Although the safety requirements specification (SRS) has been proposed in IEC61508 and IEC61511 for a long time, there are still certain gaps and deficiencies in the detailed requirements for SRS preparation in China, so some enterprises have not yet prepared SRS documents. With the increase in the frequency and depth of security inspections in recent years, the phenomenon of missing SRS files has been gradually exposed. In fact, SRS is a necessity for SIS, that is to say, if SIS is set up, it stands to reason that there must be SRS as support, to borrow a teacher's words: "If you didn't have it before, it doesn't mean that you don't need it, and if you haven't checked it before, it doesn't mean that you won't check it in the future." Today we are going to talk about what exactly is SRS?
1. Definition of SRS.
SRS is the abbreviation of Safety Requirements Specification, which translates to Safety Requirements Specification or Safety Requirements Specification, which contains all the required specifications for the safety instrumented functions (SIFs) that should be performed by the safety instrumented system. SRS is an important part of the entire safety lifecycle, and it is possible to know how to design a safety instrumented function (SIF) and how to integrate these functions (SIF) into an SIS. 2. Implementation phase The stages of implementation of the safety requirements specification are as follows (from GB T20438.)1-2017)：
The general requirements of the safety requirements specification should be implemented at the beginning of the SIS design, and it is generally believed that it can be implemented after the SIL classification, and this part of the content is the input file of the SIS engineering design. However, the specific parameter values of the instrument, SIS system and valve should be supplemented and improved after the detailed design. Purchase: Purchase equipment according to SIS design documents, equipment manufacturers provide relevant models and other data, and the automatic control professional continues to supplement SRS files; Finally, the verifier verifies the SIF loop according to the requirements of the SRS, and readjusts the content of the SRS when the SIL verification fails.
The implementation phases of SRS are described in the T CIS 71001-2021 Guidelines for the Preparation of Safety Requirements for Chemical Safety Instrumented Systems, which are consistent with GB T20438The requirements of 1-2017 are the same, which is located in the early stage of SIS engineering design and can be used as input data for SIS design. The details are as follows:
Third, the professional division of labor
The preparation of safety requirements specifications requires the collaborative preparation of process, instrument and safety professionals, and the participating units should include design institutes, evaluation units, owners and patent dealers. Usually the content of SRS will involve multiple disciplines, and some content will also involve the calculation of data, such as parameters such as engineering safety time, so it needs to be discussed together.
Fourth, the key technical documents required for the preparation of SRS.
Process related information 1, piping and instrumentation diagram (P&id) 2, process operation description 3, process control description, including basic process control system design and safety instrumented system allocation strategy, control type, operator interface, alarm management and historical data recording 4, relevant safety regulations (including national, industry, provincial, municipal and enterprise requirements at all levels) 5, reliability, quality or environmental related data operation or maintenance related technical documents;
Cause-effect diagrams can integrate safety function and integrity requirements in a single technical document (other requirements such as instrument range, setpoints, operating conditions, etc. can also be combined in a single form);
Logic diagrams are used as a supplement to cause and effect diagrams, which can describe more complex or chronological or sequential functions (those that cannot be expressed in language or causal diagrams can be expressed in logic diagrams or logical relationships);
Process Data SheetThe process data sheet provides the necessary information for the preparation of instrument selection specifications.
Fourth, content requirements.
Safety requirements specifications generally include general requirements, safety function requirements, and complete integrity requirements. SIS general requirements include design standards, operating environment, EMI RFI (Electromagnetic Interference Radio Frequency Interference) requirements, EMC (Electromagnetic Compatibility) requirements, etc., which can be obtained from the SIS Technical Specifications. The SIS needs to be designed according to the appropriate standard specifications to operate in a suitable environment to avoid electromagnetic interference affecting the performance of safety functions. The general requirements of SIFs include operating mode requirements, failure modes, MTTR MRT requirements, service life requirements, diagnostic function requirements, etc., which are common parts of each SIF. The safety function requirement describes what kind of security function the SIF loop performs, and this part is used as an input condition for the SIS design. The safety integrity level requirement describes how the SIF circuit can better perform the safety function, and puts forward the requirements for the SIL level, target failure amount, periodic detection, etc. of the SIF circuit, which is one of the input conditions for SIL verification.
5. Example of safety requirements specification template.
Part of the content is quoted from "SIL Grading and Verification" Sinopec Press).