Modeling And Simulation Of Pressure Drop Calculation For Branched Flare Networks

Flare networks for collecting excess material discharged by petrochemical process plants in the open, parking and accident conditions plays an important role in the safe production in the petrochemical industry. In the transformation or expansion process of petrochemical process units, the number of the discharge point and the vent volume of safety valve changes may result in safety relief device outlet pressure exceeds its maximum allowable backpressure,flare gas would be unable to discharge into flare networks. In response to this problem, a pressure drop calculation model of flare networks based on safety vale backpressure restrictions and gas discharge velocity restrictions was studied.The main work and achievements are listing as follows:(1) According to the flare system design standards, back-pressure limiting conditions of safety relief device and the pipeline gas flow restrictions should be considered in flare pipeline design. Romeo equation and Drwnchuk-Purvis-Robinson equation are used to improve the computational efficiency of friction coefficient and gas compression coefficient respectively. The two coefficients are contained in gas discharge pipe pressure drop calculation formula recommended in API 521 standard. Pressure drop of flare pipeline was calculated by the improved formula, the results show that the calculation results of this formula are in good agreement with other formulas within a certain length range of the pipeline.(2) Based on the abstract description of the branched flare networks, the correlation matrix is used to build the flare network topology model. At the same time, the flare networks topology model is implemented to solve the pressure drop of the flare network, combining with the pressure drop formula. In addition, a case is given to verify the accuracy of the model by comparing calculation results with that of other network software, the percentage error is within 5%.(3) In order to improve the accuracy and stability of the calculation results, the Crank-Nicolson implicit difference scheme is used to build the differential equation of transient model for gas pipeline. Moreover, transient model is built to make a dynamic analysis about the inlet pressure of flare pipeline. A comparison and analysis is made between the transient model and the network transient simulation software Pipeline Studio(PLS) on some cases. The results reveal that when the pressure of pipeline is high or the length ofpipeline is short, both of calculation results are closed to each other. So the transient model of pipeline is applicable under a certain range.(4) Based on the transient flow model of flare pipeline, a transient flow model of flare networks is built by adding flow and pressure boundary conditions at the flare network internal nodes. In addition, a case is given to verify the accuracy of the model by comparing calculation results with that of Pipeline Studio, which shows that there is a bias between two results. However, the trend is basically the same, and the relative error remains under a acceptable range.