The safety requirements shall be derived from the allocation of Safety Instrumented Function (SIF) and from those requirements identified during H&RA. The SIS requirements shall be expressed and structured in such a way that they are
• Clear, precise, verifiable, maintainable and feasible.
• Written to aid comprehension and interpretation by those who will utilize the information at any phase of the safety life cycle.
These requirements shall be sufficient to design the SIS and shall include a description of the intent and approach applied during the development of the SIS safety requirements as applicable.
There are ~ 28 requirements for Safety Requirements Specifications defined in IEC-61511-1:2016, define below.
- Description of All SIFs
A description of all the SIF necessary to achieve the required functional safety (e.g., a cause-and-effect diagram, logic narrative).
- Input and Output Devices
A list of the plant input (e.g. sensors) and output devices (e.g. actuators) related to each SIF which is clearly identified by the plant means of equipment identification (e.g., field tag list).
- Common Cause Failures
Requirements to identify and take account of common cause failures.
- Definition of Safe State
A definition of the safe state of the process for each identified SIF, such that a stable state has been achieved, and the specified hazardous event has been avoided or sufficiently mitigated.
- Combined State
A definition of any individually safe process states which, when occurring concurrently, create a separate hazard (e.g., overload of emergency storage, multiple relief to flare system). I
- Sources of Demand and Demand Rate
The assumed sources of demand and demand rate on each SIF. For example, demand is caused by basic process control system failure, and this is expected once every 10 years.
- Proof Test Intervals
Requirements relating to proof test intervals.
- Proof Test Implementation
Requirements relating to proof test implementation.
- Response Time Requirements
Response time requirements for each SIF to bring the process to a safe state within the process safety time.
- Required SIL and Mode of Operation
The required SIL and mode of operation (demand/continuous) for each SIF. Safety Integrity Level represents design targets for systematic capability of the equipment, PFDavg or PFH verification numbers and architecture constraints.
- SIS Process Measurements
A description of SIS process measurements, range, accuracy and their trip points.
- SIF Process Output Actions
A description of SIF process output actions and the criteria for successful operation, e.g., leakage rate for valves.
- Functional Relationships
The functional relationship between process inputs and outputs, including logic, mathematical functions and any required permissives for each SIF.
- Manual Shutdown
Requirements for manual shutdown for each SIF.
- Energize/De-energize to Trip
Requirements relating to energize or de-energize to trip for each SIF.
- Reset Requirements
Requirements for resetting each SIF after a shutdown (e.g., requirements for manual, semiautomatic,
or automatic final element resets after trips).
- Spurious Trip Rate
Maximum allowable spurious trip rate for each SIF.
- Failure Modes
Failure modes for each SIF and desired response of the SIS (e.g., alarms, automatic shutdown).
- Startup/Restart Procedures
Any specific requirements related to the procedures for starting up and restarting the SIS.
- Interfaces with Other Systems
All interfaces between the SIS and any other system (including the BPCS and operators).
- Plant Modes of Operation
A description of the modes of operation of the plant and requirements relating to SIF operation within each mode.
- Bypass Procedures
Requirements for bypasses including written procedures to be applied during the bypassed state which describe how the bypasses will be administratively controlled and then subsequently cleared.
- Action on Detected Faults
The specification of any action necessary to achieve or maintain a safe state of the process in the event of fault(s) being detected in the SIS, taking into account of all relevant human factors.
- Mean Repair Time
The mean repair time which is feasible for the SIS, taking into account the travel time, location, spares holding, service contracts, environmental constraints.
- Dangerous Output States
Identification of the dangerous combinations of output states of the SIS that need to be avoided.
- Environmental Extremes
Identification of the extremes of all environment conditions that are likely to be encountered by the SIS during shipping, storage, installation and operation. This may require consideration of the following: temperature, humidity, contaminants, grounding, electromagnetic interference/radio frequency interference (EMI/RFI), shock/vibration, electrostatic discharge, electrical area classification, flooding, lightning, and other related factors.
- Normal and Abnormal Process Modes
Identification of normal and abnormal process operating modes for both the plant as a whole (e.g., plant start-up) and individual plant operating procedures (e.g., equipment maintenance, sensor calibration or repair). Additional SIFs may be required to support these process operating modes.
- Survival in Major Accidents
Definition of the requirements for any SIF necessary to survive a major accident event, e.g., time required for a valve to remain operational in the event of a fire.
