Research On The Theoretical Computing Method Of Fatigue Life For Supercharged Boiler Drums

The problem of lifetime for supercharged boiler steam drums has been the pivotal technology to design,manufacture,installation,use,inspection,repair and reconstruction.Compare with the utility boiler steam drums,the various particularities make it appear outstanding and important,and make it necessary to study the applicability of the correlative standard computing methods for utility boilers.In addition,a set of theoretical computing method is being cried for testing the reliability of software for stress analysis and the accuracy of its computed result in practical application.But the researchful actuality has restricted the technology of Chinese supercharged boiler in dire need of further research and practical application.Consequently,the researches on the computing methods of low cycle fatigue lifetime,safety assessment and rupture life for supercharged boilers steam drum have important theoretic significance and applied foreground.1.In the aspect of computing method for the low cycle fatigue lifetime of supercharged boiler steam drumsThe following conclusions were determined by argumentation,analysis and comparison as:Differing from utility boilers,the fatigue checking point is at the inner corner of lengthways section of a hole on the upper half part of steam drum shell;When a supercharged boiler is under rated load,the internal pressure of a steam drum was suggested to select the setting pressure of the safety valve,and comparing with the standard computing methods of untility boilers,the numerical value considered the practical function and made the numerical value reification;The radial temperature difference stress concentration factor of fatigue checking point was suggested to select 2,and comparing with GB/T9222,the numerical value accords with practical situation all the more;The maximum temperature difference in the circumferential direction is the same as GB/T9222,and the modus operandi of not calculating this heat stress in TRD301 isn't adopted;Differing from TRD301,the strength rule of principal stress difference calculation should according to the third strength theory and a series of computing methods relating stress amplitude under the rule;The advanced Chinese low cycle fatigue design curve is regarded as the checking curve;Miner linear accumulative damage guideline is regarded as the safety one,but differing from the standard computing methods of utility boilers,the total damage fraction should be less than 1.The conclusions can be derived from calculations and analyses:The identical moment for the calculations,between membrane stress and radial temperature difference,between internal pressure stress and radial temperature difference stress at the fatigue checking point,and the superposition principle of the two stresses,should be adopted;The approximate solution based on the analytical solution of radial temperature difference applies to the same calculation of supercharged boiler steam drums,but the reasons for the methods of simplified solution and the modified method of GB/T9222 based on simplified solution cann't apply rest with besides they cann't satisfy the maneuverability of supercharged boilers,the method of simplified solution doesn't express time parameter,the modified degree to the structure of supercharged boiler steam drums or the temperature damping factor of the method of GB/T9222 is inadequate;The radial temperature difference of supercharged boilers isn't the fact that the thicker of cylindric wall,the bigger of this temperature difference,the domino offect for the collective function of start-stop time and velocity of supercharged boilers must also considered;the characteristic values of characteristic equation of Bessel function determined by the stucture of supercharged boiler steam drum shells make up of a positive slope beeline,and the characteristic values of the upper half part of cylinder are bigger,the slope is also bigger;The varying orderliness for the heat stress ratio in the computing formula of circumferential temperature difference heat stress of an imperforate supercharged boiler steam drum is that the lower of steam drum water level,the bigger of this value,and the value is selected according to the max.at this moment;It is reasonable that the linked method between a supercharged boiler steam drum shell and its linked tubes is welded on the upper half part,and is distensible on the lower half part;The educed shortcut method of alternating stress range based on the third stength rule not only reduces the calculated physical parameters and steps,but also simplifies the computing method of low cycle fatigue lifetime for supercharged boilers and the standard computing methods of ASME,BS DP 5500 and GB/T9222.In conclusion,the calculational system of ASME and BS DP 5500 can apply to this paper,but the low cycle fatigue design curve and the damage safety rule cann't apply to this paper,and it is unnecessary that there are two fatigue checking points,the calculation of radial temperature difference demands programme;TRD301 cann't apply to this paper;In GB/T9222,the computing method of radial temperature difference,the compositive extremum method of internal pressure stress and radial temperature difference heat stress,together with the damage safety rule,cann't apply to this paper.But the shortcut method of alternating stress range can simplify the standard computing methods of ASME,BS DP5500 and GB/T9222,and the method of this paper is the development of these standard methods.2.In the aspect of the computing methods for the safety assessment and rupture life of supercharged boiler steam drumsThe computing method of safety assessment was founded on the computing methods of equivalent crack size,stress intensity factor and their corresponding regulations of a 1/4 elliptical hole corner crack in the advanced standard of BS7910-2005,the advanced Chinese COD design curve and the methods of stress calculation and analysis in this paper,making the computing method apply to elastic range and plastic range,and offering the allowable crack size to the calculation of rupture life.Based on the former and adopting the testing crack growth rate,the computing method of rapture life was founded.The results of an example express that the inspection data of steam drum surface quality in JB/T1609 cannot apply to the calculated crack inspection,the existing crack will make the lifetime of this supercharged boiler steam drum not satisfy the lifetime requirement of this supercharged boiler out and away.Simultaneity,the increase of a hole size and the stopping velocity of this supercharged boiler can both reduce the rupture life,when the two act on the crack,the rupture life will prick up.3.Some suggestions for practical engineering1)Because the drop of water level of steam drum can shorten increasedly the lifetime of steam dum,some considerations must pay attention to when a supercharged boiler is designed and operated;2)The welding must be adopted to the linked tubes on the upper half part of a supercharged boiler steam drum shell,and to the linked tubes on the lower half part,it is necessary to adopt expanded joint.Furthermore,the expanded joint must be adopted to the tubes of superheater;3)The inspection data of manufacturing surface quality for steam drum in JB/T1609 cann't apply to the calculated crack,so intendance and inspection must be strengthened.Simultaneity,the big tubes have better to be avoided,and besides self-strengthening technology of pressure vessel can be adopted,the stress peak value must be reduced during the running of a supercharged boiler;4)The material of steam drum has better to select the high fracture toughness based on satisfying the low cycle fatigue lifetime of a steam drum,so as to insure the safety and reliabilityof a steam drum.