The function
Prevents liquid entering the compressor if the scrubber level control fails. Liquid slugs can break the compressor internals and lead to a gas release at the seals or casing.
Safe state: Compressor motor stopped and suction shutdown valve closed.
Process safety time: 120 s from LAHH to liquid at the compressor inlet nozzle at maximum inflow. Required response: ≤ 40 s. Setpoint: LAHH at 75 % of the scrubber height, 2oo3.
Where the target comes from (LOPA)
Scrubber level control valve fails closed → liquid carry-over into the compressor → mechanical failure, seal or casing rupture, gas release.
| Item | Value | Note |
|---|---|---|
| Initiating event frequency | 0.1 per year | BPCS control loop failure (illustrative) |
| Probability of ignition | 0.3 | |
| Probability of personnel in the compressor house | 0.3 | |
| Operator response to high level alarm from an independent transmitter on the alarm system | 0.1 | Separate transmitter and alarm panel; 20 min available; procedure trained |
| Tolerable frequency | 1e-5 per year | Company tolerable frequency for a single fatality (illustrative) |
Mitigated event frequency without the SIF: 9.0 × 10⁻⁴ per year. Required risk reduction factor: 90. Required PFDavg: 0.011. Target: SIL 1.
Inputs (illustrative values)
| Group | Voting | λDU /h | λDD /h | λSU /h | β | T1 | PTC | Partial stroke | Type | SC |
|---|---|---|---|---|---|---|---|---|---|---|
| Level transmitters LT-1201A/B/C | 2oo3 | 1.5e-7 | 7e-7 | 1e-7 | 5 % | 1 yr | 90 % | – | B | SC 3 |
| Safety PLC | 1oo1 | 5e-9 | 1e-6 | 0 | 0 % | 10 yr | 99 % | – | B | SC 3 |
| Motor trip (two contactors in series) | 1oo2 | 1.5e-7 | 0 | 3e-7 | 10 % | 1 yr | 99 % | – | A | SC 3 |
| Suction SDV XV-1201 | 1oo1 | 5e-7 | 0 | 1e-6 | 0 % | 1 yr | 85 % | – | 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 | 8.8 × 10⁻³ |
| SIL by probability | 2 |
| SIL by IEC 61511 fault tolerance | 2 |
| Systematic capability | SC 3 |
| Achieved SIL | 2 (meets SIL 1) |
| Required PFDavg | 1.1e-2 (met) |
| Mean time to spurious trip | 20.3 years |
| Group | Voting | PFDavg | Share | HFT / required | SFF |
|---|---|---|---|---|---|
| Level transmitters LT-1201A/B/C | 2oo3 | 1.1 × 10⁻⁴ | 1 % | 1 / 0 | 84 % |
| Safety PLC | 1oo1 | 2.3 × 10⁻⁴ | 3 % | 1 / 0 | 100 % |
| Motor trip (two contactors in series) | 1oo2 | 7.9 × 10⁻⁵ | 1 % | 1 / 0 | 67 % |
| Suction SDV XV-1201 | 1oo1 | 8.4 × 10⁻³ | 95 % | 0 / 0 | 67 % |
Engineer's reading
- Suction SDV XV-1201 carries 95 % of the PFDavg, so improvements there matter most.
Open this example in First Out AI: SIL verification → Examples → "Compressor suction scrubber high-high level: trip compressor".
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.