Tanker loading and unloading systems have unique surge pressure problems because the ships that berth at these systems can vary in size, capacity and design. The design pressure of the ships and (sometimes) the loading lines also tends to be low in comparison to the maximum potential surge pressures.
The Quick Release Coupling (QRC) also need to close quickly to minimise any spills which makes the surge pressures higher and less easy to control.
We also find that a significant proportion of the systems that we study can experience problems unless the start-up procedures are carefully planned using surge analysis simulations.
A combination of shutdown systems, surge relief valves and careful specification of valve closure times can usually be used to mitigate the risks. But these techniques do need to be simulated because loading and unloading can be a highly dynamic operation with high velocities and hence surge pressure problems.
Many ship loading and unloading systems have been operating for many years and are now being modified to achieve higher flows and larger ship sizes. Both of these changes can increase the transient pressure problems by raising the peak pressures or by increasing the dynamic forces. Relief valves and surge suppression techniques incorporated at design stage will be ineffective at higher flows and so do need to be re-evaluated.
The SIGTTO organisation has some useful background information of ship loading and unloading and is worth consulting if you are designing or operating loading systems.