Abstract: A method and related non-linear level controller (NLLC) for controlling surge capacity in a processing system including at least one vessel receiving an inlet flow of a fluid including a liquid subject to variation and supplying an outlet flow to at least one downstream unit includes the step of computing a first parameter relating to a difference between a current inventory in the vessel and a prevailing inventory limit for the vessel and a second parameter relating to a difference between the current inventory and an inventory set point. An unforced time to reduce the first parameter by a first percentage (T*) is compared to a first time T1. The inlet flow or outlet flow is then controlled based on the first parameter if T*?T1 and based on the second parameter if T*>T1.