The basic structural and performance characteristics of the chlorine hydrogen differential pressure T-type Annubar flowmeter should be considered

Abstract: This article introduces the significance of measuring the flow rate of hydrogen and chlorine gas, as well as the basic structure, performance characteristics, application situation, and issues to be noted of the differential pressure T-type Annubar flowmeter.

1. The significance of measuring the flow rate of chlorine and hydrogen:
The chlorine gas and hydrogen gas from the chlorine hydrogen treatment process are mixed and burned in a certain molar ratio (chlorine gas: hydrogen=1.00:1.05~1.00:1.10) through the chlorine buffer tank, hydrogen buffer tank, and hydrogen flame arrester, respectively, and enter the synthesis furnace lamp head. The generated hydrogen chloride gas is discharged from the top of the furnace and enters the cooler. The cooled hydrogen chloride gas is transported to the vinyl chloride process as raw material, while the rest is absorbed into hydrochloric acid using a falling film absorption tower and sent to the finished product tank area for sale.
In the process of hydrogen chloride synthesis, the traditional control method is to observe the flame color and manually adjust the chlorine and hydrogen feed valves. Operators need to pay special attention to observing the color of the flame, preferably a blue or white flame. It is generally believed that the flow rate has not changed, but sometimes the purity of chlorine or hydrogen may have changed, and the actual ratio may have changed. When the purity of hydrogen is low and the oxygen content is high, the flame will turn red and dark; When the purity of chlorine gas is low, the flame turns white and produces smoke, which affects the operator's correct judgment of the chlorine hydrogen ratio. In production, hydrogen should be appropriately excessive to prevent over chlorination during the synthesis of hydrogen chloride. The occurrence of chlorine phenomenon easily leads to the formation of chloroacetylene in the vinyl chloride process, which has a significant impact on safety. On the one hand, it is necessary to prevent excessive chlorine content during acid production, which can form explosive mixtures of hydrogen and chlorine. Especially when using waste chlorine produced from liquid chlorine to synthesize hydrochloric acid, the hydrogen chlorine ratio should be increased to avoid explosions in the exhaust system. On the other hand, it is necessary to prevent hydrogen chloride gas from carrying chlorine gas and reacting with acetylene to produce chloroacetylene in the vinyl chloride process, in order to avoid accidents. In addition, the temperature in the morning and evening, as well as the climate changes throughout the four seasons, can also cause changes in fluid density, which in turn affects the chlorine hydrogen ratio. Secondly, changes in the process can cause significant fluctuations in the purity and pressure of hydrogen chloride, which can greatly affect the flow rate of hydrogen chloride gas. The accurate, reliable, and stable measurement of chlorine and hydrogen flow rates in the hydrochloric acid section is the key to the operation of the hydrochloric acid furnace and the true realization of self-control in hydrochloric acid production.

2. Flow measurement method:
Chlorine and hydrogen flow detection can be achieved through orifice flow meters, rotor flow meters, vortex flow meters, and mass flow meters. However, due to the particularity of the two process media, actual measurements are not satisfactory. Due to the fast flow rate and high flow rate of chlorine and hydrogen, the rotor flowmeter may cause oscillations, making it difficult to ensure the reliability and stability of the measurement; When measuring hydrogen gas using vortex street, the accuracy cannot be met due to the low density and water content of hydrogen gas; When measuring chlorine gas, the inner wall must be treated with anti-corrosion measures. Due to the presence of water in hydrogen gas, stagnant water will form in front of the orifice plate during measurement, which will affect the flow rate and increase the differential pressure, resulting in larger and more fluctuating actual flow rates. Chlorine gas is a corrosive gas, and the orifice plate must be treated with anti-corrosion measures. The use of mass flow meters is not only expensive, but also has many interference factors when selecting.
The T-type Annubar flowmeter is a differential pressure averaging tube flowmeter developed based on Bernoulli's principle of energy conservation and pitot tube measurement principle. High precision, good stability, and high reliability. Based on years of experience in hydrogen measurement, it has been found that the amount of water released from hydrogen is not significant when the temperature and pressure are relatively stable. If there are no blind pipes, dead corners, or water accumulation in the measurement system, it will not occur. The T-shaped Annubar flowmeter has a simple structure, and as long as the instrument installation position is selected properly, the problem of water accumulation can be solved. There are certain requirements for speed, density, and Reynolds number when using a T-type Annubar flowmeter. In use, pressure loss is also an issue that needs to be carefully considered.
When measuring chlorine gas, due to its strong corrosiveness, it is easy to react with impurities inside the pressure measuring tube, pressure measuring valve, and transmitter, generating acidic blockages, resulting in inaccurate measurements. Meanwhile, the orifice plate is made of PVC material, which is prone to aging and deformation at the edges. F4, due to its lower hardness, is prone to overall deformation. These are all important factors that affect measurement. The key difficulty in chlorine gas measurement is the strong corrosiveness of chlorine gas, which can easily cause non-standard measurement systems.
The internal structure of the T-type Annubar flowmeter is simple and not easily clogged. Due to the measurement medium being dry chlorine gas, the sensor material and membrane box material are made of Hastelloy C-276. Due to the energy-saving nature of the T-type flowmeter, which results in a relatively small differential pressure, a high-performance 3051S differential pressure transmitter must be selected, and EJA and ordinary 3051CD differential pressure transmitters cannot be used. After comparison, a T-shaped Annubar flowmeter should be used to measure the flow rates of chlorine and hydrogen. 3. T-type Annubar flowmeter:
3.1 Development of T-type Annubar flowmeter:
The gas standard volume flow formula for the T-type differential pressure Annubar flowmeter is as follows: Q=0.365 84 × K × Y! In the equation Δ P/ρ× P/T: Q - Volume flow rate under standard gas conditions, m3/h; The K-flow coefficient, whose value is related to the sensor structure, fluid flow state, pipe diameter, etc., is obtained experimentally; The Y-gas expansion coefficient, which is related to gas pressure, flow rate, area ratio, differential pressure, etc., is obtained experimentally; Δ P - differential pressure, k Pa; ρ - the density of the measured fluid in working condition, kg/m3; P-gas working pressure (absolute pressure), k Pa; T-working temperature, K。
***The cross-section of the detection rod of the initial Annubar flowmeter was circular, and it was found that the flow coefficient K value of the circular Annubar flowmeter remained basically unchanged when Re<105, while when Re was between 106 and 108, The K value increases and disperses, with a dispersion degree of approximately ± 10%. Further research shows that the above phenomenon is caused by the unstable position of the separation point when the fluid flows through the circular tube; When Re<105, the separation angle between the separation point and the center of the pipeline is 78 °, and when Re>105, the separation angle is 130 °; When Re is between 105 and 108, the separation angle is at an uncertain position between 78 ° and 130 °. Due to the proportional relationship between flow rate and K value in the calculation formula of the Annubar flowmeter, a ± 10% dispersion of K value will result in an error of ± 10% in flow measurement. The Reynolds number range with a large dispersion of K value in the Annubar flowmeter corresponds to the normal flow velocity of most gases in pipelines. The outstanding advantage of the second-generation Gem Type I (diamond section) Annubar flowmeter is that the position of the fluid separation point is fixed at the sharp inflection points on both sides of the diamond, thereby solving the problem of unstable K values in circular Annubar flowmeters and improving measurement accuracy. When the fluid flows through the edge of the Annubar sensor, vortices are generated, which cause vibration of the Annubar flow sensor and distortion of the pulsating noise signal, thereby affecting the stability of the measurement. The second-generation Gemstone II (improved diamond cross-section) Annubar flowmeter solves this problem.
Whether it is the improved diamond shaped section Annubar flowmeter or the development of various averaging tube flowmeters such as elliptical, fan-shaped, bullet shaped, wing shaped, etc. based on the circular Annubar flowmeter, although there have been significant improvements compared to the circular Annubar flowmeter, there are still the following problems.
(1) The K value is still not constant enough, especially in low flow measurement, which greatly affects the accuracy of the Annubar flowmeter in low flow rate and low flow measurement.
(2) The generated differential pressure signal is relatively small, generally only 1-3 kPa. When measuring low flow rates, even only 20-50 Pa. For such a low differential pressure, the accuracy and stability of the measurement cannot be guaranteed. This limits the scope of use of the Anuba flowmeter, especially in gas flow measurement applications.
(3) It is susceptible to various interferences, resulting in low signal-to-noise ratio of differential pressure signals, which affects the stability of measurement.
(4) Due to the fact that the averaging tube flowmeter is a fluid velocity sampling flowmeter, according to the requirements of fluid mechanics research, it is necessary to sample the fluid velocity at a certain position inside the pipeline. Therefore, there are high installation requirements. In X, Y. It is generally required that the deviation in the Z-axis direction should not exceed ± 3 °, and it is quite difficult to achieve such installation accuracy on site.

3.2.1. Good intrinsic anti blocking performance:
The velocity distribution of fluid varies across different sections of the pipeline. A velocity flowmeter needs to obtain the average velocity of the fluid. In the past, a velocity tube flowmeter only had a few pressure taps, and the sampling rate for fluid velocity was only about 6.5%. The front high-pressure tapping slot of the T-shaped Annubar flowmeter spans the entire pipeline, obtaining a fluid velocity sampling area that is more than 12 times larger than that of previous uniform velocity tube flowmeters of various shapes. Its sampling rate for fluid velocity is as high as 85%, thus achieving the best average velocity measurement accuracy. The design of the T-shaped Annubar flowmeter spanning the high-pressure tapping slot of the entire pipeline also gives it better resistance to blockage, and the adsorption of some impurities will not cause measurement errors that occurred in previous averaging tube flowmeters. In the pipeline, the fluid can form a large high-pressure zone on the front of the Annubar, and the particles in the fluid are deflected to bypass this high-pressure zone and flow towards the retention zone of the low-pressure intake port behind the T-shaped Annubar, so that impurities do not enter the interior of the Annubar and achieve good anti blocking performance.

3.2.2 High accuracy and good stability:
The K value is an important characteristic coefficient of differential pressure flowmeter, which is crucial for the stability of the measured flow range and directly affects the measurement accuracy of the flowmeter. The K value of the T-type Annubar flowmeter can remain constant in various Reynolds number ranges, completely solving the problem of unstable K values in flow rate changes, especially at low flow rates, for all previous averaging tube flowmeters, ensuring measurement accuracy within ± 0 Within the range of 75%. Fundamentally ensuring that the Annubar flowmeter can obtain measurements in various fluid conditions. The improvement of the T-type Annubar flowmeter is successful, and its repeatability is excellent, at ± 0.1%.

3.2.3 Outstanding signal-to-noise ratio and range ratio:
An important condition for the effectiveness of the averaging tube flowmeter is to ensure that the differential pressure generated during fluid measurement is sufficiently large. Previous averaging tube flowmeters had poor anti-interference ability, with low signal-to-noise ratio and high distortion of the output signal at low flow rates. At the same time, the accuracy and stability of micro differential pressure measurement by previous differential pressure transmitters were not satisfactory. Under the same process conditions for fluids, The differential pressure signal generated by the T-shaped Annubar flowmeter is more than 80% larger than that of various shaped averaging tube flowmeters in the past. This not only improves the measurement accuracy, but also enables it to be used in other low flow rate scenarios, giving it a range ratio of 10:1 to 20:1. The differential pressure signal output by the T-type Annubar flowmeter has an outstanding signal-to-noise ratio, which enables the differential pressure transmitter to obtain a good differential pressure signal and thus obtain stable flow measurement.

3.2.4. Small pipeline pressure loss:
The differential pressure generated by the orifice plate itself is greater than that generated by the Annubar flow sensor, and its pressure loss inevitably reaches 60% of the differential pressure. However, the Annubar flow sensor reduces its pressure loss by several times due to the reduced blocking area, which is only 3% of the differential pressure. In actual measurements, the maximum pressure difference of hydrogen is only 144 Pa, and the maximum pressure difference of chlorine is only 1435 Pa. The pressure of the hydrogen pipeline is 110 kPa, and the pressure of the chlorine pipeline is 85 kPa. The hydrogen pressure loss is less than 0.004% of the total pipeline pressure, and the chlorine pressure loss is less than 1% of the pipeline pressure, which can be ignored.

3.2.5. Easy installation and maintenance free:
The front high-pressure tapping slot of the T-shaped Annubar flowmeter spans the entire pipeline, which not only improves the measurement accuracy of fluid averaging, but also greatly reduces the installation requirements of the T-shaped Annubar flowmeter. Allow in X, There is a deviation of ± 5 ° in the YZ three-axis direction, which can be clearly judged by human eyes, making it easier for installers to install and debug.
Since its installation in 2004, the 9 chlorine gas flow meters have never been repaired or treated for detecting high or low flow rates, truly achieving maintenance free operation. When removed for inspection during parking, the sensor and 3051S transmitter were intact and undamaged. The use of 9 hydrogen flow meters is relatively ideal, but due to the inherent characteristics of hydrogen, such as small molecules and low density, the DP differential pressure value of the hydrogen flow meters is too low; The principle of the process is that the water content in hydrogen is not fixed, causing the zero point of the hydrogen flow meter to sometimes drift, but the treatment is simple and easy. In addition, it is necessary to pay attention to the winter in the north, where the temperature is low and the water content in the hydrogen pipeline increases, forming water droplets that affect the measurement. Secondly, the phenomenon of hydrogen molecule permeation. It is ideal to use gold plating for the measurement membrane box.

4. Attention should be paid to the following issues when using the Anuba flowmeter:
The mechanical dimensions of the Anuba flowmeter are customized according to the size of the pipeline to be installed, and its flow measurement range is also calculated and calibrated based on the flow data provided by the user. Therefore, the provided flow data and pipeline data must be correct and error free, otherwise it will cause significant measurement errors. The differential pressure generated by a differential pressure averaging tube flowmeter is generally relatively small, possibly only 10-20 Pa. The magnitude of the differential pressure signal generated by the averaging tube flowmeter is closely related to the Reynolds number of the fluid operating conditions. Special attention should be paid to differential pressure measurements below 1 kPa, especially those below 0.1 kPa. The fluctuation of K in various averaging tube flow meters under such low flow rate conditions is very large, which affects the measurement accuracy. Micro differential pressure measurement below 0.1 kPa requires a very high performance differential pressure transmitter.
(1) If the fluid contains viscous impurities, careful consideration should be given when using an averaging tube flowmeter. Viscous impurities sticking to the sensor will greatly affect the measurement accuracy, and many flow meters are not suitable for this type of application.
(2) There are many impurities in the fluid, and the device operates intermittently. In the case of frequent and prolonged shutdown, impurities may dry and scale on the surface of the sensor during the interruption period, which will block the pressure tap over the years.
(3) Leakage of the pressure system. Without using a direct installation form, the leakage of the pressure pipeline system from the averaging tube flowmeter to the transmitter will cause fluid flow in the pressure chamber of the Annubar flowmeter. Although the flow rate is very small and slow, over time, impurities may enter its interior and accumulate, causing blockages.

5. Conclusion:
With the increasingly mature and perfect flow measurement technology of Anuba, its application prospects are broad. The T-type Annubar flowmeter not only has high stability and measurement accuracy, but also integrates with high-precision differential pressure transmitters for installation. It has inherent anti blocking characteristics, wide range ratio, and low operating energy consumption. Its successful application in measuring the flow rate of special media such as hydrogen and chlorine is worthy of reference and promotion in the chlor alkali industry to achieve automatic control of hydrochloric acid production.