At present, many companies in the machinery, metallurgy, building materials, and ceramics industries have built their own gas stations, producing gas for heating or firing products. There are many thermal measuring instruments for gas in gas stations, of which the detection of gas temperature and pressure has always been highly valued by the design unit and the station building company, because these two indicators are directly related to the control of gas furnace furnace conditions and gas quality. . In recent years, due to the impact of the market economy, companies have begun to pay attention to reducing energy consumption and minimizing their output costs, so the detection of gas flow has begun to receive the same attention. Through the measurement of gas flow, enterprises calculate the fuel, raw materials and power consumed for the production of unit volume of gas, and thus estimate the gas production cost for economic accounting purposes. The use of gas units, departments of gas metering control, gas cost accounting, to achieve the same control costs, energy saving purposes.
1. Features of furnace gas flow detection and corresponding requirements for its measuring instruments
First, the gas contains certain impurities and its degree of cleanliness is unstable. The gas produced by the furnace is hot and dirty gas. It must go through a series of cooling and purification processes before it can become a cleaner gas and be supplied with pressure. The cold gas still contains a certain amount of oil, dust and water vapor, and once the gas purification process has problems affecting its purification effect, the gas's net degree will decline. This requires that the flow measurement instrument is not sensitive to dirty flow.
Secondly, the diameters of the gas pipes for gas stations are generally relatively large, and most of their diameters are above DN500mm. This requires that their measuring instruments must meet the requirements for large diameter measurements.
In addition, gas belongs to flammable and explosive toxic gases. Its transmission pipelines and connection parts require strict sealing. Therefore, the selected instruments are required to be easy to maintain, and the frequent disassembly and maintenance are avoided as much as possible.
What is more important is that there is a certain safety flow rate limit for the gas transmission. This causes the gas flow rate to be low and the gas medium to be dirty, making it difficult to measure the gas.
2. Selection of gas flow measuring instruments
2.1 Instrument selection principle (1) For the gas station gas characteristics, the selected instrument should meet its basic measurement requirements.
(2) The detection of gas flow in gas stations is mainly based on the provision of enterprise cost accounting, and the accuracy of the selected instruments should also meet the requirements.
2.2 Analysis and Comparison of Instrument Selection Flow measurement instruments commonly used in the industry include differential pressure flowmeters, velocity flowmeters, positive displacement flowmeters, and other new flowmeters such as vortex flowmeters. Volumetric flowmeters are generally used for the measurement of liquid flow and are not suitable for gas measurement. Velocity flowmeters are generally required to be installed on vertical pipe sections that transport liquids, and are only suitable for applications where the pipe diameter cannot be excessively large. In line with gas station gas flow measurement requirements; and vortex flowmeter for small diameter pipeline gas measurement can also be, but the general gas station Datong pipeline gas measurement will appear less economical. So far, most of the gas flow measurement instruments in gas stations have used differential pressure flowmeters, of which orifice plate flowmeters have become more popular.
In recent years, with the increasingly perfect and mature electron target flow meter technology, in a large number of practical applications, the electronic target flow meter has shown its unique advantages in the detection of gas flow in the production furnace.
2.2.1 Orifice Plate Flowmeter (1) Basic Principle of Orifice Plate Flowmeter When the fluid filled with the pipeline flows through the throttle in the pipeline, the flow bundle will form a local contraction at the throttle, so the flow rate increases and the static pressure decreases. Therefore, a pressure difference occurs before and after the throttling member, and the greater the fluid flow rate is, the larger the pressure difference is, so the medium flow rate can be calculated based on this pressure difference.
(2) Classification and selection of throttling devices Choosing throttling devices according to their use can be classified into standard throttling devices, low Reynolds number devices, dirty flow devices, low pressure loss throttling devices, critical flow throttling devices, and miscible flow sections. There are six kinds of flow devices. The device for dirty flow is more suitable for gas flow detection.
Dirty flow devices include orifices, eccentric orifices, wedge orifices, and the like. The lack of orifice plate in the garden can avoid the accumulation of dirt at the entrance of the standard orifice plate. When horizontally installed, the solid dirt contained in the gas can pass through smoothly; the eccentric orifice plate function is the same as that of the orifice hole plate, but the outflow coefficient is taken along with the pressure port. Change in position; wedge-shaped orifices can also pass solid contaminants in the gas.
From the above analysis, it can be seen that the measured cleanliness of the gas medium is not high, the three throttling elements of the dirty flow device can be solved, but the nominal diameter range of the eccentric orifice is generally between DN50 and DN350, and the wedge-shaped orifice plate The nominal diameter is only in the range of DN25~DN250. If the gas diameter is suitable, eccentric orifice or wedge orifice can be used. However, if the diameter of the gas transmission pipe is large, only the selection of the hole is limited. The nominal pipe diameter is within the range of DN50~DN1500, but its measurement accuracy is not high and the range ratio is narrow.
2.2.2 Electron Target Flowmeter (1) Basic Principle of Electron Target Meter The electronic target flowmeter inserts a circular or square spoiler (target) in the middle of the pipeline. When the medium flows in the measuring tube, it is When kinetic energy passes through a spoiler (electron target), it has a force on the spoiler. The force is proportional to the square of the flow velocity of the medium. The mathematical expression is as follows:
F=CdAP·V2/2..................(1)
In the formula: F - Force (Kg) of the spoiler
Cd - Resistance coefficient of the object A - Axial projected area of ​​the spoiler to the measuring tube (mm2)
P - Medium density under operating conditions (Kg/m3)
v - Average flow rate (m/s) of the medium in the measuring tube
The force F received by the spoiler (electron target) is transmitted to the capacitive force sensor via the rigidly connected transmission member (the measuring rod), and the capacitive force sensor generates a voltage signal output:
V=KF..................(2)
In the formula: V - the voltage (mV) output by the capacitive force sensor, K - proportionality constant, F - the force (Kg) of the flow blocker (electron target)
Therefore, after this voltage signal is pre-amplified, AD converted, and computer processed, the corresponding instantaneous flow and accumulated total amount can be obtained.
(2)Advantages of electronic target flowmeter ·No moving parts, anti-blockage, anti-overload, easy to remove ·Low pressure loss, only about 1/2 △P of standard orifice plate ·Inserted structure, simple to install, no bulky Housing, cost saving, accurate measurement, 0.2% accuracy
The sensor is not in contact with the measured medium, there is no part wear, the use of safe and reliable. There are a variety of installation methods for selection, such as the choice of online plug-in, the installation cost is low; online demolition of the main pipe network without stopping · with integrated temperature , Pressure compensation, direct output quality or standard • With optional small signal removal, non-linear correction, filter time selectable, can accurately measure the gas, liquid flow under various normal temperature, high temperature 500 degrees, low temperature -200 degrees Reproducibility is good, generally 0.05 to 0.08%, fast and anti-interference measurement, strong anti-contamination ability, strong anti-vibration, pulsating flow can be measured within a certain range
3. Application of furnace gas flow meter The majority of domestic producers choose orifice gas flow meter furnace gas station, most of its throttling components use standard orifice plate, its advantage is that the calculation data is mature, and its differential pressure flow meter will not be calculated. be usable. However, after running for a period of time, the surface of the standard orifice plate is too dirty due to insufficient gas cleanliness, which results in an increase in the metering error of the meter, and even lead to pressure plugging. And because the gas pressure is generally not high, the pressure loss of the orifice flowmeter is too large, which is a fatal drawback of the orifice flowmeter for measuring gas.
Nowadays, there are more and more generator gas stations that use electronic target flowmeters in China. The electronic target flowmeters have many advantages over the orifice flowmeters used to generate gas flow detection in furnaces, mainly in the following aspects:
(1) The pressure loss is only 1/2 of the standard orifice plate;
(2) No moving parts, firm structure, simple installation and use;
(3) Strong anti-interference and dry impurities;
(4) After long-term operation, its stability, sensitivity, and accuracy will not change significantly;
(5) With integrated temperature and pressure compensation, direct output standard.
to sum up
Gas station gas flow detection, although it has only begun to receive attention in recent years, but the selection of detection instruments is very important, improper selection, not only increase the maintenance work of the gas station, but the measurement is not accurate will mislead the enterprise, resulting in energy waste. The reasonable choice of gas flowmeters is crucial for the control of energy consumption in an enterprise. It is of important guiding significance for enterprises to carry out energy-saving and energy-saving technological transformation and perfect management.
Related Information: Electronic Target Flow Meter