Managing static electricity during the transfer of flammable liquids is based on the fluid’s conductivity, measured in picosiemens per meter (pS/m). High-conductivity liquids dissipate charge quickly, while low-conductivity liquids (static accumulators) hold onto the charge, creating a spark hazard.
The following table summarizes the maximum flow velocities recommended by NFPA 77 and API 2003 to prevent electrostatic ignition.
Maximum Flow Velocities for Flammable Liquids
| Liquid Type | Conductivity Range | Phase 1: Initial Fill (Until Submerged) | Phase 2: Full Flow (After Submerged) | Examples |
| Nonconductive (Static Accumulator) | < 50 pS/m | 1 m/s (3.3 ft/s) | 7 m/s (23 ft/s)* | Toluene, Benzene, Gasoline, Heptane, Hexane |
| Semiconductive | 50 – 104 pS/m | 1 m/s (3.3 ft/s) | 7 m/s (23 ft/s) | Chloroform, Methylene Chloride |
| Conductive (Non-accumulator) | > 104 pS/m | Unrestricted (Practically up to 7-10 m/s) | Unrestricted (Within pipe design limits) | Acetone, Methanol, Ethanol, Water |
*Note: For nonconductive liquids in tanks smaller than 10 meters in diameter, many standards (like IEC 60079-32-1) recommend a more conservative full-flow limit of 3.5 m/s.
Critical Safety Rules for Transfers
The “1 Meter Per Second” Rule
Regardless of the liquid type, the velocity should never exceed 1 m/s until the fill pipe or splash-loading nozzle is submerged by at least two pipe diameters (or approximately 6 inches). This prevents “splash filling,” which is the most aggressive generator of static charge.
Relaxation Time
For nonconductive liquids (static accumulators), a “relaxation period” is required after the flow has stopped before any metal objects (like sampling tapes or thermometers) are lowered into the tank.
- Standard liquids: 30-second wait.
- Large tanks/high-purity fuels: Up to 30-minute wait (depending on specific industry RAGAGEP).
Multi-Phase Flow
If the liquid contains a second phase—such as water droplets in fuel or suspended solids—the flow velocity must be restricted to 1 m/s for the entire duration of the transfer, regardless of whether the pipe is submerged. The friction between the two phases generates charge at a rate that standard grounding cannot always dissipate in real-time.
Grounding and Bonding
Velocity control is ONLY effective if the system is properly bonded and grounded. If the receiving container is isolated (e.g., a plastic drum or a metal drum on a wooden pallet without a ground strap), even low-velocity flow can accumulate enough voltage (often exceeding 20,000V) to cause a discharge.