Since 2003, we have conducted a lot of researches on risk based assessment technology in China according to standards API 581, API 571 etc., mastered the risk distribution conditions of in-service pressure equipments covering all kinds of units of about 80 large-scale oil refinery plants, chemical plants and chemical fertilizer plants in China, and formed the failure trees and technical guide documents for RBI of typical petrochemical units fit to Chinese national conditions. However, we find there are interactions of various failure modes and mechanisms of many petrochemical equipment during their actual running due to the diversity of corrosive media and the complexity of influential factors in the process of risk assessment implementation process, which has not been taken into account in the existing API 581, if we cannot determine the possible dominant failure modes and failure mechanisms as well as influential factors of secondary mechanisms in the actual service environment of pressure equipment, the risk assessment analysis results obtained tend to have big difference from actual conditions. In this paper, researches are conducted with respect to the judgment methods of dominant mechanisms in case of joint action of multiple failure mechanisms of pressure equipment and influential law of secondary mechanisms in several typical complicated medium environment of petrochemical enterprises based on the survey and analysis of failure accidents, risk assessment and experimental research of pressure equipment, and suggestions on improvement with respect to failure likelihood calculation methods in risk assessment in API 581 are given.

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