Burner Types & Operation
The burner is the only component in a fired heater that converts fuel energy to heat. Everything else — draft, tubes, refractory — serves the burner or reacts to it. Understanding what your burners are, how they work, and what they look like when they're not performing correctly is fundamental to safe heater operation.
Burner anatomy — what every burner has
Regardless of type, every process heater burner performs the same three functions: it introduces fuel into the firebox, mixes it with combustion air, and provides an ignition zone where stable combustion can occur. The mechanical components that achieve this are consistent across designs.
| Component | Function | Operator relevance |
|---|---|---|
| Fuel gas tip / nozzle | Injects fuel into the mixing zone at the correct velocity and spray pattern | Blockage or wear changes flame shape and heat distribution. Inspect during shutdowns. Plugged tip ports cause uneven or asymmetric flame. |
| Air register | Controls the quantity of primary combustion air entering the burner | Primary air-adjustment tool. Opening increases air; closing reduces. Position should be recorded at each round. Stuck or seized registers are a common maintenance issue. |
| Burner tile (quarl) | Refractory-lined throat that shapes the flame and stabilises the combustion zone | A cracked or eroded tile destabilises flame and can allow flame impingement on surrounding refractory. Inspect at every turnaround. Visible damage warrants engineering review before restart. |
| Pilot burner | Small continuously-burning flame that provides ignition for the main burner and proves burner status to the BMS flame scanner | Pilot loss = BMS flame failure signal. Pilots should be checked during every round. If a pilot is out, the main burner is unproven — do not assume it is stable. |
| Flame scanner | UV or IR detector that confirms a flame is present and communicates status to the BMS | Scanner fouling (soot, oil mist) causes false "flame loss" signals and nuisance trips. Clean per maintenance schedule. A scanner that never trips may be failed-safe — verify during each maintenance opportunity. |
| Fuel shutoff valve (SSOV) | Automatic isolation valve on the fuel supply, operated by the BMS on flame failure or trip | Two valves in series (double-block) are typical on modern installations. Verify valve closure on BMS test. A passing SSOV allows fuel to enter the firebox during a trip — a serious hazard. |
Main burner types
Process heater burners are classified primarily by how they introduce and mix combustion air. The distinction matters operationally because it determines how you adjust the flame and what problems to expect.
Fuel gas is injected through a nozzle at sufficient velocity to entrain primary air before reaching the burner tile. Fuel and air are partially mixed before combustion begins. The remainder of the combustion air (secondary air) is drawn in around the flame by natural draft.
Flame appearance: Short, intense, blue with minimal luminosity. Premix burners burn hot and compact.
Operational characteristics: Less tolerant of fuel gas composition changes than raw-gas burners — a significant shift in fuel heating value can destabilise the flame. Noise is common at high firing rates (combustion roar is normal; sharp popping or rumbling is not).
Watch for: Flame lifting off the burner tile at high air or low fuel pressure. Flashback into the premix chamber if fuel velocity drops too low — a hazardous condition that requires immediate burner shutdown.
Fuel is injected directly into the firebox without premixing. Combustion air enters through the air register around the fuel nozzle and mixes with the fuel in the firebox itself. This is the most common burner type in natural-draft process heaters.
Flame appearance: Longer and more luminous than a premix burner — typically a yellow-orange core surrounded by a blue outer envelope. The yellow luminosity comes from incandescent carbon particles that form and burn as the flame develops. A healthy gas diffusion flame has a bright, clean, attached appearance with a defined shape.
Operational characteristics: More tolerant of fuel gas composition changes than premix burners. Air register position is the primary operator adjustment. Because mixing happens in the firebox rather than the burner, flame length responds directly to air register and draft — closing the register lengthens the flame; opening it shortens it.
Watch for: Lazy, luminous flame touching tubes (air register too closed or fuel pressure too high). Flame lifting at excessive draft. Uneven flame between adjacent burners indicating fuel distribution issues or tip blockage.
Designed to reduce nitrogen oxide (NOx) emissions by controlling flame temperature. This is achieved by staging the combustion — either staging the air (fuel-rich primary zone followed by air addition) or staging the fuel (burning a portion of the fuel in the primary flame and introducing the remainder further downstream). Both approaches create a longer, cooler flame that generates less NOx.
Flame appearance: Longer and less luminous than a conventional diffusion burner. The staged combustion creates a more distributed heat release. The flame can appear "softer" and less well-defined, which can be disconcerting if you're used to conventional burners — this is normal for this type.
Operational characteristics: Narrower stable operating range than conventional burners. More sensitive to turndown — at low firing rates, staged combustion can become unstable. Air-fuel ratio control is more critical. Low-NOx burners are typically more sensitive to fuel gas composition changes than raw-gas diffusion burners.
Watch for: Combustion instability (pulsing, rumbling) at low firing rates — this indicates the flame is approaching the stability limit and the firing rate should be increased or the air-fuel ratio adjusted. CO rise at low turndown is common and may require operating at a higher minimum firing rate than a conventional burner.
Designed to fire on fuel gas, fuel oil, or a blend of both. A central fuel oil gun (atomiser) is mounted coaxially with the gas tip. Oil is atomised using steam or compressed air before injection into the firebox. The burner can be switched between fuels during operation, though this requires a defined changeover procedure.
Fuel oil flame appearance: More luminous, yellower, and typically longer than a gas flame from the same burner at the same heat input. Good atomisation produces a clean, well-defined flame. Poor atomisation (low steam pressure, worn atomiser tip) produces a ragged, smoking flame with visible unburned droplets.
Operational characteristics: Fuel oil firing requires more active management — atomising steam pressure must be maintained, oil temperature must be within the correct viscosity range for good atomisation, and the oil gun tip must be clean and undamaged. Soot deposition in the convection section is more rapid when firing oil.
Watch for: Black smoke from oil firing — indicates poor atomisation or insufficient air. Check atomising steam pressure first. Also watch for oil gun tip erosion, which progressively worsens flame quality.
Air register operation
The air register is the operator's primary tool for controlling combustion air to individual burners. On natural-draft heaters it controls how much of the available draft is admitted to each burner. On forced-draft heaters it trims the air distribution between burners supplied by a common fan.
Air register adjustment principles
Register adjustments should be made one burner at a time, in small increments, with observation of flame response before proceeding. On a heater with many burners, adjustments to one affect the overall draft balance and therefore the air supply to adjacent burners.
Record register positions at every round. If a register has drifted from its last recorded position without operator input, this indicates a mechanical problem — a seized register, a loose locking mechanism, or thermal distortion of the register housing.
Burner turndown
Turndown ratio is the ratio of maximum to minimum stable firing rate for a burner. A burner with a 10:1 turndown can fire stably at 10% of its maximum rated capacity. This matters operationally because low-load conditions — unit startup, reduced throughput, partial-load operation — require firing at reduced rates.
At very low firing rates, flame stability decreases. The flame can become irregular, pulse, or extinguish. Rather than pushing a single burner to minimum turndown, the preferred approach is to take burners out of service as load decreases — keeping the remaining burners in their stable operating range while reducing total heat input. The BMS must permit individual burner isolation for this to be possible.
| Burner type | Typical turndown | Minimum firing consideration |
|---|---|---|
| Raw gas diffusion (natural draft) | 4:1 to 6:1 | Good stability at low rates; flame becomes very small and may lose scanner signal |
| Premix (natural draft) | 3:1 to 5:1 | Flashback risk increases approaching minimum; pressure-sensitive |
| Low-NOx staged (natural draft) | 3:1 to 4:1 | Instability and CO rise at lower end; minimum rate is typically higher than conventional burners |
| Forced draft (gas) | 8:1 to 12:1 | Better turndown due to controlled air supply; fan can modulate with load |
| Combination (gas + oil) | 4:1 to 6:1 (gas) / 3:1 to 4:1 (oil) | Oil mode has lower turndown; minimum oil rate constrained by atomiser performance |
Individual burner flame failure
A single burner losing flame is different from a firebox-wide flame-out. It is a more common event and often has a simple cause. The response, however, must be systematic — an unlit burner with fuel gas flowing to it is a hazard regardless of its size.
| Symptom / observation | Likely cause | Initial response |
|---|---|---|
| Flame out, pilot also out, low fuel pressure on that burner | Low fuel pressure — supply issue or SSOV partially closed | Close that burner's manual fuel valve immediately. Check fuel header pressure. Investigate before re-lighting. |
| Flame out, pilot in, main fuel valve open | Fuel tip blockage or damaged tip; low fuel gas pressure at tip | Close manual fuel valve. Check tip condition. Do not attempt re-ignition until cause is confirmed. |
| Flame unstable / pulsing then out | Excessive draft; air register too open; low fuel pressure | Close manual fuel valve. Check draft at that burner location. Re-light per procedure with adjusted air register. |
| BMS trips fuel — scanner loss, no visible flame loss | Fouled or misaligned scanner; scanner cable fault | Verify flame is actually out via peephole before treating as flame-out. If flame present, treat as scanner fault and notify instrument technician. Do not bypass scanner without engineering authorisation. |
What operators can observe during rounds
Burner condition deteriorates gradually. The peephole provides the most direct view of burner health available during normal operation. Operators should know their heater's normal flame appearance well enough to notice deviation.
| Observation | What it suggests | Action |
|---|---|---|
| One burner noticeably shorter or dimmer than adjacent burners at the same register position | Partial blockage of fuel tip ports — lower fuel flow to that burner | Log and report. Schedule tip inspection at next opportunity. Monitor for continued degradation. |
| Asymmetric flame — one side longer or brighter | Uneven fuel tip port condition; tip mounted off-centre; partial blockage on one side | Log and report. Uneven flame risks localised tube overheating on the long-flame side. |
| Bright white spot visible on burner tile / quarl | Tile erosion or cracking — flame is impinging on exposed refractory | Report to maintenance. Severe tile damage can allow flame to bypass the designed combustion zone. |
| Flame colour change — previously blue-orange now more yellow / lazy | Air register drift; fuel composition change; tip wear increasing port area | Check register position against last recorded. Adjust to restore normal flame appearance. Log change. |
| Pilot flame colour unusual — orange/red rather than blue | Pilot tip fouling or contamination in pilot gas supply | Monitor closely. A weak pilot increases flame-out risk. Schedule cleaning at next opportunity. |