Proper Grounding Reduces Static Electricity Hazard

Codes of practice outline best practices for reducing risks when loading ethanol tank cars.
By Mike O’Brien | November 18, 2014

Static electricity is virtually always generated at hazardous locations––commonly called hazloc–– in process industries. Static accumulators are materials known to be powerful attractors of electrons from other materials that also resist letting go of those electrons. Ethanol is among a host of chemicals and petroleum products categorized as static accumulators. In a typical operation, ethanol is transferred from a storage tank via a rack loading system into a receiving tank car. As the ethanol makes its way through the transfer system, the molecules become electrostatically charged. If the tank car is not grounded, contact with the charged ethanol will cause it to become electrified, presenting a potentially serious source of ignition in the presence of flammable ethanol atmospheres.

As the tank car builds up electrostatic charges on its surface, the voltage present on the tank car can rise very quickly and dramatically. This excess potential energy seeks the most efficient way of discharging excess electrons, with the best object being the earth or something directly connected to it—something grounded.

Grounded objects in close proximity to charged objects are good targets for electrostatic sparks. Allowing the uncontrolled accumulation of static electricity in a hazloc atmosphere is no different than having an engine’s spark plug exposed to a flammable atmosphere.

The energy in a static spark is a product of the capacitance of the tank car and the voltage present on the tank car. The electrostatic voltage on the tank car is a combination of the charging current generated by the flow of the liquid, the capacitance of the tank car and the tank car’s isolation from ground.

Increased flow rates and turbulence can increase the size of the charging current, but even at safe, recommended flow rates the electrostatic voltage of the tank car can build up to hazardous levels in less than 20 seconds if the transfer system is not grounded. The potential spark energy from a tank car charged to 20,000 volts can be as high as 1,000 millijoule (mJ). When compared with the minimum ignition energy for ethanol, which is 0.23 mJ, it’s easy to see why the tank car and any equipment connected to it, like flexible hoses and piping, should be bonded and grounded.

Grounding and bonding are not simply achieved by connecting alligator clips on wires back to the loading rack. Because of the serious ignition hazard that static electricity presents to a wide range of hazloc operations, industry codes of practice aim to control the risk of fires and explosions.

Codes of Practice
The National Fire Protection Association and the American Petroleum Institute publish codes of practice. NFPA 77 “Recommended Practice on Static Electricity” (2014) and API RP 2003 “Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents” (2008) are publications written by committees of hazloc industry professionals who are recognised experts.

In the codes, grounding is defined as the act of ensuring the tank car is connected to an object that has a verified connection to the general mass of the earth. Article 250 of NFPA 70, the National Electrical Code, describes these points as “ground electrodes.” Typical ground electrodes include metal rods buried up to 8 feet in the ground, pipes in direct and continuous contact with the earth for more than 20 feet and structures such as loading racks, which should be grounded for electrical fault protection and lightning protection purposes. Grounding provides a continuous and uninhibited path for charges generated during the transfer operation to flow to earth.

What is clear from the recommendations of NFPA 77 and API RP 2003 is that 10 ohms in the grounding and bonding circuit is the maximum resistance recommended for equipment at risk of electrostatic charging in hazloc atmospheres. API RP 2003 goes one step further in recommending 1 ohm or less, but if a grounding system with signal lights is in use, 10 ohms is satisfactory. This is because the grounding system continuously monitors the resistance and notifies the operator of the potential hazard if it rises above 10 ohms. Another important recommendation is to use interlocks wherever possible, to ensure the transfer does not take place if grounding is not present. By halting the movement of product, the charge generation source is eliminated, thus preventing additional charging of the tank car.

To comply with the recommendations found in NFPA 77 and API RP 2003, grounding systems should do the following:
1.    Monitor the grounding circuit to 10 ohms or less.
2.    Provide operators with a visual reference that indicate a GO/NO GO action via red and green indicators.
3.    Provide dry contacts to interlock the grounding system with the loading rack pump or control system.
4.    Display the full range of hazloc approvals with the mark of a nationally recognized testing laboratory.

One of the main problems with static electricity is that it is not something the operators can see, smell or hear which, unfortunately, can foster an attitude of “it can’t happen to me” or “it doesn’t exist” amongst personnel operating skids and loading rack systems. An effective means of controlling the risk is a grounding system that combines a simple visual indicator utilizing a traffic-light style of GO/NO GO communication along with interlock control capability. Interlocking the transfer system with the grounding system is probably the ultimate layer of protection one can take to ensure a tank car is properly grounded.

Author:  Mike O’Brien
Head of Product Marketing, Newson Gale Inc.

Contributing author: Richard Puig, Regional Manager