Author: Phil Henry, P.E., Process Technology Team Leader, Principal Engineer II

Pressure relief systems (PRS) play a critical role in the safe operation of industrial plants, particularly those handling hazardous chemicals. Regulatory frameworks such as OSHA’s Process Safety Management (PSM) require organizations to document their relief system design and ensure compliance with Recommended and Generally Accepted Good Engineering Practices (RAGAGEP) standards. However, gaps in documentation, improper system modifications, and inadequate audits can lead to safety risks and regulatory violations.

A structured audit process helps organizations verify the integrity of their pressure relief design basis, update documentation, and ensure that overpressure protection remains effective. This article explores the key aspects of PRS audits, including their purpose, scheduling, execution, and follow-up actions.

When and How to Conduct a PRS Audit

At a minimum, a PRS audit should be conducted at the same frequency as the PSM-mandated Hazard and Operability (HAZOP) study.  In particular, this is because one of the key inputs into an HAZOP study is specifying if the design of the relief system takes all hazards into consideration. A detailed HAZOP also identifies the most hazardous items, which prompts owner-users to conduct a Layer of Protection Analysis (LOPA) risk assessment. LOPAs demand a certain reliability from the pressure relief device (PRD), which can only be determined if the PRD inspection and testing program is underway and performing adequately.

When major revamps to a process occur or where capital investments have been made, PRS audits may be instigated to ensure new equipment and their PRDs are assimilated into the current work processes.  When company acquisitions occur, a PRS gap assessment or audit is an absolute must as part of the due diligence process.

Specific, targeted PRS audits should also be performed whenever the industry experiences a catastrophic failure that may be related to a pressure relief deficiency.  Post-incident investigations conducted by the Chemical Safety Board (CSB), such as the 2005 BP Texas City explosion, the 2013 Williams Geismar heat exchanger rupture, and the 2018 Kuraray fire, can and should trigger a plant to review its process for identifying areas of potential vulnerability.

To conduct an effective audit, assembling the right team is crucial. The team should include corporate PRS subject matter experts (SMEs), third-party consultants, process engineers, operators, maintenance personnel, and reliability specialists.  By avoiding bias and challenging conventional practices, the plant will achieve a comprehensive evaluation.

Key Focus Areas in PRS Audits

There are several key focus areas that need to be considered when developing the scope of work for any PRS audit.

1. Establishing the Audit Framework

The first step in a PRS audit is to understand all applicable regulations and standards. The key references include:

• OSHA 29 CFR 1910.119 – PSM Legislation
• API Pressure Relief Standards (520, 521, 526, 2000)
• ASME Boiler and Pressure Vessel Code (Sections I, VIII, XIII)
• API 510, API 576, and the NBIC Inspection Codes and Standards
• ASHRAE and NFPA guidelines

It is important to note that although these Codes and Standards seem as though they might be comprehensive due to their volume, there are still many aspects of relief design that need clarity or are vague (gray areas) where consensus between member companies cannot be obtained.  It is therefore critical that owner-users have their own PRS design guide to bridge gaps in API standards and ensure their organization’s relief system philosophy is applied consistently across the company.

Developing and maintaining a corporate PRS design guide, implementing safe operating best practices, and creating PRD inspection and testing program documentation form the framework from which to conduct the PRS audit.

2. Selection of Pressure Relief Devices (PRDs)

Determining which PRDs to audit can follow different methodologies depending on the goal of the audit.  The audit may encompass every aspect of:

• Random Sampling: Select random PRDs (~5-10%) throughout the plant to identify systemic issues.  Based on the results from the random sampling, a larger PRD audit may be warranted.
• Systematic Selection: Evaluate a cross-section of equipment and PRD types to address industry incidents and determine if the plant is subject to the same issues. For example, the 2015 BP Texas City incident may lead an owner-user to assess all the columns and vessels to identify cases where the risk of liquid overfill has not been adequately mitigated. The Williams Geismar incident may motivate operators to perform an audit of administrative procedures associated with isolation valves located in the pressure relief path. 
• Block Selection: Review all PRDs within a specific unit before major revamps are made to a process unit.
• Risk-Based Selection: Focus the sample on high-risk PRDs identified through LOPA, HAZOPs, or a risk-based inspection (RBI) assessment.

3. Documentation Review

Comprehensive documentation is fundamental to PRS compliance; thus, a good quality PRS audit will review PRD design calculations to ensure that:

• Calculation packages are complete, accurate, and meet the latest RAGAGEP (including the corporate relief system design guide)
• All credible overpressure scenarios are addressed
• Equipment protection details are documented
• PSV sizing calculations are clear and correct
• Pressure drop calculations are well-documented and repeatable
• Systemization drawings are available and up to date

It is useful to refer to a documentation checklist to ensure all necessary data points are reviewed and deficiencies identified.

4. Installation and Field Verification

PRS audits in accordance with API RP 576, Inspection of Pressure-Relieving Devices, should include visual inspections of the PRD installation to confirm that the installed PRDs match what is shown in the design documentation. The key focus areas include:

• Ensuring correct valve orientation (typically vertical for PRVs)
• Verifying that atmospheric relief discharges to a safe location
• Checking that block valves are in correct positions and locked
• Reviewing car seals and locks credited in calculations
• Assessing the accessibility of relief devices for maintenance and inspections

5. Inspection Program Evaluation

Pressure relief valves (PRVs) must undergo regular inspections to maintain mechanical integrity. OSHA’s 1910.119 regulation mandates documented maintenance procedures, staff training, and corrective actions for deficiencies. A PRS audit should evaluate:

Reporting and Mitigation Strategies

Once an audit is completed, the findings should be compiled into a detailed report which highlights deficiencies and recommends corrective actions. The key post-audit steps include:

• Prioritizing mitigation strategies for deficiencies based on risk severity
• Investigating systemic failures and outdated sizing methodologies
• Updating PRS design guides and corporate engineering practices
• Reassessing unprotected equipment and implementing necessary safeguards
• Working with valve shops to improve PRD testing and maintenance schedules

Conclusion

A well-structured PRS audit helps organizations proactively address safety concerns while also ensuring compliance with industry regulations and best practices. By conducting periodic audits, maintaining comprehensive documentation, and implementing corrective actions, owner-users can safeguard personnel, assets, and the environment. Owner-users should establish a PRS auditing program that includes planning, execution, reporting, and continuous improvement to strengthen their last line of defense against overpressure incidents.

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