Planning, Design, and Specification
Define the design basis before hardware
Planning starts with a documented design basis: process or facility description, operating envelope, hazards, and acceptance criteria. Hazard analysis methods such as HAZOP identify deviations; layer-of-protection analysis (LOPA) tests whether independent protection layers are sufficient. The output is a safety requirements specification (SRS): each safety function stated with its sensor, logic, final element, required response time, integrity target (e.g., SIL or PL), operating mode, proof-test interval, and reset behavior. Design must address independence, common-cause failure, human factors, environmental conditions, and interfaces with other systems. Verification confirms that the design meets the specification before implementation.
Traps that appear in questions
- Specifying a device ("SIL 2 valve") rather than a complete function with performance target.
- Omitting the final element or proof-test interval from the specification.
- Using "fail-safe" without defining the safe state for the specific process.
- Treating vendor packages or utility interfaces as outside the design scope.
Worked example
Weak: "Install a high-pressure trip." Stronger: "On high pressure at transmitter PT-101, the logic solver shall close inlet valve XV-101 within 2 seconds; integrity target SIL 2; proof-test interval 12 months; reset requires manual acknowledgment after pressure is below setpoint." That statement is testable, verifiable, and traceable.