The API and ASME guidelines and standards for emergency relief systems both state that total nonrecoverable inlet pressure losses between protected equipment and a spring-loaded relief valve should be limited to 3% of the relief valve set pressure. Inlet pressure losses above this limit may experience ‘chatter,’ which can lead to damage of equipment, valves, and piping. The API standards allow for inlet pressure loss beyond 3%, with a proper engineering analysis. Consistent with the performance-based nature of these guidelines, operating companies can define what a proper engineering analysis is and often such an analysis can be experience based. The requirement for inlet pressure loss, be it 3% or higher, is not sufficient to guarantee pressure relief valve (PRV) stability.
This issue is a hot topic which is currently being debated within industry and regulators. Regulatory authorities in the United States have recently imposed fines on companies which had relief valves which did not comply with these criteria. The outcome of these discussions will have widespread implications affecting most petrochemical companies.
This paper presents a summary of the history and application of the 3% rule, and presents practical mitigation options for companies trying to follow this rule. It will benefit anyone involved in safety and pressure relief systems.
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