Dynamic Reliability Of District Heating Pipeline And Network System

District heating system is an important lifeline infrastructure to guarantee local people's daily life and productions in northern urban areas of China.Heating pipeline and the network are vulnerable parts in district heating system.With the expansion of the systems' scale and the increase of elapsed service life,heating pipeline failure occurs frequently,and the reliability of heating pipeline degrades gradually in the long-term service process.Besides,the dynamic operation characteristic of district heating system can not be inigored in district heating system reliability research with the advance of delicacy management.This paper studied dynamic reliability of district system from the view of the pipelines and the network system,and proposed dynamic reliability evaluation method.According to the force analysis of buried pipeline,based on generalized stressstrength interference theory,the limit state equations of four failure modes for pip eline which are strength failure,vertical instability,radial instabilit y and local buckling are established.Failure probability of heating pipeline,caused by corrosion is studied.The power function corrosion rate model is introduced into heating pipeline failure probability analysis.According to the stochastic characteristics of the relevant factors,the Monte Carlo method is used to simulate the stress state of heating pipeline.Whether the state of the pipeline is safe or failure is determined by judging whether the limit state equation is positive or negative.The failure probability is obtaind by dividing the total number of failure events with the total simulation times.The sensitivity index of the relevant random variable is analyzed.The failure probability of the typical pipeline from DN100 to DN1000 is studied using Monte Carlo simulation method.The results show that the strength failure is the main mode of the heating pipeline failure,and the hot water temperature in the pipeline is the main heating parameter which affects the failure probability.With the increase of the elapased life and the corrosion rate,the failure probability of the heating pipeline is increasing yearly.According to the statistical data of the failure rate of heating pipe,the updating model and method of the failure rate simulation of heating pipeline based on Bayesian theory are established.Taking the district heating network of a city in Heilongjiang as case study,the failure probability and the average failure rate consider-ing dynamic supply water temperature in heating period are calculated using Monte Carlo simulation method.According to the heating pipeline failure rate updating model based on Bayesian theory,combing the failure statistical data of the pipeline network during a ten-year operation period,the simulation results of the failure rate of heating pipeline are updated.The updated failure probability function is used as the basic data in district heating network reliability evaluation.In order to study the thermal regimes of heating network and the variation of heating parameters under failure condition,the hydraulic model and dynamic thermal model of the network is established,according to the quasi dynamic assumption.The two-dimensional model and simplified one-dimensional model describing the radial and axial heat transfer processes in pipeline are established.Then the corresponding state space models are studied,and the Laplace method is used to solve the state space model and provide the discrete method of nume rical computation.The simulation error between the two-dimensional model and simplified onedimensional model are studied under different heating parameters.The supplying water temperature of a district heating project is tested to verify the established dynamic thermal model.The test data of supply water temperature at the heat source are used to predict the supply water temperature at the substations using simplified one-dimensional model with one node.The relative error between the tested and simulated supply water temperature at the substations is verified to be no more than 10%.After constructing and verifying the simplified one-dimensional dynamic thermal model of heating network,based on the topology reconfiguration of the fault network,the dynamic thermal model of the network system are constructed by combing the substation and building's thermal model.Then,the variation of supply temperature,heat gain and indoor temperature under failure condition of the circular trunk section in a large-scale district heating network is studied.The senarios of failure occurs under design outdoor temperature and three outdoor temperature levels are studied respectively.It is found that there is time lag between the states of district heating nework system and the elements failure from the dynamic thermal r egime study of district heating network under failure condition.The Markov repairable system theory considerign failure response delay is proposed to descrive the failure process of district heating network.The states of district heating network system are aggregated ac-cording to the heat users' indoor temperature level.Allowed intermittent heating time and allowed limited heating time are introduced to analyze the failure r esponsedelay time of district heating network system.The failure judgement standards basing on heating users' indoor temperature is further improved.The dynamic reliability assessment method of district heating network system is established and a set of reliability index are proposed,including availability,reliability,expected failure/ accident time,functional reliability.The dynamic reliability a district heating network is studied and it is found that the reliability of district heating network system increases with the increase of outdoor temperature during one heating period and decreases with elapsed service life.The dynamic reliability reserach explores degrading characteristics of heating pipe reliability and the influence of dynamic thermal characteristics on network system reliability.It is suitable to evaluate the district heating system with diversified structure and operation conditions in China.It will provide theoretical support for delicacy management of district heating system and intelligent scheduling under failure condition.