Selection and application of pulverized coal mass flowmeter

0 Introduction

For pulverized coal transportation using high-dense phase conveying technology, accurate measurement of pulverized coal flow is of great significance to gas consumption, smooth operation of the system, maintenance of equipment and pipelines, verification of relevant indicators such as dense phase conveying of the entire system, and control of process parameters such as conveying volume. In the process control of pulverized coal pressurized gasification equipment, the measurement of pulverized coal density and flow rate is the top priority of the entire coal transportation system. In the Shell pulverized coal pressurized gasification technology introduced by Shell in the Netherlands in recent years, the pulverized coal mass flowmeter has always used the nuclear density meter and velocity meter provided by Thermoelectric of the United States. In the use of similar equipment in China, the accuracy of its measurement needs to be adjusted by empirical data. It takes at least 2 to 3 months to collect empirical data. The calibration time period of pulverized coal is relatively long, and its velocity meter has a slow response time. This phenomenon has been verified in the production process of Puyang Aerospace Furnace. The flowmeter using microwave technology from Germany's Swell is relatively simple to adjust, the debugging time is about one week, and the measurement can meet the process requirements. In the construction of the aerospace furnace project, two sets of American thermal current meters and one set of SWR flowmeters were used for simultaneous measurement on each coal powder pipeline of the gasification device of Puyang Longyu Chemical Co., Ltd., which not only met the process requirements, but also verified the accuracy and reliability of the flowmeters of the two manufacturers in coal powder flow measurement under the same working conditions. Anhui Linquan used two sets of SWR flowmeters on each coal powder pipeline of the aerospace furnace device, which were well applied in the measurement and control of coal powder.

1 SWR (Swell) flowmeter

1.1 Measurement principle

The DensFlow flowmeter produced by SWR is a measuring instrument specially developed for gas-solid two-phase flow, suitable for dense phase or super dense phase conveying process, and measures the average flow velocity and concentration of solid materials through the pipeline cross section by generating a high-frequency, AC, uniform electromagnetic field in the measuring tube, thereby calculating the flow velocity (m/s), concentration (kg/m3), and mass flow (kg/h or t/h) of the solid materials in the pipeline, and outputting 3 independent 4~20mA current signals corresponding to the above measurement values.

The integration of flow, density and flow velocity measurement greatly improves the measurement accuracy.

DensFlow solid material density measurement requires calibration and calibration, because the instrument can measure all solid particles or powdered materials in dense phase and super dense phase conveying, and the bulk density of different materials is different, so the concentration value of the conveyed material must be actually calibrated. The normal accuracy is ±1% to ±3%, and the system can achieve a measurement and control accuracy of less than ±1% under a uniform and stable spraying state. Under the

existing dense phase and super dense phase conveying technical conditions, the following calibration methods can be used:

(1) Use SWR's special test rod for calibration; you can also order the SWR flow online calibration system for calibration.

(2) Before installing the instrument, the user places the solid flow meter vertically in an open container in the workshop and preheats it with power. After 15 minutes, put the solid material to be measured into the measuring tube for calibration.

(3) After the user installs the instrument, power on and preheat for 15 minutes, and start normal material transportation after the built-in accumulator of the flow meter is cleared to zero. The material mass flow value accumulated by the instrument is compared with the display value of the existing instrument, and then the correction coefficient is input into the instrument.

(4) After the user installs the instrument, power on and preheat for 15 minutes, and start normal material transportation after the built-in accumulator of the flow meter is cleared to zero. The material mass flow value accumulated by the instrument is compared with the daily/weekly/monthly material consumption, and then the correction coefficient is input into the instrument.

1.2 Correction scheme for the calibration process

The factors that affect the flow meter measurement results are pressure, suspension density, and temperature. Among them, the influence of pressure is very stable. After on-site observation and calculation, the influence of pressure is 0~5MPa, and the influence of zero drift is 0.4%~0.5%; the influence of concentration is only related to the characteristics of the sensor; the influence of temperature, Anhui Linquan and Puyang have different influences due to different installation locations, but it is observed that in an environment where the temperature does not change much, the influence on the flow meter detection is very small. The correction schemes for these three influencing factors are as follows:

(1) Concentration correction scheme. The DCS calculation equation is as follows:

The density correction curve is shown in Figure 1, where the horizontal axis represents density (kg/m3) and the vertical axis represents the correction coefficient.

Figure 1 Density correction curve

(2) Pressure correction. Pressure affects the actual displayed concentration value, and the influence value gradually decreases with the increase of pressure. Corresponding to the pressure change of 0~5MPa, the zero drift is 0.4%~0.5%. The

pressure correction formula is:

f(p)=1.7P

, where f(p) is the corrected pipeline pressure; p is the pressure at the pressure sampling point near the flowmeter; and P is the measured pipeline pressure.

(3) Temperature correction. The influence of temperature on the sensor is directly reflected in the axial stress of the sensor. The reason for the inconsistent temperature effect at the Linquan and Puyang sites is analyzed to be related to the installation location at the site. When the temperature change is not large, the influence of this parameter can be basically ignored.

(4) Total correction plan.

The final correction equation of DCS is:

A=(original density-f2(P)+f3(T))

DCS display density=f1(A)·A·K2·K3

, where K2 is the pipeline coefficient; K3 is the coal type coefficient, which is calibrated separately for each pipeline; f1(A) is the characteristic equation of the sensor. [page]

After calibration, the historical curve of the SWR flowmeter in actual working conditions is shown in Figure 2.

Figure 2 Historical curve of SWR flowmeter in actual working conditions

1.3 Existing problems and improvements

(1) The circuit board of the field sensor is installed in the form of connectors, and the wiring is relatively hard. During the operation of the instrument, the vibration of the coal powder pipeline can easily loosen the circuit board connector, resulting in poor contact and no display of coal powder flow. This problem has been submitted to the SWR headquarters. After the circuit board has been improved, this type of fault has been effectively eliminated.

(2) The SWR flowmeter has weak anti-interference ability. The central processor faces interference from the magnetic field, which manifests as speed fluctuations, thereby affecting flow measurement. Therefore, the installation of the secondary meter central processing unit should be away from the magnetic field to avoid the occurrence of interference signals.

2 American thermal current flowmeter

2.1 Measurement principle

The gamma rays generated by radioactive isotopes have the ability to penetrate matter. For a beam of collimated gamma rays, after passing through the measured medium with a diameter of d and a density of ρ, the ray intensity I incident on the gamma detector is exponentially related to the ray intensity Io when ρ=0. By measuring the change in ray intensity, the density of the medium can be known. The radiation source and the radiation detector (scintillation detector or ionization chamber detector) are installed on both sides of the measured pipe respectively. The output signal of the radiation detector is processed by a computer to display the density or concentration of the measured medium.

If the pipeline flow signal is sent to the transmitter, the mass flow rate and the integrated value of the measured medium can be displayed after computer processing.

2.2 Composition of the instrument system

DensityPRO density meter consists of two main parts: radiation source and lead tank, integrated transmitter/detector.

The radiation source used in the density meter generally uses Cs137, and Co60 is used in special cases. Both Cs137 and Co60 radiation sources are sealed radiation sources and will not cause environmental pollution.

The radiation source is fixed in a container, which consists of a lead layer, a steel shell and a source switch device. There is a radiation channel (collimation hole) in the source container. When the source switch device is in the closed state, the gamma rays coming out of the collimation hole are shielded; when the source switch is in the open state, the rays pass through the collimation hole through the measured pipe and shoot onto the detector. The thickness of the lead layer of the source tank ensures that the radiation dose leaked from the source tank does not exceed the safety protection standard.

2.3 Calibration

During the calibration cycle, the densitometer averages the detector signal. The default cycle time for calibration is 17 minutes. The averaged detector signal provides a very reproducible signal measurement in the standard configuration.

Once the standard measurement has been completed, any changes such as increasing attenuation caused by the medium depositing on the tube wall can be compensated for repeatedly. The densitometer can then adjust the calibration values ​​based on the new calibration values. Since the calibration values ​​are stored as a scale factor relative to the calibration values, no recalibration is required. Whenever a new calibration value is calculated, the calibration values ​​are automatically adjusted.

2.4 Calibration

Unless calibration has been completed in the "tube filled with carrier" state for suspension type media ("tube filled with solvent" for solution type media, "tube filled with liquid 1" for emulsion type media), calibration must be completed using the "densitometer calibration" menu in the "Set density,..." menu group. If a calibration measurement is required, the display shows the message "Unit has not been calibrated". When calibration is required, a single point calibration is suitable for many applications. Calibration measurements should be performed on actual media with a density close to the normal media density expected during normal operation. The media must be sampled to determine the media density during calibration measurements. The densitometer can use information such as the source tank (geometric condition factor), pipeline size and media type to measure other density values ​​by calculating the density change corresponding to the detector signal change. When more accurate measurements are required, a two-point calibration can be performed. The second calibration value applies the "slope" correction factor to convert the detector signal to the media density. When applying a two-point calibration, try to complete the first point calibration at one end of the working media density range, and then complete the second point calibration at the other end.

3 Comparison

Under the same working conditions, the application of American Thermoelectric and SWR flowmeters in pulverized coal pressurized gasification units has the following advantages:

(1) The instrument does not contain any radioactive materials, and on-site technicians do not have to worry about being harmed by nuclear radiation.
(2) The instrument calibration is simple and easy to operate, which reduces the maintenance cycle of instrument personnel.
(3) The GFK (reinforced glass fiber) lining is resistant to high pressure, wear and corrosion. SWR promises that the sensor life is more than 3 years and provides a corresponding 3-year sensor quality assurance.
(4) The velocity measurement range is optional (0~10m/s or 0~20m/s), and it can accurately measure in both dense and dilute phases, and there is no restriction that the velocity measurement range of the electrostatic flowmeter must be greater than 4m/s.
(5) One instrument can be used to complete the online measurement of the flow rate and concentration of coal powder.
(6) The measuring tube of DensFlow does not have any throttling components, no pressure loss, and maintenance-free.
(7) There is no problem of source attenuation, and the measurement accuracy will not be affected after the instrument is used. (end)