Stress Analysis And Strength Design Of Quench Boiler With Special Structure

At present,the market demand for ethylene continues to increase.In order to expand the production capacity of ethylene,domestic new ethylene production equipment is being added and at the same time,the old equipment is being upgraded,among which steam cracking is still the main production process.As one of the key equipment for producing ethylene by steam cracking,the safe and stable operation of quench boilers has a direct impact on the economy of ethylene production.The tubesheet of a quench boiler is formed by welding flat-round tubes side-by-side.Since the structure of the tubesheet connected with the jacketed heat exchanger tubes and the loadings are special and complicated,it is hard to design the quench boiler as no suitable design code is available.In this thesis,finite element models were established for a vertical quench boiler and stress analysis and assessment was performed based on the standard of design-by-analysis for pressure vessels.Rational pre-tension for the heat exchanger tubes and reinforcement structures for the non-tube layout zone of the tubesheet were studied in order to complete the design of the quench boiler.The main contents and conclusions are as follows:(1)Finite element analysis model of the quench boiler was established with ANSYS Workbench.The stress analysis of the quench boiler under six load combinations specified in GB/T 151-2014 Heat Exchangers was conducted,and the strength was assessed according to the stress classification method of JB4732 Steel Pressure Vessels —Design by Analysis.Results show that the quench boiler could meet the strength requirements only under the tube-side design pressure,or the tube-side working pressure and temperature load.Under other load cases,however,the peak stress intensity mainly appeared at the inner arc of flat-round tubesheets,and the primary and secondary stress at this position exceeded the stress limit.(2)In order for the quench boiler to pass the strength assessment,the heat exchanger tubes need to be pre-stretched,which should satisfy the strength demand of the axial stress and pull-off stress of heat exchange tubes according to GB/T 151.Therefore,the axial stress and pull-off stress of heat exchange tubes were assessed under four operating conditions and seven pre-stretching amounts of heat exchange tubes.Results show that the pull-off stress of the heat exchange tube could meet the requirements in the pre-stretching range of0～13mm,and when the pre-stretching amount reaches 11 mm,the axial stress has exceeded the assessment limit.The pre-stretching range to meet the requirements of axial stress and pull-off stress of heat exchange tubes is 0～9mm.(3)With different pre-stretching amounts of the heat exchange tubes,the strength of the flat-round tubesheets under two dangerous load cases were assessed respectively.Results showed that when considering the temperature load,the pre-stretching amount of the heat exchange tubes needs to reach 5mm for the stress of the tube layout zone of flat-round tubesheets to meet the strength requirements;Without considering the temperature load,when the prestretching amount of the heat exchange tube reaches 13 mm,the stress at the joint between the heat exchange tubes and the flat-round tubesheets is too large to pass the strength assessment.In order to meet the strength requirements of the tube layout zone of flat-round tubesheets,the pre-stretching amount of heat exchange tube should be 5～11mm.(4)In the non-tube layout zone of flat-round tubesheet,the reinforcement structure is needed which could be an annular flat plate or an annular vertical plate.For the annular flat plate,if the thickness of the plate reaches 15 mm and the pre-stretching amount is 5～9mm,the improved flat-round tubesheet could meet the strength requirements.For the annular vertical plate,however,if the thickness of the plate is 85 mm and the pre-stretching amount is 9mm,the improved flat-round tubesheet could also pass the strength assessment.