Figure 2.1 Example of Emergency Shutdown system
Source: https://cdn.instrumentationtools.com/wp-content/uploads/2019/06/Purpose-of-Emergency-Shutdown-ESD-System.png
The model of the Emergency Shutdown or ESD system shall take into account the requirements resulting from common operation and shall also fulfill the necessities that may appear throughout other possible (and likely to occur) abnormal or down-graded configurations. Depending on the type of operating plant and functions, ESD system design will vary.
ESD is typically completed by observing the state of field mounted sensors, valves, trip relays and inputs to a control system as alarms (Figure 2.2).
Figure 2.2 ESD components
Source: https://instrumentationtools.com/purpose-of-emergency-shutdown-esd-system/
The control system performs a cause and effect analysis of the parameters of field mounted sensors, valves, trip relays, and inputs to a control system as alarms to regulate plant health. Switches are installed within the emergency shutdown systems for components like compressors, pumps, turbines, or computer control systems that send a signal to trigger a shutdown. The system will reduce the effects in case of anomalous behavior by decreasing the number of plant items available or shutting down part of the systems. For example, in case of a fire hazard, a Fire Damper control system may override the existing controls to open or close vents as needed, and close fire doors.
In Figure 2.3, the causes are listed in left section while the effects are listed on top section, both are described in form of tag number with their descriptions. The marked intersection between both means that they are related as cause-effect. Therefore, here the cause could be a vessel high pressure alarm and effect could be open the vessel inlet valve.
Figure 2.3 Cause and effect sheet
Source: https://youtu.be/Z4jvHVaZH4Q
Analysis of credible scenarios helps to improve understanding of the necessary functions of a safety system. For example, as shown in Figure 2.4, fire detection in the cargo area would most likely be the result of loss of containment of flammable cargo. In this circumstance, a suitable system should lead to full shutdown of cargo transfer operations and any gas burning in the engine room.
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