Protection layers shutdown systems

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

In addition to the barriers and protection layers, the reactor is also designed with safety system. The three primary objectives of nuclear safety systems are (1) shut down the reactor, (2) maintain it in a shutdown condition, and (3) prevent the release of radioactive material during events and accidents. These objectives are accomplished using a variety of equipment, which are part of different systems, of which each performs specific functions. The safety systems are provided for each NPP and are explained below. 

Safety instrumented system (SIS) SIS is meant to prevent, control, or mitigate hazardous events and take the process to a safe state when predetermined conditions are violated. An SIS can be one or more SIFs, which is composed of a combination of sensors, logic solvers, and final elements. Other common terms for SISs are safety interlock systems, emergency shutdown (ESD) systems, and safety shutdown systems (SSDs). So, SIS is used as a protection layer between the hazards of the process and the public. SIS or SIF is extremely important when there is no other non-instrumented way of adequately eliminating or mitigating process risks. As per recommendations of standards lEC 61511 2003 (or ANSI/ ISA-84.00.01-2004), a multi-disciplinary team approach following the safety life cycle, conducts hazard analysis, develops layers of protections, and implements an SIS when hazardous events cannot be controlled, prevented, or mitigated adequately by non-instrumented means. 

BPCS is a system which handles process control and monitoring for oil and gas facilities. SIS is a system composed of sensors, logic solvers and final elements to keep people , environment , assets in safe conditions during operation periods often oil and gas facility. Physicd mitigation system is a system which consists of active fire protection and fire/blast walls. Among the various multiple protection layers, SIS is the most important and critical protection layer to prevent or reduce the risk of abnormal process condition which may be hazardous. A variety of SISs are installed in the oil and gas plants such as fire gas system, emergency shutdown system and process shutdown system. SIS is the next layer of protection following BPCS and alarm/operator intervention. 

The final protection layer is the safety shutdown system in which automatic and independent action is taken by the shutdown system to protect the personnel and plant equipment against potentially serious harm. The essence of a shutdown system is that it is able to take direct action and does not require a response from an operator. 

It may be noted that the hot gas filters not only minimized deposition of dust particles in the catalyst beds but also increased the heat capacity of the conversion system. In the case of a long plant stoppage, there could be more heat retention in the plant when blinds are inserted in the gas inlet line of the HGF. The unit would also protect the top layer of the first pass catalyst from any soot/unbumt carbon particles during plant start-up (during commissioning or after an annual shutdown.)... 

The economic impact of a spurious or nuisance trip of an ESD system can be disastrous. An ESD system is an important layer of protection to prevent and prevent hazardous situations from occurring. So, it is needless to mention that the ESD system must be extremely reliable and function on demand. During an emergency, it must put the process in a safe state in orderly fashion. Also ESD systems design shall be based on a fail safe independent system, that is, ESD systems are such that during a failure of a component the process reverts to a condition considered safe and not a vulnerable serious hazardous event. Reliability and availability are major parameters for ESD system performance. Reliability is a function of system failure rate (its reciprocal) and mean time between failures. Spurious trip conditions may initiate a so-called fail safe incident that may result in accidental shutdown of equipment or processes. However, undetected process design errors or operations may initiate dangerous incidents that may disable the safety interlock and may even cause accidental process... 

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