Bonding a Tank Does Not Remove Static From the Liquid

How a normal transfer can create an ignition source

Flow through a pump, valve, pipe or filter separates electric charge. Splashing, entrained air or water, and disturbed sediment can increase charge generation. In a low-conductivity liquid, that charge can remain in the liquid and build up as filling continues. If the tank headspace contains an ignitable vapour–air mixture, a discharge to a metal fitting or other conductive object can ignite it.

Bonding joins conductive components at a common electrical potential; grounding provides a path from conductive equipment to earth. Both must be part of the transfer design. They do not necessarily dissipate charge carried in a nonconductive liquid. The U.S. Chemical Safety Board (CSB) made this distinction explicit in its Barton Solvents case study.

The Barton Solvents warning

On 17 July 2007, explosions and fire occurred during transfer of VM&P naphtha into a storage tank at Barton Solvents in Valley Center, Kansas. The CSB’s final case study, released on 26 June 2008, found that the tank headspace likely held an ignitable mixture. Stop–start filling, air in the line, and likely water and sediment contributed to rapid static accumulation. A loose linkage in a level-gauging float was the most likely spark location, though the CSB did not rule out another type of static discharge.

Witnesses described bonding and grounding of the tanker, pump, piping and tank. Fire damage prevented the CSB from determining whether those connections were effective. Even if conductive equipment is properly connected, the CSB warned that static-accumulating liquids can require further precautions. The explosion led to evacuation of thousands of residents and offsite projectile damage.

Critical controls before the pump starts

Know the product. Check the current safety data and obtain technical advice when conductivity, static accumulation or headspace flammability is unclear. Do not assume a liquid is safe from static ignition because its flashpoint or transfer temperature appears familiar.

Verify the complete conductive path. Confirm that the tanker, hose, nozzle, piping, tank and internal metal parts are suitable for flammable service and connected as the approved design requires. Check continuity and the condition of clips and connections by the site procedure; paint, corrosion or a disconnected component can defeat an intended bond.

Control charge generation and accumulation. Follow the engineered filling method and approved flow limits, particularly at startup. Review the effect of filters, line air, splash filling, water and sediment. Where the assessment requires it, use a suitable submerged or bottom-fill arrangement, relaxation time, inerting or another engineered safeguard. These controls depend on the liquid and installation; they are not interchangeable rules of thumb.

Protect the receiving tank. Inspect internal devices such as floats and gauge components for secure electrical continuity and mechanical condition. Confirm tank venting, level protection and emergency response arrangements. Treat any abnormal flow, air entrainment or equipment change as a reason to stop and reassess.

A question for today’s team

For our next flammable-liquid receipt, what evidence shows that we have assessed charge in the liquid itself—not only checked the tank’s grounding clamp—and that our startup flow, fill method and internal fittings are suitable?