Field Reference / Reference / Operating Limits
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Operating Limits

Consolidated normal / alarm / trip limits for all key parameters. Generic typical values — your unit's DCS setpoints and approved operating envelope always take precedence.

Important: These are generic industry-typical values for training and orientation purposes only. Always refer to your site's approved operating envelope, DCS alarm setpoints, and cause & effect matrix. Limits vary by heater duty, tube metallurgy, and process fluid. When in doubt — reduce load and call your supervisor.
Legend
Normal operating range
Alarm — operator action required
Trip / Hard limit — do not exceed
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Process Parameters
COT · Flow · Pass Balance
Parameter Normal Alarm Trip / Action Notes
Coil Outlet Temp (COT)
TAHH-xxx / TAH-xxx		 Set-point ± 5°C SP + 10°C Alarm SP + 20°C Trip Most critical process limit. Reduce firing at alarm; BMS trips fuel on TAHH.
Feed Flow — Low Low
FSLL-xxx		 Above FSLL + 15% FSL setpoint Alarm FSLL setpoint Trip Low flow = dry firing risk. Trip closes fuel SDV immediately. Confirm in field after trip.
Pass Flow Deviation < 5% between passes > 10% deviation > 20% — rebalance now Maldistribution at elevated COT causes coking in the low-flow pass within minutes.
Coil Inlet Pressure Per design envelope Significant drop Low pressure = possible tube leak Sudden drop in coil inlet pressure with no flow change is a tube failure indicator.
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Firebox Parameters
Bridgewall · Radiant Zone · Flame
Parameter Normal Alarm Trip / Action Notes
Bridgewall Temperature 750 – 870°C > 900°C Reduce firing immediately Bridgewall is the flue gas temperature leaving the radiant section. Sustained high = refractory damage.
Radiant Zone Temperature 800 – 1100°C Approaching 1100°C > 1150°C — reduce firing Peak flame zone. Exact limit depends on refractory design rating. Observe via peepholes.
Stack Temperature 150 – 250°C typical > 20°C above baseline > 300°C (fouling) Rising stack temp = convection section fouling. Each 20°C rise ≈ 1% efficiency loss.
Firebox Pressure (Draft) −0.1 to −0.25 in WC Near zero Positive pressure — stop Positive firebox = flame/hot gas escaping through seals. Immediate risk of injury. Reduce firing, check damper.
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Combustion Parameters
O₂ · CO · Excess Air · Flame
Parameter Normal Alarm Trip / Action Notes
Flue Gas O₂ (stack, dry) 2 – 4% < 1.5% or > 6% < 1% — adjust air urgently Below 2% = incomplete combustion risk (CO, soot). Above 5% = excess heat loss. Trim burner registers.
Excess Air 10 – 20% < 5% or > 30% < 2% — risk of CO buildup Excess air = (O₂% / (21 − O₂%)) × 100. Keep minimum 10% to ensure complete combustion.
CO in Flue Gas < 200 ppm > 200 ppm > 500 ppm — add air High CO = insufficient air or air distribution issue. Also check for burner tip fouling or damage.
Flame Appearance Stable, bright orange-blue Yellow tips, lazy flame Black smoke — shut down burner Yellow/lazy = rich mixture (low air). Black smoke = severe air deficiency or fuel contamination. Observe via peephole.
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Fuel Gas Parameters
Header Pressure · Pilot · Purge
Parameter Normal Alarm Trip / Action Notes
Fuel Gas Header Pressure Design setpoint ± 10% High or low deviation PAHH / PALL → BMS trip High pressure = over-firing risk. Low pressure = flame instability / flameout. BMS monitors both.
Pilot Gas Pressure Per burner spec (low pressure) Pressure drop detected Low pilot pressure → BMS trip Pilot must be stable before main fuel is admitted. Loss of pilot = loss of ignition source.
Fuel Gas Temperature Above hydrocarbon dew point Approaching dew point Liquid carryover — isolate Liquid in the fuel system causes severe flame instability and burner tip damage. Superheat minimum 15°C above dew point.
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Draft Parameters
Firebox · Stack · Fan
Parameter Normal Alarm Trip / Action Notes
Firebox Draft (at bridgewall) −0.10 to −0.20 in WC Near zero or unstable Positive — reduce load / open damper Always negative. Positive firebox means hot gas is escaping — immediate safety risk.
Stack Draft −0.3 to −1.0 in WC Less than −0.2 in WC Zero or positive — check ID fan / damper Insufficient stack draft = poor flue gas removal, CO risk, back-pressure on burners.
ID Fan Vibration < 2.5 mm/s (typical) 2.5 – 5.0 mm/s > 7.0 mm/s — reduce speed / stop High vibration = rotor imbalance (fouling, blade damage). Do not run through the trip limit.
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Tube Metal Temperature (TMT)
By Metallurgy · MAMT · Wall Thickness
Material Normal TMT Range Alarm (approach) Hard Limit (MAMT) Notes
Carbon Steel (CS) < 400°C 380 – 400°C 450°C Lowest temp limit. Rarely used in radiant section; common in convection.
5Cr – 0.5Mo < 550°C 560 – 600°C 620°C Common alloy in medium-duty radiant coils. Creep begins above 580°C.
9Cr – 1Mo < 600°C 620 – 650°C 700°C High-temp duty. Used in cracking service. Check unit datasheet for exact limit.
347 Stainless Steel < 700°C 720 – 750°C 800°C High-duty reformer/cracker tubes. Sigma phase embrittlement risk above 850°C.
Tube Wall Thickness (any alloy) > 90% of min. design wall 80 – 90% < 80% — engineer disposition required Below 80% = API 530 / API 579 hard stop. Must not restart without engineering sign-off.
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Heat-up & Cool-down Rates
COT Rate · Refractory · Startup
Operation Target Rate Maximum Rate Notes
Normal startup — heat-up rate (COT) 25 – 35°C / hr 50°C / hr Check every 15 min. Do not set TIC to target and walk away during heat-up.
After refractory repair / new refractory 10 – 15°C / hr 20°C / hr Slow dry-out required. Hold plateaus at 100°C, 200°C, 350°C. Follow refractory cure curve.
Planned shutdown — cool-down rate 25 – 50°C / hr 60°C / hr Refractory cracking risk if cooled too fast. Do not force cooling with air until below 200°C.
Online decoking — steam/air velocity Per decoking procedure Do not exceed tube design pressure Monitor COT during decoking pass. Coke combustion front = localised temperature spike.
Firebox purge (pre-ignition) ≥ 5 complete volume changes Cannot be shortened NFPA 86 minimum. BMS will not permit ignition until purge timer completes.
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