| Sewers and septic tanks are essential urban infractures. Serious explosion accidents from sewers and septic tanks have happened many times in many cities in recent years, seriously threatening to people's lives and properties. So to study on risk assessment and early-warning mechanism of gas explsion from urban sewers and septic tanks are very important to ensure urban infractures safety and community safety.Aiming to solve frequent gas explosion accidents and serious safety problems, based on field investigation and data collection, comprehensive analysis on distribution characters of occurred explosions as well as monitoring and controlling system alarms, and change characters of flammable gases concentration, flammable gases generation and accumulation mechanism was studied, then gas explosion inducements were ananysied. Therefore, risk assessment model and early-warning mechanism of gas explosion from urban sewers and septic tanks was established, to support the safe management of urban sewers and septic tanks in chongqing. So the following conclusions can be reached:①Acoording to flammable gases concentration change rules,and its acuumulaition mechanism , three accumulation phenomena in sewers and septic tanks were discovered:stability state, perturbation state and accumulation state. HRT, sludge accumulation stage in sewers , exhaust condition of viccinal septic tanks and discharge probability of exogenous flammable gases were screened out to be main influence factors of flammable gases accumulation in sewers. Exhaust condition of septic tanks, load condition and discharge probability of exogenous flammable gases were main influence factors of flammable gases accumulation in septic tanks. Pressure and folw were main factors of flammable gases concentration.②Acoording to risk assessment theory, semi-quatative risk assessment model of gas explosion from urban sewers and septic tanks was established. Gas explosion risk was comprehensive reflection of flammable gases generation and accumulation possibility combined with gas explosion consequence. According to established risk assessment indicators, grade of flammable gases generation and accumulation(L) and grade of gas explosion concequence(C) was separately divided to five grades. Gas explosion risk value was quatanted by product L and C, which was representated by R. Semi-quatititive risk assessment matrix was established in this dissertation. According to the area of risk value(R), gas explosion risk was determined to severe risk, moderate risk, and slight risk of three risk levels. For sewers and septic tanks of severe risk level, online flammable gases monitoring and controlling system should be established.This semi-quatative method was relatively flexible, which can solve problems of lack of primary standards. Managers can ajust the area of risk value(R)of three risk level in semi-quatititive risk assessment matrix, according to specific requirements,to determine the objects of severe risk, to sceern out main monitoring objects from a complex range of sewer and septic tanks.③According to inducements ananysis, gas explosion risk assessment indexes were established, then combined with experts survey and AHP, weight coefficient of each index was determined. Indicators of flammable gases generation and accumulation possibility grade of sewers include: average hydraulic retention time, index of pipeline intersection in manhole, and index of source of exogenous flammable gases, which weight coefficient was: 0.304, 0.145, and 0.551. Indicators of sewer gas explosion concequence grade include: population density surrouding sewer, and possibility of secondary disaster, which weight coefficient were: 0.804, and 0.196. Indicators of flammable gases generation and accumulation possibility grade of septic tanks include: septic tank volum, index ofseptic tank loads, and index of source of exogenous flammable gases, which weight coefficient was: 0.150, 0.619, and 0.231. Indicators of septic tank gas explosion concequence grade include: population density surrouding septic tank, and possibility of secondary disaster, which weight coefficient were: 0.810, and 0.190.④According to change charaters of flammable gases concentration, indicators for gases safety monitoring of sewers and septic tanks was proposed. The indicator was percentage concentration of lower explosion limit, which was representated by %LEL. Three early-alarming modes were eatablished: stability mode, perturbation mode and accumulation mode. Their discrimination standard was also established. The discrimination standard was based on concentration change trends of flammable, which was discriminated by fomula: ?1Δ=?C t i CtiCti. If since some sampling time, the concentration of front moment was continuously three times higher than the next moment, flammable gases concentration had increasing trend. According to this discrimination standard, the frequent alarm and fault alarm phenomenon existed in gas monitoring system of sewers and septic tanks can be significantly reduce, to improve reliability and validity of monitoring system. On this basis, three early warning grades were proposed: security, attention and vigilance. Different early warning modes were corresponding to different early warning grades, and different early warning grades were corresponding to different management measures. The accumulation mode was identified to vigilance grade, and treatment measures should be taken immediately. Urban sewers and septic tanks should be treated as an organic whole, to establish their linkage contorling mechanism. |
