【作者】 米莉；

【导师】 彭绪亚；

【作者基本信息】 重庆大学 ， 环境工程， 2010， 硕士

【摘要】 下水道和化粪池是城市必不可少的基础设施。近年来,全国许多城市都先后发生过严重的下水道和化粪池爆炸事故,严重威胁着人民生命和财产安全。因此开展城市下水道和化粪池气体爆炸风险评估及预警机制方面的研究对保障城市基础设施安全以及城市公共安全十分重要。本论文针对重庆市城市下水道和化粪池气体爆炸事故频发、安全隐患严重等问题,通过现场调研与数据收集,对比分析已发生气体爆炸事故与已有监控系统报警的时间及地域分布特征和可燃气体浓度变化特征,从机理上研究可燃气体的积聚现象,分析气体爆炸诱因,构建了城市下水道和化粪池气体爆炸风险评估模型,提出了气体爆炸预警模式,可为城市下水道和化粪池按风险级别分级管理,以及提高预警系统的安全性和有效性提供技术依据。主要得到了以下结论:①通过分析可燃气体浓度变化规律及可燃气体积聚机理,发现下水道和化粪池内可燃气体存在三种积聚结果:稳定状态、扰动状态和聚集状态。并筛选出HRT(水力停留时间)、管道内污泥沉积状态、相邻化粪池排气条件、外源性可燃气体泄露概率是影响下水道内可燃气体积聚的主要因素;化粪池排气条件、负荷状态、外源性可燃气体泄露概率是影响化粪池内可燃气体积聚的主要因素。压力和流量的变化是影响下水道和化粪池内可燃气体浓度变化的主要因素。②根据风险评估理论,建立了城市下水道和化粪池气体爆炸半定量风险评估模型。城市下水道和化粪池气体爆炸风险是可燃气体积聚可能性与爆炸后果的综合体现。通过建立风险评估指标体系,将可燃气体积聚可能性等级(L)与爆炸后果等级(C)分别分为五个级别,并建立半定量风险矩阵。气体爆炸风险值R为L与C的乘积,根据风险值R在半定量风险矩阵中的区域将气体爆炸风险确定为重度风险、中度风险及轻度风险三个级别。对确定为重度风险级别的下水道和化粪池,应建立可燃气体在线监控系统。这种半定量评价的方法比较灵活,可解决缺乏安全风险基准标准的问题。管理人员可根据具体要求,调整半定量风险矩阵图中三个风险等级所对应的风险值R的区域,确定应按重度风险级别管理的对象,从复杂繁多的下水道和化粪池中筛选出重点监控对象。③通过诱因分析,构建了城市下水道和化粪池气体爆炸风险评估指标体系,并通过专家调查法和层次分析法,确定了各指标的权重系数。下水道内可燃气体积聚可能性等级评价指标包括:平均水力停留时间指数、检查井内管网交汇指数、外源性气体聚集指数,其权重系数分别为:0.304、0.145、0.551;下水道气体爆炸后果等级评价指标包括:所在区域人口密度分值、次生灾害可能性分值,其权重系数分别为:0.804、0.196。化粪池内可燃气体积聚可能性等级评价指标包括:化粪池容积指数、化粪池负荷指数、外源性气体聚集指数,其权重系数分别为:0.150、0.619,0.231;化粪池气体爆炸后果等级评价指标包括:所在区域人口密度分值、次生灾害可能性分值,其权重系数分别为:0.810、0.190。④根据可燃气体浓度变化特征,提出下水道和化粪池气体安全监控系统的监控指标为爆炸下限的百分比浓度,以%LEL表示。并建立了三种预警模式:稳定模式、扰动模式和聚集模式,以及对应的判别标准。预警模式判别标准基于可燃气体浓度变化趋势判别式:?1Δ=?C t i CtiCti。若从某一采样时刻起,连续3次出现后一时刻的采样浓度大于前一时刻的现象,判定为可燃气体浓度有增加趋势。通过这种判别标准,可大幅度减少现有下水道和化粪池气体安全监控系统的频繁报警与“误报”现象,提高气体监控预警系统的可靠性和有效性。在此基础上,提出了三种预警等级:安全、关注和警戒。不同预警模式对应不同预警等级,不同预警等级对应不同管理对策。其中聚集模式确定为警戒等级,应立即采取处理措施。并将下水道和化粪池作为一个统一的有机整体,建立联动预警调控机制。更多还原

【Abstract】 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.更多还原