If we estimate a low value of 1 nano-farad for the truck’s capacitance value (1×10-9 F) combined with a voltage of 30,000 volts on the truck-hose system at the time of ignition, we can assume a total potential energy of 450 milli-joules, which is far in excess of the MIE of toluene which is 0.24 milli-joules. A certain percentage of the energy would be given off in the form of sound and radial heat, but a significant percentage would have been released in the spark gap, igniting the toluene in the process.
Grounding: getting it right.
It’s pretty obvious that not grounding the truck was the single biggest contributor to this fire. However, grounding is not simply connecting the truck to something that looks like it’s entering the ground. Careful consideration needs to be placed on the method of grounding and what parameters can be implemented so that a constant connection to ground is maintained for the duration of the operation. As stated earlier there are many guidance documents that can be followed in relation to providing effective grounding of equipment. In relation to vacuum trucks there are two guidance documents that should be adhered to.
- API 2219 “Safe Operation of Vacuum Trucks in Petroleum Service” (2005).
- CENELEC TR 60079-32-1 “Explosive atmospheres. Electrostatic hazards, guidance” (2014).
The code of practice outlined in API 2219 recommends the following:
- “Before starting transfer operations, vacuum trucks should be grounded directly to earth or bonded to another object that is inherently grounded such as a large storage tank or underground piping.”
- “A designated, proven ground source is preferred.”
- “This system (grounding) should provide an electrical contact resistance of less than 10 ohms between the truck and a grounded structure.”
- “To assure proper bonding, the continuity should be verified with an ohmmeter following connection and prior to operation.”
In a nutshell API 2219 recommends that the truck be connected to a “proven” ground source and that the connection to the ground source is not greater than 10 ohms resistance. In order to ensure the connection resistance does not exceed 10 ohms, this connection should be verified with an ohmmeter or some other device capable of indicating a 10 ohm or less connection resistance.
The code of practice in CENELEC TR 60079-32-1 recommends the following:
- “Vacuum trucks should be connected to a designated site earth before commencing any operations.”
- “In areas where site earths are not present, i.e. where portable earthing rods are required, or there is doubt regarding the quality of site earths, the resistance to earth should be verified prior to any operation.”
- “When the truck is connected to a verified earth, the connection resistance between the truck and verified earth should not exceed 10 Ω for pure metallic connections or 1 MΩ for all other connections.”
- “This requirement should be verified with a truck mounted earthing system or portable ohmmeter.”
In summary CENELEC TR 60079-32-1 states that the truck should only be grounded via “designated” grounding points. “Designated” means that the point has been tested and verified as having a direct connection to earth. The primary function of designated grounding points is to provide grounding protection for electrical circuits, lightning strikes and static electricity. The resistance from the truck to the designated grounding point should not exceed 10 ohms and should be verified with a truck mounted grounding system or ohmmeter. Metallic systems include electrical circuits inside grounding systems as the static charge should pass directly from the truck to the grounding clamp via copper tracking located on the grounding system’s PCB.
If a truck mounted grounding system had been used by the contractor providing waste removal service at the site in this case study, this fire would have been avoided. There are several benefits of using a truck mounted grounding system. One such example is the Earth-Rite® MGV.
- The Earth-Rite MGV automatically verifies if the truck is connected to a ground source that is connected to the mass of the earth.
It will monitor the resistance between the verified ground source and the truck so that if exceeds 10 ohms the green LED indicators will switch form pulsing green to red.
- Internal dry contacts can be utilized to shut down the movement of liquid or powder if the Earth-Rite MGV detects that ground connection is not present.
- Site electricians do not need to perform a one-time check with a meter. The Earth-Rite MGV will not only perform this check automatically for the driver, it will continue to monitor the connection for the duration of the transfer operation.
If you would like to learn more about the Earth-Rite MGV follow this link to the product webpage.
Please note this case study is referenced from a third party source and is not in any way linked to the operations of Newson Gale customers.
If you have any questions relating to the topics discussed in this article, please contact Newson Gale.