## 3 Materials and Methods  
In this section, the operation philosophy of the unloading system is explained and the followed approaches are summarized. In general, the truck unloading system is a piping system that absorbs liquid fuel from trucks and measures the quantity as accurately as possible before transferring it to the storage tanks. The characteristics of the devices are listed in Table 1.  
In the following section, a process sequential overview is discussed to understand the role of main components in the system. Table 2 shows the list of components of the truck unloading system and their associated symbols. Figure 1 shows the truck unloading system showing the symbols. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{Features} & \multicolumn{2}{c}{Low pressure drop—less than 3.2 psi at extended maximum flow} \\ \cline{2-3}  & \multicolumn{2}{c}{Relatively low initial cost and minimum maintenance} \\ \cline{2-3}  & \multicolumn{2}{c}{High accuracy} \\ \hline \multirow{4}{*}{Accuracy} & Repeatability & Less than or equal to \(\pm\)0.02\%. \\ \cline{2-3}  & \multirow{2}{*}{Linearity} & \(\pm\)0.15\% over the normal flow range \\ \cline{2-3}  & & \(\pm\)0.25\% over the extended flow range \\ \cline{2-3}  & Stability & Better than \(\pm\)0.05\% per 10 million gallons (38 million liters). \\ \hline \multirow{2}{*}{Range} & Normal Range & 17 m\({}^{3}\)/h–170 m\({}^{3}\)/h \\ \cline{2-3}  & Extended Range & 10 m\({}^{3}\)/h–204 m\({}^{3}\)/h \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Device characteristics. \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Symbol** \\ \hline Unloading Arms & UA \\ \hline Ball Valves & BV \\ \hline Strainers & ST \\ \hline Differential Pressure Transmitter & DPT \\ \hline Positive Displacement Pump & PDP \\ \hline Check Valve & CV \\ \hline Air Eliminator & AE \\ \hline Level Switch & LS \\ \hline Gate Valves & GV \\ \hline Pressure Relief Valve & PRV \\ \hline Positive Displacement Flow Meter & FM \\ \hline Pressure Transmitter & PT \\ \hline Plug Valve & PV \\ \hline Flow Control Valve & FCV \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Unloading system components list. Figure 1: Unloading system piping and instrumentation diagram. The unloading arms are connected to the bottom of a truck and the vapor arm is connected to the top of a truck to prevent a vacuum, which might cause the truck to collapse. The fluid starts flowing slowly from the truck to the unloading arms due to the gravitational force and difference in the elevation, which can be noticed through the visual flow glass in each of the two arms. The ball valves in each arm will be normally open so that fluid can pass through the two inlet streams. After that, the fluid will merge into the main streamline via a tee joint. The fluid then flows through a strainer that includes a differential pressure transmitter to monitor the difference in pressure between the inlet and outlet. This is mainly designed to filter the liquid in order to protect the rotating parts of the unloading pump against solid particles. Once the unloading pump starts working, the fluid will flow in the desired predefined rate in the pre-set controller, which is considered as the brain of the system. Downstream of the pump there is a check valve to prevent fluid from flowing in the opposite direction. Up to this point of the process, the fluid contains air, which needs to be removed to have an accurate measurement of the liquid quantity. Therefore, the fluid will pass through an air eliminator vessel where the air and liquid are separated. The air will flow back to the top of the truck through the vapor arm, while the liquid will continue its flow in the mainstream. There are two pressure relief valves; one valve is located in the vent line of the air eliminator and the other valve is available downstream of the air eliminator. The function of these valves is to protect the system from excess pressure with a predesigned pressure set point.