Authors: Nicholas Plentovich, Staff Engineer I; Phil Henry, P.E., Process Technology Team Leader, Principal Engineer II

While the requirement for pressure relief is rooted in the ASME code, it is the US CFR 29 1010.119 ^[1] that requires the proper documentation of relief system design and design bases.  This is typically taken to mean that proper overpressure analysis has been completed using Recommended and Generally Accepted Good Engineering Practice (RAGAGEP).  This has been a recent focus of targeted enforcement through OSHA’s National Emphasis Program (NEP)^[2], with the stated goal to “reduce or eliminate workplace hazards associated with the catastrophic release of highly hazardous chemicals.”  During these inspections, OSHA found instances wherein owner’s written Process Safety Information (PSI) did not contain enough detail to fully document the pressure relief design basis or had other deficiencies such as missing or undersized safety relief devices, incorrect relief device set pressures, high inlet and outlet pressures, and poor installation and inspection practices that were not corrected in a timely matter.   

Deficiencies in relief systems can be costly when found by OSHA, but this is still far preferable to a loss of containment, or worse – a fatality.  If these deficiencies can be located and corrected on our own, facilities can be operated more cleanly and safely, minimizing the risk of overpressure failures and avoiding potential scrutiny from OSHA.  The best means of locating these deficiencies is performing periodic audits of relief systems, and ensuring not only that documentation meets PSM regulations, but that relief systems are designed, maintained, inspected, and tested to adequately protect against dangerous overpressure events.  

There are many triggers for a pressure relief system audit.  Major industry incidents like the explosions at BP Texas City ^[3] and Williams Geismar ^[4] justifiably provoke increased scrutiny from OSHA and the Chemical Safety Board (CSB), and should encourage owner-users to evaluate operations and potential vulnerabilities.  Even minor incidents or near misses such as the recent incident at Kuraray Pasadena could trigger review of aspects of a relief system to ensure that similar mistakes are not repeated.  An audit could be also triggered by the revamp of a unit or a large capital project.  These audits could even be a regular occurrence, coinciding with other PSM audits.

The scope of the audit should be clearly defined.  This could be a unit or a block selection of pressure relief devices (PRDs) on a specific equipment type.  The scope may only include a select sample of PRDs intended to represent a cross-section of all devices at a facility, in which the goal is to identify systemic problems that may warrant a wider audit of the relief system.  Alternatively, risk-based methods could be used to target audit of high-risk PRDs to ensure that they are designed, installed, and maintained appropriately.

Begin the audit by reviewing the relief system design guide (RSDG).  This is the guiding document in a site’s relief system that provides the corporate relief system philosophy and fills in the gaps in design methodology where RAGAGEP documents (such as API) are unclear or leave decision up to engineering judgement.  The RSDG ensures that all calculations are performed consistently and engineers have the same philosophy with which to consider relief scenarios.  Not only should this document exist, but it should be updated periodically to reflect any technological developments or modifications to pressure relief RAGAGEP.  

Next, documentation for individual PRDs should be audited, which should be easy to locate and well-organized.  An audit should ensure that all credible scenarios are addressed, and the PRD is sized appropriately for the largest individual scenario.  All scenarios deemed non-credible should have proper justification for the designation, and it should also be ensured that the appropriate sizing methods were used.  Two-phase sizing methods have improved over time, though calculations are often not updated to reflect this.  An audit should also confirm the appropriate pressure relief valve (PRV) coefficient of discharge and orifice area were used in calculations, because sizing calculations are often completed preliminarily using effective values provided in the API 520^[5] standard.  Final calculations should be redone once a valve model is selected using the valve’s certified coefficient of discharge and actual orifice area.  If the valve model is changed, the manufacturer’s values should also be checked to ensure adequacy.  The inlet and outlet pressure drop must be documented and within acceptable ranges, and this suggests that the piping isometrics of the inlet and outlet piping for each PRD should be included in the PRD documentation.  For PRDs discharging to atmosphere, reaction forces and consideration of the fluid and discharge location should also be documented.  API STD 521^[6] section 4.7 provides examples of good documentation.  

To facilitate calculations, support process hazard analysis (PHAs), and assist with audits, the documentation for each pressure vessel and its PRD should have a systemization drawing (highlighted P&ID) that identifies the scope of the protected system, sources of overpressure, and discharge locations. 

A good set of systemized P&ID drawings will assist the audit in confirming the following: 

• The set pressure of each PRD is at or below the maximum allowable working pressure (MAWP) of each vessel in the protected system
• All inlet piping to the PRDs is at minimum the size of the PRD inlet
• No equipment is left unprotected
• Any block or isolation valves in the pressure relief path have been identified and appropriate rules surrounding their use have been applied

As changes to processes are implemented, the systemized P&ID drawings of the relief system need to be updated as well, likely as a part of the management of change (MOC) process.

The audit should then review the installation of the PRD.  The goal is to confirm that the installation does not compromise the operation of the device matches the documentation.  Be sure to confirm the following:

• Is the correct relief device installed?  
• Is it set at the correct pressure?  
• Are block valves locked or car sealed in the appropriate position?

Also consider adding a photo of the PRD nameplate and its installation to the documentation file.  PRDs should be installed vertically unless otherwise approved by the manufacturer.  If a rupture disc is installed underneath the PRV, the pressure in the space between the disc and the PRV should be monitored and should be zero.  If the PRD is discharging to the atmosphere, confirm that it is discharging to a safe location.

Finally, the audit should include a detailed review of the PRD inspection and testing program to ensure the mechanical integrity of the device is maintained.  An audit should confirm that the PRD is a part of an inspection program, with inspection performed on a regular basis.  Ensure that the inspections are adequate to determine the condition and functionality of the PRD.  Check the documentation from the inspection shop for completion and detail.  Examine at the results of the inspections and ask yourself the following questions:  

• Are any issues found during inspection addressed?  
• Are there indications of PRD instability or chatter?  
• Are inspection intervals appropriately adjusted based on the results of prior inspection? 
• Are intervals shortened with failed pop tests?  
• How are inspection deferrals handled?

The results of the audit should be compiled into a report to provide readers a clear description of the state of the pressure relieving system.  It should clearly list any deficiencies found so that a plan to remediate these issues can be formed and the deficiencies addressed.  The report should consider systemic issues including those related to installation or deficiencies in the inspection program.  The need for further audits should also be indicated, which could be a simple scheduled audit or an audit of certain aspects of the relief system to review systemic issues.

Do you currently have a program in place to audit your relief system?  Pressure relieving systems serve as the last line of defense in our facilities, so ensure their integrity.  Plan a timeframe, assemble a team, create a strategy, perform the audit, and create a report.  Are there gaps in your pressure relieving system? 

If you have any questions for the authors, please submit the form below:

References:

1. OSHA 29 CFR 1910.119 13, Process Safety Management of Highly Hazardous Chemical
2. OSHA Directive CPL 03-00-021 PSM Covered Chemical Facilities National Emphasis Program
3. CSB (March 20, 2007). Investigation Report: Refinery Explosion and Fire (15 Killed, 180 Injured) (Report No. 2005-04-I-TX). Washington, D.C.: U.S. Chemical Safety and Hazard Investigation Board.
4. Williams Olefins Plant Explosion and Fire. U.S. Chemical Safety and Hazard Investigation Board. October 19, 2016.
5. The American Petroleum Institute, API Standard 520, Sizing, Selection, and Installation of Pressure-Relieving Devices, Part II -- Installation,  7^th Edition, 2020
6. The American Petroleum Institute , API Standard 521, Pressure-relieving and Depressuring Systems, 7^th Edition, 2020