Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation

Risk Based Inspection (RBI) implementation and revalidation efforts require process engineers to provide process information on representative fluids, operating temperatures, operating pressures, and toxic fluids to be used in consequence models of RBI software. To streamline the work process and minimize the effort expended by the process engineer in gathering, scrubbing, and tabulating the required information for each piece of piping and equipment covered in the RBI effort, this process data is usually also used as the primary data for performing a Damage Mechanism Review.

Taking into consideration that process conditions in a unit may vary significantly due to breaches in operating conditions, cyclic service, process upsets, and frequency of shutdowns, it is sometimes difficult to determine the actual values to use for the process data. Yet the impact of the values that are selected to be used can be significant in the risk that is calculated by the RBI software (due to the impact on the consequence model), the damage mechanisms that apply, and to the inspection plans that are generated.

As an example, an insulated 304SS vessel from the 1960's had an operating temperature that varied from 320°F to 480°F and was assigned an average operating temperature of 400°F. Taking into consideration the 400°F operating temperature that was provided by the process engineer, the vessel had not previously been assigned a chloride stress cracking (Cl-SCC) environmental damage mechanism since external wetting is not expected at these high temperatures. However due to actual operating conditions of frequent shutdowns and cyclic service operation where the temperature cycled from ambient to 480°F, the vessel experienced external Cl-SCC and had to be replaced.

It is more straightforward to assign process data in some units and equipment than in others and the required level of sensitivity in providing process data may vary with the process unit being evaluated or with the particular equipment being assessed. Therefore, it is important for the process engineer and other RBI team members work closely to ensure there is a good level of understanding of how the process information provided will affect the damage mechanisms that are assigned and the risk levels that are calculated by the RBI software since the risk levels and damage mechanisms will drive the types of inspections that are to be carried out as well as the frequency and extent of inspection.

This series of short articles will provide guidelines on assigning process conditions for RBI efforts. Over the course of the next few weeks / months we will be discussing guidelines for each of the following:

1. Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation (this introduction)
2. Corrosion Under Insulation (CUI) and How it Relates to Risk Based Inspection
3. Process Fluids and Consequence Models
4. High Temperature Damage Mechanisms
5. Low Temperature Damage Mechanisms
6. High Temperature Hydrogen Attack
7. Environmental Cracking Damage Mechanisms
8. Concluding Remarks

Stay tuned for the next entry in this eight-part series.

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