flow, m3/s; N is the number of turns, cycles/s; V is the volume of discharge flow at a certain time, m3/time

the leakage has a great influence on the measurement accuracy. The leakage is related to the gap between the components of the flowmeter. The larger the gap between the components, the greater the measurement error. The formula for calculating the flow error caused by the leakage is as follows:

when selecting the flowmeter, pay attention to the gap parameters of the instrument components to ensure that all parameters are within the allowable range of accuracy, so as to meet the error measurement requirements

3.3 measurement accuracy analysis of differential pressure flowmeter

differential pressure flowmeter is mainly orifice flowmeter. What has a great impact on the measurement accuracy of the instrument are the fluid characteristics of natural gas, the performance of the instrument itself, and the installation and use conditions. The specific analysis is as follows:

(1) pressure and temperature. Changes in ambient temperature and pressure will affect the density, compressibility and viscosity of natural gas. Impurities in the measuring fluid may cause scouring and corrosion at the corners of components and pipe bends

(2) instrument performance. The thickness of the orifice plate of the flowmeter, the flatness of the end face, the axial degree of the components, the setting of the pressure taking position, the position of the impulse pipe and the length of the impulse pipe will all have an impact on the convection volume liquid, so the accuracy of the measurement results will be reduced

(3) working conditions. The rationality of the installation of pipelines is directly related to the accuracy of the flowmeter's deviation from the center and the measurement of the straight pipe section. At the same time, environmental temperature, humidity, electromagnetic interference, etc. have an impact on the measuring instrument

4 conclusion

it is very important to correctly select the type of flowmeter and understand the use requirements of various flowmeters to improve the measurement accuracy of natural gas and reduce the measurement deviation. Operators should improve their professional skills and knowledge, be familiar with the technical characteristics of instruments and meters, reduce operating errors, improve measurement accuracy, and ensure the safe operation of equipment

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