Module 03 · Abnormal Operations
Upset Recognition
How to identify abnormal conditions before they escalate — using instrumentation, visual observation, and process trends. Covers the most common upsets encountered in day-to-day operation.
Escalation principle
Every significant incident on a fired heater was preceded by recognisable warning signs. The value of this page is in training the operator to identify the early signal — not just respond to the alarm. If something looks wrong, it probably is.
First principles
A fired heater upset is any deviation from normal operating conditions that, if left uncorrected, will damage equipment, harm personnel, or cause unplanned shutdown. Upsets can originate from the process side (flow, composition, pressure), the fireside (fuel supply, combustion, draft), or the mechanical / safety system side (instrumentation, valves, BMS).
Recognition has three layers. Instrumentation gives the numbers — trends matter more than single readings. Visual observation at the heater gives information that no instrument captures: flame colour and shape, smoke colour, refractory condition, sounds and smells. Process context tells you whether what you're seeing is expected given recent changes to feed rate, fuel composition, or ambient conditions.
An operator who only watches the DCS screen will be slower to recognise upsets than one who also makes regular field rounds.
Quick-scan indicators — field round priorities
During every field round, check these in sequence. Each takes seconds. Together they give a complete picture.
Stack colour
Clear to pale grey = normal. Black smoke = unburned carbon. White/grey haze = poor atomisation or excess water/steam injection.
Draft gauge
Should read negative (below zero). Positive or zero = immediate investigation. Cross-check at multiple elevations if fitted.
Peephole — flame
Blue/blue-orange = correct excess air. Long lazy yellow = rich, low air. Short lifting flame = excess air / fuel pressure low.
O₂ analyser
Target typically 2–4% in stack. Below 1% = risk of CO, smoke, tube overheating. Above 6–8% = excess air, efficiency loss.
Bridgewall temp
Rising BWTemp with no firing rate increase = early sign of coking or maldistribution. Sudden drop = possible flame loss.
COT / TMT
Coil outlet or tube metal temperature. Any reading above design limit requires immediate reduction in firing. Check spread between passes.
Process flow
Low flow is the single most dangerous condition for tube integrity. Any unexplained flow reduction must be investigated and corrected immediately.
Fuel pressure / flow
Instability suggests supply issues, slugs of liquid, or regulator failure. Correlated with flame instability.
Upset severity overview
The table below groups common upsets by severity. Severity is defined by how quickly the condition can cause injury, tube failure, or uncontrolled fire if not corrected — not by how frequently it occurs.
Upset
Severity
Time window to act
Loss of process flow
High
Minutes — tube failure risk is rapid once flow stops
Positive firebox pressure
High
Immediate — burn hazard to any personnel near openings
Tube metal temperature exceeding limit
High
Minutes to hours depending on degree of exceedance
Fuel gas liquid slug
High
Sudden — flameout and firebox flooding risk
High excess air / O₂
Medium
Hours — efficiency degradation, flame instability
Low excess air / black smoke
Medium
Hours — CO risk, environmental breach, tube fouling
Uneven pass outlet temperatures
Medium
Hours — indicates maldistribution or partial coking
Unstable / noisy combustion
Medium
Monitor closely — can escalate to flameout
Gradual bridgewall temp rise
Low–Med
Days to weeks if slow — review trend and adjust
Soot in convection section
Low–Med
Days — stack ΔT rises, efficiency falls; schedule sootblowing
Detailed upset profiles
High Severity
Loss of process flow — low / no flow through coil
How to recognise
- Low flow alarm on pass flow indicators
- Coil outlet temperature rising with no increase in firing
- Tube metal thermocouple readings rising
- Differential pressure across coil changes unexpectedly
- Downstream unit starved of feed
Causes
- Feed pump failure or cavitation
- Control valve failure (closed position)
- Upstream vessel empty / level low
- Blocked strainer on suction
- Inadvertent valve closure by operations or maintenance
Immediate response
- Reduce firing immediately — do not wait to confirm cause
- Restore flow via backup pump, bypass, or alternative feed path
- If flow cannot be restored within minutes: initiate controlled shutdown or ESD
- Monitor TMT closely throughout
- Do not allow TMT to reach or exceed design limit
Danger — low / no flow
A fired heater with no process flow and burners still firing will fail in minutes. The coil is heated by both radiation and convection with no fluid to absorb the heat. Reduce firing the moment low flow is confirmed — do not wait for the cause to be identified.
High Severity
Positive firebox pressure
How to recognise
- Draft gauge reads zero or positive
- Flames / hot gas visible at burner front openings or peepholes
- Smoke or heat discharge from casing seams or inspection doors
- Burner management system draft alarm (if fitted)
- Unusual sound — roaring or pulsing at firebox openings
Causes
- Stack damper closed unexpectedly or failed shut
- ID fan trip (forced-draft / induced-draft heaters)
- Convection section heavily fouled / plugged
- Sudden large increase in firing rate
- Cold start before adequate draft established
Immediate response
- Do not approach peepholes or burner fronts
- Reduce firing rate immediately
- Open stack damper if accessible and safe from a distance
- Start ID fan if applicable and safe to do so
- If firebox remains positive: initiate controlled shutdown
- Isolate area — positive firebox is a burn hazard to all nearby
High Severity
Tube metal temperature (TMT) above design limit
How to recognise
- TMT thermocouple high alarm on DCS
- Infrared gun reading on tube surface above limit during round
- Tube glowing visibly red / orange through peephole (severe)
- Rising trend despite constant or reducing firing rate
- Spread between pass outlet temperatures widening
Causes
- Coke deposition reducing internal heat transfer
- Low process flow through affected pass
- Flame impingement on tubes
- Burner maldistribution — uneven heat release
- Loss of refractory directing flame toward tubes
Immediate response
- Reduce firing rate to bring TMT within limits
- Check and restore process flow if low
- Inspect burners for flame impingement through peephole
- Redistribute firing load if multi-pass (reduce affected burner zone)
- If TMT cannot be controlled: shutdown and investigate
- Log all readings — tube life assessment required post-event
High Severity
Fuel gas liquid slug / condensate carryover
How to recognise
- Sudden erratic fuel pressure fluctuation
- Burner flames surge or pulsate then flameout
- BMS flame detector loss alarm (one or more burners)
- Unusual sound at burner fronts — liquid discharge noise
- Fuel gas KO drum level rising without drainage
Causes
- KO drum high level / liquid not being drained
- Failure of fuel gas scrubber or separator
- Cold weather — condensation in fuel lines
- Composition change in refinery fuel gas system
- Valve or drain failure on fuel knock-out system
Immediate response
- Reduce firing rate — do not attempt to re-light during slug
- Drain KO drum immediately
- Monitor for multiple burner flameouts
- If multiple burners out: ESD / fuel isolation — risk of firebox flooding with unburned fuel
- Do not re-light until fuel quality is confirmed stable
Medium Severity
Low excess air — incomplete combustion / black smoke
How to recognise
- Black or dark grey stack smoke visible from ground
- O₂ analyser reading below 1–2%
- CO analyser high reading (if fitted)
- Flames appear long, lazy, yellow-orange with luminous tips
- Soot accumulating in convection section over time
Causes
- Air registers closed too far
- Stack damper over-closed reducing total airflow
- Fuel gas composition change — higher heating value
- Air supply obstruction (debris at air registers)
- Firing rate increased without adjusting air
Immediate response
- Open air registers on affected burners gradually
- Open stack damper to increase draft if needed
- Reduce firing rate if smoke persists
- Check fuel gas analysis / BTU value if composition suspected
- Do not accept sustained black smoke — environmental and tube fouling risk
Medium Severity
Uneven pass outlet temperatures — coil maldistribution
How to recognise
- One or more pass outlet temps significantly higher than average
- High-temperature pass shows rising trend over days/weeks
- Flow balancing valves may show changing differential
- Infrared inspection shows localised hot spots on tube runs
Causes
- Partial coking in one pass — increased resistance, reduced flow
- Pass flow control valve or restriction orifice issue
- Flame impingement localised to one area of radiant section
- Fouling at inlet distributor
Immediate response
- Re-balance pass flows using manual valves if available
- Reduce overall firing to protect hot pass
- Review trend data — rate of temperature divergence indicates coking rate
- If coke buildup suspected: plan decoking within operating window
- Notify process engineer — may require throughput reduction
Trend recognition — reading the DCS history
Most serious incidents on fired heaters are preceded by gradual trends that are visible days or weeks before a crisis. The following patterns are worth knowing.
DCS Trend Patterns and What They Indicate
| Trend pattern observed | Most likely cause | Action |
|---|---|---|
| Bridgewall temperature creeping up over days while firing rate is constant | Convection section fouling (soot/scale) — reduced heat extraction | Schedule sootblowing; increase frequency of visual checks |
| Stack O₂ rising while air register positions and damper are unchanged | Air in-leakage — casing crack, failed refractory, open seal | Physical inspection of casing, seals, and sight glass areas |
| Fuel flow required to hold target COT increasing over weeks | Coking in radiant coil — more firing needed to achieve same outlet temp | Monitor closely; plan decoking outage before tube damage occurs |
| Outlet temperature of one pass rising while others remain stable | Partial coking in that pass — flow reducing | Re-balance flows; reduce firing; plan offline decoking of affected pass |
| Fuel pressure instability at a single frequency / regular pulsing | Pressure regulator hunting or fuel gas KO drum filling | Check regulator; drain KO drum; sample fuel gas for liquid |
| Firebox pressure trending toward zero over hours | Stack damper drifting closed, soot buildup, or draft fan issue | Check and correct damper; inspect stack; investigate fan |
Visual and auditory indicators — field observation
These are what the operator sees and hears during a field round. No DCS alarm covers most of these. They are only caught by an operator who is present and looking.
Field Observation Reference
| What you observe | Normal / abnormal | Investigate if |
|---|---|---|
| Stack colour | Clear to pale grey — normal | Black smoke (incomplete combustion) or white steam plume (liquid in fuel or steam system issue) |
| Flame colour through peephole | Blue with orange tips — normal for gas fuel | Fully yellow/luminous = rich; short blue lifting = very lean or low fuel pressure; intermittent = instability |
| Flame length and geometry | Consistent, symmetrical across similar burners | One burner significantly longer or shorter; any flame contacting tubes |
| Casing temperature (hand check) | Warm but touchable in insulated areas | Hot spot that cannot be held — refractory failure behind casing at that point |
| Combustion sound | Steady low roar | Pulsating, rumbling, or popping — combustion instability; high-frequency hum — possible pressure oscillation |
| Smell near heater | Minor combustion odour acceptable | Strong hydrocarbon smell = process leak; rotten egg / mercaptan = fuel gas leak at connections |
| Refractory visible through peephole | Even white/cream colour | Dark or black patches (soot/carbon deposits); any cracking or fallen pieces |
| Tubes visible through peephole | Uniform dull-red to black at operating temp | Any tube glowing bright orange or red — immediate action; localised bright spot = impingement or coke hot spot |
Peephole discipline
Always approach a peephole from the side, not head-on, and confirm draft is negative before opening. Open slowly. Close the plug immediately after inspection — open peepholes introduce false air and affect combustion readings. Do not look into a peephole without appropriate face and eye protection.