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Due to the large cross section of the flue, the large-scale coal-fired boilers have difficult and cumbersome sampling of smoke and dust and require a lot of manpower. The soot dust automatic constant velocity sampling device developed by using the static pressure balanced sampling principle can greatly reduce the workload of the sampling process.
1 Working principle and functional module of the device 1.1 Principle of work Static pressure equalization The constant velocity sampling is achieved by maintaining the static pressure difference between the airflow of the sampling nozzle and the air flow of the flue gas where it is located. Therefore, in the sampling process, the exhaust flow should be constantly adjusted according to the change of the flue gas flow to keep the static pressure difference to zero. In actual sampling, one person is usually required to adjust the flow while others operate the sampling gun. The sampling process is performed at multiple sampling points on the sampling section, and the sampling time is the same for each sampling point. When sampling is completed at one point, the sampling point must be replaced at the shortest possible time. The purpose of the automatic smoke equalization speed sampling device is to replace a large number of manual operations in the original sampling process, so that the control is more accurate, fewer personnel, and more convenient work.
1.2 Function modules In order to enable the device to replace a large number of manual operations in the sampling process, it must have the following basic functions:
(1) accept the instruction of sampling personnel;
(2) Monitor the static pressure difference, and according to the monitoring results, timely adjust the pumping flow and maintain the static pressure difference to zero;
(3) Instruct the sampling personnel to change the sampling point at a predetermined sampling time;
(4) When the static pressure difference cannot be maintained for any reason, notify the sampling personnel for troubleshooting.
In addition, the device can also be used as an electronic micro-pressure gauge, thereby reducing the overall flue gas testing equipment. The entire automatic isokinetic sampling device includes the following functional modules (Figure 1): (1) differential pressure test module; (2) flow regulation module; (3) display and parameter setting module; (4) control module; Carrying module.
After the device is powered on, the display and parameter setting modules complete the sampling time setting for each measurement point. After the setting is completed, the air pump is started, the sampling gun is inserted into the predetermined sampling point, and the sampling instruction is issued by the carrying module to start sampling. At this time, the control module regulates the flow according to the differential pressure measured by the differential pressure measurement module to the flow adjustment module, so that the static pressure difference between the inside and outside of the test gun is zero, and the isokinetic sampling is realized. After a point sampling is completed, the control module sends a switching sampling point signal to the carrying module, and then the control module continues to maintain the constant speed sampling. After all the points have been sampled, the sampler sends a sample end signal to the control module through the carrying module.
2 Function Implementation 2.1 Software Fig. 2 and Fig. 3 are flow charts of the function setting and sampling process, respectively. The software design adopts the standard PLM language, uses each interrupt service to complete the main function, the main program finishes the calculation function regularly. The keyboard scanner and display program components are large. Although various interrupt service programs are relatively small, they are the core parts. The entire program has two flag control bytes, DP-ID and IDEN, which are the core of the program flow.
Software design follows a structured specification, and all software consists of 21 subroutines. The main program is an infinite loop program that performs PID calculations based on a fixed frequency. Five of the interrupt service routines are core programs that perform the corresponding functions, while other programs are called separately.
2.2 Hardware The system control core is the 80C196KC microcontroller (CHMOS high-performance 16-bit microcontroller), which has an on-chip oscillator and the state cycle is obtained by dividing the oscillator signal by two. The entire controller works around the 80C196KC microcontroller and its hardware is shown in Figure 4.
In addition, in the hardware design, the use of anti-jamming, universal peripheral interface chip, non-volatile ROM interface circuit and other technologies. The differential pressure sensor is a light diffusion silicon type, which allows positive and negative input of differential pressure signal. The measurement range is -2~+2kPa, and the maximum deviation is 5Pa. The random solid-state relay output circuit with high isolation controls the air pump. Its maximum allowable current is 10A, and it can be connected with various air pumping devices.
The carrying module is a bi-directional control module and is connected to the control device using an air outlet. The module can be carried by the sampling personnel, and the sampling personnel can control the start and stop of the device through this module, and can send various information through the module receiving device in the sampling process, switching the sampling point at any time and processing the inconstant speed problem in the sampling process. . The introduction of this module is an important feature of this device, which is different from other automatic isokinetic sampling instruments. It is of great significance in reducing sampling personnel and improving the friendliness of human-machine interface.
In summary, the device has the following features in hardware and software design:
(1) using software control algorithms;
(2) High-resolution AC signal zero-crossing detection;
(3) High-resolution output control, output control resolution of 7000 codes;
(4) using a good man-machine interface, through the membrane keyboard control program variable parameters, through the start-stop operation of the control module carrying the module and get the controller's various alarm information;
(5) Online adjustable control parameters;
(6) High stability control accuracy and fast dynamic response process;
(7) High reliability and anti-jamming capability using integrated devices;
(8) All setting parameters have a non-volatile memory function.
3 Experimental Verification 3.1 Test Method After the hardware and software design was completed, various performances of the device were adjusted in the laboratory, and compared with the traditional static pressure balance smoke test method under field conditions. In order to try to eliminate the possible impact of other operational errors, in order to facilitate the comparison of test data, the test takes a single sampling point. At the same time, in order to avoid changes in the working conditions of the boiler in the sampling process, two methods are alternately used for sampling.
3.2 Test results Table 1 shows the amount of soot collected at 1 min using the two methods, respectively. The data corresponding to the YA is obtained by the method of controlling by the device; YB is the data obtained by the control by the conventional method. The table also shows the differences in the amount of dust collected by the two methods and the mean and variance of the columns. As can be seen from Table 1, the average deviation of the two methods is only 0.5%.
The homogeneity of the variance of the data obtained from the two methods can be tested using the F-test; the t-test method can be used to determine if there is a difference in the arithmetic mean between the two sets of data. Based on this, the data in Table 1 is calculated as follows:
1.690. Obviously, when the significance level is 0.1, the assumption that the variance and average value of the two methods are acceptable can be accepted. There is no significant difference between the two groups of data. Therefore, it is believed that sampling using the device developed in this paper is reliable.
4 Conclusion The automatic isokinetic sampling device for smoke dust has introduced the carrying module, which has greatly enhanced the human-machine interface characteristics of the device. It is especially suitable for smoke and dust tests under strong on-site noise conditions, reducing manpower and greatly improving labor efficiency. Moreover, the device can also be used as an electronic micro-pressure gauge, which reduces the equipment for flue gas testing. Under field conditions, a comparative test of the newly developed device and the conventional soot sampling method showed that there is no significant difference between the two when the significance level is 0.1. The field application of several power plants proves that the device has the characteristics of anti-vibration, anti-interference, convenient operation, and easy transportation.
Development of smoke and dust automatic constant velocity sampling device
Abstract: Under the condition of fully considering the situation of the site, the automatic isokinetic sampling device of smoke and dust was developed using the principle of static pressure balance sampling. On the basis of the design of the functional module of the device, the hardware is controlled by the single-chip microcomputer, the output of the solid-state relay adjusts the exhaust flow, and a portable module is specially designed to make the human-machine interface more friendly.