Explosion Control and Deflagration Venting in BESS Enclosures

Explosion control has emerged as one of the most critical and least understood aspects of Battery Energy Storage System (BESS) design. While most fire protection discussions focus on preventing or managing thermal runaway, the secondary hazard—deflagration—often poses the greatest risk to system integrity, first responders, and adjacent equipment. As lithium-ion batteries release flammable gases under failure conditions, these gases can ignite rapidly, producing violent pressure surges within enclosed containers. Understanding and controlling that hazard is essential for any BESS project.

Why Explosion Control Matters

When a lithium-ion cell experiences thermal runaway, it emits a mixture of hydrogen, carbon monoxide, and light hydrocarbons. In the confined volume of a BESS container, these gases can accumulate to concentrations well above their lower flammable limit (LFL). A single spark from electrical equipment or hot surface can then trigger a deflagration—a fast-moving combustion wave that creates a rapid rise in pressure.

In early BESS incidents, such explosions caused catastrophic enclosure ruptures and projectile hazards for firefighters and maintenance personnel. These events revealed a major gap between conventional fire protection design and the real behavior of energy storage systems. Simply installing sprinklers or gas suppression is not enough; designers must also ensure that the enclosure can either prevent or safely relieve the pressure from an internal explosion. This is the purpose of the explosion control requirements in NFPA 855, which references NFPA 68 (Standard on Explosion Protection by Deflagration Venting) and NFPA 69 (Standard on Explosion Prevention Systems).

Deflagration Venting vs. Explosion Prevention

NFPA 855 allows two recognized approaches to explosion control: deflagration venting and explosion prevention. While both are acceptable, they serve different engineering goals and are often applied in combination.

Deflagration venting accepts that ignition may occur and provides a safe outlet for the expanding gases and flames. Vent panels or blow-out sections are designed to open at a specified internal pressure, releasing combustion products upward or in a controlled direction. When designed properly, this prevents the enclosure from rupturing or ejecting debris. Vent sizing and placement must be based on expected explosion pressures, flame propagation behavior, and venting efficiency, as described in NFPA 68. The goal is not to stop the explosion, but to make it survivable.

Explosion prevention, on the other hand, aims to stop the event from ever occurring. This approach is governed by NFPA 69, which specifies that flammable gas concentrations must be maintained below 25 percent of the LFL under all foreseeable conditions. Prevention can be achieved through forced ventilation, continuous air dilution, or inert gas injection systems. Gas detection sensors trigger these systems automatically when a buildup is detected. In industrial process environments—such as chemical plants—explosion prevention is standard practice. In BESS, however, it is more challenging because gas generation during thermal runaway is unpredictable, and the release rates vary by module design and chemistry. For this reason, most containerized BESS designs rely on venting rather than full inerting systems.

Design Challenges for Containerized Systems

Applying NFPA 68 and NFPA 69 principles to BESS containers is far from straightforward. These systems are densely packed with electrical components, battery racks, and cabling, leaving little free air space. Internal obstructions can significantly alter gas flow and flame propagation, resulting in nonuniform pressure distribution during a deflagration.

Furthermore, many BESS units are not designed as traditional rooms but as prefabricated steel enclosures with complex ventilation, cooling, and access pathways. This geometry complicates the placement and effectiveness of vent panels. Small deviations in vent sizing, orientation, or strength can produce large differences in pressure relief performance.

Another challenge lies in gas distribution. During thermal runaway, gases typically vent from a single module or rack, leading to localized accumulation zones rather than even mixing. Traditional vent sizing formulas, which assume homogeneous gas concentrations, often underestimate pressures in these scenarios. The result can be an undersized vent area, incapable of relieving the initial overpressure.

Common Pitfalls and Misinterpretations

Many explosion control designs fail because of overreliance on prescriptive formulas or assumptions derived from unrelated applications. Designers may underestimate the vent area needed or place vents where flame fronts are obstructed. In some cases, vents are oriented toward other containers or walkways, inadvertently increasing risk. Another frequent issue is assuming that fire suppression agents—such as Novec 1230 or nitrogen—can double as explosion prevention systems. These agents are typically designed to control flaming combustion, not prevent gas-phase deflagrations. Without proper inerting or dilution, they provide little benefit once ignition occurs.

A more rigorous approach requires empirical validation through testing or modeling. This is where UL 9540A test data becomes invaluable. By quantifying gas generation rates, flame temperatures, and venting behavior, UL 9540A results help determine whether a proposed venting or prevention strategy will perform as intended. Projects that fail to integrate such data often face challenges during code review or field commissioning.

The Role of Performance-Based Design and CFD

Because BESS containers vary widely in layout, prescriptive vent sizing from NFPA 68 rarely fits every scenario. Instead, many engineers now use Computational Fluid Dynamics (CFD) to simulate gas dispersion, ignition, and pressure wave dynamics. These simulations can model obstructions, internal geometry, and multi-vent interactions, providing a more accurate representation of real behavior.

NFPA 68 and the new Annex G of NFPA 855 (2025 Edition) both support performance-based methods when validated by qualified professionals. CFD analysis allows teams to justify alternative vent placements, combined venting and dilution strategies, or nonstandard container geometries. When documented as part of the Hazard Mitigation Analysis (HMA), these studies provide AHJs with the confidence that the design meets the intent of the standard, even if it departs from prescriptive tables.

Conclusion

Explosion control is not just a code checkbox—it is a cornerstone of safe BESS design. The confined and obstructed nature of containerized systems makes them uniquely vulnerable to deflagration hazards. Whether through properly engineered venting under NFPA 68 or through prevention and dilution measures under NFPA 69, the objective remains the same: to ensure that if ignition occurs, it happens safely, predictably, and without catastrophic failure.

As energy storage systems grow in capacity and density, understanding explosion dynamics and integrating performance-based design tools will be essential. The most effective BESS designs are not those that ignore the possibility of ignition, but those that accept it—and are built to withstand it.

For any further inquiries regarding this topic, as well as for code consulting and fire engineering design support related to your project, please don’t hesitate to contact us via email at contact@engineeringfireprotection.com.