To minimise the interruption to production caused by a safety shutdown, the safety system should be divided into 5 hierarchical levels, with progressively wider impact on production. These levels of emergency shutdown are as follows:
• Level 1: Emergency shutdown and depressurisation of the overall plant
• Level 2: Emergency shutdown for a process unit within the plant
• Level 3: Process shutdown for a process unit within the plan
• Level 4 Process train shutdown within a unit
• Level 5: Shutdown of Individual equipment and utilities
The safety system for some plants may not require all five levels of shutdown due to the plant being of a small physical size or a single stand alone process;making five levels of shutdown impractical.
In this situation the lower hierarchical levels of shutdown may be omitted with the devices associated with their levels reassigned to a higher level.Each shutdown level shall be initiated either automatically due to: an ESD/PSD field instrument, intertrip from the Fire & Gas system or manually from a hard-wired push-button in the CCR or in the field.
When a shutdown level is activated all lower shutdown levels hierarchically connected to this level shall also be activated.Each shutdown level shall have a manual shutdown push-button and a reset push-button.A level reset will only be enabled when all the trip initiators have returned to a safe condition.When a level is reset all lower shutdown levels hierarchically connected to this level shall also be reset.
Level 1 Shutdown – Emergency shutdown and Depressurisation of the Overall Plant
A Level 1 shutdown shall be initiated by detection of gas or fire by the F&G system in more than one unit area.
A Level 1 shutdown shall shutdown and depressurise the overall plant.
This shall include:
• Isolation of all process units and blowdown all hydrocarbon inventory
• Isolation of pipelines
• Tripping of all electrical equipment
• Initiation of audible and visual alarms at the CCR and throughout the plan
Level 2 Shutdown – Emergency Shutdown for a Process Unit within the Plant.
A Level 2 shutdown shall be initiated by detection of gas or fire by the F&G system in the area local to the unit.
The level 2 shutdown will shutdown the entire process unit by:
• Isolation of the process unit and blowdown its hydrocarbon inventory
• Tripping of all electrical equipment within unit
• Initiation of audible and visual alarms at the CCR and in the area local tothe unit
Level 3 Shutdown – Process Shutdown for a Process Unit within the Plant
A Level 3 shutdown shall be initiated by major process or utility failure that may affect all trains within a unit.
A Level 3 shutdown will shutdown common equipment for the unit in which the abnormal condition occurs and initiate audible and visual alarms at the CCR.
Level 4 Shutdown – Process Train Shutdown within a Unit
A Level 4 shutdown will shutdown the entire process train in which the abnormal condition is occurring and initiate audible and visual alarms at the CCR, where there is no expected potential for the abnormal condition to propagate to a parallel train
Typical examples include high-high levels in vessels, where carry-over of liquid can have adverse consequences on downstream equipment.
Level 5 Shutdown – Shutdown of Individual Equipment and Utilities
A Level 5 shutdown will shutdown the piece of equipment required to bring an abnormal situation under control and initiate audible and visual alarms at the CCR, where the situation has a low potential for escalation. A typical example isa ‘low-low’ level in a process vessel, which shuts an ESD valve in the vessel outlet line to prevent gas blow by to a lower rated vessel downstream.