Risk Based Approach to Integrity Assessment of a Large Spherical Pressure Vessel

Abstract

The risk based approach has been applied, in its simplest form, i.e. by using the risk matrix to illustrate how the water proof test can shift risk from high to very high level in the case of large spherical pressure vessel (ammonia storage tank). Having in mind the basic definition of risk, being the product of the probability and consequence, and fixing the consequence at the highest level, only probability of unfavourable event (leakage and/or failure) has been evaluated. Toward this end, the failure assessment diagram (FAD) has been used here as another simple engineering tool to estimate probability of the failure, as the function of the position of the operating point, i.e. defining probability as the ratio between the distance of the operating point from the zero point, and the appropriate distance between the point on the limiting curve and zero point. This simple engineering tool to assess structural integrity showed clearly that water proof test is not always recommended, because it disregards possible stable growth of cracks, which might reach critical size for unstable growth, i.e. it does not prove that failure will not happen in future under the same conditions.