Frequently Asked Questions
The assessment of corrosion damage due to loss of thickness at nozzles has been a subject of work as part of TWIs' Core Research Programme[1] . Several methods have been proposed for the assessment of corroded nozzles. These include methods based on the area replacement rules commonly used for the design of nozzle reinforcement, empirical rules based on methods devised by Rodabaugh[2] , and computational or finite element analysis. None of the methods has been fully validated by reference to extensive, large-scale tests of corroded nozzles. However, API RP579[3] has proposed methods based on area replacement and the empirical method for the assessment of locally thinned regions near nozzles, and limited experimental trials have been conducted. TWI has proposed a simple design method based on BS PD5500[4] . This method has the advantage of being less conservative in many applications[1] , but it lacks extensive validation.
When assessing corrosion damage at nozzles, consideration should be given to local collapse of the ligament underneath the defect as well as global collapse of the opening. Many of the existing methods do not adequately consider both failure modes. Nor do the proposed methods address local loading of nozzles.
Nozzles can also be considered using finite element analysis. A purpose-built program (available through TWI) exists for the assessment of corroded nozzles, but this program only evaluates the general stress and is not at present capable of determining whether the nozzle is fit for purpose. However, provided an appropriate criterion can be agreed with the operator, finite element analysis can be used to assess corrosion damage.
References