Plant Safety, Risk Management and HSE
This domain covers hazard identification, risk assessment, layer protection, relief systems, fire and explosion prevention, and HSE management. The exam tests whether you can distinguish hazard from risk and select credible safeguards.
Core rules
- HAZOP examines deviations from design intent using guide words such as no flow, more temperature, and reverse flow. It identifies causes, consequences, and safeguards.
- Risk is frequency times consequence. A risk matrix must use defined severity and likelihood scales; changing either scale changes the result.
- LOPA evaluates independent protection layers. Each IPL has a probability of failure on demand; the mitigated frequency is the initiating frequency multiplied by the PFDs.
- ALARP means risk is reduced as low as reasonably practicable, not necessarily to zero. Relief devices protect equipment from overpressure but do not by themselves prevent a runaway reaction.
Worked example
An initiating event occurs at 0.1 yr⁻¹. Two independent protection layers have PFDs of 0.1 and 0.01. The mitigated frequency is 0.1 × 0.1 × 0.01 = 1×10⁻⁴ yr⁻¹. If the target is 1×10⁻⁵ yr⁻¹, the gap is a factor of 10, so an additional independent layer or a design change is needed. The reasoning: LOPA multiplies PFDs only when the layers are truly independent; common-cause failure can invalidate the multiplication.
Traps
Treating a procedure as an IPL without verifying independence, confusing hazard with risk, relying on relief alone for runaway control, and using inconsistent risk-matrix scales.