Study On On-line Monitor System To Detect Leakage Of Boiler Tubes

Introduction With the quick development of country economy, the importance of energy sources problems is more and more obvious, especially the requirement for electric power is larger and larger. Many large thermal power stations have been built and run. However, because boiler furnace tubes of power plants work in the environments with high temperature, high pressure, corrosion and abrasion, the blasting and leakage happen frequently. This affects the safety and economical efficiency of power plants seriously and becomes the key obstacle of power plants for running safely and the increase of economic benefit. Especially with the increase of the parameters and capacity of thermal power generating units, the economic loss and society influence brought by furnace tubes blasting and leakage of large thermal power stations are larger and larger. Thus, if we could monitor on-line and diagnose furnace tubes earlier, track and measure the extent and trend of leakage, judge furnace tube leakage and the location of leakage in time, then make it alarm automatically, on the one hand, we could prevent the enlargement of accidents efficiently; on the other hand, we could make power plant declare the closedown in time and do the repair preparation in advance, so the repair period could be shortened. Because of the reasons above, both domestic researchers and researchers abroad have studied early monitoring techniques on boiler furnace tubes leakage of power stations. One of the key problems is that the difference of background noise and leakage noise. Because of the complexity of the noise in boilers and the frequency range difference between water, steam and leakage noise, when the light leakage happens, it is hard to distinguish between boiler leakage noise and background noise. This results in failing to report and false alarm. To solve these problems, it is necessary to process the gathered leakage noise and background noise further. This paper spreads from the signal processing of background noise and leakage noise, and makes use of the electric power carrier communication technique with low cost and good reliability for transmission, through the software process of host computer, realizes better monitoring ability on-line. 1. Power Line Carrier Communication Technique Power line carrier communication technique utilizes the existing low voltage network as transmission media and realizes data transmission and information exchange, avoid the construction and investment of new communication network, so get broad attention from researchers. Now, power line carrier communication technique is mainly used in automatic meter reading system. In this system, because the boiler works in the bad environments with high temperature and pressure, etc., so we need a large cost for the span of the communication network. By using power line carrier communication technique, we could economize the installation cost and improve the installation efficiency, so it is significant to use this technique. The proposed system utilizes the KQ-100F electric power carrier wave module. It makes use of FSK modulation technique and does the frequency modulation to the signal or data with the carrier wave between 50 KHz and 350 KHz. The difference between this module and traditional FSK is that the frequency of two carrier waves has large difference. This could make their data transmission quality independent relatively and don't disturb each other. In order to avoid the inherent interference of the power lines themselves, this module utilizes zero crossing transmission technique, measures the zero crossing point at the zero crossing moment quickly and exactly, sends the data out in the 5 degrees of zero very quickly. Because of the above characters, we choose KQ-100F carrier wave module. This module has better communication effect. 2.Active Power Bandpass Filter Technique When boiler tubes leak, the high pressure medium in the tubes eject through the crack or the broken hole, which result in the vibration of tube and the current of high pressure. The center frequency is about 20 KHz. Though the boiler works well, it makes the strong noise. One of the key problems is that the difference of background noise and leakage noise for leakage detector. In this system, we adopt 8th-order continuous-time active filters. It can implement any high-level bandpass or lowpass filter response, such as Butterworth, Chebyshev, and Bessel, and is programmed by several external resistors. Based on the characteristic of the frequency of leakage noise, we need the bandpass filters which should make the prime attenuation at stopband approaching the pass-band in the design of the system. So we select the 8th-order Chebyshev filter, which is the pass-band frequency from 16KHz to 24KHz,and the center frequency is 20KHz. Filter...