Why Delayed Ignition Matters

Hydrogen has several unique properties that make it promising as a clean fuel — but also challenging to manage safely. It is buoyant and disperses quickly, both of which reduce risk in open air, but it also has an exceptionally low ignition energy, increasing the probability of ignition in flammable mixtures.

For decades, industry practice has assumed that if hydrogen leaks and ignites, it will do so immediately, creating a jet flame that can be managed with proper system design.

But more recent real-world experience and research have shown that this is not always the case. Ignition can be delayed by fractions of a second, several seconds, or even longer.

In those cases, hydrogen has time to form an unignited jet, accumulate in overhead or enclosed spaces, and potentially form a combustible mixture. When ignition finally occurs, the consequences may include jet explosions, flash fires, or vapor cloud explosions (VCE), which may produce damaging overpressures.

“For a long time, people assumed that if hydrogen ignites, it happens right away. But recent incidents have disproved that myth. We’re now seeing the real possibility of delayed ignition — and the consequences can be far more severe, especially in confined or congested areas.”  — Dani Murphy, WHA

What Happens During a Delayed Ignition

When hydrogen is released but doesn’t ignite right away, three different outcomes are possible:

1. Dissipation without ignition
   Under the right conditions, the gas can disperse harmlessly. This is the best-case scenario, but by no means guaranteed.
2. Ignition in an unignited jet
   The gas ignites within a turbulent jet, accelerating through the mixture and potentially generating damaging overpressures. This can occur after just a fraction of a second or longer.
3. Accumulation and ignition in a volume
    Hydrogen disperses and mixes with air to form a flammable concentration. Ignition may then result in a flash fire or vapor cloud explosion (VCE) with confinement and/or congestion increasing the overpressure hazard.

“If gas doesn’t ignite immediately, one of three things happens: it may dissipate harmlessly, it may ignite within a turbulent jet, potentially producing damaging overpressures, or it may accumulate and ignite as a flash fire or even a vapor cloud explosion.”  — Dani Murphy, WHA

The following high-speed video demonstrates a simulation of hydrogen deflagration, replicating an outdoor or unconfined release of hydrogen into ambient air with congestion.

Categories of Delayed Ignition Hazards

There are three major hazard categories for delayed ignition:

• Jet plume explosions: ignition within a turbulent, high-flow jet, leading to localized blast effects.
• Flash fires: ignition of a dispersed flammable gas cloud, producing severe thermal hazards but little overpressure.
• Vapor cloud explosions (VCEs): ignition of a flammable gas cloud resulting in a damaging blast wave and overpressure. VCEs may occur outdoors with congestion, in semi-confinement, or fully confined indoors or within an enclosure where reflected pressures amplify damage.

Each hazard type presents different risks to people, equipment, and structures — and requires different mitigation strategies. The key to understanding and mitigating delayed ignition hazards is a thorough risk assessment using tools like WHA’s Hydrogen Combustion Risk Assessment (HCRA).

Codes and Standards: A Gap That Must Be Closed

Current codes and standards provide guidance primarily for immediate ignition scenarios — jet fires and thermal hazards. They often fail to directly address the explosion overpressures that can occur after delayed ignition.

Setback distances, firewall requirements, and vent system designs are all typically based on fire hazards. A performance-based approach is needed to capture the real risks of delayed ignition.

WHA and its partners are actively working through codes and standards bodies to help close these gaps, but change takes time. In the meantime, engineers and facility owners must take a proactive approach by considering delayed ignition hazards in their own designs and hazard analyses.

“We need better awareness about delayed ignition and the potential overpressures associated with it.”  — Dani Murphy, WHA

Learn More About Delayed Ignition of Hydrogen

A full technical paper on delayed ignition hazards will be published by the Hydrogen Safety Panel at h2tools.org.

As the industry seeks to better understand these hazards, WHA encourages system designers, safety professionals, and regulators to take them seriously and to partner with experienced experts who can help analyze and mitigate the risks.

Contact WHA below to learn more about hydrogen safety training, hazard analysis, and system design reviews.