Application of Precession Vortex Flowmeter in Hydrogen Metering

1 Introduction to the process

The brine sent from the upstream salt and nitrification plant is purified and then refined into the ion membrane electrolysis device for electrolysis to produce caustic soda, chlorine and hydrogen. The wet hydrogen produced by the electrolysis device is sent to the hydrogen treatment device through pipelines for The dry hydrogen obtained by drying is sent to the downstream device. The characteristics of the production process determine that the hydrogen pressure is low and the flow rate is relatively low. Usually, the hydrogen is controlled at about 25% ℃ at room temperature, and the pressure is controlled at about 0.1 MPa in gauge pressure.
2 Choice of measurement methods
Due to the small molecular weight and low density of hydrogen, hydrogen is an industrial gas that is not easy to measure accurately. At present, according to the different hydrogen pressure and process pipe diameter, the instruments used for measurement mainly include thermal mass flowmeter, Coriolis mass flowmeter, vortex flowmeter and orifice flowmeter. The production process of our factory has resulted in the low pressure of the hydrogen pipeline, and the differential pressure flowmeter (orifice plate and Proba) and vortex flowmeter have been used successively, but the actual use effect is not satisfactory. In the end, only the two parties can negotiate and resolve, and the measurement data of the flowmeter is for reference only. Through continuous learning, the understanding of low-pressure hydrogen flow measurement has been continuously improved. Regardless of orifice flowmeter, vortex flowmeter or Coriolis mass flowmeter, there are certain requirements for hydrogen pressure, and thermal mass flow has been considered. However, the working principle of this flowmeter determines that there are strict requirements for the water content in the gas. I also happened to learn that a gas-alkali enterprise supplies chlorine gas to surrounding users with a thermal mass flowmeter of a well-known brand, resulting in a measurement dispute. Because the hydrogen drying device cannot be guaranteed to operate normally, the thermal mass flowmeter is not considered.
After many investigations and demonstrations, it was decided to try to use the precession vortex flowmeter, which is widely used in the natural gas industry, to measure the hydrogen flow. This flowmeter has the advantages of strong signal and low flow rate characteristics. And it can measure the flow rate, temperature and pressure of hydrogen at the same time, and then automatically perform temperature and pressure compensation and convert it into standard flow rate. All data can be displayed on site and transmitted to the central control room through RS485 communication.
3 Measurement principle and advantages
The flow profile of the flow sensor is similar to that of a Venturi tube. A set of helical guide vanes are placed on the inlet side. When the fluid enters the flow sensor, the guide vanes force the fluid to produce a violent vortex flow. When the fluid enters the diffusion section, the vortex flow is affected by the backflow and begins to rotate twice, forming a gyroscopic vortex precession phenomenon. The precession frequency is proportional to the flow rate, and is not affected by the physical properties and density of the fluid. The detection component can obtain a good linearity in a wide flow range by measuring the fluid secondary rotation precession frequency. The signal is amplified, filtered and shaped by the preamplifier and converted into a pulse signal proportional to the flow rate, and then sent to the microprocessor together with the detection signals such as temperature and pressure for integration processing, and finally displayed on the liquid crystal display. Results (instantaneous flow, cumulative flow and temperature, pressure data). Its working principle is as follows:
Working principle diagram of precession vortex flowmeter
Although the precession vortex flowmeter and the vortex flowmeter are both fluid vibration flowmeters, the precession vortex flowmeter has the following advantages:
The vortex signal generated by 3.1 is stronger than that of the vortex flowmeter, so the lower limit Reynolds number is lower. When measuring gas flow, the lower limit flow rate can be as low as (1.5~2)m/s.
3.2 Because the vortex signal generated by the flowmeter is stronger than that of the vortex flowmeter, and is less affected by the upstream blocker than the vortex flowmeter, the length of the upstream straight pipe section is required to be much shorter.
3.3 The instrument coefficient K is 1~2 times larger than that of the vortex flowmeter.
4 Usage and improvement measures.
After the precession vortex flowmeter is put into use, the operation is relatively stable, and the measurement data is basically consistent with the theoretical data of the process calculation. However, due to many disturbance factors in the on-site environment, some problems also appeared in the process of use, and the problems were solved by taking improvement measures.
4.1 Select the appropriate range meter according to the flow range
When the flowmeter often works near the lower limit flow, it is difficult to guarantee the measurement accuracy. In order to achieve the best performance of the flowmeter, it is appropriate to ensure that the common flow rate of the flowmeter is within 20% to 80% of the range. The flow range provided by the user is generally the standard flow, but the range provided by the manufacturer is the working flow, which needs to be converted according to the working pressure, working temperature and the actual local atmospheric pressure to select the appropriate flowmeter.
4.2 The flowmeter has flow display problems during the shutdown period
Some users have reported that the flowmeter still displays the flow rate during the shutdown period. By checking the DCS historical trend in the central control room, the analysis shows that: ① The DCS curve has a rectangular wave, which is judged to be caused by vibration, and is generally displayed as (200~300)m/ h about h, and the lower limit of the DN200 flowmeter used is 360m2/h. By adjusting the small signal cutoff value, all the parts below 300m2/h are cut off; ②The DCS curve sometimes has a large jump. ground. After taking measures, it works well.
4.3 The influence of hydrogen water on the flowmeter
In the process of use, it is found that the precession vortex flowmeter also has certain requirements for the water content of hydrogen. If it is relatively high, the displayed value of the meter will be seriously distorted. Technicians are required to strengthen the monitoring of hydrogen drying treatment.
4.4 The problem of setting the small signal cutoff value
Setting the small signal cutoff value is to determine the starting flow of the instrument according to the actual situation on site. If the small signal cut-off value is set too large, the initial flow of the instrument will increase accordingly, which will result in no measurement at low flow; if the small signal cut-off value is set too small, the sensitivity of the instrument will be improved accordingly , Some weak vibration or electrical interference of the process pipeline may cause the flowmeter to start working without the passage of gas, thus causing the flow of "things out of nothing" and causing unnecessary disputes. Our factory selects the appropriate small signal cutoff value based on the historical trend curve of the DCS system. It not only avoids on-site interference, but also ensures the normal measurement of the flowmeter.
5 Conclusion
The precession vortex flowmeter has a good application effect in the flow measurement of low-density and low-pressure hydrogen in our factory, and has a certain reference for gas measurement in similar working conditions.