Shutdown level hierarchy is a classification of shutdown into a group which is categorized by intention priority. These shutdown levels will be implemented in Safety Instrumented System and will function to protect the personnel, equipment and environment as per their specific purpose. The higher number of shutdown level shall initiate the lower level groups.
Shutdown level hierarchy may differ among each plant and platform. The number of level may vary and shall be determined by considering all safety factors to suit the needs and condition of each plant or platform. The following is an example only of general shutdown level that may exist within a plant or platform in an ascending order of risks severity:
Level 1 Unit Shutdown (USD)
Unit Shutdown (USD) is the shutdown of individual process or utility system or subsystem and intended to prevent equipment from operating outside process limits that may cause damage to the equipment or adversely affect the process. USD can be initiated automatically by various process sensors or manually through means of push button on the panel or Human Machine Interface (HMI). A unit shutdown shall not impair the operation of other system in the plant or platform. Shutdown of one vendor package is also classified into this level of shutdown.
Level 2 Process Shutdown (PSD)
Process Shutdown (PSD) is shutdown of all process system. PSD is activated automatically by various process sensors. PSD will shutdown and isolate all related process equipment or systems, to limit the probability of an abnormal operating condition leading to an emergency situation.
Level 3 Emergency Shutdown (ESD)
Emergency Shutdown (ESD) is shutdown to minimize the consequences during emergency. ESD can be automatically initiated by load shedding functions or manually by the operator through push button on the panel. ESD will shutdown and isolate all designated process related equipment, including inlet and outlet ESD valves. Depends on each project some plant will allow the power generation, utilities, and fuel gas system keep running during this condition.
Level 4 Emergency Depressurize Shutdown (EDP)
Emergency Depressurize Shutdown (EDP) will shutdown, isolate and depressurize all equipments by opening the de-pressuring valves to flare. In this condition, main power generation system will be shutdown and the emergency power generator will start. EDP is manually activated by the operator through push button.
As mentioned before, the number of level shutdown in the hierarchy may differ among each project. In some platform, there are other levels of shutdown i.e.total process shutdown and abandon facility shutdown. On platform abandonment, all source of electrical power will be totally isolated except DC battery system for the navigation aids.