Liquid Refined Product Pipeline Operations
tainer. Given their prevalence, this text provides additional information on cargo tank truck loading racks.
Cargo Tank Truck Loading Racks
Cargo tank trucks used for the transportation of refined petroleum products must be constructed to U.S. Department of Transportation (DOT) specifications for the MC-306/DOT-406 atmospheric pressure cargo tank trucks. However, these products may also be transported in MC-307/DOT-407 low-pressure cargo tank trucks.
Cargo tank trucks may be loaded in two manners:
• Bottom loading is the most common method of loading cargo tank trucks and is used for the transfer of flammable liquids, such as gasoline. It is a closed loop system that incorporates a vapor recovery system. See Figure 4.13.
Each compartment on the tank truck has a vent that is connected to the
truck’s vapor recovery piping system. These vents are connected either mechanically or pneumatically to each compartment’s internal safety valve; when the internal safety valve is opened, the vent will also open. This pro- vides both pressure and vacuum protection during product transfer opera- tions without the vehicle operator having to open the dome cover. As liquid product is transferred from the loading rack into the cargo tank truck, vapors are collected in the vapor collection header on the tank truck and are piped to either a vapor recovery unit (VRU) or a vapor combustion unit (VCU).
Responders should remember that most MC-306/DOT-407 cargo tank
trucks that are bottom loaded will have product in their discharge piping prior to making their first delivery. Any mechanical damage to the piping, such as a vehicle going under the unit, can result in 20 to 40 gallons of product being spilled.
• Top loading is primarily used for the transfer of combustible liquids and low vapor pressure products, such as fuel oils and distillates. A fill tube from the loading rack is inserted into each compartment and product is then transferred. Because it is typically used for low vapor pressure products, there is no vapor recovery. See Figure 4.13.
There are a number of safeguards that are installed on loading racks and
MC-306 /DOT-406 cargo tank trucks in order to prevent spills and fires. These include
• Electrical connection between the loading rack and the cargo tank truck. Often referred to as the Scully connection, no product can flow until this electrical connection is made. This connection provides both bonding and grounding between the rack and the cargo tank truck, and provides a com- munications link between sensors on the vehicle and computerized moni- toring devices that are part of the loading rack. Additional grounding equipment may be used at some facilities.
• “Deadman switches” that control product flow. These are switches that must be actuated constantly in order for product to flow from the rack to the cargo
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