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Vacuum trucks provide an important contribution to the transportation and recovery of flammable and combustible products within the hazardous process industries. Their efficiency and versatility means they can fulfill a broad array of duties ranging from the transfer of chemicals in manufacturing production, to removing waste deposits from storage tanks or performing hazardous material recovery at the site of road & rail traffic incidents.
Equally, truck deliveries within retail gas & petroleum distribution and the food & beverage industry require transportation to locations where grounding systems may not be installed or verified grounding points may not be present to ground the tanker while it is transferring material.
Vacuum truck used for site chemical transport and recovery
In the recovery and transportation of flammable & combustible products the generation and build of electrostatic charges can pose a significant hazard to personnel and equipment if correct static grounding precautions are not put into action. As you may have read in previous ETTG articles, the relative motion and interaction of different materials leads to the instantaneous combination and separation of positive and negative charges. If these charges do not have a means to dissipate from the objects or materials they come into contact with, i.e. flow to true earth (ground) or share charge with available opposite charges, they become “static” and raise the electrical potential difference of the object or material on which they are accumulating.
In essence, this potential difference is equivalent to a stored source of energy which is immediately seeking to discharge itself in order to return the object to a natural state of electrical equilibrium (0V). If the energy is allowed to discharge in an uncontrolled manner it will do so, in the majority of cases, in the form of an incendive electrostatic spark. Should such an event occur in the presence of a vapour or dust, when they are within their respective ignitable flammable and combustible thresholds, there is a high probability that ignition of the material will occur.
The potential energy stored on an object that can be released in the form of an electrostatic spark is equivalent to:
W = 1/2(C).(V)2
The total energy available for discharge, (W), is equal to the product of the object’s capacity to store charge (capacitance, C) and the square of the voltage, (V), of the body. The voltage of the object is increased by the generation and accumulation of electrostatic charges. To illustrate, a small object like a metal bucket has a capacitance of around 20 pico-farads. If electrostatic charges are permitted to accumulate on the bucket, raising its voltage by just 10 kilo-volts, 1 mJ of spark energy can be discharged by the object.
1 mJ is capable of igniting the majority of flammable vapours and gases. In real world processes the larger charge storing capacity of equipment like tanks, hoses, lances and trucks (up to 5000 pico-farads), when combined with high potential differences caused by the rapid interaction of liquids and solids, can generate much more significant levels of stored energy ready for uncontrolled discharges.
(a) In 1998 an explosion, and one fatality, occurred when granular polypropylene was being vacuumed from a dust collector into a large vacuum truck. The cause of the explosion was a static spark that discharged from the lance to the dust collector. The cause was a non-conductive hose that was used to connect the lance to the vacuum truck.
Because the hose was non-conductive, instead of static charges flowing through the hose to the grounded / bonded truck, static charges accumulated on the metal lance, raising its potential difference relative to the duct collector. In order to equalize the potential difference of the lance, the static spark discharged to the dust collector, igniting the combustible atmosphere in the process.
(b) A fire in a toluene sump was caused when a static spark discharged from the conductive metal windings of a rubber hose to the metal rim of the sump. Although the conductive windings of the hose were bonded to the truck, the truck itself was not grounded. This caused static charges to accumulate on the windings of the hose, raising its potential difference relative to the sump.
The common denominator for these incidents is that the rate of electrostatic charge generation on the components of the system were permitted to exceed the rate of charge dissipation resulting in the accumulation of static charges on some part of the transfer system.
The transfer system includes the lance, hose, hose connections, truck collection chamber and the chassis of the truck itself. To remove the risk of an incendive static spark discharges causing a catastrophic accident these components must be correctly bonded and grounded.
API 2219, entitled “Safe Operation of Vacuum Trucks in Petroleum Service”, is probably the most relevant standard to address the hazards of static electricity in vacuum truck operations directly, although valuable information and recommendations can be sourced from CLC/TR: 50404 and NFPA 77. Of the many recommendations outlined in API 2219, the most relevant instruction is to fully ground the truck by connecting it to “a designated, proven ground source”, before commencing with transfer operations.
Is grounding point (rod or buried metal structure) capable of dissipating electrostatic charges?
The “ground source” describes an object with a low resistance connection to true earth (ground). It is this connection to true earth that will guarantee the rapid dissipation of electrostatic charges from the equipment ensuring that personnel and the equipment being used are protected from the risk of fires or explosions. The API standard provides some examples of potentially suitable grounding sources, including large storage tanks or piping that is known to run underground.
The standard also states the importance of confirming that the connection resistance between the truck and the designated grounding point is less than 10 ohms and that this resistance should be verified with the use of an ohmmeter. All conductive and semi-conductive components of the transfer system must be bonded together with a connection resistance not exceeding 10 ohms.
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