Protection layers interlocks

By attempting to maintain process conditions at or near their design values, the process controls so attempt to prevent abnormal conditions from developing within the process. Although process controls can be viewed as a protective layer, this is really a by-product and not the primaiy func tion. Where the objective of a function is specifically to reduce risk, the implementation is normally not within the process controls. Instead, the implementation is within a separate system specifically provided to reduce risk. This system is generally referred to as the safety interlock system. 

Are there other safeguards or protection layers to prevent and/or mitigate the hazard, e.g., BPCS interlocks, SIFs, or other non-SIS protection layers ... 

SIS This is the first automatic protection layer to BPCS and second overall layer of protection. It is desired that this shall be independent of BPCS. Even if these are combined it is necessary to ensure that single failure does not take toll of safety. SIS may stop part of plant operation and/or diverts some flow safely, etc. It may have separate set of instrumentation to detect and take safety action in the event of instrument/system failure. It has to be more aggressive than BPCS for safety functions. Under SIS, there will be several interlocks and protections to save the system and in many places like off shore design, ESD is considered as last resort or emergency plan achievable through PEs. 

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. 

The reactor will be shut down twice a year for off-line maintenance and safety interlock testing. All protection layers identified in the LOPA that provide risk reduction must be tested at this same frequency. Note Since this is a batch operation, some SIS components could be tested more frequently (e.g., the vent valves could be tested before each batch is started) if necessary to meet the target PFD. Presently, the SIL verification calculations described in Clause 5.3.4.3 indicate that the higher test frequency is not required. However, if operating experience shows that SIF component failure rates are aotually higher than assumed in the PFD calculations, higher frequency testing of some components could be implemented. 

From Joe s and Mary s viewpoints, each may be correct. Joe s diverse interlock system is indeed inherently safer as a layer of protection than the alternative using identical sensors, but it is still part of a process which is inherently less safe than alternatives which... 

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. 

Activation of layers of protection such as relief valves, interlocks, rupture disks, blowdown systems, halon systems, vapor release alarms, and fixed water spray systems... 

If the plant is small enough, the maximum possible accident may not pose a significant hazard to people, the environment, or property. This may result in an additional reduction in the equipment needed for the plant—it may not require as much (or any) safety equipment, emergency alarms and interlocks, or other layers of protection to manage risk. Even if the small plant still requires safety equipment, this equipment will be smaller and cheaper. Installation and ongoing operation of safety equipment is often a major expense if it can be eliminated or reduced in size and complexity, there will be cost savings. Safety need not cost money—safer can also be cheaper if a small, efficient, inherently safer process can be invented. 

If active layers of protection are needed, provide detection and response for all the failure modes and ensure the personnel and interlocks can respond in time to prevent the undesired event. 

Insuloc. A system for protecting water-cooled pipes in steel reheating furnaces. An inner layer of refractory fibre is covered by a tough outer skin of interlocking refractory tiles. 

FEED stage At the initial FEED stage, PFDs, certain equipment, etc. are fixed and it is in the process of development of a P. ID. At this stage, preliminary hazard analysis, checklists, etc. are utilized for PHA. Then, slowly instrumentation is developed based on layers of protections desired as a safeguard and interlock. 

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... 

Safety Layers - An approved system or function specific to a hazardous event that prevents an event from occurring or protects against the consequences of an event. Examples AIB S, BPCS Interlocks, procedures, relief valve discharge collection systems, etc. 

Interlocks provide logical constraints within control systems and often provide a safety related function. These functions may be embedded within the basic control system in the form of software or they may be relay or mechanical interlocks directly linked to the equipment. They can therefore be considered as providing a layer of protection and their degree of independence must be evaluated in each particular application - in each application decide if the interlock is part of the control system, an independent safety device or part of the SIS. 

Table 5 is a partial list of accident events and the associated prevention strategy used to propose interlock strategy and actions to help further identify or create additional independent layers of protection. 

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