The significance of flow measurement
Published on：2015-1-21 9:24:57 Hits：
According to information on the development of the world's energy in 1920 was the peak of the coal, the percentage of the energy of more than 70%, 70's to 90's petroleum replace coal, oil accounts for energy percentage is 30% to 40%, to 21 the early centuries of natural gas will gradually replace petroleum, natural gas accounted for the percentage of energy projected to more than 50%. Alternate energy to replace its own internal and external reasons, internal reasons in the last 20 years, the slow rise in oil production, while natural gas has high growth from 1970 to 1996 more than doubled the world's annual production of natural gas and oil production base remain unchanged. Allegedly the world still contains extremely abundant natural gas resources, it is the development of the natural gas industry has laid a solid foundation.
External causes alternate energy replacement for natural gas is an efficient, clean fuel and high-quality chemical raw materials, which greatly attracted users. Rapid deterioration of the urban environment is the biggest driving force for change in the energy mix. Automobile exhaust emissions have become intolerable to the city of acid rain occurs frequently warned people can no longer maintain the original structure of the energy.
Our country is rich in natural resources, big country, according to the evaluation, natural gas reserves of 38 trillion cubic meters vision, has proven reserves of 1.6 trillion stand alone master meters. For a long time the development of the natural gas industry has seriously lagged behind, but so far the annual output of more than 200 billion cubic meters, accounting for only 2% of the energy consumption structure (and the world for more than 20%), the situation with natural gas resources in the country disproportionate. Recently, the government attaches great importance to the development of the natural gas industry to make major decisions on policy, "natural gas" to start to pull out a major adjustment of China's energy structure curtain. The city is the natural gas end-users, both industrial enterprises, but also urban public industrial users in the city, it is a heterogeneous group of users. In the use of natural gas is an important issue that must be addressed, it is the need to adapt to the socialist market economy provides gas flow metering systems. Both the supply and demand of natural gas flow measurement is based on bilateral trade settlement, is also the main technical gas production sector efficiency. In the production and management of flow measurement is carried out a routine, important technical foundation work. Accurate flow measurement can improve timely production processes, improve product quality, reduce product costs, ensure safe production, rational management and improve the economic and social benefits. Now we have realized that compressed natural gas as a vehicle fuel and urban enterprises and public utilities. Substitution of gas for coal as a fuel is a basic fundamental measures to improve the quality of the urban environment. These departments need to fit user requirements flow measurement instruments. Because the user is extremely complicated, the requirements for a variety of flow meter, flow meter product diversification is inevitable. Attendant is how to choose the appropriate instrument in such diverse types of flowmeters in this issue. Gas Flowmeter city natural gas meter and long-distance pipelines used for trade settlement have different requirements, which need to be resolved measurement object roughly the low pressure (1.6MPa or less), small and medium flow (DN200 below), poor installation conditions (straight pipe insufficient length), weak management and maintenance functions is straightforward, easy to operate, maintenance-free, affordable and so on. To meet these requirements, although there are many sophisticated instruments, to meet the requirements of users still need a lot of development work, "adapt to the market, always strive to" should be the motto of the production plant.
II. Gas Flowmeter Introduction
Has been used or will be put into use in the meter to the following types: differential pressure flowmeter, turbine flow meters, fluid vibration meter, volumetric flow meter, ultrasonic flowmeter, Target Flowmeter and Coriolis mass flowmeter.
1. Differential pressure flowmeter: Includes standard throttling device for detecting member and the average rate and so on. It is the application of a long history, rich practical experience mature, standards improve (all except speed pipe), a class full of different specifications meter, ten 50 years ago twentieth century it may be the only natural gas meter. It has many deficiencies: If the measurement accuracy in general, on-site installation conditions require high pressure loss, and so narrow range of degrees; for these shortcomings that the recent launch of a series of ways to improve production plant, such as a differential pressure flowmeter asanas, value cutting pieces, interchangeable orifice throttling device, using the standard nozzle, etc., plus 80 years after the two components of the differential pressure flowmeter: differential pressure transmitter and flow indicator with outstanding progress, such estimate the future will continue to occupy an important position flowmeter.
2. Turbine flowmeter: In Europe and the United States following the turbine flowmeter orifice flowmeter second statutory gas flow meter, has been developed for multi-species, the whole series, multi-standard, mass-scale natural gas meter. Standard specification is also very complete, but also inadequate: cleanliness requirements for measuring medium high, life is limited. However, according to the use of foreign department said: 240 had an 8 to 15-year cycle of flow meter calibration, instrument accuracy deviation found still within the specified range. In addition, the use of an efficient rectification rectifier coupled with the turbine flow sensor integrated into the structure of the gas turbine flowmeter, on the upstream side Baffles for elbow or half-open shutoff valve condition, only 2D straight pipe length, for cities with narrow installation space, it is a very prominent feature.
3. Fluid vibration meter: This meter has two types: Vortex and precession (vortex precession) meter.
Vortex has many features, such as the output pulse frequency signal, the signal is a linear relationship with the flow, easy measurement of the total, the wide range of degrees, pressure loss, no moving parts, simple structure, easy installation and maintenance, inadequate for higher flow field rule requirements, the need for long straight pipe, pipe stress style mechanical vibration sensitive, should not be used in strong vibration. Years of practice has proved that the vortex flowmeter selection (suitable measuring range) is critical, the installation requires special attention should sometimes live appropriate adjustments are necessary, these are the key to improve the use of effects.
Precession vortex flowmeter features and basically the same, but there are two differences: the meter much pressure loss and a shorter length of straight pipe requirements. Production plant launched direct-reading (live show) flowmeter welcomed by the users, by the way, all cities should develop natural gas are direct reading flowmeters live shows, it should be used as one of the basic varieties, is a measurement of the total amount based instruments.
4. Volumetric flowmeter include: Roots Flowmeter, film flowmeter (domestic gas meter) and so on. It is a long history of use, a huge amount of natural gas flow meter, a comprehensive standard specification. Accuracy is moderate, special wide range of degrees (150: 1), suitable for small and medium flow range, direct-reading, no external energy and without straight sections and so on.
5. Ultrasonic flowmeter
90s twentieth century began gas ultrasonic flowmeter applications in the natural gas industry, with its excellent features caused great international importance, becoming the plate, the vortex of an important class of natural gas flow meter. Its features are: high accuracy, a very wide range of degrees, can be adapted to a very low velocity (0.5m / s or less), the short length of straight pipe installation (multi-channel varieties) and so on. Currently shortcomings: only suitable for medium and large caliber, expensive and so on.
6. Target Flowmeter
The new target flowmeter uses strain gauge force sensors, has many excellent features. Such as high accuracy (up to the total measurement of ± 0.5% ~ ± 0.1%), the scope of the wide (10: 1) for small and medium caliber (DN15 above), no moving parts, simple structure, easy maintenance, and can be implemented dry check (hanging weight method) simplify periodic verification and so on.
7. Coriolis mass flowmeter
Compressed natural gas as a vehicle to promote the use of fuel metering problems must be solved car stations. Coriolis mass flowmeter in liquid successful application extended to encourage people to want to go to the gas, but encountered a problem for low gas density, the instrument can not meet the characteristics required to be `. High-pressure natural gas a substantial increase in the density of the gas just to meet this requirement.
Three. Gas Flowmeter Selection
Faced with so many varieties of natural gas flow meter, for the average user selection became a problem. How to scientifically and objectively select the best flowmeter is an issue of concern. We believe that the selection should follow the appropriate rules to avoid misleading publicity firm, to find a right flowmeter for themselves, it is their ideal flowmeter.
Selection according to five areas: instrumentation performance, fluid characteristics, installation conditions, environmental conditions, and economic factors. Consider various factors as follows:
1. Instrument performance: accuracy, repeatability, linearity, range, pressure loss, upper and lower flow signal output characteristics, response time;
2. Fluid properties: the fluid pressure, temperature, density, viscosity, lubricity, chemical properties, abrasion, corrosion, fouling, dirt, gas compression factor, such as entropy, specific heat capacity, electrical conductivity, speed of sound, mixed-phase flow, pulsating flow, etc. ;
3. Installation conditions: the piping direction, flow direction, upstream and downstream pipe length, pipe diameter, maintenance space, pipe vibration, ground, power, auxiliary equipment belongs to (filtration, sewage), explosion-proof;
4. Environmental conditions: temperature, humidity, safety, electromagnetic interference;
5. Economic factors: acquisition costs, installation costs, maintenance fees, check fees, service life, operating costs (energy), spare parts and so on.
Selection procedure is as follows:
1. Factors based on five primary types of available instruments;
2. Elimination method using selected types in Comparative 2-3, the discharge order;
3. By careful appraisal five again, and finally eliminated until one instrument type.
Selection success depends largely on the selection of personnel know exactly the quality and performance of the instrument measurement object characteristics. For instrument performance quality manufacturers should pay particular attention to the false propaganda and misleading. The exact measurement of the object is very important to understand, not the user object has its own measurements accurate understanding of many selection failure is caused because the provider parameter is not accurate. Some object properties are needed in order to figure out the in-depth investigation.
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