In the world of flammable liquids processing, we have two (2) types of flammable liquids:
- Conductive
- Non-Conductive
Members can read all my articles on how these flammable liquids get classified as “conductive” vs. “non-conductive. This post is to explain HOW and WHY the traditional “bonding and grounding” used in the transfer of flammable liquids may be inadequate when the flammable liquid is Non-Conductive. Let’s jump into it…
Bonding and grounding are designed to provide a low-resistance path for static electricity to flow from a conductor (i.e., a metal tank or pipe) to the earth (i.e., the ground). However, these methods are ineffective for non-conductive liquids (also known as static accumulators).
The primary reason is that the liquid’s physical properties prevent the charge from “migrating” to the grounded metal walls of the container/pipe.
In a conductive liquid, electrons move freely. When the liquid touches a grounded metal tank, the charge quickly flows into the tank and dissipates along the tank’s path to ground.
In a non-conductive liquid (such as toluene, heptane, or many refined petroleum products), the electrical resistance is so high that the static charges generated during movement (pumping, splashing, or filtration) remain “trapped” within the body of the liquid. Even if the tank is perfectly grounded, the charge in the middle of the liquid cannot reach the walls to be neutralized.
Non-conductive liquids have a VERY LONG relaxation time. This is the time required for a charge to dissipate to 37% of its original level.
- Conductive liquids: Dissipate in fractions of a second.
- Non-conductive liquids: Can retain a high-voltage charge for several minutes or even hours after the transfer has stopped.
Because the charge stays in the liquid, a high electrical potential builds up on the liquid’s surface. If a grounded object (such as a sampling dipstick, a metal float, or even the fill pipe) is brought near the surface of the charged liquid, the potential difference can be high enough to bridge the gap. This creates an incendiary spark through the flammable vapor space, leading to an explosion, regardless of how well the tank itself is grounded.
Since bonding and grounding only protect the container and not the liquid, other safety layers MUST be used:
- Relaxation Times: Increasing the length of piping between a filter (a high static generator) and the receiving tank to allow time for the charge to begin dissipating.
- Velocity Limitation: Reducing the transfer speed (typically to 1 m/s or less) until the fill pipe is submerged to minimize turbulence and “splash” charging.
- Anti-Static Additives: Chemically increase the liquid’s conductivity to ensure standard bonding/grounding is effective.
- Inerting: Displacing the oxygen in the tank with an inert gas, such as Nitrogen or Carbon Dioxide, so that even if a spark occurs, there is no “fire triangle” to support combustion.
Summary Table
| Feature | Conductive Liquids | Non-Conductive Liquids |
| Charge Movement | Moves to the tank walls immediately. | Stays trapped inside the liquid. |
| Grounding Effect | Highly effective. | Protects the tank, but NOT the liquid. |
| Primary Danger | External sparks. | Internal surface-to-tank sparks. |
| Examples | Alcohols, water-based solutions. | Hexane, Toluene, Benzene, Naphtha. |