蠕变-疲劳载荷下高温法兰连接系统的紧密性及评价方法研究

Considering the problems of the long-term security operation of high-temperature flanged connections under creep-fatigue loadings in petrochemical industry, with the goal of establishing the tightness theory of high-temperature flanged connections with flexible graphite corrgated metal gasket(FGCMG) and its safety assessment methods based on integrity theory under extreme conditions. Two main questions will be investigated intensively. Firstly, to reveal the sealing mechanism and failure mode of high-temperature flanged connections under creep-fatigue loadings, material characteristic of connecting parts and sealing performance of real systems under creep-fatigue conditions are studied by experimental tests. The time-frequency dependent leakage rate, bolting loosening mechanism produced by self-loosening and stress relaxation interaction under the compatibility condition of connection systems and the complex leakage model for FGCMG will be stressed. Secondly, to ensure the integrity of high-temperature bolted-flange connections, the key control parameters affecting safety operation are deeply investigated. The safety assessment methods based on the shakedown theory, creep-fatigue damage as well as time-frequency dependent tightness theory are established. This project involves in the realms of Solid Mechanics,Permeation Fluid Mechanics, Heat Transfer Theory, Materials and so on. Breakthroughs about failure mechanism and integrity evaluation method of high-temperature flanged connections under creep-fatigue conditions are expected. This investigation will provide technical support for reliability and safe operation of high parameters flanged connections under extreme conditions.

针对石化行业高温法兰连接系统在蠕变-疲劳载荷下长周期安全运行的难题，以柔性石墨金属波齿复合垫片连接为研究对象，以建立极端条件下法兰连接系统的紧密性理论及基于完整性的安全评价方法为目标，主要从两个方面取得突破：其一，开展法兰连接系统及各连接部件的蠕变-疲劳实验，重点探索变形协调下法兰连接系统泄漏率的时间-频率相关性、自松弛和应力松弛耦合作用下螺栓松动机制和柔性石墨金属波齿复合垫片的复杂泄漏模型，揭示其在蠕变-疲劳载荷下的密封机理和失效模式；其二，研究高温法兰连接系统完整性的关键控制参数，建立以安定性准则、蠕变-疲劳损伤及时间-频率相关的紧密度理论为基础的安全评价方法。本项目内容涉及固体力学、渗流力学、传热学和材料学等多学科领域，可望在蠕变-疲劳载荷下高温法兰连接系统的失效机理及完整性评估等方面取得突破，为极端条件下高参数法兰连接系统的可靠性和安全运行提供技术支持。

该项目围绕蠕变—疲劳载荷下高温法兰连接系统的紧密性问题，展开了如下研究：（1）系统测试了高温螺栓及法兰的蠕变、松弛、循环松弛、疲劳—蠕变性能，构建了材料的蠕变、松弛及蠕变—疲劳本构方程，提出了基于蠕变回复的变形预测模型。（2）设计了高温垫片的压—压蠕变—疲劳测试装置，研究了垫片材料在压—压蠕变疲劳下的循环压缩回弹性能及棘轮变形的时间相关性。（3）搭建了高温法兰连接系统泄漏率测试平台，获得了高温法兰系统在保温和不保温条件下的热耗散率、温度场分布和泄漏率。（4）考虑材料参数的温度相关性及温度分布不均匀性，对法兰连接系统进行热—结构耦合及蠕变分析，研究了循环热—机载荷下螺栓、垫片应力变化规律。（5）提出了综合考虑焊接接头形式、材料参数的高温管道与法兰对焊焊接接头安定评定方法，并构建了高温法兰连接管道在循环热—机械双轴载荷下的棘轮极限评定方法，为蠕变—疲劳载荷下法兰接头的局部安全评估提供了依据。（6）提出了平均松弛时间和平均松弛速率的概念，构建了松弛—蠕变转换模型和基于短时螺栓材料松弛曲线的稳态蠕变速率预测模型，为高温法兰连接系统的长时运行评估提供了依据。（7）基于柔性石墨金属波齿复合垫片的高温压缩回弹非线性属性，推导了高温法兰连接系统工作压力、垫片压紧力与螺栓预紧力的对应关系，结合ASME VIII-I中法兰紧密性条件，提出了螺栓预紧力的安全范围和设计方法。（8）推导了蠕变条件下法兰转角及垫片应力时间相关的解析解，提出了以安全运行时间为设计指标的高温法兰系统蠕变设计方法、基于时间相关的紧密性评定方法、基于设计寿命和螺栓应力松弛比例的碟形弹簧设计方法。本项目的法兰连接系统蠕变设计理论、泄漏测试方法、局部高温管道法兰系统局部棘轮与安定评定方法等部分科研成果弥补了目前设计标准的不足，具有良好的应用前景。