Protection layers defined

Process risk is defined by the frequency of the occurrence and the potential consequence severity of the process hazard. To define the frequency, the initiating causes (e.g., single causes or multiple causes and conditions) are identified for each process hazard, and their frequency of occurrence is estimated. The consequence severity is the logical conclusion to the propagation of the process hazard if no protection layers are implemented as barriers to the event. 

The gap between the process risk and the owner/operator s risk criteria establishes the requirements for risk reduction. The risk gap can be managed by a single safety function or by multiple functions allocated to protection layers. The team defines the risk reduction that must be provided by each safety function and allocates the safety function to a protection layer that is designed and managed to achieve the allocated risk reduction. 

In the air-sandwich structure, which consists of two disks bonded face to face and separated by a spacer-defined air gap, the substrate becomes the protective layer. Very high quality optical substrates are necessary because the write and read functions take place through the disk. The air gap is advantageous for ablative media. Other related structures have been disclosed in which a thin polymer film is supported physically or aerodynam-ically some distance above the recording surface (12). 

In higher organisms epithelia are covered by a protective gel layer defined as mucus. By weight, mucus consists mainly of water (95-99.5%) in which the mucous glycoprotein mucin (0.5-5 /o) is dispersed. Mucins are the major components responsible for the gel-like structure of the mucus. They possess a linear... 

Because of the processing of aggressive fluids or of the conditions under which a heat exchanger is operated, the heat transfer surface may be subject to corrosion, which may be defined as the deterioration and loss of material because of some form of chemical attack. The products of corrosion on the surface offer a resistance to heat transfer. The corrosion is often because of the impurities in the fluid being processed, and it may be accelerated by the presence of other deposits, such as scale or biofllm. It has to be remembered, however, that in many instances a layer of corrosion product, e.g., a thin oxide layer, is necessary to protect the metal from further deterioration. If this protective layer is removed, there could be serious implications for the continued integrity of the equipment involved. 

MUCILAGES Mucilages are substances based on less well-defined polysaccharide derivatives, which locally exert an inhibition of irritation by covering the mucosa with a protective layer. 

The lEC 61508 standard defines safety as "freedom from unacceptable risk" (Ref. 1). Functional safety has been defined as "part of the overall safety relating to the process and the Basic Process Control System (BPCS) which depends on the correct functioning of the SIS and other protection layers." The phrase "correct functioning of the SIS" identifies the key concern. A high level of functional safety means that a safety... 

In the passive oxidation regime, the ability of Si based ceramics to form a continuous protective layer of silica depends upon their silicon content and the volume change occurring during oxidation. It is often characterized with a parameter. A, defined as ... 

Because of the complexity of chemical processes, the response selection should be tailored to the characteristics and requirements of each plant and safety function. A hazard and risk analysis defines for each hazard scenario when action should be taken, giving the initiating causes, consequence severity, and protection layers. The potential for common cause should also be evaluated to ensure the actions can be implemented in the presence of the initiating cause and the SIF device fault. 

This annex defines the requirements for assessing, establishing, and maintaining SIS setpoints for instrument segments. The safe operating limit is not related to the SIS setpoint it is a function of the process hazard and identified protection layers. Safe operating limit is beyond the scope of this annex. 

Independent protection layer In addition to conditional modifier and mitigating factor (influencing the likelihood of event), IPLs, which reflect the reliability of the protection layer, are responsible for performance of LOPA. In LOPA, the concept of the IPL is important, and should be well-defined and understood. CCPS definition of IPL is An IPL is a device, system or action which is capable of preventing a scenario from proceeding to its undesired consequence-independent of the initiating event or the action of any other layer of protection associated with the scenario. The effectiveness and independence of an IPL must be auditable. ... 

In natural water, the M alk, the CO2 content, the pH and the TH are the basic parameters (among others) chat define the carbonic equilibrium or aggressivity of the water. The aggressivity is related to limestone or "marble" CaCOy) which is the major component in the natural protective layer laid down in piping that carries nonaggressive water. The equilibrium depends on the following balance ... 

The same principles of independence, effectiveness and auditability which apply to protection layers also apply to conditional modifiers. It is important to make sure that the conditional modifier, as defined in the LOPA, is effective in its own right in preventing the consequence without relying on the performance of another conditional modifier or protection layer. Where the performance of a proposed conditional modifier is conditional on the performance of a protection layer or another conditional modifier, it cannot be considered independent. Instead it should be considered part of another protection layer or conditional modifier. The risk reduction should only be claimed once and the LOPA team will need to deoide where best to include it. 

Note that management system and standard operating procedures cannot be claimed as a protection layer in their own right. On their own, procedures do not meet the requirement of effectiveness for a protection layer because they cannot identify a hazard or perform an action. Instead, procedures are incorporated in the performance claimed for a protection layer because they define requirements for the conduct of activities and therefore are included implicitly rather than explicitly within the analysis. 

Before proceeding with the analysis of the performance of the alarm function, the required function should be carefully defined. It is not enough simply to identify an instrument and consider that as a protection layer. The protection layer will need to make up a complete loop and should therefore include ... 

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