Emergency Relief System Design

Reducing costs and increasing accuracy in the design or revalidation of relief systems.

Effective Emergency Relief System (ERS) design helps companies meet risk-management goals, compliance requirements, and sound business practices. ioMosaic provides a total ERS solution with our comprehensive ERS design services, from reactivity testing for design basis determination to calculations for Z-axis deflection from dynamic loads.

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How We Can Help You

Our team has decades of experience performing PRFS analysis and design.

Our risk-based approach helps mitigate near-unventable scenarios to a tolerable level of risk.

Better evaluate hazards in your facility with an accurate process simulation.

Delivering properly designed pressure relief systems that save both money and time.

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Featured Resources


Performance Requirements for Flame Arresters

Understanding Flame Propagation and Flame Arresters

Flame arresters are used to protect equipment from overpressure caused by internal flames. Read this paper for a basic understanding of flames, flame arresters, and the multitudinous designs of flame arresters to help an Emergency Relief System (ERS) designer in selecting an appropriate flame arrester.

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RAGAGEP Considerations for Overtemperature Protection in Relief Systems

A properly sized reclosing pressure relief valve (PRV) can protect process equipment against a variety of overpressure scenarios. Fire exposure scenarios leading to overpressure are particularly challenging, especially where a reclosing pressure relief device provides the only means of pressure relief.

It is widely known, that if the fire duration is long enough, the process equipment will ultimately yield or fail at the reseat pressure of the reclosing PRV. A reclosing PRV can only provide overtemperature protection up to a maximum allowable duration of fire exposure. We define the maximum allowable duration of fire exposure as the expected time to failure (ettf). Another useful maximum allowable duration of fire exposure criteria is based on equipment deformation or yield but not failure or expected time to yield (etty). 

The maximum allowable fire duration depends on many variables including but not limited to type of fire (pool fire or flame jet), type of fuel, size and geometry of process equipment, equipment wall thickness, equipment pressure and temperature rating, initial liquid fill level, etc. As a result, expected ettf or etty values can range from few minutes to a few hours. 

Because the hazard of a long enough fire exposure scenario is already recognized, relief systems design basis documentation should provide calculated best estimates for etty or ettf or both. A hazard is ”recognized” under the OSHA General Duty Clause where (a) the employer has identified it, (b) it is known in the industry, or (c) it is blatantly obvious. 

Reasonable estimates of etty or ettf are required for strict OSHA compliance. They are also necessary for effective risk management as well as effective emergency and fire protection and response. This is particularly important for systems that contain reactive chemicals, chemicals with high boiling points, and process equipment that are gas filled or process equipment containing liquids where the vapor space can be engulfed / impinged by fire or exposed to flame thermal radiation. 

A reclosing pressure relief device can only be considered adequate for overtemperature protection if the calculated ettf or etty exceeds the estimated fire duration or the estimated failure time for vessel structural supports, whichever is less. Vacuum protection may also be necessary if a reclosing relief device is the only mean of overpressure and overtemperature protection.

Time to failure and consequence of failure

Time to failure and consequence of failure

Overpressure vs. Overtemperature

Fire exposure scenarios are probably the most widely used as the dominant pressure relief design basis, especially in the hydrocarbons industries. Standards such as API-521/520[1, 2] are used to determine the relief requirements by first determining the heating rate or heat absorption rate from fire exposure into the liquid vessel contents. Only heating through the wetted surface area is considered. The heating rate is then divided by the latent heat of vaporization of the vessel liquid contents to generate a rate a vapor generation. The volumetric rate of vapor generation determines the relief requirement and relief device sizing assuming all vapor flow.

Software Tools

Modeling system dynamics using our software Process Safety Office® SuperChems™ can be very useful and can provide insight into the proper selection of insulation thickness, actuation time of the depressuring system, water spray density requirements, etc. More importantly, dynamics are very useful to study the sensitivity of the final design to key mitigation parameters or system characteristics.


Our Team

Georges A. Melhem, Ph.D., FAIChE

President & CEO The founder of ioMosaic and internationally renowned expert in the areas of pressure relief and flare systems design, chemical reaction systems, process safety and risk analysis. Read more...

Neil Prophet

Senior Vice President and Partner He brings over 20 years of experience in pressure relief and flare systems design project management and engineering expertise for chemical, pharmaceutical and petrochemical companies. Read more...

John Barker, Ph.D.

Director The Director of our U.K. office and an expert in risk management for the international oil and gas and transportation industries. Read more...

Marcel Amorós Martí

Director and Partner His expertise consists of a diverse range of industries from chemical and petrochemical to oil and gas and utilities. Read more...

Charles Lea, P.E.

Director, Minneapolis Office Lead He directs a number of large technical projects across multiple offices and is also responsible for all project management in our Minneapolis office. Read more...

Matthew LeVere, P.E.

Senior Safety and Risk Management Consultant Experienced in PRFS design and analysis for clients in the petrochemical, chemical, and pharmaceutical industries. Read more...

Neal Dahlheimer, CPPM

Senior Safety and Risk Management Consultant Technical lead on PRFS projects for chemical, petrochemical and oil facilities as well as QA/QC reviews and training on advanced techniques for complex systems. Read more...

Justin Phillips, PE

Senior Director Responsible for emergency relief system design services with expertise in supporting oil & gas, refining, and chemical process sectors. Read more...

Featured Videos


Emergency Relief System Design Workflow

This PSE module performs efficient tracking of process safety related data and analysis. A customized workflow allows for a specific operating unit or the entire facility to be studied and evaluated for compliance.

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Featured Case Studies

Validate Relief System Performance and Flare System Capacity for Increased Unit Charge Rate

A major petroleum company recently increased production capacity and required an analysis of its existing relief systems to validate performance and design. As a result of increasing production capacity and debottlenecking studies, several refinery units were found to be operating at charge rates higher than the design basis for the relief systems documentation.

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A refinery approached ioMosaic for the purposes of ensuring that pressure relief capacity was adequate for the loss of liquid seal scenario in a high-pressure separator (2,000 psig). They were also concerned about the pressure waves that would occur in the high-pressure separator’s outlet lines on rapid closing of the isolation valves and sought our expertise.
A large oil refinery with a very complex flare network had become so complex that the tools the refinery was using to evaluate the flows through the flare network could not adequately model the system. Management no longer had confidence that their model results reflected the actual network performance and therefore, could not be sure the system would perform properly in the event of a global relief scenario at the facility.
A multinational energy company wanted to complete an evaluation of a PRV system in order to comply with the PSM standard OSHA 29 CFR 1910.119 which requires that employers compile information pertaining to the equipment in the process, including relief system design and design basis. 


Featured Services

Pressure Relief and Flare System Design

Our risk-based approach helps mitigate near-unventable scenarios to a tolerable level of risk and develop economical designs for more credible events. Read more...

Relief Header and Flare Analysis Systems

Delivering properly designed pressure relief systems for refineries and chemical plants that save both money and time. Read more...

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