Top loading begins with the loading rack attendant or the driver opening hatch covers on the top of his vehicle's cargo compartments. To try to use a hose-and-nozzle arrangement to fill these compartments would be both awkward and dangerous. It would require that the driver or attendant crawl around on top of the vehicle tank.
Instead of the use of a hose and nozzle, the filling operation is accomplished through use of rigid tubes, 6 feet or more in length.
The tubes, called loading arms, are connected through a piping network to large storage tanks at the terminal. Although the loading arms are rigid, they are designed to swivel and to move up and down through use of counterweights. The arms also have a telescoping feature, which permits them to be shortened or lengthened.
The loading rack attendant can quickly and easily swing a loading arm out over the top of the waiting transport vehicle, and position its discharge end above the open hatchway of the compartment to be filled. When the arm is in position, the attendant activates a pump and the loading process begins.
For bottom loading, the hatch covers remain closed and the driver or loading rack attendant connects the loading arm to a drybreak connector at the bottom of the transport vehicle's tanks. The loading arm used in bottom loading is a flexible tube or hose with a special drybreak coupler on the connection end.