彈簧式安全閥之失效分析與可靠度評估

Abstract

舉凡工業系統、化工設備、鍋爐、壓力容器或管路之中，使用高壓流體甚為頻繁、普遍，惟因壓力設備結構體之耐壓強度設計仍有極限，必須採取超壓事故之防範措施，彈簧式安全閥係裝置於各該壓力設備中，為防止流體超壓不可或缺之安全裝置。由於安全閥可紓解設備過高之流體壓力，具有瞬時洩放、全開急洩之功能，然而溫度、壓力、負荷等環境應力，或動作頻率等操作條件為動態性質，而非固定不變，具有偶發性、隨機性或變異性，實難完全準確預測，且安全閥在現場使用時，因流體性質、環境應力之作用與效應，使其特性或功能受影響，產生疲勞、腐蝕、磨損等常見誘發失效因素，又長期使用下，時間因素使其構造減弱、強度退化、損傷等，此時，功能將產生異常，使安全閥不能準確動作，甚或無法立即發揮洩放功能。但實務上安全閥在任何時間均不容失效，否則後果嚴重，易釀成重大災害事故，故對安全閥之性能及可靠度，自應有一定要求。因此，本研究特針對安全閥之失效分析與可靠度評估，加以探究。 本研究對於安全閥之構成、功能及所處工作條件等，將進行特性參數及失效機制分析，逐項研析環境應力與導致失效現象之因果關係，探討衍生疲勞、腐蝕、磨耗等失效現象相關物理化學原因；在定量分析方面，利用可靠度機率統計理論中之強度--應力干涉模型理論，以可靠度指標法（β-method）估算可靠度，並由試驗驗證可靠度值，以量化評估可靠度能否達到需求。實驗方面，建立實驗室模擬之試驗方法，以評估安全閥之彈簧在使用過程中疲勞失效現象；在定性分析方面，採用失效模式分析及FMEA方法探討安全閥各部位元件失效模式、效應及危害性，可提供現場應用或後續研究參考。

Generally speaking, high-pressure fluid is commonly and frequently employed in industrial systems, chemical equipment, boilers, pressure vessels and pipes. As a result that the anti-pressure design of the pressurization equipment still has its limitation, it is necessary to take prevention from over-pressurized accidents. The Spring-loaded Safety Valve (SLSV) is one of the indispensable devices, which is installed in pressure equipment to prevent from over fluid pressurization. Although SLSV relieves high fluid pressure as well as helps to exhaust and pop action, instead of being static, the operational factors, such as temperature, pressure, and load vary with environmental stress and frequency of movement. Moreover, affected by the nature of fluid and environmental stress, SLSV often comes into being fatigable, corroded, and exhausted, or it may even cause structural degradation and functional degenerate if it is continually used over a long period of time. Hence, it is difficult to precisely predict the safety of SLSV due to its fortuity, randomness or variability. SLSV is not allowed to fail since it plays one of the key roles to prevent high-pressure accidents. Therefore, the functions and reliabilities of the SLSV should be seriously concerned. The purpose of the study is to explore SLSV by discussing its failure analysis and reliability assessment. To study the function, structure and operational condition of SLSV, the characteristic variables and failure mechanism will be analyzed and the environmental stress and the cause-effect relationship of failure phenomena will be explored. In additions, those failure phenomena in respect of fatigue, corrosion, exhaustibility will be investigated from the perspectives of Physics and Chemistry. In this study, the reliability index (β-Method) based on Strength-Stress Interference Model will be used to predict and evaluate reliability quantitatively to meet safety demands. The simulation method will be applied to distinguish fatigue deterioration during operating processes. Failure Modes and Effects Analysis (FMEA) technique will be used to understand the failure model and effect of different parts of SLSV, which may shed a light for practical application or further researches.