/According to the information, the development of energy in the world was the peak of coal in the 1920s, accounting for more than 70% of the energy. From the 1970s to the 1990s, oil replaced coal, accounting for 30% ~ 40% of the energy. By the early 21st century, natural gas will gradually replace oil, and the percentage of natural gas in the energy is predicted to be more than 50%. There are internal and external reasons for the alternation of energy. The internal reason is that the oil production has increased slowly in the past 20 years, but the natural gas has increased rapidly. From 1970 to 1996, the world's annual natural gas production has more than doubled, while the oil production has basically remained unchanged. It is said that the world is still rich in natural gas resources, which has laid a solid foundation for the development of natural gas industry.
The external reason of energy alternation is that natural gas is an efficient, clean fuel and high-quality chemical raw material, which greatly attracts users. The rapid deterioration of urban environment is the biggest driving force to promote the reform of energy structure. Automobile exhaust emission has reached the point of unbearable. The frequent occurrence of urban acid rain reminds people that they can no longer maintain the original energy structure.
China is a big country rich in natural gas resources. According to the evaluation, the potential reserves of natural gas are 38 trillion cubic meters, and the proven reserves are 1.6 trillion cubic meters. However, for a long time, the development of natural gas industry has lagged behind seriously. By this year, the output is only over 20 billion cubic meters, accounting for only 2% of the energy consumption structure (compared with more than 20% in the world). This situation is not commensurate with the large natural gas resource countries. Recently, the government attaches great importance to the development of natural gas industry and makes major policy decisions. The start of "west to East Gas Transmission" has opened the curtain of major adjustment of China's energy structure. The city is the end user of natural gas, in which there are both industrial enterprises and urban public industriesIt's a huge user base. In the use of natural gas, we must solve an important problem, which is to provide a natural gas flow measurement system to meet the requirements of the socialist market economy. Flow measurement is not only the basis of trade settlement between supply and demand sides of natural gas, but also the main technical index of gas consumption efficiency of production department. In the production and management of enterprises, flow measurement is a daily and important technical basic work. Accurate flow measurement can timely improve the production process, improve product quality, reduce product costs, ensure safe production, reasonable management and improve economic and social benefits. Now we have realized that compressed natural gas as vehicle fuel and urban enterprises and public utilities. Using gas instead of coal as basic fuel is the fundamental measure to improve the urban environmental quality. All these departments need flow meters suitable for users' requirements. Due to the complexity of users, there are various requirements for flow meters, and the diversification of flowmeter products is an inevitable trend. Then there is the problem of how to choose the right instrument in such a variety of flowmeter. There are different requirements between natural gas flowmeter used in city and natural gas flowmeter used in trade settlement of long-distance pipeline. The measuring objects it needs to solve are low pressure (below 1.6Mpa), medium and small flow (below DN200), poor installation conditions (insufficient length of straight pipe section), weak management and maintenance force, simple and easy to understand function, convenient operation, maintenance free, moderate price, etc. In order to meet the above requirements, although there are many mature instruments, it still needs a lot of development work to meet the requirements of users. "Adapt to the market, always strive" should be the motto of the manufacturer.
2、 Introduction of natural gas flowmeter
The types of flowmeters that have been used or will be used are as follows: differential pressure flowmeters, turbine flowmeters, fluid vibration flowmeters, volumetric flowmeters, ultrasonic flowmeters, target flowmeters and Coriolis mass flowmeters.
1. Differential pressure flowmeter: including standard throttling device and averaging tube. It is a kind of flowmeter with a long history of application, rich and mature practical experience, perfect standards and specifications (except for averaging tube) and complete varieties and specifications. It may be the only natural gas flowmeter before 1950s. It has many shortcomings, such as general measurement accuracy, high requirement of field installation conditions, large pressure loss, narrow range and so on; In recent years, domestic manufacturers have introduced a series of improvement methods, such as integrated differential pressure flowmeter, constant value throttling element, replaceable orifice throttling device, standard nozzle, etc. in addition, two components of differential pressure flowmeter, differential pressure transmitter and flow indicator, have made outstanding progress since 1980s. It is estimated that this kind of flowmeter will still occupy an important position in the future.
2. Turbine flowmeter: in Europe and the United States, turbine flowmeter is the second statutory natural gas flowmeter after orifice flowmeter. It has developed into a variety, full series, multi specification, mass production scale natural gas flowmeter. The standards and specifications are also very complete, but there are also shortcomings: the requirements for the cleanliness of the measuring medium are high, and the service life is limited. However, according to the foreign user department, 240 flowmeters that have been used for 8-15 years have been checked periodically, and it is found that the deviation of instrument accuracy is still within the specified range. In addition, a highly efficient rectifier and turbine flow sensor are coupled to form an integrated gas turbine flowmeter, which only needs 2D straight pipe length under the condition of elbow or half open stop valve on the upstream side. It is a prominent feature for the narrow installation space of urban use.
3. Fluid vibration flowmeter: this kind of flowmeter has two types: vortex flowmeter and precession flowmeter.
Vortex flowmeter has many characteristics, such as output pulse frequency signal, linear relationship between signal and flow, convenient for total measurement, wide range, small pressure loss, no movable parts, simple structure, easy installation and maintenance, etc, It should not be used in strong vibration place. Many years of practice has proved that the selection of vortex flowmeter (appropriate measurement range) is very important, special attention should be paid to the installation requirements, and sometimes it is necessary to adjust properly on site, which are the key to improve the use effect.
The characteristics of the precession flowmeter are basically the same as that of the vortex street, but there are two differences: the pressure loss of the flowmeter is much larger and the length of the straight pipe is shorter. The direct reading (on-site display) flowmeter introduced by the manufacturer is welcomed by users. By the way, all natural gas used in cities should develop direct reading on-site display flowmeter, which should be one of the basic varieties, and is a kind of instrument based on total quantity measurement.
4. Volumetric flowmeter: including waist wheel flowmeter, membrane flowmeter (household gas meter), etc. It is a natural gas flowmeter with a long history and huge consumption, and has complete standards and specifications. Moderate precision, wide range (150:1), suitable for medium and small flow range, direct reading, no external energy and no straight pipe section, etc.
5. Ultrasonic flowmeter
In the 1990s, gas ultrasonic flowmeter began to be used in natural gas industry. It has attracted great attention in the world because of its excellent characteristics. It has become an important type of natural gas flowmeter after orifice plate and vortex street. It is characterized by high precision, wide range, and can adapt to very low velocity (0.5m / s)The length of straight pipe section is short (multi-channel varieties are used), etc. Current deficiencies: only suitable for medium and large caliber, high price, etc.
6. Target flowmeter
The new target flowmeter adopts strain gauge force sensor and has many excellent characteristics. If the precision is high (the total amount of measurement can reach ± 0.5%~ ± 01%), wide range (10:1), suitable for small and medium caliber (above DN 15), no movable parts, simple structure, convenient maintenance, and dry calibration (hanging weight method) simplified periodic verification.
7. Coriolis mass flowmeter
To promote the use of compressed natural gas as vehicle fuel, it is necessary to solve the measurement problem of vehicle filling station. The successful application of Coriolis Mass Flowmeter in liquid makes people hope to extend it to gas, but a problem is that the gas density is low, which can not meet the requirements of instrument characteristics. High pressure natural gas makes the gas density increase greatly, which just meets this requirement.
3、 Selection of natural gas flowmeter
Facing so many kinds of natural gas flowmeter, it is a difficult problem for general users to select the type. How to select the best flowmeter scientifically and objectively is a problem to be concerned. We believe that the selection should follow the appropriate rules, try to avoid the manufacturer's misleading propaganda, and find a suitable flowmeter for ourselves, which is our ideal flowmeter.
The type selection can be carried out in five aspects: instrument performance, fluid characteristics, installation conditions, environmental conditions and economic factors. The considerations are as follows:
1. Instrument performance: accuracy, repeatability, linearity, range, pressure loss, upper and lower limit flow, signal output characteristics, response time, etc;
2. Fluid characteristics: fluid pressure, temperature, density, viscosity, lubricity, chemical properties, wear, corrosion, scaling, fouling, gas compressibility, isentropic index, specific heat capacity, conductivity, sound speed, miscible flow, pulsating flow, etc;
3. Installation conditions: pipeline layout direction, flow direction, upstream and downstream pipeline length, pipeline diameter, maintenance space, pipeline vibration, grounding, power supply, auxiliary equipment (filtration, sewage), explosion-proof, etc;
4. Environmental conditions: environmental temperature, humidity, safety, electromagnetic interference, etc;
5. Economic factors: purchase cost, installation cost, maintenance cost, calibration cost, service life, operation cost (energy consumption), spare parts, etc.
The selection steps are as follows:
1. According to five factors, the types of available instruments are preliminarily selected;
2. Select 2-3 types in comparison by elimination method and arrange them in order;
3. Carefully evaluate again according to five aspects, and finally eliminate to one instrument type.
To a great extent, the success of type selection depends on the exact understanding of the instrument performance and quality and the characteristics of the measuring object. For the performance and quality of instruments, special attention should be paid to the false propaganda and misleading elements of manufacturers. It is very important to know the measured object exactly. Not all users have an accurate understanding of their own measured objects. Many types of failure are caused by inaccurate parameters. Some of the object characteristics need to be thoroughly investigated< BR>