Safety requirements specifications

The design basis includes the process requirements specification and the safety requirements specification. The process requirements specification is typically developed by process engineering, with input from operations personnel. The process requirements are provided to the instrumentation, electrical, or controls systems personnel to develop the safety requirements specification with input from operations and maintenance personnel. 

Safety Requirements Specification The instrumentation and electrical (I E) requirements are developed to meet the intent of any H RA findings and the process requirements. The design documentation should establish a clear connection between each process hazard and the design of its SIFs. I E personnel should meet with the process engineering representative responsible for the process requirements to ensure that the intent is understood. 

Validation—The activity of demonstrating that the safety-instrumented system under consideration, after installation, meets in all respects the safety requirements specification for that safety-instrumented system. 

The safety requirements specification defines the safety requirements necessary for risk education... 

Allocation of the safety requirements to the safety instrumented functions and development of safety requirements Specification... 

A review of the SIS performance should be periodically undertaken to ensure the original assumptions made during the development of the safety requirements specification (SRS) are still adhered to. For example, a periodic review of the assumed failure rate of different components In a SIS should be carried out to ensure that it remains as originally defined. If the failure rates are worse than originally anticipated, a design modification may be necessary. Likewise, the demand rate on the SIS should be reviewed. If the rate is more than that which was originally assumed, then an adjustment in the SIL may be needed. 

The outcome of the hazard and risk assessment and allocation process should be a clear description of the functions to be carried out by the safety systems, including potential safety instrumented systems together with safety integrity level requirements (along with mode of operation, continuous or demand) for any safety instrumented function. This forms the basis for the SIS safety requirements specification. The description of the functions should be clear as to what needs to be done to ensure that safety is maintained. 

The development of the SIS safety requirements specification is one of the more important activities of the whole safety lifecycle. It is through this specification that the user is able to define how he wants the Safety Instrumented Functions (SIF) to be designed and integrated into a SIS. 

The SIS safety requirements specification may be a single document or a collection of several documents including procedures, drawings or corporate standard practices. These requirements may be developed by the Hazard and Risk Assessment team and/or the project team itself. 

Safety requirements specifications for individual subsystems may also be derived from this overall specification. 

Some considerations with respect to the safety requirements specifications are as follows ... 

When operator action is needed during emergency conditions, the update and refresh rates of the operator display should be carried out in accordance with the safety requirements specification. 

For each diagnostic implemented, testing interval and resulting action on fault detection should meet the safety requirements specification. 

The detailed functional safety requirements specification should include all necessary functions during all modes of operation of the process being protected. Additionally, the periodic testing of all the safety instrumented functions should be provided. This typically requires the definition of maintenance override capabilities so the sensors and final elements can be tested without shutting down the process. The same methodology described in the paragraph above can be used to document these requirements. 

Prior to development of the application software, the user provides a process risk and hazard assessment which is used to identify the software safety requirements in terms of the safety instrumented functions and their SIL. Once the decision to implement the safety instrumented functions in software is made, any conflicts, discrepancies and omissions in the safety requirements specification which come to the attention of the software designers should be addressed. One example might be the effect of the order of execution of the safety instrumented functions within the software. Another example would be the response of the application software as it relates to energy outages. 

The application software safety requirements should be developed as a traceable response to the SIF safety requirements specification. Factors to be addressed include ... 

Determine the proper execution order of the networks and logic, within each program and the execution sequence and desired execution rates of all the application programs. Confirm that the execution rates of the application programs are consistent with the required process response times from the software safety requirements specification. 

Components and devices used in SIF applications that comply with this standard should be provided with documentation that details all known aspects of installation, maintenance, configuration, programming and operation that should be observed if the component or device is to meet the safety requirements specification of the application. 

The application software safety requirements specification will include ... 

The objective of the SiS safety vaiidation is to vaiidate that the SIS achieves the requirements stated in the safety requirements specification. Validation activities should be completed prior to the placing of the SIS into operation. 

The proof test interval should be selected to achieve the average probability of failure on demand as required in the safety requirements specification. 

B.2.3.1 Available information from LOPA safety requirements specification and SIL for the... 

In this step, all functions of the safety requirements specification are allocated to system components, functions or software. Safety integrity requirements will determine the appropriate... 

Safety requirements specification if reactor pressure exceeds 10 bar, close off steam to the reactor jacket within 20 seconds to avoid exothermic reaction. There is no operator action necessary. The required SIL is 3. 

Step 5 - Documentation Summarize the findings of the LOPA study and, if appropriate, document that the proposed change meets the corporate risk tolerance criteria/local code targets. Safety Requirements Specification (SRS) is defined to undertake appropriate cost-effective designs for SISs in order to achieve optimal test and maintenance strategies for the lifespan of the plant... 

Each safety instrumented function is documented in the safety requirements specification. That document (or collection of documents) includes all functional information, logic, performance information, timing, b q)ass/maintenance requirements, reset requirements, the safety integrity level for each safety instrumented function, and any other requirement information that the designers may need. 

In the safety lifecycle, a safety requirements specification is done after ... 

A failure occurs when a device at some level (a system, a unit, a module, or a component) fails to perform its intended function. To many, the definition is clear. Disagreement may occur, but when this happens it is usually a matter of properly defining "intended function." For safety instrumented systems, the definition of intended function is usually clear and should be properly recorded in the safety requirements specification. 

The conceptual design process for a safety instrumented function begins with a Safety Requirements Specification (SRS). The SRS is an important document. It should contain complete specifications for designing all the safety instrumented functions. For each SIF the following information should be included ... 

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