Fire Exposure Modeling Considerations

A classic scenario in risk assessments is the exposure of process/storage vessels and piping to an external pool fire or a jet fire. The heat from a fire causes the temperature of the metal walls to increase and subsequent heat transfer from the metal walls causes the pressure and temperature of the vessel and piping contents to increase. The heating rate from fire exposure determines the pressure relief rate requirements for both boiling liquids and all-gas systems. Venting requirements can be estimated using, for example, the methods of API Standard 521 (2014) or the Design Institute for Emergency Relief Systems (DIERS) “Guidelines for Pressure Relief and Effluent Handling Systems” (2017).

For pressure vessels that are only protected from overpressure with pressure relief valves (PRVs), the PRVs continue to open and reseat at the PRV opening pressure and reseating pressure, respectively. As the metal wall temperature increases, the metal strength decreases. When vessels are exposed to fires for extended periods of time the metal wall will weaken enough to fail at the PRV reseat pressure. Thus PRVs cannot protect a vessel from an extended fire exposure if the actual wall stresses exceed the material strength (Melhem and Gaydos, 2015).

There is a substantial difference in the likelihood of vessel wall failure between vessels exposed to jet fires and those exposed to pool fires. Flame jet impingement causes localized high intensity heating. If the flame jet impinges on a dry vessel wall segment then wall failures can occur within a few minutes. Heating rates from pool fires depend on whether the vessel is total engulfed, partially engulfed, or heated from a distance by thermal radiation. In vessels containing liquids and exposed to pool fires, failures typically occur at the vapor/liquid interface because of increased thermal stress between the dry wall hot metal temperature and the wetted wall cooler metal temperature (Melhem and Gaydos, 2015).

Reasonable estimates of the estimated time to failure are an element of risk management (Melhem, 2021). A PRV can only be considered adequate for PRV overtemperature and overpressure protection when the estimated time to failure exceeds the fire duration. Also, the estimated time to failure is important for emergency response and risk analysis. The response time of risk mitigation measures must be less than the estimated time to failure to be effective. Finally, in the case of a fire, the emergency response team staging is influenced by knowing whether or not there is risk of vessel rupture.

In fire scenarios, the dynamic response of a pressure relief valve in vapor service depends upon transient heating effects of the vessel inventory. Fire heating of the vessel walls causes superheating of the vapor phase and thermal stratification of the liquid phase (Hendrickson, 2023). Consequences of these phenomena include more rapid pressurization and more frequent pressure relief valve cycling than is predicted using models based on thermodynamic equilibrium. These effects are considerations when evaluating pressure relief system performance.

The cases considered in this paper were modeled using the commercial software package SuperChems™ provided by ioMosaic. SuperChems™ solves the time dependent material, momentum and energy balances along with thermodynamic equilibrium phase behavior. The model allows user input for fire parameters, such as flame temperature and emissivity, and the relevant heat transfer parameters. Vessel wall segmentation is provided to allow the user to specify which vessel wall segments are exposed to flames and hot gases. Model results include estimated time to failure and the estimated time to first relief. 

Additional Resources

What is a Safe Discharge Location Webinar

When it comes to emergency relief systems, what is meant by safe discharge? How do you assess it and if the discharge location presents minimal risk? Although required in some API standards, safe discharge from both an operational and compliance standpoint is not accurately defined within the industry. This one-hour webinar examines the RAGAGEP requirements for safe discharge.

DIERS Technology Fundamentals II: VLS Webinar

How confident are you when it comes to evaluating emergency relief systems design for runaway reactions? Are you aware of the factors that impact reaction rates and the role of thermal stability? What methodology is most effective for analyzing process chemistry and thermochemistry? Understanding process chemistry is key to reducing the risk of runaway reaction incidents. Failure to adequately understand and assess process chemistry and thermochemistry can result in erroneous scale-up as well as erroneous sizing/evaluation for relief systems. This one-hour webinar will cover runaway reactions characterization methods and provide a useful summary of thermal stability indicators.

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