Reliability and Risk Management of Pressure Relief Valves in Oil Refineries

Introduction

Properly functioning pressure relief valves (PRVs) are essential to safeguard plants, people, and the environment, and are considered to be the last layer of protection for pressurized systems. PRVs need to be inspected and tested periodically to ensure functionality on demand. Many plants follow a time-based approach, most likely aligned with API 510, without considering the associated risk of each PRV. The main goals for implementing a risk-based approach to PRV inspections are inspection interval determinations that include identifying and reducing operational risk in the process unit, optimizing inspection intervals, and determining the necessity of installing isolation valves, rupture discs, or 100% spare capacity.

The main objectives of this assessment are as follows:

• Determine the relevant overpressure demand cases for each PRV and utilize them in the risk analysis; consideration will be given to all applicable overpressure scenarios for each PRV.
• Evaluate the pressure relief valve risk ranking and criticality in order to set the optimum inspection/testing frequency.
• Identify areas where isolation valves or other mitigation action is needed to reduce the risk and to satisfy a minimum inspection interval of five years to align with the process unit’s shutdown cycle

Risk-based inspection (RBI) analysis for PRVs is conducted after completing an RBI assessment for the protected static equipment. The adapted methodology is based on the API Recommended Practice 581 that uses input data, such as operating conditions, and the calculated damage factor data of the protected equipment in order to compute the risk associated with the PRV. The assessment assumes that PRVs are properly sized, and is conducted in the following phases:

• Phase 1: Pre-Assessment Preparation – This phase involves the selection of process and inspection engineers, the selection of an RBI facilitator to develop the work schedule and scope, and the identification of the sources of data needed for the assessment.
• Phase 2: Data Collection – This phase includes a team kick-off meeting, plant walk-through, interviews with plant staff, completing the data gathering, review of PFDs, P&IDs, etc., a thorough review of inspection records for the relief valves, and finalizing the data entry in the software.
• Phase 3: Analysis and Inspection Planning – This phase includes producing an initial risk assessment, validating the assessment assumptions and data output, and presenting preliminary findings.
• Phase 4: Final Reporting – This phase involves the issuance of the final report including all findings and recommendations.

Creating Value with an RBI Program

The greatest values that RBI programs provide are better management of plant operating risk and improved integrity. This is normally associated with optimization of inspection activities’ cost, affecting both turnaround and on-stream inspection (OSI) activities. The primary work products of the RBI assessment are inspection plans that address ways to manage risks on an equipment or component level. These equipment plans highlight risks from a safety perspective. Implementation of these inspection/risk mitigation plans will provide the ability to measure and manage relative risks of pressure equipment and tanks. The creation and implementation of effective equipment strategies, using risk as a primary metric, provides the ability for the users to identify and manage risks to acceptable levels. It does require an accurate understanding of the impact of risk drivers upon risk (relative risk in the case of RBI).