With the surge of natural gas use in China, gas companies pay more and more attention to the selection and use of natural gas flowmeter. The measurement difference of the gas flowmeter directly affects the management of the company. This paper will discuss the use of different types and principles of flowmeter in different metering occasions, and analyze the advantages of each flowmeter.
Natural gas metering flowmeter and its accuracy
Common types of natural gas metering flowmeters include membrane flowmeter, gas waist wheel (roots) flowmeter, gas turbine flowmeter and gas ultrasonic flowmeter. Among them, membrane and gas waist wheel flowmeter belong to volumetric flowmeter, and gas turbine flowmeter and gas ultrasonic flowmeter belong to velocity flowmeter. According to the national requirements for energy measurement, the measurement accuracy of membrane, gas waist wheel flowmeter, turbine flowmeter and gas ultrasonic flowmeter is required to be within the range of ± 1.0% ～ ± 2.0%.
Main application scope of natural gas metering flowmeter
Membrane flowmeter
Scope of application: for the measurement of domestic and catering users, the gas pressure is in the range of 0.5kpa-20kpa, and the gas consumption is: the maximum flow of gas load of gas equipment is ≤ 40m3 / h, so membrane gas meter should be preferred.
Technical indicators: working environment temperature: - 10 ℃ ~ + 40 ℃; Basic error: ± 3% when qmin ≤ Q ＜ 0.1qmax; ± 1.5% when 0.1qmax ≤ Q ≤ Qmax.
Gas waist wheel (roots) flowmeter
Application scope of gas waist wheel flowmeter (with temperature and pressure compensation): generally, the measurement range of gas waist wheel flowmeter is 0.8 ~ 480m3 / h. However, considering the actual installation requirements, when the maximum flow of gas load of gas equipment of industrial and commercial users is greater than 25m3 / h and less than 250m3 / h, gas waist wheel flowmeter should be preferred. The range ratio can reach 1:200, which is suitable for the measurement of gas flow in medium and low pressure pipelines. It is mostly used for the measurement of gas consumption in small boiler rooms, commercial gas consumption points or large load changes. Temperature and pressure compensation devices are required during measurement.
Installation conditions: the temperature range of metering medium gas is - 20 ℃ ~ + 55 ℃, and the ambient temperature is - 20 ℃ ~ + 55 ℃; Avoid being placed in the open air, long-term direct sunlight, easily soaked or wet places; Compared with the membrane meter of the same specification, the minimum flow and working pressure are relatively high and the volume is small; The upstream of gas waist wheel (roots) flowmeter must be equipped with filters of corresponding specifications and cleaned regularly. Installing appropriate filter is an effective way to reduce faults and prolong service life of gas waist wheel flowmeter.
Gas turbine flowmeter
It is applicable to the measurement of natural gas, LNG liquefied natural gas and LPG liquefied petroleum gas. It is mainly used for gas equipment with relatively constant heat load. It is mostly used for large customers such as industrial furnaces, large boilers and chemical gas equipment. It has the characteristics of high precision, small volume, moderate range ratio and large flow range. It can be used for the measurement of gas flow in high, medium and low pressure pipelines.
As the gas turbine flowmeter belongs to the velocity type flowmeter and has high requirements for the front and rear straight pipe sections, when selecting the gas turbine flowmeter, the length of the front and rear straight pipe sections of the flowmeter shall be determined according to the technical requirements of the selected flowmeter samples, usually 5D and 3D.
Compared with membrane meter and gas waist wheel flowmeter of the same specification, it has large minimum flow and wide working pressure range. Because the range ratio is not too large (generally 1:20 under normal pressure), it is not suitable for catering users with large fluctuation of gas consumption, and it is suitable for industrial and commercial users with relatively stable gas consumption.
High pressure gas ultrasonic flowmeter
The high-pressure gas ultrasonic flowmeter is suitable for the trade handover and metering comparison of long-distance natural gas pipelines and urban gate stations. In such applications, the pressure is particularly high and the flow is particularly large.
Compared with other flowmeters, the high-pressure gas ultrasonic flowmeter has a high range ratio, up to 1:100, which is suitable for the measurement of high-pressure natural gas. Due to the probe principle, the ultrasonic probe works more stably and reliably under high pressure, and the measurement accuracy is high, up to 0.5 meter accuracy, which is especially suitable for large flow measurement occasions.
However, due to the high technical threshold, the current mainstream brands are imported from abroad, so the price is very expensive and is not suitable for ordinary urban gas metering users. In addition, due to the design principle of high-pressure gas ultrasonic flowmeter, it is required to install a long front and rear straight pipe section (usually it is recommended to install 20d before and 10d after installation), and it is necessary to install a special rectifier plate for gas flow rectification. If necessary, it is also necessary to install π noise reducer to prevent the noise of on-site voltage regulator from interfering with the signal of ultrasonic probe. The on-site installation requirements are very high.
Medium and low pressure gas ultrasonic flowmeter
This kind of product is different from high-pressure gas ultrasonic flowmeter. Although the working principle is similar to that of high-pressure ultrasonic flowmeter, there are essential differences in structural design, probe type and rectification mode. Different manufacturers of medium and low voltage ultrasonic flow meters have designed different rectification methods. Some use external rectifiers and some use built-in rectifiers. The high-pressure ultrasonic flowmeter needs to be used under high pressure, and the probes are special alloys, while the medium and low-pressure gas ultrasonic flowmeter uses the probe with ceramic surface, and the working pressure is less than 1.0MPa, which is suitable for industrial and commercial users of urban gas.
The accuracy of medium and low pressure gas ultrasonic flowmeter is grade 1, which meets the national accuracy requirements for natural gas measurement. The range ratio can reach 1:80, which is much higher than the commonly used gas turbine flowmeter, and is similar to the range ratio of gas waist wheel flowmeter.
Traditional mechanical flowmeter, such as gas turbine flowmeter and gas waist wheel flowmeter. During the operation of the equipment, due to the continuous rotation of mechanical parts, the bearings need to be lubricated regularly. Otherwise, the mechanical parts will be worn to a certain extent, resulting in the slow measurement of the flowmeter, which will cause the measurement loss of the gas company. The gas ultrasonic flowmeter used in medium and low pressure has no moving parts and does not need to be lubricated regularly.
The gas ultrasonic flowmeter can remotely diagnose the working state of the flowmeter by using the sound wave principle of the ultrasonic probe.