edVirtus Reference

What Are Hazards of Electromagnetic Radiation to Fuel (HERF)?

How Could an RF-Induced Spark Ignite Flammable Vapor?

Hazards of electromagnetic radiation to fuel, abbreviated HERF, concern the possibility that RF-induced electrical energy could ignite a flammable vapor or gas mixture. The hazard is associated with ignition conditions around fuel handling, not with RF energy causing the liquid fuel itself to burn directly.

Conductive objects such as pipelines, hoses, nozzles, aircraft, ships, vehicles, tanks, and nearby structures can intercept RF fields and develop voltages or currents. A spark may occur when charged or energized conductors touch, separate, or approach closely enough for a discharge to bridge a gap.

Ignition requires several conditions at the same time: a flammable mixture within its combustible range, sufficient oxygen, a discharge at the relevant location, and energy above the mixture's ignition threshold. Removing any one of these conditions prevents RF-induced ignition.

HERF risk depends on transmitter frequency and power, antenna direction and distance, field strength, the dimensions and electrical continuity of conductive structures, contact geometry, fuel-vapor properties, and the location of vents or transfer points.

Potential HERF situations include aircraft and ship refueling, fuel-transfer operations, tanker loading, petroleum terminals, storage facilities, and industrial work involving volatile solvents or gases. The credible RF sources and conductive structures must be identified for the actual installation.

Assessment may use RF field surveys, analytical coupling calculations, measurements of induced voltage or current, and the separation criteria specified by the applicable fuel-handling or transmitter standard. Ordinary personnel exposure limits do not by themselves demonstrate HERF safety.

Controls include maintaining specified transmitter separation, reducing power or suspending transmission during fuel transfer, controlling antenna orientation, managing conductive structures and connections, maintaining bonding and grounding required by the governing procedure, and controlling flammable-vapor releases.

Electrostatic discharge is often a more familiar ignition source, but HERF is assessed where strong RF transmitters and flammable atmospheres coexist. Safe practice must follow the applicable fuel, fire, hazardous-area, and RF operating requirements for the site.