Selection and comparison of several characteristics for measuring natural gas flowmeters
As a clean energy source, natural gas is being increasingly valued by the users. Under the general trend of energy conservation and emission reduction, the country is adjusting its energy structure, and the proportion of natural gas in energy use is increasing year by year. With the expansion of China ’s proven natural gas resources, especially the construction of the West-to-East Gas Transmission Project, the area covered by natural gas is becoming more and more extensive. Therefore, the measurement and settlement of natural gas flow has become an important link, and Opportunity for the development of natural gas flow meter manufacturers.
The flow measurement of natural gas is currently mainly used in trade settlement. It is common to measure natural gas trade in China by volume or energy under the quality indicators required by law. At this stage, volume measurement is the main method.
At present, China's natural gas flowmeter products are roughly: gas waist flowmeter, gas turbine flowmeter, precession vortex flowmeter, vortex flowmeter, ultrasonic flowmeter and orifice plate flowmeter. Below we make a simple comparison of the use of these flow meters.
I. Gas Waist Wheel Flowmeter
Gas waist flowmeter is also called Roots flowmeter, which can be used not only to measure dry gas, but also to measure moisture (that is, associated gas). Orifice flowmeters and turbine flowmeters are not suitable for measuring associated gas containing liquid droplets, and because gas flowmeters have no strict requirements, they have certain advantages. In addition, another advantage is that it is not sensitive to convective arterial motion.
But pay attention to the following points when measuring natural gas:
1. Generally, a certain number of filters should be installed upstream of the flowmeter to prevent the flowmeter from being blocked and blocked, and the sewage should be regularly discharged and the filter screen should be inspected and cleaned.
2. In order to prevent the flowmeter from accumulating fluid in the measurement chamber, the flowmeter should be installed vertically as far as possible, or higher than the process pipeline, and the fluid should be discharged regularly.
3. Before the flowmeter is put into operation, it must be ensured that there are no impurities such as welding slag in the pipeline, and the bypass is opened to prevent the flowmeter rotor from running at high speed and damaging the flowmeter.
4. It is necessary to observe the pressure before and after the flowmeter frequently, and judge the fault in time.
In addition, the gas waist wheel flow meter also has a big disadvantage, which is that the waist wheel is noisy when rotating at high speed.
Second, the gas turbine flowmeter
The gas turbine flow meter
is a natural gas flow measuring instrument that is frequently used at present. The advantages are simple structure and convenient installation; relatively small dimensions; high degree of repeatability; good repeatability; wide range can reach 15: 1 to 25: 1. In the case of high-pressure gas transmission, the range can be further increased; its output is a pulse frequency signal, so when it is used with a programmable flow display meter, it is easy to obtain a lower system uncertainty.
The disadvantages are the high-speed rotation of the turbine, the mechanical friction between the bearing and the shaft, and the service life is not very long. Therefore, attention should be paid to the lubrication. The lubrication means provided by the manufacturer can be used to replenish the lubricant regularly. In addition, if the high-speed flowing gas contains large solid particles, it is easy to damage the turbine blades. Therefore, a filter should be installed on the pipe in front of the turbine flow meter.
Third, precession vortex flowmeter
The precession vortex flowmeter is the current flowmeter product of some small natural gas distribution stations in China. It has a wide operating temperature range; a large range; the Reynolds number is within a certain range and is not affected by fluid temperature, pressure, density and viscosity Impact; strong applicability, except for containing larger particles or longer fiber impurities, generally do not need to install a filter; lower requirements for upstream and downstream straight pipe sections, take the upstream 4D and downstream 2D straight pipe sections; the output frequency is the same as the volume flow Into a linear relationship.
The disadvantage is that the pressure loss is large. Secondly, the precession vortex flowmeter is a fluid vibration flowmeter, which is more sensitive to pipeline vibration and electromagnetic interference, so it can only be used in environments with small vibration and no electromagnetic interference.
Four, vortex flowmeter
are currently widely used in the internal evaluation of natural gas and energy consumption measurement in industrial and mining enterprises. They have reasonable overall structure design, wide dynamic measurement range, and small pressure loss. The main body of the meter can be made of stainless steel, which is suitable for corrosiveness. Medium measurement; the instrument has no moving parts, and is easy to install and maintain; on-site liquid crystal display, pulse, 4-20mA output or 485 serial communication interface, can be connected with industrial automation systems and many other advantages.
Its biggest disadvantage is that it is afraid of vibration and electromagnetic interference.
Five, ultrasonic flowmeter
Gas ultrasonic flowmeter is currently the latest technology of flowmeter products. Its application in China has just begun. There are no manufacturers producing gas ultrasonic flowmeters in China. Due to its wide measurement range, high measurement accuracy, no pressure loss and moving parts, and low installation and use costs, it is used by hundreds of users in Europe, America and other countries for natural gas trade measurement. So far, the governments of 12 countries including the United States, the Netherlands, the United Kingdom, and Germany have approved gas ultrasonic flowmeters as the legal measuring instruments for natural gas.
The accuracy of the gas ultrasonic flowmeter is not only related to the flow rate, but also to the instrument caliber. For the small-caliber instruments, the accuracy of the small-diameter gas ultrasonic flowmeter is difficult to improve due to the short channel length and it is difficult to measure the propagation time of sound waves in turbulent gas.
Orifice flowmeters are the most widely used flow meters for measuring natural gas flow. For decades, people have summarized dozens of special research and practical applications for natural gas measurement, and a qualitative leap has occurred on the basis of quantity. The symbol is standardization. Even if a standard orifice plate flowmeter is used, it can be used without real flow Calibration to determine the relationship between the signal (differential pressure) and the flow rate, and to estimate the measurement error, is the only one of all flowmeters that has reached this standard. In order to eliminate the major shortcomings of the output signals being analog signals, low repeatability, narrow range, and large pressure loss, microelectronic technology, computer technology, fixed value throttles and standard nozzles are used to make them The technical level has been further improved.
In short, these types of flowmeters have their own advantages in the measurement of natural gas flow, but also have their own shortcomings. Users can consider their actual conditions when choosing, and analyze specific problems and choose appropriate flowmeter products To ensure the correct measurement.