Application Spotlight:
Static grounding protection for Tank Cars

 

Tank Car

In the hazardous process industries, more commonly referred to as the HAZLOC industries, static electricity is generated virtually all of the time. Various grades of crude oil, refined petroleum products like LPG, and a host of chemicals fall into a category of materials that are often referred to as “static accumulators”. Materials in this category are known to be powerful attractors of electrons from other materials and resist “letting go” of electrons they come into contact with. In other words they “accumulate” static charge.

In a typical LACT unit or rack loading operation, the static accumulating product is transferred from, say, a truck, via the LACT unit or from a storage tank via a rack loading system into a receiving tank car. We can refer to the equipment involved in the transfer of product collectively as the product “transfer system”. As the product makes its way through the transfer system to the customer side of the transfer, the molecules in the product become electrostatically charged.

As the tank car builds up electrostatic charges on its surface, the voltage present on the tank car rises dramatically in a very short space of time. Because the tank car is at a high voltage, it is seeking to find ways of discharging this excess potential energy and the most efficient way of doing this is to discharge the excess electrons in the form of a spark.

Energy discharged in static sparks.

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

If the transfer system is not grounded, the electrostatic voltage of the tank car can build up to hazardous levels in less than 20 seconds. Table 1 illustrates how much energy can be discharged by a spark from a tank car charged to 20,000 volts.

Potential Energy Table

Table 1. Potential energy of sparks from various objects.

When the energy of the sparks discharged by static electricity is compared with the minimum ignition energies of a wide range of both raw and refined petroleum sourced products, 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.

MIES Petroleum Producs

Fig. 1. The MIEs of common petroleum products. (Source NFPA 497).

As can be seen in Table 1, isolated tank cars can discharge sparks with a huge amount of energy. At these static spark energy levels the prevention of electrostatic shocks to workers is an important safety consideration. Involuntary physiological reactions caused by electrostatic shocks could lead to trips and falls and could be particularly hazardous when personnel are working above ground level.

Of the several factors that contribute to static charging, the one variable that must definitely be controlled is the grounding of the tank car. Grounding the tank car ensures that the tank car’s resistance to the general mass of the earth is maintained at a level that does not impede the safe transfer of static charges from the tank car to ground.

Grounding and bonding is not simply achieved by connecting alligator clips on wires back to the loading rack or LACT unit. Because of the serious ignition hazard static electricity presents to a wide range of HAZLOC operations, there are industry guidelines in place designed to control the risk of fires and explosions caused by static electricity.

Industry codes of practice related to the static grounding of tank cars in HAZLOC atmospheres:

The National Fire Protection Association and the American Petroleum Institute each publish codes of practice for controlling the risks associated with static electricity in hazardous locations. 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 area of static control for hazardous locations. Understanding and implementing the recommendations of these guidance documents will ensure the risk and liability attached to static hazards is well controlled.

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. While API RP 2003 goes one step further in recommending 1 ohm or less, if a grounding system with signal lights is in use, 10 ohms is satisfactory. This is because the grounding system is continuously monitoring the resistance in the grounding circuit, so that if it rises above 10 ohms, the grounding system can signal this potential hazard to the operator of the loading rack or LACT unit. 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.

Specifying a Static Grounding System for tank car loading/unloading operations.

One of the main problems with static electricity is that it is not something the operators can see, smell or hear. This characteristic of static electricity can, unfortunately, promote an attitude of “it can’t happen to me” or “it doesn’t exist” amongst personnel operating LACT units and loading rack systems. A grounding system that combines a simple visual “GO / NO GO” communication via a traffic light model of indication with interlock control capability is the most effective means of controlling the risk of ignitions caused by static electricity during tank car product transfer operations. Interlocking the transfer system with the grounding system is probably the ultimate layer of protection equipment specifiers and designers can take to ensure the tank car is grounded.

Tank Car Loading InstallationFig. 2 Typical Earth-Rite® PLUS tank car loading rack installation. The tungsten carbide teeth on the grounding clamp are the contact points for the signal that verifies if the tank car is bonded to the loading rack (or LACT unit) to 10 ohms or less.

Newson Gale recommends the Earth-Rite PLUS for bonding tank cars to LACT units and loading rack superstructures. Along with demonstrating the full range of Class I, Div. 1 approvals for all gas and liquid vapor groups, it also ensures there is a 10 ohm, or less, connection between the tank car and the product transfer system. This provides equipment specifiers with the ability to demonstrate full compliance with the recommendations of NFPA 77 and API RP 2003. By simply connecting the grounding clamp to the tank car the Earth-Rite PLUS automatically verifies if the tank car is connected to the LACT unit or loading rack by delivering an Intrinsically Safe monitoring circuit to the system’s Factory Mutual approved grounding clamp. The Factory Mutual approved stainless steel heavy duty grounding clamp ensures that a strong initial connection, via a pair of tungsten carbide teeth, is made to the tank car, and is then maintained for the duration of the product transfer operation, resisting movement caused by vibration or accidental dislodging.

Unlike standard grounding systems that rely on the their non-monitored electrical ground connection to dissipate the static charges generated by the transfer, the Earth-Rite PLUS ensures that its dedicated static grounding connection to the LACT unit or loading rack is always monitored, via the static ground connections G1 and G2 (ref. Fig. 2). This ensures there is a monitored connection directly between the tank car and the LACT unit or loading rack. This is an important feature as we are depending on the LACT unit / loading rack’s verified ground connection to dissipate static charges from the tank car to the general mass of earth.

Earth-Rite PLUS System

Fig. 3 Earth-Rite PLUS ground status indicators pulse continuously when grounding is in place.

When the Earth-Rite PLUS verifies the tank car is bonded to the LACT unit or loading rack superstructure, a cluster of attention grabbing green LEDs pulse continuously to inform the operator that the system is actively monitoring the integrity of the ground loop.

A pair of dry contacts can be interlocked with the power delivered to the pump or PLCs to halt the product transfer operation if the Earth-Rite PLUS detects a resistance of more than 10 ohms in the ground loop between the tank car and the product transfer system.

The Earth-Rite PLUS can be powered off a 110 or 230 volt AC source or 24 volt / 12 volt DC source and is cCSAus approved Class I, Div.1 for gas groups A, B, C, D and all combustible dust and fibre groups.

Codes of Practice:

NFPA 70, the National Electrical Code.
NFPA 77, Recommended Practice on Static Electricity.
API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents.