An industrial safety system is a countermeasure crucial in any hazardous plants such as oil and gas plants and nuclear plants. They are used to protect human, industrial plants, and the environment in case of the process going beyond the allowed control margins.
As the name suggests, these systems are not intended for controlling the process itself but rather protection. Process control is performed by means of process control systems (PCS) and is interlocked by the safety systems so that immediate actions are taken should the PCS fail.
Process control and safety systems are usually merged under one system, called Integrated Control and Safety System. Industrial safety systems typically use dedicated systems that are safety integrity level (SIL) certified. SIL is defined as a relative level of risk‐reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a safety instrumented function. SIL applies to both hardware and software requirements such as cards, processors redundancy, and voting functions. Under any circumstances, however, safety must not be compromised with cost (CCPS 2012; Crowl and Louvar 2011; Das and Prabhudesai 1996).
Types of Safety Systems
There are two main types of safety systems in chemical and allied products industries:
- Process safety system or process shutdown system (PSS)
- Safety shutdown system (SSS): This includes emergency shutdown (ESD) and emergency depressurization (EDP) systems
A third system also exist which acts as a barrier and contains the spray out of hot oil and gases from flanges, valves, and pipe joints. These systems are popularly known as safety spray shields and flange guards. The use of spray guards is mandated by OSHA.
Emergency Shutdown System
These systems may also be redefined in terms of ESD/EDP levels as follows:
- ESD level 1: In charge of general plant area shutdown, can activate ESD level 2 if necessary. This level can only be activated from main control room in the process industrial plants.
- ESD level 2: This level shuts down and isolates individual ESD zones and activates if necessary EDP.
- ESD level 3: This level provides “liquid inventory containment.”
Safety Shutdown System
The SSS shall shut down the facilities to a safe state in case of an emergency situation, thus protecting personnel, the environment, and the asset. The SSS shall manage all inputs and outputs relative to ESD functions (environment and personnel protection). This system might also be fed by signals from the main fire and gas system (FGS).
Fire and Gas System
The main objectives of the fire and gas system (FGS) control are to protect personnel, environment, and plant (including equipment and structures). The FGS shall achieve these objectives by
- detecting at an early stage, the presence of flammable gas
- detecting at an early stage, the liquid spill (LPG and LNG)
- detecting incipient fire and the presence of fire
- providing automatic and/or facilities for manual activation of the fire protection system as required
- initiating environmental changes to keep liquids below their flash point
- initiating signals, both audible and visible as required, to warn of the detected hazards
- initiating automatic shutdown of equipment and ventilation if 2 out of 2 or 2 out of 3 detectors are triggered
- initiating the exhausting system
Emergency Depressurization Systems
Due to closing ESD valves in a process, there may be some trapped flammable fluids, and these must be released in order to avoid any undesired consequences (such as pressure increase in vessels and piping). For this, EDP systems are used in conjunction with the ESD systems to release (to a safe location and in a safe manner) such trapped fluids.
Pressure Safety Valves
Pressure safety valves or PSVs are usually used as a final safety solution when all previous systems fail to prevent any further pressure accumulation and protect vessels from rupture due to overpressure by their designed action.
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