Flare separation distance is one of the most common questions raised during the design of biogas plants, anaerobic digestion facilities, and other process installations that use flaring systems.

Many operators are looking for a simple answer or a fixed minimum distance. Unfortunately, the reality is far more complex.

The appropriate flare separation distance depends on factors such as thermal radiation, gas dispersion, flare design, site layout, prevailing wind conditions, and the consequences of abnormal operating scenarios.

Rather than relying on a universal distance, flare siting should be based on a structured risk assessment and supported by appropriate engineering calculations.

Is there a minimum flare separation distance?

Some standards provide baseline expectations.

For example, BS ISO 22580 includes guidance on:

• Minimum separation distances
• Clearance from combustible materials
• Thermal radiation considerations
• Safe operation and maintenance access

For enclosed flares, minimum distances of around 5 metres are often referenced as a starting point.

However, these values should not be treated as fixed rules.

The standard allows alternative distances where supported by suitable engineering justification and thermal radiation assessments.

This means that the final flare separation distance may be greater or smaller depending on the specific circumstances of the installation.

Why is thermal radiation important when determining flare separation distance?

Thermal radiation is one of the primary factors influencing flare siting.

A flare produces heat that can affect:

• Personnel
• Buildings
• Plant equipment
• Electrical systems
• Adjacent process areas

Thermal radiation assessments are typically used to determine whether acceptable exposure limits are achieved at nearby locations.

The results help define appropriate exclusion zones and influence the required flare separation distance.

Without understanding thermal radiation impacts, it is difficult to demonstrate that a flare has been positioned safely.

Does flare positioning only depend on thermal radiation?

No.

While thermal radiation is important, it is only one part of the assessment.

Other factors that influence flare separation distance include:

• Gas composition
• Flare operating conditions
• Wind speed and direction
• Site topography
• Nearby occupied buildings
• Critical plant equipment
• Potential flame out scenarios
• Gas dispersion characteristics

A flare that appears acceptable under normal operating conditions may present different risks during abnormal events.

This is why consequence based design approaches are often used.

What does API 521 say about flare siting?

API 521 places significant emphasis on understanding the consequences of flare operation rather than relying solely on fixed separation distances.

The guidance encourages consideration of:

• Thermal radiation effects
• Vapour dispersion
• Wind effects
• Terrain and topography
• Flame stability
• Potential flame out events

Particular attention should be given to scenarios where unburned gas may be released and dispersed following flame failure.

The objective is to ensure that flammable concentrations do not reach occupied areas or other locations where ignition could occur.

Are there any flare separation distance rules of thumb?

Industry guidance and textbooks often reference typical flare separation distances.

In hydrocarbon processing facilities, distances of approximately 60 to 90 metres or more may sometimes be used as an initial design reference.

However, these values should be treated with caution.

Rules of thumb are often:

• Conservative
• Facility specific
• Based on historic designs
• Unsuitable for direct application without assessment

A distance that is appropriate for one facility may not be suitable for another.

What are the key principles of flare siting?

Although flare separation distance varies from site to site, several common principles apply.

Good flare siting should:

• Minimise thermal radiation impacts
• Consider prevailing wind conditions
• Account for gas dispersion behaviour
• Maintain adequate separation from occupied buildings
• Protect critical equipment
• Consider maintenance access requirements
• Be integrated into plant layout design from an early stage

Crosswind siting is often preferred where practical because it can help reduce the impact of prevailing winds on occupied areas.

Why should flare separation distance be considered early in design?

Flare relocation can become difficult and costly once plant layouts have been finalised.

Considering flare separation distance during the early design stages helps:

• Avoid layout constraints
• Improve plant safety
• Reduce redesign costs
• Simplify hazardous area assessments
• Improve overall risk management

Early consideration also allows thermal radiation and dispersion studies to influence the wider site arrangement rather than being treated as a late design check.

What is the best approach to determining flare separation distance?

The most effective approach is a risk based assessment that considers both normal and abnormal operating conditions.

This typically includes:

• Thermal radiation modelling
• Dispersion modelling
• Consequence assessment
• Review of occupied buildings
• Evaluation of critical equipment locations
• Assessment of site specific environmental conditions

The outcome should be a flare separation distance that is justified by engineering analysis rather than a generic rule.

What is the key takeaway?

There is no universal flare separation distance that can be applied to every facility.

The appropriate distance depends on the flare design, process conditions, site layout, and surrounding environment.

A robust flare siting assessment should be based on thermal radiation and dispersion modelling, supported by a structured risk assessment, and integrated into plant design from the earliest stages.

If you are designing a new biogas facility, reviewing an existing flare installation, or assessing compliance with process safety requirements, our team can help determine an appropriate flare separation distance for your site. Contact us