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

RAGAGEP Considerations for Overtemperature Protection in Relief Systems

How to Calculate ETTF or ETTY in Fire Exposure Scenarios

Reasonable estimates of the expected time to failure (ettf) or expected time to yield (etty) are required and necessary for effective risk management as well as effective emergency and fire protection and response. Read this paper for a demonstration of calculating ettf or etty in fire exposure scenarios with Process Safety Office® SuperChems™.

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Heat of Vaporization Considerations for Relief Systems Applications

A common scenario that is encountered in pressure relief systems design centers around the calculation of vapor generation rates from liquids under external heating, internal heating, or fire exposure.

Pressure relief design is all about a volume balance. As the heating increases the liquid temperature and generates more vapor (volume) in a vessel, the pressure increases to fit the additional vapor generation (volume created) within the confines of the vessel. Relieving the vapor at a specific pressure, removes the additional vapor volume and keeps the pressure in the vessel in check.

This relief design scenario shares some commonalities with batch distillation when the liquid being heated is a chemical mixture. Similar to batch distillation, mixture light ends are preferentially depleted first. The resulting volumetric vapor generation rate depends on the vapor composition. Initially the vapor composition will be rich in light components.

As the light components are preferentially depleted, the vapor composition will become rich in heavier components. Thermodynamic, physical, and transport properties change as the mixture is fractionated for both the liquid and the vapor. The maximum relief requirement may occur anywhere along the fractionation curve.

For reactive mixtures where all vapor venting occurs, special care must be taken to ensure that materials that are preferentially concentrated do not spontaneously decompose or deflagrate.

Prevailing Practices

Simple design equations, such as those provided by API, continue to be widely used. How- ever, these simple equations require a value of the latent heat of vaporization which varies with composition for a liquid mixture as venting is occurring.

Limited guidance provided in API-521 includes the following statements: (a) “The latent heat and relative molecular mass values used in calculating the rate of vaporization should pertain to the conditions that are capable of generating the maximum vapor rate” and (b) “The vapor to be relieved is the vapor that is in equilibrium with the liquid under conditions that exist when the PRD is relieving at its accumulated pressure".

Dynamic software tools such as Process Safety Office® SuperChems™ can automatically identify the mixture conditions leading the the maximum vapor generation rate under equilibrium conditions and can also calculate the vessel wall temperatures and expected time to failure or time to yield as heavies are concentrated. A reclosing pressure relief device can only protect from overpressure and not overtemperature.

Several operating companies have also resorted to rules of thumb on how to calculate a representative latent heat of vaporization value. While it is relatively simple to establish relief requirements for vessels containing a single liquid component, there are many pitfalls associated with vessels containing multicomponent liquid mixtures. The value of the heat of vaporization for mixtures (enthalpy or internal energy) is not straight forward to calculate. The vapor composition is only equal to the bubble point liquid composition at the dew point.

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 head of our international offices in the U.K. and the Kingdom of Bahrain and an expert in risk management for oil, gas and transportation. 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...

James Close

Safety and Risk Management Analyst Mr. Close is focused on pressure relief and flare system design and analysis for large chemical and petrochemical companies in Europe and the United States. 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 pressure relief valve 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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