The Risk Assessment And Early Warning Study Of Sewers Gas Explosion Of Mountain City

Along with the development of the scale of cities, the enhancement of theawareness of environment, the more and more antention and input of the governmentand societies, setting up a set of effective sewers harmful gas monitoring and earlywarning system is not only an important content of the city environment monitoring butalso a necessity requirement of the modern construction of the municipal establishment.The paper is sponsored by national important water pollution control andmanagement item of Chongqing main city drainage system safey and city non-pointsource pollution control technology research and comprehensive demonstration（2008ZX07315-001）.This paper finished the component analysis and the reason studyof accumulation rule of sewers harmful gas of main typical area of Chongqing JiangbeiDistrict and Shapingba District. Built a model of sewers’ two dimensional turbulence ofmultiple gas and studied on Numerical Simulation of the explosion. Developed thesewers gas safety threshold research facility to conduct a series of explosion tests, toanaylse the explosion reason of the sewers gas and to decide the gas explosion limit.Found the sewers gas explosion risk evaluation model. By means of technique reachand comprehensive demonstration, set up the mountain city sewers harmful gasmonitoring and early warning system. It has the practical and positive meaning toensure the safety running of sewers. The main achievements include:①In accordance with the characteristic of gas explosion in confined space,builtthe basic physical and maths model of the mountain city sewers gas explosion.Constructed a set of energy equation、momentum equation、mass equation、stateequation、 turbulence equation、 component equation and chemical reation rateequation.Using the Fluent6.3software,simulated many explosion tests with CH₄concentration on0.8%、2%、5%、8%、9.5%、10%in the pipe diameter of300mm.Testsfound that when the CH₄concentration on8%、9.5%and10%，the ignition end pressureis steep than that of the CH₄concentration on5%. Along with the adding of inertia CO₂,reduced the reaction activity of mixture gas, leading to the increase of the ignition delaytime and the minishment of the combustion reaction velocity.②Constructed the sewers gas explosion simulation experiment system,which couldchecking the sewers’ two dimensional turbulence module of multiple gas.The facilityhas four parts, gas distribution system, the explosion cavity, ignition device and monitoring system. The explosion cavity is a transparent hose with the inner diameter of25mm and0.5m long.During the explosion experiment, the oncentration of CH₄is from4.4%to5.9%.The concentration of H₂S is0to0.02%. The concentration of CO₂is from0.03%to0.8%. The concentration of O₂is20.9%.The oncentration of N2isfrom72.93%to73.55%. The test results show that CH₄with the oncentration of5.5%has theexplosion wave appeared earlier than that of5.6%.CH₄with the oncentration of5.5%can not produce an explosion。The orthogonal test L₂₅（5⁶） show that when theoncentration of CH₄is5.6%，CO₂with the oncentration of2%can inhibit theoccurrence of explosion,while with the oncentration of CH₄is5.7%and5.8%, CO₂withthe oncentration of2%has no obviously effect.When the oncentration of CH₄is over5.5%，H₂S with the oncentration of0.01%can promot the temperature of explosionwave slightly，with the explosion temperature peak value appeared a little on the highside and advance.H₂S is a kind of toxicity gas in sewers. With trace concentration,it hasnegligible effect on the explosion threshold of CH₄.③By means of numerical simulation and the system test through the sewers gassafety threshhold study facility,confirmed the explosion threshhold of CH₄in sewers is5.5%.The facility has such characteristics as gas distribution accurately.The gascomposition and content can be precisely formulated into a predeterminedrequirement.Can exactly determine the explosion temperature pressure changes beforeand after and can accurately determine the mixed gas explosion lower limit.④Elaborate the necessary and the characteristic effect of risk assessment onMountain City sewers gas explosion.Promote the risk assessment module on MountainCity sewers gas explosion based on risk matrix. The risk matrix is composed of riskfactors, risk probability, risk effect, risk level、 Borda sequence value and riskweight.With the example of Luyun Shangdu international fashion town No.3inspectionwell,caculated the risk weight of0.4295、0.0246、0.2895、0.0379、0.1435and0.0750,which is respectively on behalf of risk factors of CH₄,CO₂,H₂S,temperature,regional enviorment and inspection well position. Done the riskassessment to sewers of Chongqing typical area and proposed a series of managementdecision.⑤Set up the mountain city sewers harmful gas monitoring and early warninginformation system based on the technology of GIS and GPRS.The sewers explosionthreshhold has the low Level of30%LEL and the other high level of50%LEL.Monitoring probe is located under the manhole cover in the distance of0.3-0.5m.With the demonstrative field trial operation in Chongqing Jiangbei District,the system’s dataacquisition and transmission meet the design requirements and the terminal equipmentruns normally and with stable situation.