A brief discussion on the application of Wilba flowmeter in factories

1 Overview

Wellbar flowmeter is a new type of flow meter designed and produced using the differential pressure working principle. It consists of a sensor, a differential pressure transmitter or a flow integrator. The sensor is shaped like a head and can quickly and accurately detect the differential pressure signal generated by the average flow velocity of the fluid. The special design of the pressure-taking hole on the low-pressure side effectively reduces the blockage of the low-pressure hole, thereby improving the accuracy of the flowmeter measurement. It can measure gas, liquid, steam and corrosive media, and is suitable for the installation of round and rectangular pipes of various sizes. It has low requirements for the straight pipe sections before and after the measuring pipeline, and can be installed and maintained online. It is widely used in the metallurgical and chemical industries.

2 Measurement principle of Verabar flowmeter

Wilba flow probe is generally installed perpendicular to the pipeline. When the fluid flows through the probe, a high and low pressure distribution area is generated in front and back of the measuring probe. The average differential pressure △p generated by the average velocity of the fluid can be accurately detected through the pressure holes regularly distributed on the probe. The corresponding relationship between flow q and △p in gas measurement is:

Where, q: standard volume flow rate, nm3/h;

△p: differential pressure, kpa;

ρ: Density of the measured fluid in working state, kg/m3;

Pa: absolute pressure of fluid in working state, kPa;

ta: absolute temperature of fluid in working state;

n: unit conversion constant (0.36584), which is related to the sensor structure, fluid temperature and pressure, and is obtained by experiment;

k: flow coefficient, its value is related to the sensor structure, fluid flow state, and pipe diameter, and is obtained by experiment;

y: Fluid expansion coefficient, its value is related to gas pressure, velocity-area ratio, and differential pressure, and is obtained through experiments;

d: Equivalent diameter of the measured pipe, mm.

3 Correction of Verabar flowmeter parameters in actual use

The Verabar flowmeter manufacturer determines the equipment model and calculates the differential pressure value for flow measurement based on the relevant process parameters provided by the user and the design values ​​of the original process design drawings to meet the flow measurement requirements. In the actual production process after the commissioning of our company, due to objective reasons, the type of coal burned in the generator, the outlet pressure and temperature have changed, which will greatly affect the measurement accuracy of the Verabar flowmeter manufactured according to the original design parameters. It is extremely important to re-verify the relevant parameters and proofread the calculation data during actual use and maintenance.

In 2009, our company used the VLB640B line-mounted Verabar flowmeter produced by ECE to measure the gas flow at the outlet of the gas plant. The working pressure parameter provided when ordering was the design value of 40kPa, and the differential pressure value calculated by the manufacturer was 0.263kPa. According to the formula: q=0.36584*k*y*d2 δp/ρ *(pa/ta), the flow rate was q==50599 m3/h, but after the production system was put into operation, it was obviously inconsistent with the actual consumption.

After carefully checking the installation method on site and various data after production operation, the problem lies in the deviation of the design parameters provided at the beginning, among which the pressure parameter has the greatest impact. The actual gas supply working pressure is only 37kpa, and the Wilba differential pressure calculated at 37kpa (correcting other error parameters) should be 0.32kpa, which is nearly 57pa different from the original differential pressure. After the modification, the flow rate displayed under the same production conditions is about 58000m3/h, which is basically consistent with the production consumption. It can be seen that although the differential pressure only differs by 57pa, the flow rate differs by about 7000m3/h. The Wilba setting parameters are adjusted in time to ensure the accuracy of the measurement. Due to the different types of coal burned in the generator, the concentration of the gas components changes. Whether the gas density (ρ) can be accurately obtained under the working state is also the key to the accuracy of the flow measurement.

By calculation, ρ (comprehensive gas density under working conditions) is 1.121 kg/m3. As the type of coal burned in the generator changes during production, the calorific value of the gas is greatly improved, and the concentration of the gas components also changes accordingly. In the actual production process, the new gas composition table obtained by manual testing and online gas composition analyzer is as follows:

By calculating ρ (comprehensive gas density under working conditions) as 1.287 kg/m3, the gas density parameter was adjusted in time in the calculation formula, and the flow rate was displayed to be about 59303 m3/h under the same temperature, differential pressure and production conditions. Due to the difference between the actual coal type used in production and the original design, the measurement deviation of the Verabar flowmeter was about 1000 m3/h. In the early stage of production, a comprehensive analysis of the deviation of the Verabar flowmeter was conducted to find out the cause of the deviation. Timely correction of the flowmeter parameters in use ensured the accuracy of the flowmeter measurement. It provides reliable calculation data for the company's control of raw fuel consumption and production indicators.

4 Factors affecting the measurement accuracy of Verabar flowmeter and solutions

(1) If the measuring medium (such as coal gas) contains a lot of dust and tar, the pressure pipe will be blocked and should be cleaned and drained regularly.

(2) Since leakage of the pressure taking pipe causes changes in differential pressure, the pressure taking pipe should be leak tested regularly.

(3) The Wilba probe generates a small differential pressure, so try to use a high-precision micro differential pressure transmitter, such as ABB, EJA, etc.

(4) Correct the static pressure error of the differential pressure flowmeter. Since the differential pressure scale of the differential pressure transmitter is usually calibrated under the condition that the negative pressure chamber is open to the atmosphere, it is often found that the zero position output is inconsistent with the zero position output when the negative pressure chamber is calibrated after installation on site. Therefore, before the instrument is put into operation, the transmitter zero position must be calibrated again using the actual static pressure on site.

(5) Temperature and pressure compensation must be performed in flow calculations to eliminate errors caused by changes in operating conditions.

(6) When calculating the flow rate of industrial gas, the influence of humidity on the measurement results should be considered to eliminate the changes in gas density caused by different humidity during the transmission process.

5 Conclusion

When using the Wilbar flowmeter to measure fluid flow, the relevant parameters in the calculation must be reliable parameters under the actual production conditions of the process. Otherwise, the parameters are incorrect and the calculation results must be biased. Of course, in the actual use process, to ensure that the measurement results of the Wilbar flowmeter are accurate and reliable, the installation method of the flowmeter and the length of the straight pipe section before and after the sensor can meet the measurement requirements are also critical. It is particularly necessary to clean the measuring parts of the flowmeter regularly.