Types of natural gas flow meters and factors affecting their accuracy

1 Natural Gas Flow Meter Type

Based on the current development status of natural gas metering instruments, from the perspective of measurement principle, natural gas flow meters can be divided into five categories, namely ultrasonic flow meters, turbine flow meters, rotary flow meters, orifice flow meters and membrane flow meters [1-4]. The specific working principles and characteristics of each type of flow meter 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 changes in pressure and temperature. When the gas volume is under standard measurement, the standard range of natural gas can be reasonably adjusted according to the temperature and pressure of the medium material and the actual natural gas operation conditions, which can effectively reduce the measurement deviation. In the north, the temperature difference between winter and summer is large, and the measurement error range of natural gas flow is 3% to 8%. If the medium pressure and temperature measurement specifications are not formulated, the gas company will suffer a certain degree of loss [1-4].

2.2 Measuring ambient temperature

The accuracy of natural gas metering is also affected by changes in ambient temperature. When the ambient temperature changes, the measurement accuracy decreases. Long-term unstable temperature conditions can cause instrument problems. One instrument in the metering device is a flow sensor , which is made of a thermal expansion material. The flow sensor is very sensitive to changes in the working environment temperature. When the measurement environment temperature is very low, natural gas metering will be slow and the metering error will be large, generally 2.6 to 3.9 times the normal measurement value, indicating that changes in the working environment temperature difference have a greater impact on the metering accuracy of the metering instrument [5].

2.3 Skills and training

The understanding of metering specifications and the improvement of skills of instrument operators are conducive to reducing human measurement deviations. For personnel engaged in flow metering, it is necessary to strengthen skill training and learning to improve the overall comprehensive level of the metering team. First, it is necessary to understand the performance of metering instruments, detect the flow of instruments under different pressures and temperature differences, and select metering instruments with appropriate models and performance according to the working environment; secondly, installation training should be provided to operators, especially to understand the working principle of the meter to avoid improper installation of metering instruments due to improper human operation, which may cause large measurement deviations. In response to the above problems, it is necessary to strengthen skill training for staff, popularize knowledge of natural gas flow metering errors, and master the working principle of metering devices, so as to effectively ensure the safety of natural gas transportation and operation of gas companies.

3 Analysis of factors affecting the accuracy of different types of flow meters

3.1 Analysis of the measurement accuracy of velocity flowmeter

The most widely used velocity flowmeter is the ultrasonic flowmeter. Velocity flowmeters also include turbine flowmeters, vortex flowmeters, and swirl flowmeters. The factors that have a greater impact on the measurement accuracy of velocity flowmeters 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 only be improved when the fluid flow rate and flow state are relatively stable.

(2) Installation requirements for turbine flowmeters. Deflection in the installation of the measuring instrument can also cause metering errors.

(3) Mechanical components. The size of instrument components will also affect the measurement results of the turbine meter. If the fluid contains impurities or the flow meter runs for a long time, it will cause wear on the bearing pressure and reduce the measurement accuracy.

3.2 Analysis of the measurement accuracy of volumetric flowmeter

The most typical volumetric flow meter is the rotary flow meter, and the flow calculation formula is as follows:

q =？n·V

In the formula: q is the volume flow rate, m3/s; n is the number of rotations, cycles/s; V is the discharge volume in a certain period of time, m3/time.

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

When selecting a flow meter, attention should be paid to checking the gap parameters of the instrument components to ensure that all parameters are within the allowable accuracy range to meet the error measurement requirements.

3.3 Analysis of measurement accuracy of differential pressure flowmeter

The differential pressure flowmeter is mainly an orifice flowmeter. The factors that have a greater 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 measured fluid may cause erosion and corrosion at the corners of components and pipe bends.

(2) Instrument performance. The thickness of the flow meter orifice, the flatness of the end surface, the axiality of the components, the pressure taking position, the setting of the pressure lead pipe position and the lead pipe length will all affect the flow volume, thereby reducing the accuracy of the measurement results.

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

4 Conclusion

Correctly selecting the type of flow meter and understanding the use requirements of various flow meters are very important for improving the accuracy of natural gas measurement and reducing 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 safe operation of equipment.