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Bonding Versus Grounding at the Fill Point
Liquid moving through pipes, valves and filters can generate static electricity. A static spark can ignite the vapor-air mixtures of many flammable and combustible liquids. For static control in hazardous areas, two connections matter: bonding joins conductive objects to equalize their electrical potentials, while grounding connects a conductive object to earth to dissipate accumulated charge. A grounded tank does not establish that its filling equipment is bonded to it.
A grounded tank can still have unbonded filling equipment
- Source
- CSB case study, Sept. 2008
- Liquid
- Ethyl acetate
The Chemical Safety Board’s (CSB) September 2008 case study, Static Spark Ignites Flammable Liquid during Portable Tank Filling Operation, describes a steel tote being filled with ethyl acetate. The tote stood on a grounded weigh scale, and witnesses reported that the operator attached a grounding clamp before filling began. However, the metal components of the nozzle, synthetic rubber fill hose and pump were not bonded to the tote.
The CSB determined that an ignitable vapor-air mixture formed near the fill opening under the conditions in which the ethyl acetate was being handled. It concluded that ungrounded metal parts on the filling equipment likely accumulated static charge and sparked to the tote’s external wall, igniting the vapors.
Bonding can use wiring or direct contact between conductive objects. In a bonded hose, conductive wire or fibers connect the nozzle to the pump, preventing static from accumulating as liquid flows through the hose and nozzle.
Choose filling equipment for flammable service
- Source
- CSB recommendations
The CSB case study recommends that:
- Fill nozzles and hoses be bonded and grounded and designed for flammable service.
- Companies purchasing equipment verify that it is designed for flammable liquids and capable of being bonded and grounded.
- Bonded and grounded metal dip pipes be used when top-filling portable tanks.
Tanker filling points under TRGS 727
- Rule
- TRGS 727
- Edition
- Jan. 2016, corr. July 2016
The German technical rule TRGS 727, January 2016 edition corrected July 2016, separately describes a limited insulating-floor exception at rarely used tanker filling points. For road-tanker and rail-tank-car filling points, it requires unavoidable ground sealing, rare use and additional measures that keep nearby people grounded or prevent dangerous charging.
Frequently Asked Questions
Bonding connects conductive objects to equalize their electrical potentials; grounding connects a conductive object to earth to dissipate accumulated electricity. The CSB’s September 2008 portable-tank-filling case study distinguishes these two connections. Bonding objects together does not by itself provide a path to earth.
No. In the incident described by the CSB in September 2008, the tote stood on a grounded weigh scale and witnesses reported that a grounding clamp had been attached, but the nozzle, hose and pump metal components were not bonded to the tote. The CSB concluded that ungrounded filling-equipment metal likely accumulated charge and sparked to the tote.
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