Incorporating Risk into the Pressure Relief System Design Process

Recent updates to pressure relief system design Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) include provision for risk based overpressure protection. Although risk based approaches are not new, the recent RAGAGEP provides additional guidance and criteria.

Risk based pressure relief and effluent handling system design begins with identification of overpressure scenarios. The risk based approach utilizes existing Process Hazard Analysis techniques (PHAs) and risk tolerability criteria already implemented at Process Safety Management (PSM) covered facilities. When PHAs use Layer of Protection Analysis (LOPA), risk matrix or other qualitative, semi-quantitative, or quantitative approaches, the consequences and likelihood of the scenarios are determined.

The consequence of an overpressure scenario can be predicted using computational techniques, which include consideration for the pressure rise, potential release of toxic and flammable fluids, and resulting vapor cloud due to atmospheric relief or loss of containment. The frequency of occurrence of the overpressure scenario can also be predicted through historical data, or analytical techniques such as fault tree analysis.

Upon determination of the consequences and frequency, the risk can be calculated; which can then be compared to the applicable tolerability criteria. Scenarios with an intolerable risk may then mitigated through a variety of enhancements or safeguards. The cost of prevention, control, and/or mitigation measures are then balanced to ensure a safe and cost effective relief system always ensuring the As Low as Reasonably Practicable (ALARP) principle.

Additional risk factors that are often overlooked during pressure relief system design are also discussed. These include dispersion analysis, thermal radiation, noise, vibration risk, reaction forces and structural supports, metal cold temperatures due to expansion cooling and two phase flow, hot temperatures due to fire exposure and/or runaway reactions, PRV stability, chemical reaction systems, and loss of high pressure/low pressure interface.

Integrating the pressure relief system design into existing PSM practices reduces the effort and cost of developing and maintaining relief system design basis documentation and focuses the designers’ intent on improving plant safety. This risk based approach to pressure relief system

design ensures adequate overpressure protection systems and sound investment in human and capital resources focused on risk reduction.

This study presents a practical risk based approach to pressure relief and effluent handling system design and a supporting case study that results in safe and cost effective design.