The loading and unloading of tank trucks containing flammable or combustible products has long been recognized as one of the most serious fire and explosion risks for hazardous industry operations. A study conducted by the American Petroleum Institute in 1967, for example, identified static discharges as being responsible for more than 60 incidents in tank truck loading operations. Since then more stringent preventive standards have been developed and significant advancements have been made in the technology of static electric discharge prevention. Yet accidents traced to static electric discharges still occur, with sometimes tragic results.

The natural presence of static electricity during product transfer operations, combined with its associated ignition hazards, ensure that regulators take static control precautions for tank trucks very seriously. The primary guidelines are detailed in three primary standards: NFPA 77, API RP 2003 and CLCTR: 50404. An understanding of these standards and of current technology in static discharge prevention can contribute greatly to preventing tank truck load transfer incidents.

Static Electricity Dynamics and Tank Truck Product Transfer Operations

Powders and liquids with low electrical conductivities are the prime sources of static charge generation because their electrical properties do not easily permit the transfer of excess charges. Instead, nonconductive and semi-conductive liquids and powders retain and accumulate charges after they make contact with conductive objects. The most common interface for charging of non-conductive and semiconductive product is contact with a wide variety of metal plant equipment including pipes, filters, pumps, valves, drums, totes, mixers and agitators. When the

electrostatically charged liquid (or powder) is deposited into a container like a barrel, tote or a tank truck, charging of that container will occur if there is nowhere else for the charges to go. In this situation the charges are “static”; they accumulate on the surface of the container and set up a potential difference with respect to ground.

Over a short time period (less than 20 seconds), potentials in excess of 50,000 volts can be induced on a tank truck’s container when it is being filled at normal flow rates with a product that is electrostatically charged. The magnitude of the voltage induced is directly proportional to the quantity of charges making contact with the container. This voltage represents the ignition source and the potential energy available for discharge via a static spark. For a typical truck a typical discharge at voltage levels of 50 kV can occur with energy levels in excess of 1250 mJ. The vast majority of flammable vapors and combustible dusts can be ignited at these energy levels.

For sparking to occur during tank truck product transfer operations, other conductive objects must come into close proximity with the charged container of the tank truck. Examples of conductive “objects” include the fill pipe entering the opening on the top of the container, fall prevention systems such as folding stairs, and drivers or operators working around the

tank truck. The charges on the tank truck’s container attract opposite charges to the surface of the object and rapidly create an electric field between their respective surfaces. It is the strength of this electric field that causes the “breakdown” of the air between the container and the object. When the air is “broken down” a conductive path for the excess charges to rapidly discharge themselves is created, leading to a static spark discharge. If a combustible atmosphere is present in this space, ignition of the atmosphere is very probable.

Under ambient conditions an average field strength of 30 kilo-volts is capable of causing the electrical breakdown of air over a spark gap of 2 cm. In addition, loose conductive items located inside the container could become charged by contact with the liquid and discharge to the container if they are capable of floating on top of the liquid. For this reason, it is important to carry out regular visual inspections of the container to ensure loose debris is not present inside the tank truck container.

Standards and Recommended Practice Governing the Static Control of Tank Truck Product Transfers

As outlined earlier, regulators are extremely cautious about the ignition hazards presented by static electricity in tank truck product transfer operations. Three standards, in particular, provide clear guidance on what precautions should be taken: NFPA 77, API RP 2003 and CLCTR: 50404. They state that grounding of the tank truck should be the first procedure carried out in the transfer process. Grounding effectively creates an electrical circuit that connects the tank truck to ground, and it is this connection to ground that prevents static charges accumulating on the tank truck’s container. The reason the charges can transfer from the tank truck to ground is that the Earth has an infinite capacity to absorb and redistribute static charges, with the positive effect of removing the ignition source from a potentially combustible atmosphere.

The electrical resistance of this circuit from the tank truck to the “ground source” (or “grounding point”), which is in contact with the earth, is a key performance indicator of the entire grounding circuit’s capacity to provide a secure and safe product transfer operation. NFPA 77 and API RP 2003 state the resistance in a healthy metal circuit should never exceed 10 ohms, therefore the entire circuit between the truck and grounding point should be measured

and be equal to, or less than, 10 ohms. If a resistance above 10 ohms is measured this will indicate problems with parts of the grounding circuit - including the tank truck connection, the ground point connection or the condition of the conductor cable.

Tank Truck Grounding Systems

The standards advise that a grounding system, which can measure and monitor resistance in the grounding circuit, can be utilized. The system should verify if the ground connection to the tank truck is complete before loading or unloading is initiated. The CLCTR: 50404 standard recommends 10 ohms or 100 ohms for “convenience” in monitoring. An additional recommendation in NFPA 77 and in API RP 2003 calls for interlocking the feed system (e.g. pump) with the grounding system so that if the grounding system is not connected to the tank truck, product cannot be transferred. This will ensure that electrostatically charged product cannot enter or leave the tank truck when the tank truck has no grounding protection in place. In general, interlocked grounding systems will complete the grounding circuit when the driver connects the clamp of the grounding system to the tank truck and the system sees a circuit resistance of 10 ohms or less.

Although the standards recommend a monitored resistance of just 10 ohms, there are many grounding systems on the market today that monitor well in excess of this level. While it may be claimed that these systems are capable of dissipating static charges, the capacity of a system to monitor at 10 ohms has significant advantages. It not only provides an opportunity to demonstrate compliance with internationally recognized recommended practice, it also means that hazardous location operators know the system’s grounding clamp is making a secure and reliable connection to the tank truck every time a product transfer is carried out. Grounding clamps should be designed to penetrate coatings, rust and dirt build up, as these contaminants are very effective at impeding secure electrical contact with the conductive metal of the tank truck.

Additionally, the grounding system must be capable of detecting minute changes in resistance when the transfer is underway and should not allow a high degree of change in resistance before shutting down the pump or alerting personnel. As soon as a resistance above 10 ohms is present in the grounding circuit, the grounding system should be capable of detecting this change and take control of the feed into the tank truck. Systems that permit resistances higher than 10 ohms have a greater degree of difficulty in detecting changes in the health and condition of the grounding circuit.

Tank Truck Recognition

Because resistance monitoring systems operate when connected to conductive metal objects, additional features can enhance the protection of drivers, product and equipment. A “tank truck recognition” feature can be utilized to ensure that drivers can only operate the feed system when the grounding system detects it is connected to a tank truck. Such a system is the Earth-Rite RTR, which will analyze the capacitance of the tank truck as part of the grounding circuit. If the capacitance presented is in the normal range for tanks trucks (1 nano Farad or 1 x 10-9 farads), the grounding system will recognize that it has made a positive connection to a tank truck.

From the site operator’s perspective, this eliminates the risk of drivers unknowingly connecting the grounding clamp to parts of the truck chassis that are electrically isolated from the truck’s container. Such isolation may be due to original design oversight such as isolated mud guards or paint coatings insulating conductive parts like truck light enclosures from the chassis. In addition drivers have been known to attach the grounding system’s clamp to the loading rack in order to obtain a permissive state for the feed system to “speed up” the transfer.

So while a permissive state for the feed system can be obtained with a standard resistance based monitoring system it does not necessarily mean the grounding clamp is electrically connected to the tank truck’s container. Specifying a grounding system with a tank truck recognition feature ensures the tank truck is, in fact, safely grounded before drivers are in a position to begin filling it with product. Once the system has verified it is connected to a tank truck it should then monitor the tank truck’s connection to the grounding point to 10 ohms or less.

The Grounding Source

When a tank truck grounding system is installed it is assumed that the grounding source (e.g. buried ground electrode) to which the system is connected has been independently verified as having a low resistance connection to earth. This connection is the foundation for secure and safe transfers, and it is incumbent on the site operator to conduct seasonal “Fall of Potential” tests to ensure these ground connections do not deteriorate due to changes in soil composition, soil resistivity or corrosion of the ground electrode.

In winter, ground temperatures can reduce dramatically and cause an exponential increase in soil resistance levels. For the ground electrode these temperatures can have a significant impact on its contact resistance with the soil potentially impeding the transfer of static charging currents. Standard grounding systems are not designed to verify this connection; however, the patented Earth-Rite RTR can remove this uncertainty. This system has a unique feature that verifies that it is connected to a grounding point that is capable of safely dissipating static charges to ground. In combination with the tank truck recognition capability, this ground verification function ensures that two vital connections in the grounding process are securely made (making one connection to the tank truck and the second connection to the ground source.)

When both of these connections are confirmed, the new system will continuously monitor the resistance of these connections at 10 ohms (or less) for the duration of the transfer process. Should either connection be opened during the transfer, the system will detect this and switch off power to the pump or valve actuators in order to stop the feed of charged liquids into, or out of, the tank truck.

Summary

In accordance with the recommendations of industry groups and fire safety associations, the static grounding of tank trucks is a key safety protocol in the loading or unloading of flammable and combustible products. Grounding ensures static charges are not permitted to accumulate on the tank truck, eliminating the risk of the container becoming an ignition source.

Additionally, national and international recommended practice advocates the adoption of static grounding parameters that will enhance the safety of the product transfer process, including monitoring the grounding circuit to 10 ohms or less and interlocking the product feed system with a dedicated grounding system.

When selecting tank truck grounding systems, specifiers should also consider additional functions that can enhance the safety of the transfer process. Grounding systems that include tank truck recognition and static ground connection verification functions provide additional guarantees that a transfer process cannot take place unless the tank truck is connected to the grounding system and the grounding system itself is connected to a verified ground source. These features enhance the secure grounding of the tank truck and enable hazardous area operators demonstrate the highest levels of compliance with NFPA 77, API RP 2003 and CLCTR: 50404.

For additional white papers and detailed technical information on hazardous location static-grounding protection, visit www.newson-gale.com/KnowledgeBase/KnowledgeBase.asp.

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