The MGV incorporates the recommendations of API RP 2219 “Safe Operation of Vacuum Trucks in Petroleum Service”, which is the most relevant standard to address the precautions that should be put in place when vacuum tankers are being used to transfer combustible materials or are carrying out transfers in potentially combustible atmospheres.
When it comes to earthing vacuum tankers the recommendations in API RP 2219 are:
- To earth the truck before commencing with any operations, where a “designated, proven ground source is preferred.”
- To ensure the electrical resistance between the truck and the earthing point does not exceed 10 ohms, “the continuity should be verified with an ohmmeter following connection and prior to operations.”
Focussing on the first requirement of the standard, it states that we need to establish that the earthing point we are connecting the tanker to, is a proven “ground source”. The “ground source”, in other words, is an object that has a direct electrical connection to a True earth ground. The MGV system matches this requirement by ensuring the earthing point to which it is connected, has a verifiable connection to True earth. This function is called “Static Ground Verification” (SGV).
To ensure the connection between the point the MGV has verified as having a connection to a True earth ground and the tanker is 10 ohms or less, the MGV will continuously monitor the tanker’s connection to the now verified earthing point. In addition, the MGV monitors its own connection to the tanker. This is especially important as it ensures both the tank and chassis are connected to earth via the MGV system. This ensures that there is a continuously monitored circuit between the tanker and the verified earthing point which will enable static electricity flow off the tanker and into the ground. Because the MGV is now monitoring the circuit between the tanker and the verified earthing point, if the clamp’s connection to the earthing point is compromised or removed while a transfer is underway, the MGV system detects this. This function is called “Continuous Ground Loop Monitoring” (CGLM).
The Green Pulsing LEDs inform the team that the MGV is continuously monitoring the health of their tanker’s static earthing circuit.
Both the SGV and CGLM checks of the Earth-Rite MGV must be positive in order for a static ground connection to be established. When both checks are positive three highly luminous green LEDs pulse to inform the drivers that the tanker is earthed. At this point, the transfer team can proceed with the next stage of the material transfer operation. Any static generated by the transfer process will immediately flow directly through the MGV to earth removing the risk of ignition of combustible atmospheres or shocks to operators caused by discharges of static electricity.
From the driver’s perspective, earthing the vehicle with the MGV couldn’t be simpler. When the LED behind the system’s window is red, it means the tanker does not have an earth connection. All the driver needs to do is connect the earthing clamp to an object that he wishes to test. If the object to which the clamp is connected has a verifiable static earth connection and the clamps’ connection resistance to the earthing point is 10 ohms or less, the LEDs will switch from red to pulsing green. The green pulsing LEDs inform the team that the MGV is continuously monitoring the health of their tanker’s static earthing circuit.
Both the Static Ground Verification and Continuous Ground Loop Monitoring checks need to be positive in order for the earth status indicators to change from red to green.
And just like the levels of security provided by a gantry mounted static earthing system, the vacuum tanker provider has the option of interlocking a pair of volt free contacts with the pumping system. If the MGV detects a loose or broken connection in the earthing circuit when a transfer is underway it immediately shuts down transfer of material, which in turn, stops the generation and accumulation of static electricity on the tanker and the overall transfer system. Another interlock option is to mount a strobe light at an elevated position on the tanker to provide an additional indicator for the recovery team especially if they need to work in a position, or at a distance, that prohibits their view of the earth status indicators on the MGV system.