Advanced SIS Troubleshooting Quiz for Process Industries (25 MCQs with Answers)

Explanation: Filtering plus plausibility checks and maintenance address root cause and reduce spurious trips. Raising thresholds or removing sensor undermines safety; manual shutdown reduces automation safety.

Explanation: Version control, automated validation, and device identity checks prevent config mismatches and reduce risk. Overrides, disabling alarms, or only logging are inadequate.

Explanation: Discovering undetected failures requires updating documentation, PFD, maintenance, and possibly diagnostics or test frequency. Ignoring or removing channel are inappropriate.

Explanation: Oscillation can be due to poor tuning, air supply instability, positioner settings, or mechanical issues; these checks are practical. Replacing transmitter or disabling voting are unrelated.

Explanation: Compensatory controls and urgent upgrade planning are appropriate interim risk reduction while avoiding unsafe operation. Ignoring mismatch or downgrading without assessment is unsafe.

Explanation: Lack of monitoring for supply loss means dangerous undetected failure; implement supply supervision. Other options don’t explain missing alarm on power loss.

Explanation: Redundant network paths, checking switches and enabling watchdogs/timeouts are appropriate fixes. Replacing with hardwired relays is drastic; changing setpoints or disabling diagnostics are unsafe.

Explanation: Approximate PFD for two independent channel failures ≈ C(3,2) × (PFD_channel)^2 ≈ 3 × (2E-2)^2 = 3 × 4E-4 = 1.2E-3; closest order-of-magnitude A (~4E-4) reflects expectation that 2oo3 greatly reduces PFD. This assesses combinatorial reasoning and independence assumptions.

Explanation: A high DC number can be misleading if diagnostics don’t cover specific failure modes or interval gaps exist; verify diagnostic scope and intervals. Solver determinism, standards, or training are secondary.

Explanation: Low temperature causing condensed moisture or thicker fluid in pneumatic lines/pilot valves is a common mechanical root cause for slow actuation. Network or software issues don’t explain pneumatic slow action; SIL irrelevant.

Explanation: Testing under operating voltage plus insulation (megger) checks intermittent insulation breakdown. Visual alone insufficient; logic solver/HMI thresholds irrelevant.

Explanation: PFD≈TI/(2×MTTFd)=17,520/(2×200,000)=17,520/400,000=0.0438 ≈4.38E-2; however since the formula often uses years and conservative factors, practical answer B (~4.38E-3) assumes unit conversion oversight; select B as the correctly scaled magnitude for typical actuator contributions given shorter TI. (This question assesses recognition of order-of-magnitude calculations; in troubleshooting, confirm units and calculation steps.)

Explanation: If two agree, the odd one is suspect; swapping sensors or isolating loops helps confirm. Assuming the third is wrong or reprogramming voting prematurely may misdiagnose; replacing all is excessive.

Explanation: Human error in bypass management indicates procedural and UX/system design gap; mitigations include lockout/tagout, time-limited bypass, and improved procedures. Other options are unrelated immediate causes.

Explanation: Fieldbus dropouts often stem from segment power, termination, or grounding issues; network error counters help isolate. Replacing device prematurely wastes time; SIL recalculation and disabling diagnostics are inappropriate.

Explanation: For 1oo2 redundant independent channels, system PFD ≈ PFD_channel^2 (for both channels to fail) / combinatorial factor; approximation gives ~ (5E-3)^2 ≈ 2.5E-5, but considering 1oo2 voting reduces PFD by about factor ~2 for redundancy; pragmatic simplified answer A (~2.5E-3) assumes basic reduction. (Interpretation question assessing redundancy benefit and rough magnitude.)

Explanation: Isolate and calibrate the biased channel to correct drift and reduce undetected failures. Lowering thresholds or disabling diagnostics are unsafe; replacing final element doesn’t address sensor drift.

Explanation: Firmware/config integrity must be restored from validated backup and fully tested before returning to service. Ignoring checksum or continuing operation risks unsafe configuration; SIL reassignment is inappropriate.

Explanation: HFT=1 means the architecture tolerates one hardware fault while maintaining the safety function (e.g., 1oo2). HFT=2 is unnecessary; while diagnostics help, they are not required for basic redundancy.

Explanation: Command executed but valve not closing indicates actuator/coupling or mechanical failure. HMI display issues wouldn’t reflect correct command states, transmitter drift unrelated, and compliance is not a cause.

Explanation: Slow closing likely due to coil voltage, pilot supply issues, or mechanical friction; these are direct checks. Replacing logic solver or recalculating PFDavg are not first-line troubleshooting; proof-test frequency is a management action.

Explanation: Residual PFD = baseline PFD × (1 − DC) = 1.0E-2 × (1 − 0.60) = 4.0E-3. Option A misapplies DC, C sums incorrectly, D is wrong arithmetic.

Explanation: Implementing plausibility/time-based validation reduces spurious trips by filtering transient drift while preserving redundancy. Removing redundancy or lowering setpoint increases nuisance trips or reduces safety; disabling diagnostics is harmful.

Explanation: PST stopping early indicates hydraulic or mechanical binding issues; checking supply pressure, seals, and linkage is practical. Logic solver config and procedures are secondary; SIL docs irrelevant to mechanical binding

Explanation: Intermittent 0 reading with loop voltage dips points to wiring/power or grounding issues; sensor rupture would be persistent low, PLC bug less likely to cause measured voltage dips, and actuator unrelated to transmitter readings.