Covers the design and operation of fuel gas and dual-fuel supply systems, including conditioning requirements, pressure control, and changeover procedures. Knowledge of the fuel system is prerequisite to safe burner operation and upset response.
Fuel Gas Supply System
Fuel gas arrives at the heater from a refinery header at controlled pressure. Before reaching the burners, it passes through a conditioning circuit designed to protect burner tips from liquids and maintain stable pressure.
Source
Refinery Fuel Gas Header
Typical: 2–6 bar(g)
→
Conditioning
KO Drum / Liquid Separator
Removes condensate & entrained liquid
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Control
Pressure Control Valve (PCV)
Maintains local supply pressure
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Safety
Block & Bleed Valves
Per BMS requirements
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Delivery
Burner Supply Manifold
Individual burner tips
KO Drum Function
The knockout drum (condensate separator) removes liquids that condense in the fuel gas header due to temperature drop or pressure swings. It is sized to give sufficient residence time for gravity separation. Liquid level is typically controlled via a level control valve (LCV) discharging to a closed drain system.
KO drum level should be monitored during operations. A high-level alarm indicates abnormal liquid accumulation — investigate the cause before it reaches the burners.
Liquid Carryover — Why It's Dangerous
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Liquid carryover into burner tips is a primary cause of flame instability and explosion risk
Liquid hydrocarbons reaching burner tips vaporise rapidly and release significantly more energy per unit volume than gas. This can cause sudden flame enlargement, burner tip erosion, and in severe cases, detonation within the firebox.
Conditions that promote liquid carryover:
Upstream pressure surges driving condensate through the KO drum
KO drum high level — liquid migrates past the outlet
Cold weather — ambient cooling of exposed pipework between drum and heater
Feed composition upsets increasing heavier hydrocarbon fractions in fuel gas
KO drum drain valve failed closed — liquid accumulates undetected
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Warning — KO drum drain frequency
Drain the KO drum at the required frequency per site procedure. In cold conditions or following upstream process upsets, increase drain frequency. Never rely on level alarms alone — level transmitters can stick or fail.
Fuel Gas Pressure Control
Stable burner operation requires stable fuel gas supply pressure. The PCV downstream of the KO drum maintains a setpoint, typically in the range of 0.5–2.0 bar(g) at the burner manifold, depending on burner design.
Fuel Gas Pressure — Typical Operating Bands
Condition
Pressure (bar g)
Alarm / Action
Consequence if Unchecked
Normal operating
0.8–1.5
—
—
Low pressure alarm
< 0.5
Investigate; reduce firing rate
Burner flame instability, potential flame-out
Low-low pressure trip
< 0.25
BMS trips fuel gas supply
Flame-out and firebox gas accumulation risk
High pressure alarm
> 2.0
Check PCV, throttle if required
Burner tip overload, increased NOx, flame instability
High-high pressure trip
> 2.5
BMS trips fuel gas supply
Flame impingement, refractory damage, tube damage
Low Pressure Response
Falling Fuel Gas Pressure
Check header pressure at battery limit
Verify PCV position — not stuck open
Check for blockage at strainer / filter
Reduce firing rate to match available pressure
If pressure drops below low-low setpoint, prepare for BMS trip
Do not allow continuous operation above high alarm
Report PCV fault to maintenance
Dual-Fuel Systems
Many refinery heaters are equipped to fire both fuel gas and fuel oil. This provides operational flexibility and security of supply when one fuel type is unavailable or uneconomical.
Fuel Type
Fuel Gas
Atomising:
Not required
Ignition:
Easier — pilot dependent
Control:
Rapid response, precise
Emissions:
Lower NOx, negligible particulates
Liquid risk:
Carryover (KO drum)
Fuel Type
Fuel Oil
Atomising:
Steam required — 3–6 bar(g)
Ignition:
Requires pre-warming of tips
Control:
Slower response, viscosity-dependent
Emissions:
Higher NOx, SO₂, particulates
Tip fouling:
Carbon deposits — more maintenance
Atomising Steam
Fuel oil burners require atomising steam to break the oil into a fine mist for efficient combustion. Steam pressure must be maintained above the minimum required by the burner design — typically 3–6 bar(g) . Loss of atomising steam while on fuel oil will result in poor combustion, sooting, and potentially flame-out.
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Warning — atomising steam pressure
If atomising steam pressure falls below minimum during fuel oil firing, the operator must immediately reduce fuel oil rate or initiate changeover to fuel gas. Continued firing with inadequate atomisation causes soot deposition on tubes and refractory, and risks flame-out.
Fuel Changeover Procedures
Changeover between fuel types must be executed methodically. Both fuels must never be introduced simultaneously through a tip designed for single-fuel operation — check the specific burner design before starting.
Fuel Gas → Fuel Oil Changeover
Type: Normal OperationsSteps: 8
01
Confirm fuel oil system is ready
Fuel oil at correct temperature and viscosity. Circulation pump running. Atomising steam available at ≥ minimum design pressure.
02
Warm through fuel oil tips
Introduce steam briefly through oil tips to preheat and purge the tip body. This prevents cold oil from extinguishing the flame on introduction.
03
Introduce fuel oil on one burner — verify ignition before proceeding
Introduce fuel oil on the first burner. Confirm stable flame visually (observation port) before switching any additional burners. Do not proceed if flame is unstable.
04
Transfer remaining burners one at a time
Switch each burner from gas to oil in turn. Confirm flame stability after each switch before proceeding to the next burner.
05
Adjust air registers for fuel oil combustion
Fuel oil requires more combustion air than gas for equivalent duty. Open air registers and adjust excess O₂ to site target. Monitor flame colour — a clean oil flame is bright orange with minimal smoke.
06
Close fuel gas supply to burners
Once all burners are confirmed stable on oil, close the fuel gas block valve to the burner manifold. Leave pilot gas supply open if pilot burners remain lit.
07
Purge fuel gas from burner manifold
Vent manifold pressure to safe disposal. Confirm manifold is depressured before any maintenance on gas-side components.
08
Monitor and confirm stable operation
Check COT, stack O₂, atomising steam pressure, and fuel oil flow over the first 15 minutes following changeover. Log the changeover event in the shift log.
Fuel Oil → Fuel Gas Changeover
Type: Normal OperationsSteps: 7
01
Confirm fuel gas supply is available and at correct pressure
Verify KO drum level is normal. Confirm fuel gas supply pressure within operating range at the heater battery limit.
02
Confirm KO drum has been drained — no liquid carryover risk
Drain KO drum to confirm liquid-free supply. Do not introduce fuel gas to burners until this is verified.
03
Light gas on one burner — confirm stable flame
Introduce fuel gas on the first burner. Pilot must be proven. Confirm stable flame before proceeding.
04
Transfer remaining burners from oil to gas, one at a time
Switch each burner individually. Confirm stable gas flame before closing oil supply on that burner.
05
Close fuel oil supply and steam atomising valves
Once all burners confirmed stable on gas, close the fuel oil block valve. Close atomising steam to all oil tips.
06
Steam-purge fuel oil tips
Pass steam through the oil tips briefly to clear residual oil. If not purged, residual oil will carbonise and block tips on next use.
07
Adjust air registers and confirm steady-state
Gas firing requires slightly less air than fuel oil. Trim air registers. Monitor stack O₂ and COT. Log the changeover in the shift log.
Fuel Gas Composition Variability
Refinery fuel gas header composition is not constant. It receives contributions from multiple process units — crude distillation, catalytic reforming, hydrocracking, coking — and composition shifts with unit throughput and operating mode.
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Wobbe Index
The Wobbe Index (WI) is a measure of fuel gas interchangeability — it combines calorific value and specific gravity. Burners are designed for a range of Wobbe Index values. Large shifts in WI require burner tip changes or orifice adjustments. A sudden change in header composition (e.g., from a unit trip) can cause the WI to shift rapidly, altering the heater heat release without any change in control valve position.
Operators should be alert to unexplained COT changes, flame appearance changes, or O₂ fluctuations that could signal a composition shift. Coordination with the control room on upstream unit events is essential.