In order to reduce air pollution from the automobile exhaust, compressed natural gas (CNG) has been widely used as automotive fuel [1]. With the dramatic increase in the use of CNG, its measurement problems have become more prominent. Coriolis Mass Flowmeter (hereinafter referred to as Coriolis Mass Flowmeter) is the core component of CNG dispenser metering, and it achieves a truly accurate direct mass flow measurement. CNG Dispenser is an application field with mass market prospects to be occupied by mass flowmeters. The author actively cooperated with foreign instrumentation technicians and proposed Coriolis flowmeters in the measurement of CNG dispensers based on the information obtained at the site. The technical points of the technology and the effective technical measures and structural solutions for the main problems in the measurement process have been put forward, which provide guidance for the extensive application of the Coriolis mass flow meter in the natural gas measurement industry.
1 The use of mass flow meters in CNG dispensers
1.1 Principle of Electrical Control of CNG Dispenser
The inlet of the CNG dispenser is connected to the main valve of the filling station. After entering the dispenser, the medium enters the mass flowmeter through the filter and the gas-liquid separator to accurately measure. The measured gas passes through the safety valve and the aeration handle to the gas. In the car. The amount of air-injection is sent to the CPU by the mass flow meter, and the amount of air refueling and refueling amount are displayed on the display screen after calculation, as shown in Fig. 1 [2].
Mass flow meter flow signals are also used to control high, medium and low pressure solenoid valves. The control system diagram is shown in Figure 2. The setting on the dispenser's CPU is: the flow value converted from the low pressure valve to the medium pressure valve is 4.00kg/min; the flow value converted from the medium pressure valve to the high pressure valve is 2.8kg/min; the stop is added by the high pressure valve. The gas conversion flow rate is 1.5 kg/min. The flow signal of the mass flow meter is not always displayed by the dispenser display. When the fill key is pressed, the flow signal is accepted by the CPU and displayed on the filler screen.
2.2 Technical points
(1) The pressure sensor is used as the overpressure protection of the entire dispenser. When the line pressure is greater than 22MPa, the CNG dispenser starts overvoltage protection, and the three solenoid valves are closed to stop filling. When this pressure sensor fails, the refueling opportunity presents some malfunction that appears to be caused by the flowmeter.
(2) For the cost reduction of domestically-made dispensers, solenoid valves are generally used in high, medium, and low valves, rather than pneumatic valves. This is different from imported dispensers (eg, Italian Saifu dispensers), and solenoid valves. If the quality is not good, there will be problems such as lax, which will also cause some seemingly caused by the flowmeter fault, such as zero drift.
(3) When the filling machine fills the car with full gas, the pressure between the main pressure of the filling machine and the pressure of the car cylinder will basically reach a balance.
(4) A taxi can fill air when it is empty. When the air source pressure is 20MPa, the full air is usually about 13m3, and the bus is usually about 80m3. Of course, it also depends on the size and number of cylinders of a vehicle.
(5) The mass flow meter measures the gas medium, its upper limit of measurement is no longer the mass flow, but the flow rate, general requirement is less than 0.5 Mach number.
(6) Natural gas dehydration at some gas stations is not good enough. After the raw material gas enters the station, it is filtered, metered, and then entered into a low-pressure dehydration unit to remove its moisture so that the dew point reaches or falls below the national automotive standard (that is, -55°C in the standard state), and then enters the buffer. Natural gas compressors, after multi-pressurization and pressures up to 25 MPa, enter the gas cylinder group and are used. If the control is not good during this process, there may be water or oil in the gas.
2 Coriolis mass flow meter measuring principle
The Coriolis force mass flow meter measures the mass flow rate of a fluid according to the Coriolis force principle. The Coriolis force refers to the force that an object experiences when it performs a linear motion in a rotating system [3]:
among them, 
Coriolis force, N; m—the mass of the moving object, 
Angular speed, 
Radial speed in rotation or vibration, m/s. The Coriolis force is proportional to the mass m and velocity v of the moving fluid, ie, proportional to the mass flow of the fluid. The Coriolis force mass flowmeter replaces the constant angular velocity with the vibration of the measuring tube. This system can be seen as two rotating systems with equal angular velocity but opposite directions, as shown in Fig.3.
When the fluid flows through two parallel measuring tubes, the measuring tube is subjected to the anti-phase vibration due to the Coriolis force. The Coriolis force generated in the measuring tube causes the tube to be deformed to generate the phase difference between the inlet and the outlet, as shown in Fig. 4 Show. When the flow is zero, that is, when the fluid is stagnant and does not flow, the phase difference between the inlet and the outlet is zero, as shown in Fig. 4(1). When the fluid flows, the vibrations at the inlet and outlet of the measuring tube are not synchronized, and there is a phase difference, such as Figure 4 (2) and Figure 4 (3).
As the mass flow increases, the phase difference AB also increases, and the vibration phase of the tube can be measured through the inlet and outlet electromagnetic phase sensors. The counter-vibration of the two measuring tubes is used to ensure system balance. The mass flow measured by the Coriolis force principle is independent of the fluid temperature, pressure, viscosity, conductivity, and flow characteristics. The phase difference is proportional to the mass flow:
Among them, △ φ - phase difference; K - constant; E - elastic modulus; qm - mass flow, kg / s.
The prerequisite for the normal measurement of the flowmeter is that the two measuring tubes vibrate at the resonant frequency.
3 Main Problems, Causes and Solutions of Mass Flow Meters
3.1 The drift of the flowmeter
The zero drift of the flowmeter is a concern for manufacturers and users of dispensers. Zero drift is also called zero stability. It is a measure of the true zero flow capacity of the meter. That is, when the flow rate is zero, the flowmeter has no rules. The amplitude of the output signal, zero point stability is expressed in units of mass flow in kg/min or kg/h. In the application of the dispenser, it means that the flowmeter has a flow display when no air is added. When the zero drift phenomenon occurs, manufacturers of gas dispenser manufacturers or flowmeter manufacturers generally do frequent zero corrections after the sale of the flowmeter. However, blind zero calibration is not correct, depending on the structure and performance of the flowmeter. set. For the U-tube flowmeter, the vibration frequency is low and it is easily affected by the external vibration, installation stress and severe changes in temperature and pressure. This kind of flowmeter requires frequent zero correction; for the straight tube or microbend flowmeter, it With the characteristics of high vibration frequency, a strong second chamber of the measuring tube, process connection and measurement tube deformation, etc., this type of flow meter generally does not need to be zero, only in some special cases: if it needs a very high Measurement accuracy; the flow rate is very low; extreme operating conditions (high temperatures, for example) require a zero point [4]. Therefore, when the zero-drift phenomenon occurs during the use of the dispenser, it must be determined according to the actual situation on the site.
According to the actual situation found on the site, when the measurement tube contains a certain amount of liquid, there will be zero drift failure. Take E+H Promass 80M mass flowmeter as an example. Its structure is double straight pipe. The installation on most CNG gas generators is horizontal installation, and the outlet of the flowmeter process connection is less than two measuring pipes, that is to say when the CNG gas source When water or oil is not clean, there may be water or oil in the flowmeter measuring tube. At this time, the measuring tube cannot reach resonance. There will be a phase difference at both ends of the measuring tube and there will be a flow signal output, as shown in FIG. 5. According to the on-site experience, the flow rate will reach 300kg/h, which is equivalent to the normal flow rate.
Because the liquid in the measuring tube may be carried away by the air flow when the air is added, such a malfunction sometimes occurs. In addition, when the measuring tube contains liquid, a sudden increase in the flow rate occurs when the air is added, and the dispenser count increases. For this fault, the installation method of the mass flow meter on the dispenser can be designed to be more reasonable. For a straight tube type flow meter, the measurement tube can be installed at an incline; for U-tubes and micro-bend flowmeters, the tube will be measured. The elbow section is installed upright. This facilitates the easy discharge of liquids in the gas or is not in the flowmeter measuring tube.
3.2 Reproducibility of Flowmeters
Among the numerous performances of flowmeters, manufacturers, users, and measurement departments of CNG dispensers care more about the repeatability of the flowmeters, which is more of a problem encountered at the site. Reproducibility refers to the degree of agreement between the measurement results for multiple consecutive measurements of the same measured value under the same measurement conditions. For this problem, many methods have been tried in the scene, such as re-doing zero, modifying K-factors and other unfair methods, but generally can not work. According to some experiences, it is really effective to optimize the flow field of the flow meter.
Case 1: A county-produced gasifier was used in a county in Sichuan Province. The meter on the meter was Promass 83M15 from E+H Company. The reproducibility of the measurement in this area was poor. According to the on-site installation situation, it is recommended to adjust the inlet pipe of the flowmeter to increase the length of the straight pipe section. After the adjustment, the length of the straight pipe section shall be checked again. The repeatability and precision shall be qualified. Fig. 6 and Fig. 7 are on-site pictures, which show the installation of the poor repeatability problem and the installation of improved repeatability.
Case 2: A dispenser manufacturer uses a CNGmass 8FF15 type mass flow meter from E+H. Users have poor repeatability at the time of the factory calibration. When on-site service, no adjustments are made to the flowmeter, only the flowmeter is used. After the entrance trachea made an investigation, the accuracy and repeatability of the two tables were very good. As shown in Fig. 8, the inlet pipe was placed directly downwards when the user performed the test himself.
This shows that in the application of the dispenser, the design of the straight pipe section of the flowmeter can not be ignored, but the dispenser manufacturers generally do not easily modify the formed flowmeter installation pipeline.
3.3 Discussion on Mass Flow Meter Installation Straight Pipe Section
In the materials we can find and in the marketing of manufacturers, there is no need to describe the straight pipe section for mass flow meter installation. Actually, there is a premise that the measurement fluid must be incompressible and full. As mentioned before, the mass flowmeter is a dual-tube structure, and its normal operating condition is that the two measuring tubes resonate. When the fluid is incompressible (most liquids) and the measuring tube is full of no gas, the mass of the fluid in the two measuring tubes remains essentially the same. At this point, the measurement of the flowmeter is independent of the flow field distribution of the fluid, ie, it is not required. Straight section.
However, when the fluid is a compressible fluid (gas and a few liquids) and a gas-liquid two-phase flow, the importance of the flow field on the flowmeter measurement cannot be ignored [5]. From the above two cases, we can see that for high-pressure gas, if the installation conditions are not good, the flow field will be very turbulent, and the quality of the fluid entering the double tubes will be different. When the difference between the fluid quality of the two tubes reaches a value, the measuring tube will be Will not work properly. Figure 9 shows the effect of fluid flow through the elbow on the distribution of the flow field [6].
For a compressible gas such as CNG, the working pressure is about 20MPa. Figure 9 not only reflects the flow velocity distribution, but also reflects the gas mass distribution. Therefore, the mass of the fluid flowing through the two measuring tubes under the disordered flow field will not be equal, and the vibrations of the two measuring tubes will no longer vibrate at the resonant frequency, which will affect the measurement accuracy and repeatability of the flow meter. In extreme cases, the flow meter measures The council will stop working [7∼9]. Therefore, at the beginning of gas filling (a pressure difference of about 16 MPa at this time), the flow meter will experience a similar alarm that the measuring tube is not working properly.
4 Conclusion
Coriolis mass flow meter is the most rapid flow measurement instrument with the most advantages. With the widespread application of Coriolis mass flow meters to CNG dispensers, the issues that arise in CNG metering have received increasing attention. The zero drift problem of the mass flowmeter may be caused by the phase state of the measuring medium. When the CNG gas source contains water or oil and other liquids, the measuring tube cannot achieve resonance. This problem can be solved by purifying the gas and optimizing the CNG dispenser. Structure starts. The reproducibility of the flow meter is the most concerned performance of CNG manufacturers, users and metering parts. The actual solution of the situation and flow simulation found that optimizing the flow field distribution of the flow meter is an important method to solve the problem of poor repeatability. A sufficient straight section can achieve the purpose of optimizing the flow field. By installing enough straight pipe sections, it is found that for compressible fluid and gas-liquid two-phase flow, the flow field distribution is uniform. For the two measuring tubes to work at the resonant frequency, normal measurement of the flow meter is crucial, and this technique can also be applied to other similar. Conditions.
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