The safety requirements specification (SRS) turns the hazard analysis into requirements that designers, programmers, testers and operators can follow. A SIF that is verified but badly specified will be built, tested and maintained wrongly.
Section by section
| Section | Questions it must answer |
|---|---|
| Identification | Which function, which hazard, which drawing and hazard study reference? |
| Hazard and risk reduction | Which scenario, what IEF and layers, what required PFDavg or PFH? |
| Integrity | Target SIL, demand mode and why, required risk reduction, spurious trip requirement. |
| Safe state | What exactly is safe for this hazard? Which valves close, which stay open? |
| Timing | Process safety time and its basis; required response time of the whole function. |
| Inputs | Devices, voting, trip points with units and margin to the safety limit. |
| Logic | Trip condition, time delays, latching, first-out, degraded voting on faults. |
| Outputs | Final elements, action on trip, stroke time requirement, tight shut-off if needed. |
| Energise / de-energise | Behaviour on loss of power, air or hydraulics; monitoring of energise-to-trip circuits. |
| Diagnostics | Response to detected faults, maximum repair time, compensating measures. |
| Manual shutdown | Where, and whether it acts independently of the logic solver. |
| Reset | Manual or automatic, from where, under which conditions. |
| Bypass and overrides | Who, how long, indication, compensating measures; start-up overrides and their removal. |
| Proof testing | Interval and coverage per subsystem; partial stroke; reference to the procedure. |
| Reliability basis | Result, failure rate sources, assumptions (MTTR, β, mission time). |
| Common cause | Measures assumed in β (separation, diversity). |
| Application program | Programming rules, review, testing, version control. |
| Environment | Area classification, temperatures, ingress protection, vibration. |
Usual gaps
- A safe state that is a single word ("trip").
- Response time not related to the process safety time.
- Bypass rules missing or "as per procedure" without the procedure.
- Failure rate sources not stated; assumptions in the calculation not repeated in the SRS.
- The SRS not updated after the design changed.
In First Out AI
The SRS draft is built from the function data, its result, its linked LOPA scenario and HAZOP rows that mention its tag. Every field still empty appears in [brackets] and in the "Open items" section, so you can see at a glance what remains. Download it as Word and complete it in your company template.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.