PLC-Course

🏆 Capstone — Industrial Boiler Burner Start-Up Sequence

Bridges: Sprints 4 (SFC), 5 (timers), 6 (safety) — and is directly applicable to real-world burner management systems and kedelpasser-style industrial training.

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🎯 The Brief

Design and document a complete start-up sequence for a small industrial gas-fired boiler. Your deliverable is a portfolio piece: SFC + structured text + safety logic + documentation.

This is the kind of project a control engineer might present in a job interview. Take your time.

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📋 System Description

A natural-gas-fired steam boiler with the following components:

Inputs (sensors):

• PB_Start — Start push button (NO)
• PB_Stop — Stop push button (NC, fail-safe)
• PB_EStop — Emergency stop (NC, dual-channel)
• PSL_Gas — Low gas pressure switch (NO when adequate)
• PSH_Gas — High gas pressure switch (NC)
• LSL_Water — Low water level switch (NO when adequate)
• LSH_Water — High water level switch (NC)
• FT_Air — Forced-draft fan running feedback
• FS_Flame — Flame detection (UV scanner, NO when flame present)
• PT_Steam — Steam pressure transducer (4–20 mA, 0–10 bar)
• TT_Stack — Stack temperature (4–20 mA, 0–500 °C)

Outputs (actuators):

• Y_Fan — Forced-draft fan contactor
• Y_Pilot — Pilot gas valve
• Y_Main — Main gas valve
• Y_Igniter — Spark igniter
• Y_Alarm — General alarm horn
• HMI_Status — Status word to HMI

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🧭 Required Sequence

flowchart TD
    S0[Step 0: STANDBY] -->|PB_Start ∧ permissives OK| S1
    S1[Step 1: PURGE<br/>Run fan for 30s<br/>at full speed] -->|T#30s elapsed ∧ FT_Air ✓| S2
    S2[Step 2: PILOT TRIAL<br/>Open Y_Pilot<br/>Energize Y_Igniter<br/>10s window] -->|FS_Flame ✓ within 10s| S3
    S2 -->|10s timeout ∧ no flame| F1
    S3[Step 3: MAIN IGNITION<br/>Open Y_Main<br/>Igniter off after 5s] -->|FS_Flame ✓ stable 5s| S4
    S3 -->|FS_Flame lost| F1
    S4[Step 4: RUN<br/>Modulate to setpoint<br/>Monitor all permissives] -->|PB_Stop| S5
    S4 -->|Any permissive fault| F1
    S5[Step 5: POST-PURGE<br/>Close gas valves<br/>Fan runs 60s] -->|T#60s elapsed| S0
    F1[FAULT<br/>Close all valves<br/>Hold fan<br/>Sound alarm<br/>Lock out] -->|Manual reset ∧ fault cleared| S0

    style S0 fill:#94a3b8,color:#fff
    style S1 fill:#3b82f6,color:#fff
    style S2 fill:#f59e0b,color:#fff
    style S3 fill:#f59e0b,color:#fff
    style S4 fill:#22c55e,color:#fff
    style S5 fill:#3b82f6,color:#fff
    style F1 fill:#ef4444,color:#fff

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✅ Deliverables (in your /capstone/ folder of the PR)

1. README.md — system description, sequence of operations, assumptions
2. io-list.csv — every tag, type, address, fail-safe state
3. sfc-diagram.png, .svg, or a Mermaid source — the SFC drawn out
4. main.st — the SFC + actions in Structured Text
5. safety.st — separate safety POU (would normally run on a safety PLC)
6. fmea.md — at least 10 entries with severity / likelihood / detectability scoring
7. test-plan.md — how you’d FAT and SAT this system
8. reflection.md — what was hardest, what surprised you, what would change in real life

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🚦 Grading Rubric (self-assessed)

Criterion	Weight
Sequence correctness — all steps and transitions covered	20%
Safety logic — fail-safe behavior on every fault	25%
Code clarity — comments, naming, structure	15%
FMEA — coverage and quality of mitigations	15%
Documentation — could maintenance read it?	15%
Reflection — honest, specific, useful	10%

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💡 Hints

• The pilot trial is the most safety-critical step. If the flame doesn’t establish within the trial-for-ignition time (TFI), you must lock out — multiple ignition attempts with unburned gas in the firebox cause explosions.
• The pre-purge and post-purge are not optional. Five air changes through the firebox is the standard.
• Emergency stop is hardwired to the gas valves’ shutoff coils — it does not rely on the PLC alone. The PLC is informed of the E-stop state but cannot override it.
• Burner management systems in industry are usually IEC 61508 SIL 2 or SIL 3 certified. Your code is for portfolio purposes, not actual installation.

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📚 Real-World References

• EN 12952 / EN 12953 — water-tube and shell boilers
• NFPA 85 — boiler and combustion systems hazards (US)
• IEC 61508 — functional safety
• ISO 13577 — safety of industrial furnaces

This capstone is illustrative. A real installation requires a certified burner management system from a vendor like Honeywell, Siemens LMV, or Fireye — never a custom PLC program in the safety path.

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