scope:

Purpose

This recommended practice provides quantitative procedures to establish an inspection program using riskbased methods for pressurized fixed equipment including pressure vessel, piping, tankage, pressure relief devices, and heat exchanger tube bundles. API RP 580 [1] provides guidance on developing a risk-based inspection program for fixed equipment in the refining and petrochemical, and chemical process plants. The intent of these publications is for API RP 580 to introduce the principles and present minimum general guidelines for RBI while this recommended practice provides quantitative calculation methods to determine an inspection plan.

Introduction

The calculation of risk in the Risk-Based Inspection (API RBI) methodology involves the determination of a probability of failure combined with the consequence of failure. Failure in API RBI is defined as a loss of containment from the pressure boundary resulting in leakage to the atmosphere or rupture of a pressurized component. As damage accumulates in a pressurized component during in-service operation the risk increases. At some point, a risk tolerance or risk target is exceeded and an inspection is recommended of sufficient effectiveness to better quantify the damage state of the component. The inspection action itself does not reduce the risk; however, it does reduce uncertainty thereby allowing better quantification of the damage present in the component.

Risk Management

In most situations, once risks have been identified, alternate opportunities are available to reduce them. However, nearly all major commercial losses are the result of a failure to understand or manage risk.

API RBI takes the first step toward an integrated risk management program. In the past, the focus of risk assessment has been on on-site safety-related issues. Presently, there is an increased awareness of the need to assess risk resulting from:

On-site risk to employees,

Off-site risk to the community,

Business interruption risks, and

Risk of damage to the environment

The API RBI approach allows any combination of these types of risks to be factored into decisions concerning when, where, and how to inspect equipment.

The API RBI methodology may be used to manage the overall risk of a plant by focusing inspection efforts on the process equipment with the highest risk. API RBI provides the basis for managing risk by making an informed decision on inspection frequency, level of detail, and types of NDE. In most plants, a large percent of the total unit risk will be concentrated in a relatively small percent of the equipment items. These potential highrisk components may require greater attention, perhaps through a revised inspection plan. The cost of the increased inspection effort can sometimes be offset by reducing excessive inspection efforts in the areas identified as having lower risk. With an API RBI program in place, inspections will continue to be conducted as defined in existing working documents, but priorities and frequencies will be guided by the API RBI procedure.

API RBI is flexible and can be applied on several levels. Within this document, API RBI is applied to pressurized equipment containing process fluids. However, it may be expanded to the system level and include additional equipment, such as instruments, control systems, electrical distribution, and critical utilities. Expanded levels of analyses may improve the payback for the inspection efforts.

The API RBI approach can also be made cost-effective by integrating with recent industry initiatives and government regulations, such as Management of Process Hazards, Process Safety Management (OSHA 29 CFR 1910.119), or the proposed Environmental Protection Agency Risk Management Programs for Chemical Accident Release Prevention.

Organization and Use

The API RBI methodology is presented in a three part volume.

Part 1 - Inspection Planning Using API RBI Technology

Part 2 - Determination of Probability of Failure in an API RBI Assessment

Part 3 - Consequence Modeling in API RBI

The methods used to obtain an inspection plan are provided in Part 1 for fixed equipment including pressure vessels, piping, atmospheric storage tanks, pressure relief devices and heat exchanger tube bundles. The pressure boundaries of rotating equipment may also be evaluated using this Part. The probability of failure for fixed equipment is covered in Part 2. The probability of failure is based on the component type and damage mechanisms present based on the process fluid characteristics, design conditions, materials of construction, and the original construction code. Part 3 provides methods for computing the consequence of failure. Two methods are provided. The first method, or Level 1, is based on closed form solutions generated for a limited set of reference fluids or fluid groups. The second method, Level 2, is a general, more rigorous method that can be used for any fluid stream composition.

An overview of the probability of failure and consequence calculation procedures and the associated paragraph in this recommended practice for fixed equipment is provided in Table 1.1.