CSP-11 CSP-11 D2.06 · Domain 2 · Program Management · 25%
System Reliability
A refinery vapor-control exhaust train must run for a 500-hour campaign. Either of two full-duty fans can carry the flow, but the flame arrestor and pressure switch are both required. A reviewer adds the fan reliabilities and calls the result “better than 100%.” You are the CSP: draw the success logic first, then calculate the mission reliability and challenge the independence assumption.
Updated . Independent CSP-11 practice. Not BCSP-endorsed. No pass guarantee.
Decision rule
Reliability is whether the function works when demanded.
MTBF is not proof of independence.
In 20 seconds
- Reliability is whether the function works when demanded.
- Reliability: availability, failure rates, redundancy, and common-cause failures.
- Series systems fail if any critical element fails; parallel improves availability if independent.
- Proof tests and MTBF/MTTR concepts appear — read definitions carefully.
How to apply System Reliability
THEORY: Availability, redundancy, common cause. Deep availability is Class 104. CALC: If λ and time are given, use the stem model only. FIELD: Two pumps, one suction. Not independent. ASSESS: Counting nameplates as reliability is the trap.
Worked case
Wrong first move: Buy a second identical pump on the same header. Correct move: Fix common cause; then quantify.
Why the rule holds
Related: 104, 23.
Same rule, different setting
A gas-handling package has two critical exhaust fans in active parallel redundancy, followed by a flame arrestor and pressure switch. One fan can carry the required flow while the other remains available. The CSP question is whether the train meets the required mission reliability and which arithmetic error most changes the answer.
A paint-process exhaust train uses two duty-capable fans in active parallel redundancy, then a flame arrestor and pressure switch in series. The field team has mixed up MTBF, lambda, and block reliability. Your job is to identify the logic and calculate the mission reliability correctly.
Picture / model
Reliability block model: [Fan A || Fan B] → [Flame arrestor] → [Pressure switch]. For independent active-parallel branches, Rparallel = 1 − ∏(1−Ri). For required series blocks, Rseries = ∏Ri. For a constant failure-rate model, λ = 1/MTBF and R(t) = e^(−λt).
Exam traps
Original SafePath contrasts for System Reliability. These are study traps, not recalled BCSP items.
| Trap | Correct read |
|---|---|
| Series | Every block must survive → multiply reliabilities |
| Active parallel | At least one independent branch works → 1 − product of branch failure probabilities |
| MTBF | Mean time between failures; under the constant-rate exponential model, lambda = 1/MTBF |
| lambda | Failure rate, with inverse-time units |
| R(t) | Mission reliability for constant failure rate = exp(−lambda t) |
Field move
Field decision: 0.9802..
Scenarios
Scenario 1 · Applied Scenario · CSP-11 D2.06
You are the CSP reviewing a live decision at an offshore fabrication night shift. Operations proposes a convenience path. Under a constant failure-rate model with λ=0.00004 h⁻¹, what is mission reliability at 500 hours?
Best action: 0.9802.
Why: Evaluate the best response. Pick the method that matches the question: combinations to a top event, deviations in a process, a task sequence, or a component failure mode. A score is not a safeguard. You are the CSP reviewing a live decision at an offshore fabrication night shift. Operations propose…
Scenario 2 · Advanced Engineering · CSP-11 D2.06
Dual fans have R1 = 0.98 and R2 = 0.97. The flame arrestor has R = 0.99 and the pressure switch has R = 0.98. Both fans are capable of carrying the required exhaust duty. What is the full train reliability?
Best action: 0.9696 after calculating the redundant fan block, then multiplying the flame arrestor and pressure-switch reliabilities.
Why: The parallel fan block is R_p = 1 − (1−0.98)(1−0.97) = 1 − 0.02×0.03 = 0.9994. The train is 0.9994 × 0.99 × 0.98 = 0.969618, reported as 0.9696. Multiplying the fans as series blocks, or ignoring the downstream devices, uses a different architecture.
Scenario 3 · Applied Scenario · CSP-11 D2.06
A reliability model has MTBF = 10,000 h and the mission is 1,000 h. Which pair is closest?
Best action: lambda = 0.0001 h⁻¹ and R(1000) ≈ 0.9048.
Why: Lambda = 1/MTBF = 1/10,000 = 0.0001 per hour. For the exponential model, R(1000) = exp(−0.0001 × 1000) = exp(−0.1) = 0.904837, about 0.9048. Using MTBF as lambda inverts the rate.
If the stem says this
| Stem signal | Best direction |
|---|---|
| shared utility feeds both redundant trains | treat as common-cause — not fully independent |
| proof test overdue | restore proof testing |
| series safety chain | any link failure fails the function |
| two answers work | higher hierarchy / system / design |
| hazard can be eliminated | prefer elimination over more redundancy alone |
Check before you leave
- ✓ Reliability: availability, failure rates, redundancy, and common-cause failures.
- ✓ Series systems fail if any critical element fails; parallel improves availability if independent.
- ✓ Proof tests and MTBF/MTTR concepts appear — read definitions carefully.
- ✓ Independence assumptions fail with shared utilities/humans.
- ✓ Design redundancy for critical safety functions.
- ✓ Maintenance quality drives achieved reliability.
Blueprint mapping
System Reliability is the SafePath study class for CSP-11 D2.06 inside CSP-11 Domain 2, Program Management (25%). A high-weight management domain focused on measuring performance, investigating incidents, managing change, building systems, leading people, controlling records and resources, and interpreting data.
SafePath uses the publicly posted CSP-11 outline as a study map. It does not reproduce official exam content, and it is not affiliated with or endorsed by BCSP or Pearson VUE.
Practice / flashcards / formulas
More classes in Program Management
Nearby lessons mapped to the same CSP-11 domain.
CSP-11 D2.06
Other classes on this task
Guides that use System Reliability
- Oil and gas safety study guideProcess safety, change, and investigations
- CSP application and exam guideEligibility, scheduling, and exam method
- CSP-11 exam prep guideDomain weights, sample items, and a study sequence
- CSP-11 blueprint and weightsSeven published domains and the class map
- Seven-domain study directoryScope, tasks, and links into each lesson
System Reliability study questions
- What decision does the System Reliability class train?
- Reliability is whether the function works when demanded. This is independent SafePath CSP practice for CSP-11 D2.06. It is not an official BCSP item and it is not a pass guarantee.
- What exam trap shows up in System Reliability?
- A common miss treats this as sufficient: Series The stronger read is: Every block must survive → multiply reliabilities
- How does System Reliability map to the CSP-11 blueprint?
- SafePath maps this class to CSP-11 D2.06 in Domain 2, Program Management (25% of the published blueprint). Use the public outline for study planning. SafePath is not affiliated with or endorsed by BCSP.
Verify
Verify: Confirm current certification requirements with BCSP and technical requirements with the cited authoritative standard.