Safety Systems, Relief Devices, and Risk Assessment
What this domain tests
Safety-system questions cover relief device selection and sizing, overpressure scenarios, HAZOP/LOPA thinking and RBI. You must distinguish set pressure, MAWP, accumulation and backpressure.
Core rules and concepts
- A pressure relief valve set pressure must not exceed MAWP. Non-fire accumulation is usually 10% of MAWP; fire case may allow 21% for vessels under API 521 rules.
- API 520/521 sizing needs relieving rate, fluid properties, orifice coefficient, backpressure correction and inlet/outlet line losses. Excessive built-up backpressure reduces capacity.
- Inlet line loss should generally stay below 3% of set pressure for conventional valves; otherwise use a pilot-operated or balanced valve.
- Rupture discs are fast but do not reseat; they may be combined with a relief valve.
- HAZOP identifies deviations; LOPA tests whether independent protection layers reduce risk enough; RBI prioritizes inspection by probability and consequence.
Worked example
Vessel MAWP = 1.20 MPa. Proposed PSV set pressure = 1.05 MPa. Non-fire allowable accumulation is 10% of MAWP above MAWP, so the vessel limit is 1.20 x 1.10 = 1.32 MPa. The set pressure is below MAWP, so it is acceptable, but the valve must have enough capacity before 1.32 MPa. Inlet loss limit = 3% of 1.05 = 0.0315 MPa. If calculated inlet loss is 0.05 MPa, the conventional valve is unacceptable.
Traps that cost marks
Setting PSV pressure above MAWP; applying 10% to set pressure instead of MAWP; ignoring built-up backpressure; missing the fire-case accumulation rule; using a rupture disc without checking reseating and downstream effects.