Lab 19: Burner Pre-Purge and Ignition Permissive

Application Scenario

A gas-fired boiler in a factory heats water for the whole plant. The most dangerous moment in a burner's life is ignition. If unburned gas has collected in the firebox — from a leaking valve, or from a failed ignition attempt — lighting the burner can cause an explosion. Burner control codes (such as NFPA 85 and NFPA 86), therefore, require a strict start-up sequence:

1. Pre-purge: the combustion-air fan blows fresh air through the firebox for a set time, to sweep out any gas. No gas is allowed during the purge.
2. Trial for ignition: after the purge, the gas valve and igniter are energized for a limited time. A flame sensor must detect (prove) the flame within that time.
3. Run: if the flame is detected, the burner runs for as long as the flame is present.
4. Safety shutdown: if no flame is proven by the end of the trial, or if the flame is lost later, the gas valve closes, and the burner shuts down. Every new start begins again with a full purge.

In this lab (times are shortened for the lab):

Learning Outcomes

After this lab you will be able to:

• Build a timed start-up sequence with two timers working together.
• Use an on-delay timer as a permissive (no gas until the purge is complete).
• Use an interval timer as a limited-time window in which a proof signal must appear.
• Explain why a safety shutdown must lead to a completely new sequence, not a quick retry.

Background

How does the circuit sequence? The whole sequence is controlled by one burner RUN relay, CR1, and two timers:

• TR3 (interval, 10 s) is energized as soon as START is pressed. Its N.O. contact TR3A is closed for the first 10 s. TR3A is in series with the CR1 coil, and the flame sensor PB3 is in parallel with TR3A. So CR1 can stay ON for the first 10 s without a flame — but after 10 s, only a proven flame keeps it ON.
• TR1 (on-delay, 6s) starts when CR1 turns ON. Its N.O.T.C. contact TR1A opens the gas valve only after the purge is complete.

So the gas valve is ON from 6 s to 10 s (the ignition trial), regardless of whether there is a flame. After 10 s, the gas valve stays ON only because CR1 stays ON, and CR1 stays ON only if PB3 (flame) is held. If the flame is lost at any time after 10 s, CR1 drops, the fan and gas stop, and the seal-in opens; the operator must press START to begin a new purge.

Why is TR3's coil connected to N3? TR3A is in series with the CR1 coil, so CR1 cannot energize until TR3A has closed. TR3's coil is therefore connected before TR3A, at N3. When START is pressed, TR3 energizes first; its contact closes a moment later, allowing CR1 to pull in and seal in.

Timing Diagram

• Successful start: the fan runs for the 6 s purge; then gas and igniter come ON; the flame is proven 2 s into the trial; when the window closes at 10 s, the flame keeps the burner running.
• Failed start: no flame during the trial; when the window closes, CR1 drops out, and the fan and gas stop. The next START begins with a full purge.

Functional Requirements

1. Pressing PB1 starts the fan (CR1, G1). The gas valve (R1) stays OFF during the 6 s purge.
2. After the purge, R1 turns ON for the ignition trial.
3. If PB3 (flame) is pressed and held for 10 s, the burner continues running.
4. If there is no flame at 10 s, the burner shuts down: G1 and R1 OFF.
5. If PB3 is released while running (flame failure), the burner shuts down at once.
6. Pressing PB2 stops the burner at any time. Every new START begins with a full purge.

Ladder Diagram — Step-by-Step Drawing Guide

This design uses 7 grid lines. The order of the rungs matters for a neat drawing: draw the rungs fed from N4 before the rung fed from N3.

Step 1 — Rungs 1–3: burner RUN relay

1. Grid line 1: L1 → PB2 (N.C.) → N2 → PB1 (N.O.) → N3 → TR3A N.O. (1–3) → N4 → CR1 coil (2–7).
2. Grid line 2: CR1A N.O. seal-in (1–3) from N2 to N3.
3. Grid line 3: PB3 (N.O.), the flame sensor, from N3 to N4, in parallel with TR3A.

Step 2 — Rungs 4–6: fed from N4 (burner running)

1. Grid line 4: from N4, G1 (fan) to L2.
2. Grid line 5: from N4, the TR1 coil (2–7) to L2 (pre-purge timer).
3. Grid line 6: from N4, TR1A N.O.T.C. (1–3) → N5 → R1 (gas valve) to L2.

Step 3 — Rung 7: proving-window timer

1. Grid line 7: from N3, the TR3 coil (2–7) to L2.

Step 4 — Trace your drawing with a pencil

1. START is pressed. Put these events in order: TR3 coil ON, TR3A closes, CR1 ON, CR1A seals in, TR1 starts timing.
2. t = 3 s. Is R1 ON? Why not?
3. t = 8 s, PB3 not pressed. Is R1 ON? What keeps CR1 energized?
4. t = 11 s, PB3 was never pressed. What happens, and why does the fan stop too?
5. The burner has been running with PB3 held for 1 minute. PB3 is released. What happens? Can the burner be restarted without a new purge?

Step 5 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them, and fill in the wiring table. The N4 row is started for you.

Get instructor approval before wiring.

Key Wiring Points

1. No gas before purge: the gas valve is fed through the on-delay contact TR1A.
2. Limited trial: the burner may run without a flame only while the interval window TR3A is closed.
3. Flame supervision: after the window closes, only the flame (PB3) keeps CR1 energized. Losing the flame shuts down everything.
4. Every start is a full sequence: after any shutdown, the seal-in is open and both timers are reset.
5. Feed the window timer before its own contact (N3), or the circuit can never start.

Wiring Procedure

1. Power OFF. Set TR1 to on-delay 6 s and TR3 to interval 10 s.
2. Wire the circuit from your approved wiring table, one node at a time.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:3 to TR3:1 and CR1:3 to TR3:2 should read near 0 Ω (N3).
   • TR3:1 to TR3:3 should read high, and near 0 Ω while PB3 is pressed.
   • TR3:3 to TR1:1 should read near 0 Ω (N4).
   • TR1:1 to TR1:3 should read high (TR1A open).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Why must the fan purge the firebox before the gas valve opens? What could happen without a purge?
2. Why is the trial for ignition limited to a few seconds? What would happen if the gas valve stayed open for a long time without a flame?
3. After a failed ignition attempt, why must the next attempt start with a full purge rather than simply trying to ignite again?
4. In Test 7, the "flame" was present before any gas was admitted. What could cause a flame sensor to report a flame when there is none? What should a real burner control do in that case?
5. Real burner codes require a lockout after a failed ignition attempt: the burner cannot be restarted until someone presses RESET at the boiler. How does this lab's circuit differ, and why is a lockout safer?

Applying This Circuit Pattern

The sequence with a proving window pattern: a command starts a sequence; a permissive timer holds back the next step; a window timer limits how long the next step may run without proof; a proof signal takes over from the window.

Design question for the future: How would you add a real lockout to this circuit, so that a failed ignition latches a FLAME FAILURE lamp and blocks START until a separate RESET is pressed? Which relay and which ideas from Lab 14 would you use?

Extension: Flame Before Gas

A real burner control refuses to start if the flame sensor reports a flame before any gas has been admitted, because the sensor may be faulty. Using CR2 (and PB3 through a relay, since PB3 has only one contact), design a change that blocks START if the flame sensor is ON when START is pressed. Draw it and explain it; wire it if time allows.