Fitness-for-service assessment for thermal hot spots and corrosion in pressure vessels

Tantichattanont, Pattaramon (2006) Fitness-for-service assessment for thermal hot spots and corrosion in pressure vessels. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (39MB)

Abstract

Corrosion and thermal hot spots are typical of damage occurring in pressure vessels, piping or storage tanks used in industrial processes. Structural integrity of such components needs to be evaluated periodically to determine the acceptability for continued service. The ability to demonstrate structural integrity of an in-service component containing damage is termed as Fitness-for-Service (FFS) assessment. -- In the present thesis, the Remaining Strength Factor (RSF) is chosen to quantify FFS assessments. Three alternative methods for Level 2 FFS assessments (according to API 579) of pressure components are proposed. Damage due to corrosion and thermal hot spots is considered. The methods are based on variational principles in plasticity, the limit load multiplier mα-method, reference volume and the concept of decay lengths in shells. The mα-method has been shown to provide acceptable approximations to limit load of various mechanical components and is thus employed as one of the bases of the recommended calculations for RSF in the current study. -- The effects of local damage on a shell structure are normally restricted to a limited volume in the vicinity of the damage termed as reference volume. The use of reference volume instead of the entire volume of the structure gives a better approximation of limit load multipliers. In the present study, reference volume is characterized by using decay lengths of shells. Decay lengths for spherical and cylindrical shells are derived based on elastic shell theories. The derived decay lengths are also used to specify the limit of what can be called “local” damage. They allow one to examine the interaction between decay effects in two perpendicular directions. The local damage limit is defined as the maximum size of damage beyond which pure equilibrium stress occurs at some place inside the damaged area. If the damage in a component is larger than this limit, it can be deemed as “global.” For global damage, the “failure” load is as if the damage extends to the entire component and the classical lower bound RSFL is used. -- The stretching and bulging effects due to the damage are studied. For highly localized damage, the effects of stretching action dominate the behavior of the damage and surrounding areas. The damaged spot tends to open up instead of forming a bulge. The limit defining the threshold to dominance of stretching action is proposed by using an approximate equilibrium calculation and yield-line analysis. -- The usefulness and effectiveness of the three recommended methods are demonstrated through illustrative examples. The recommended approaches are validated by detailed inelastic finite element analysis. Recommendations and relevant observations for general shapes have been made. Detailed procedures and useful aids for FFS assessments are provided.

Actions (login required)

View Item