The function
Prevents accumulation of unburnt fuel gas in the firebox after a flame-out, which could ignite explosively on the hot refractory or at re-light.
Safe state: Both fuel gas shutdown valves closed, vent between them open, pilot gas isolated.
Process safety time: 4 s (flame-out to an explosive mixture at the burner, vendor data). Required response: ≤ 3 s including scanner response. Setpoint: Loss of flame on either scanner (1oo2 trip voting); scanner flame failure response time ≤ 1 s.
Where the target comes from (LOPA)
Burner flame-out with fuel gas valves open → unburnt fuel accumulates in the firebox → explosion on re-ignition.
| Item | Value | Note |
|---|---|---|
| Initiating event frequency | 0.1 per year | Flame instability and fuel pressure upsets, site history (illustrative) |
| Probability of delayed ignition producing an explosion | 0.5 | |
| Probability of personnel at the heater | 0.1 | |
| Tolerable frequency | 1e-5 per year | Single fatality (illustrative) |
Mitigated event frequency without the SIF: 5.0 × 10⁻³ per year. Required risk reduction factor: 500. Required PFDavg: 2.0 × 10⁻³. Target: SIL 2.
Inputs (illustrative values)
| Group | Voting | λDU /h | λDD /h | λSU /h | β | T1 | PTC | Partial stroke | Type | SC |
|---|---|---|---|---|---|---|---|---|---|---|
| Flame scanners BE-1301A/B | 1oo2 | 3e-7 | 1.5e-6 | 4e-7 | 10 % | 1 yr | 90 % | – | B | SC 3 |
| Burner management logic solver | 1oo1 | 5e-9 | 1e-6 | 0 | 0 % | 10 yr | 99 % | – | B | SC 3 |
| Fuel gas SSOVs XV-1301/1302 | 1oo2 | 6e-7 | 0 | 1e-6 | 10 % | 1 yr | 90 % | – | A | SC 3 |
Mission time 20 years, low demand (PFDavg). Failure rates are illustrative values written for First Out AI, not taken from any certificate or database.
Result (First Out AI engine v0.3.0)
| Measure | Value |
|---|---|
| PFDavg | 1.4 × 10⁻³ |
| SIL by probability | 2 |
| SIL by IEC 61511 fault tolerance | 3 |
| Systematic capability | SC 3 |
| Achieved SIL | 2 (meets SIL 2) |
| Required PFDavg | 2e-3 (met) |
| Mean time to spurious trip | 12.0 years |
| Group | Voting | PFDavg | Share | HFT / required | SFF |
|---|---|---|---|---|---|
| Flame scanners BE-1301A/B | 1oo2 | 4.0 × 10⁻⁴ | 27 % | 1 / 0 | 86 % |
| Burner management logic solver | 1oo1 | 2.3 × 10⁻⁴ | 16 % | 1 / 0 | 100 % |
| Fuel gas SSOVs XV-1301/1302 | 1oo2 | 8.2 × 10⁻⁴ | 57 % | 1 / 0 | 63 % |
Engineer's reading
- Fuel gas SSOVs XV-1301/1302 carries 57 % of the PFDavg, so improvements there matter most.
- With a perfect proof test (the Free plan assumption) the PFDavg would be 6.3 × 10⁻⁴; the difference to 1.4 × 10⁻³ is the effect of the realistic proof test coverage.
Open this example in First Out AI: SIL verification → Examples → "Fired heater flame failure: close fuel gas double block".
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.