Risk-Based Inspection Technology

API RECOMMENDED PRACTICE 581 SECOND EDITION, SEPTEMBER 2008

Risk-Based Inspection Technology

Downstream Segment

API RECOMMENDED PRACTICE 581 SECOND EDITION, SEPTEMBER 2008

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Copyright © 2008 American Petroleum Institute

Foreword

This publication provides quantitative procedures to establish an inspection program using risk-based methods for pressurized fixed equipment, including pressure vessel, piping, tankage, pressure relief devices, and heat exchanger tube bundles. This document is to be used in conjunction with API 580, which 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 580 to introduce the principals and present minimum general guidelines for RBI while this publication provides quantitative calculation methods to determine an inspection plan using a risk-based methodology.

The API Risk-Based Inspection (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 making informed decisions on inspection frequency, the extent of inspection, and the most suitable type of NDE. In most processing plants, a large percent of the total unit risk will be concentrated in a relatively small percent of the equipment items. These potential high-risk components may require greater attention, perhaps through a revised inspection plan. The cost of the increased inspection effort may sometimes be offset by reducing excessive inspection efforts in the areas identified as having lower risk.

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PART 1

INSPECTION PLANNING USING API RBI TECHNOLOGY

PART CONTENTS

1. SCOPE 5

   1. Purpose 5

   2. Introduction 5

   3. Risk Management 5

   4. Organization and Use 6

   5. Tables. 7

2. REFERENCES 8

3. DEFINITIONS 8

   1. Definitions 8

   2. Acronyms 10

4. API RBI CONCEPTS 11

   1. Probability of Failure 11

      1. Overview 11

      2. Generic Failure Frequency 11

      3. Management Systems Factor 11

      4. Damage Factors 11

   2. Consequence of Failure 12

      1. Overview 12

      2. Level 1 Consequence Analysis 12

      3. Level 2 Consequence Analysis 13

   3. Risk Analysis 14

      1. Determination of Risk 14

      2. Risk Matrix 15

   4. Inspection Planning Based on Risk Analysis 15

      1. Overview 15

      2. Risk Target 15

      3. Inspection Effectiveness – The Value of Inspection 16

      4. Inspection Effectiveness – Example 17

      5. Inspection Planning 17

   5. Nomenclature 18

   6. Tables. 19

   7. Figures 21

5. PRESSURE VESSELS AND PIPING 26

   1. Probability of Failure 26

   2. Consequence of Failure 26

   3. Risk Analysis 26

   4. Inspection Planning Based on Risk Analysis 26

6. ATMOSPHERIC STORAGE TANKS 27

   1. Probability of Failure 27

   2. Consequence of Failure 27

   3. Risk Analysis 27

   4. Inspection Planning Based on Risk Analysis 27

7. PRESSURE RELIEF DEVICES 28

   1. General 28

      1. Overview 28

      2. PRD Interdependence with Fixed Equipment 28

      3. Failure Modes 28

      4. Use of Weibull Curves 29

      5. PRD Testing, Inspection and Repair 30

      6. PRD Overhaul or Replacement Start Date 30

      7. Risk Ranking of PRDs 30

      8. Link to Fixed or Protected Equipment 30

   2. Probability of Failure 31

      1. Definition 31

      2. Calculation of Probability of Failure to Open 31

      3. PRD Demand Rate 31

      4. PRD Probability of Failure on Demand 32

      5. Protected Equipment Failure Frequency as a Function of Overpressure 39

      6. Calculation Procedure 40

   3. Probability of Leakage 41

      1. Overview 41

      2. Calculation of Probability of Leakage 41

      3. Calculation Procedure 43

   4. Consequence of PRD Failure to Open 44

      1. General 44

      2. Damage State of the Protected Equipment 44

      3. Overpressure Potential for Overpressure Demand Cases 44

      4. Multiple Relief Device Installations 45

      5. Calculation of Consequence of Failure to Open 45

      6. Calculation Procedure 46

   5. Consequence of Leakage 46

      1. General 46

      2. Estimation of PRD Leakage Rate 47

      3. Estimation of Leakage Duration 47

      4. Credit for Recovery of Leaking Fluid 47

      5. Cost of Lost Inventory 47

      6. Environmental Costs 48

      7. Costs of Shutdown to Repair PRD 48

      8. Cost of Lost Production 48

      9. Calculation of Leakage Consequence 48

      10. Calculation Procedure 49

   6. Risk Analysis 49

      1. Risk from Failure to Open 49

      2. Risk from Leakage 50

      3. Total Risk 50

      4. Calculation Procedure 50

   7. Inspection Planning Based on Risk Analysis 50

      1. Risk Based Inspection Intervals 50

      2. Effect of PRD Inspection, Testing and Overhaul on Risk Curve 50

      3. Effect of PRD Testing without Overhaul on Risk Curve 51

   8. Nomenclature 52

   9. Tables. 55

   10. Figures 70

8. HEAT EXCHANGER TUBE BUNDLES 77

   1. General 77

      1. Overview 77

      2. Background 77

      3. Basis of Model 77

      4. Required and Optional Data 77

   2. Methodology Overview 77

      1. General 77

   3. Probability of Failure 78

      1. Definition of Bundle Failure 78

      2. Probability of Failure Using Weibull Distribution 78

      3. Exchanger Bundle Reliability Library or Seed Database. 79

      4. POF using the Owner-User Supplied Weibull Parameters 81

      5. POF using the User Supplied MTTF 81

      6. POF calculated using Specific Bundle History 81

   4. Consequence of Failure 81

      1. Calculation Method 81

      2. Example 82

   5. Risk Analysis 82

      1. General 82

      2. Risk Matrix 82

   6. Inspection Planning Based on Risk Analysis 83

      1. Use of Risk Target in Inspection Planning 83

      2. Example 83

      3. Inspection Planning Without Inspection History (First Inspection Date) 83

      4. Inspection Planning with Inspection History 84

      5. Effects of Bundle Life Extension Efforts 86

      6. Future Inspection Recommendation 87

   7. Bundle Inspect/Replacement Decisions using Cost Benefit Analysis 87

      1. General 87

      2. Decision to Inspect or Replace at Upcoming Shutdown 87

      3. Decision for Type of Inspection 88

      4. Optimal Bundle Replacement Frequency 88

   8. Nomenclature 90

   9. Tables. 92

   10. Figures 102

1. SCOPE

   
   

   1. Purpose

      This recommended practice provides quantitative procedures to establish an inspection program using risk- based 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.

      
      

   2. 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.

      
      

   3. 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:

1. On-site risk to employees,

2. Off-site risk to the community,

3. Business interruption risks, and

4. 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 high- risk 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.