Terms, abbreviations,
and definitions.

Adjustable vanes or dampers at the base of each burner that control the volume and swirl of combustion air entering the firebox. Setting register position is the primary means of controlling excess air at individual burners. Over-opening cools the flame and raises stack temperature; under-opening causes fuel-rich combustion and CO formation.

Low-pressure steam injected into liquid fuel oil burner tips to break the oil into fine droplets, improving air-fuel mixing and combustion quality. Loss of atomising steam causes poor flame shape, smoking, and incomplete combustion. Typical pressure 2–4 barg above fuel oil pressure.

The minimum temperature at which a fuel-air mixture will spontaneously ignite without an external ignition source. Relevant to firebox safety because a firebox that is hot enough from a previous fire can ignite unburned fuel without a pilot — creating explosion risk if fuel accumulates before ignition.

The mass or volumetric ratio of combustion air to fuel supplied to a burner. At stoichiometric combustion, exactly enough air is provided to completely burn the fuel. In practice, heaters operate with excess air (AFR above stoichiometric) to ensure complete combustion. Measured indirectly via flue gas O₂ or CO analysis.

The primary industry design standard for fired heaters in the petroleum, chemical, and gas industries (American Petroleum Institute). Covers design, materials, fabrication, inspection, and testing. Referenced throughout this library for design limits and inspection criteria.

The flue gas temperature at the point where gases leave the radiant section and enter the convection section — measured at the "bridgewall" between them. BWT is a key indicator of radiant section heat release and a leading indicator of convection section fouling. Typical range 700–900°C depending on heater type. Rising BWT at constant duty may indicate radiant section or convection section fouling.

A dedicated safety instrumented system that controls the startup, shutdown, and interlock functions for heater burners. The BMS enforces the required purge, pilot-proving, and fuel trip logic. It is separate from the basic process control system (DCS) and cannot be overridden by operators during safety-critical sequences. Governed by IEC 61511 / NFPA 86.

A condition where flames or hot gases are forced backwards through the burner air register into the windbox, caused by excessive positive firebox pressure (loss of draft). Creates a burn hazard for operators approaching burner fronts and indicates a serious draft or combustion upset requiring immediate response.

The theoretical maximum radiation emitted by a perfect radiator at a given temperature. Refractory-lined firebox walls approach black body behaviour and re-radiate absorbed heat to the process tubes. This refractory re-radiation contributes significantly to total radiant heat transfer, which is why refractory condition directly affects heater efficiency.

The final orifice through which fuel gas or oil is injected into the combustion zone. Tip size (orifice diameter) determines fuel flow at a given pressure. Worn, plugged, or incorrectly sized tips cause uneven flame distribution, incomplete combustion, or reduced turndown capability. Fuel gas tips are periodically removed and cleaned or replaced.

British Thermal Unit — the quantity of heat required to raise one pound of water by 1°F. MMBtu = one million BTU, the standard unit for expressing fired heater duty in US practice. SI equivalent: 1 MMBtu/hr ≈ 293 kW. Heater duties range from a few MMBtu/hr (small treaters) to several hundred MMBtu/hr (CDU charge heaters).

Solid carbonaceous deposits that form on the inner wall of process tubes when process fluid temperature or residence time causes thermal cracking and polymerisation of heavy hydrocarbons. Coke has low thermal conductivity, so it acts as insulation — the tube wall must run hotter to maintain process outlet temperature, eventually causing tube overheating and failure. Decoking removes these deposits.

The upper section of a fired heater where flue gas from the radiant section passes across rows of closely-spaced process tubes, transferring heat primarily by convection (forced gas flow) rather than radiation. Also houses steam generation, process preheat, and combustion air preheat coils in some designs. Fouling of convection tubes raises stack temperature and reduces efficiency.

The temperature of the process fluid leaving the heater coil. The primary controlled variable in most fired heater operations. COT is set by the downstream process requirement and controlled by adjusting fuel firing rate. High COT is the most common cause of tube coking in cracking and distillation services. COT alarms and trips are a core BMS function.

Air supplied to the burners to support combustion. Consists of approximately 21% oxygen and 79% nitrogen by volume. The nitrogen passes through the firebox as an inert, diluting the flame and carrying heat to the flue gas — this is the primary mechanism of stack heat loss. See also: excess air, stoichiometric combustion.

A toxic, colourless, odourless combustion product formed when insufficient oxygen is available to fully oxidise carbon to CO₂. In a fired heater, CO in the flue gas indicates incomplete combustion — either from low excess air, poor air-fuel mixing, or flame impingement. High CO is both a safety hazard and an efficiency loss. Flue gas CO is measured continuously on modern heaters.

A fired heater that heats the process feed ("charge") to a distillation column or reactor to the required inlet temperature. The term is common in crude distillation (CDU charge heater) and vacuum distillation units. Functionally identical to any other process heater — the name describes its role in the unit rather than its design.

Corrosion of convection section tubes, stack internals, and air preheater surfaces caused by condensation of sulphuric or sulphurous acid when metal temperatures fall below the acid dew point of the flue gas. Most severe during startup/shutdown and at low-load operation. Prevented by maintaining stack temperature above approximately 140°C (sulphur fuels) or by using corrosion-resistant materials.

The negative pressure (below atmospheric) that exists inside the firebox and stack of a natural-draft heater, created by the buoyancy of hot flue gases in the stack. Draft is the driving force that draws combustion air through the burner registers and pulls flue gas through the heater. Measured in millimetres or inches of water column (mmWC / inWC). Correct draft is essential for safe, stable combustion.

A movable plate or set of vanes located in the stack or flue duct used to regulate the flow of flue gases and thereby control firebox draft. Opening the damper increases draft and airflow; closing it reduces both. Stack dampers are the primary draft control device on natural-draft heaters. A stuck or failed damper can prevent safe operation.

The process of removing coke deposits from the inside of process tubes. Two main methods: steam-air decoking (burning coke out by passing steam-air mixtures through hot tubes) and mechanical pigging (physical scraping). Decoking is a planned maintenance operation requiring careful temperature management to avoid tube damage. See the Decoking Operations procedure for full sequence.

The temperature at which acidic components in flue gas (sulphuric acid, sulphurous acid) begin to condense from vapour to liquid. For sulphur-containing fuels, the acid dew point is typically 120–150°C — substantially higher than the water dew point. Metal surfaces below this temperature are subject to cold-end corrosion. Stack temperatures are maintained above the acid dew point during normal operation.

The total rate of heat transfer from combustion gases to the process fluid, expressed in MMBtu/hr or MW. Duty = process fluid mass flow × specific heat × temperature rise across the heater. Heater duty is set by the downstream process requirement and determines the required fuel firing rate. Design duty is the maximum continuous rating; operating above this damages tubes and refractory.

The amount of combustion air supplied above the theoretical (stoichiometric) requirement, expressed as a percentage. Example: 15% excess air means 15% more air than needed for perfect combustion. Excess air is necessary to ensure complete combustion, but too much excess air raises stack temperature and reduces efficiency. Typical target: 10–20% excess air (2–4% O₂ in flue gas).

An automatic or manually-initiated trip that immediately closes all fuel supply valves and, where applicable, initiates steam smothering. Triggered by the BMS on confirmed loss of all flame, high coil outlet temperature, low process flow, high firebox pressure, or manual operator activation. After an ESD, the BMS enforces a mandatory purge and lockout sequence before re-ignition is permitted.

A dimensionless factor (0–1) describing how efficiently a surface absorbs and emits thermal radiation compared to a perfect black body. Refractory walls have high emissivity (~0.9), making them effective re-radiators to process tubes. Coke deposits on tube surfaces reduce effective emissivity and alter heat flux distribution, contributing to local overheating.

A calculated measure of tube or refractory life consumption that accounts for the accelerated damage caused by high-temperature excursions. Operating at temperatures above design consumes life disproportionately due to the exponential relationship between temperature and metal creep rate. EOH is used to schedule tube replacement and set inspection intervals.

The refractory-lined enclosure where combustion takes place and heat is transferred to process tubes primarily by radiation. Synonymous with radiant section. The firebox contains the burners, the radiant process coil, and the hot refractory walls that re-radiate heat to the tubes. Operating conditions inside the firebox are extreme: temperatures of 750–1100°C, negative pressure (draft), and direct flame exposure.

Unintended extinction of one or more burner flames while fuel gas continues to flow. The most serious immediate hazard in heater operation — unburned fuel accumulates in the firebox, forming an explosive mixture. The BMS detects flameout via flame scanners and initiates fuel trip. Reignition without a full firebox purge after a flameout is forbidden. See the Flameout Response procedure.

An optical sensor (UV, IR, or UV/IR) mounted in or near each burner position that detects the presence of a flame by sensing the characteristic radiation emitted by combustion. Flame scanner signals are the primary input to the BMS flame detection logic. Scanner fouling or malfunction is a significant cause of spurious trips; scanners require regular cleaning and testing.

The combustion products leaving the firebox, consisting primarily of CO₂, H₂O (vapour), N₂, and excess O₂. Flue gas composition is the primary tool for assessing combustion quality — particularly O₂ % (excess air indicator) and CO (incomplete combustion indicator). Continuous flue gas analysers are standard on modern heaters and are monitored as part of normal operations.

The rate of heat transfer per unit area of tube surface, expressed in BTU/hr·ft² or kW/m². Average heat flux is total duty divided by total tube area. Local (peak) heat flux can be significantly higher than the average due to flame proximity, non-uniform firing, or flow maldistribution. Tube design limits are expressed as maximum allowable average and peak heat flux. Exceeding peak flux limits causes coking and overheating.

The piping, valves, pressure regulators, and instrumentation that supply fuel gas from the refinery fuel header to the heater burners. Includes the main fuel header isolation valve, pressure control valve (PCV), fuel gas knockout drum (to remove liquids), and individual burner block valves. Fuel gas pressure and composition variations are a common source of firing instability and must be monitored during operations.

A measure of the overall rate of heat transfer across the tube wall from flue gas to process fluid, expressed in BTU/hr·ft²·°F or W/m²·K. The overall coefficient depends on the external film coefficient (flue gas side), tube wall conductivity, and internal film coefficient (process side). Coke deposits and scale reduce U, requiring higher tube metal temperatures to maintain the same heat transfer rate.

Operating condition where the amount of combustion air significantly exceeds the stoichiometric requirement. Causes: large air register openings, air infiltration through refractory cracks (tramp air), or low firing rate. Effects: reduced flame temperature, higher stack temperature, lower thermal efficiency, and potential flame instability. Typical maximum operating target is 25–30% excess air (≈4–5% O₂ in flue gas).

The total heat released by complete combustion of a fuel, including the latent heat recovered by condensing the water vapour in the flue gas. Used as the basis for thermal efficiency calculations in most refinery heater applications (gross efficiency basis). Compare with Lower Heating Value (LHV) which excludes latent heat and is the practical basis for efficiency since flue gas exits as steam.

The temperature at the inner (firebox-exposed) surface of the refractory lining. Hot face temperature determines the refractory grade required and the rate of thermal degradation. Refractory failure (cracking, spalling) is accelerated by exceeding the material's hot face temperature limit. Measured by embedded thermocouples or pyrometer during inspections.

An automated control action triggered when a process variable reaches a predetermined limit, typically resulting in a fuel trip or other protective response. Heater interlocks are implemented in the BMS (safety interlocks) or DCS (process interlocks). Safety interlocks are hardwired and cannot be bypassed during normal operation. See fh-safety-interlocks.html for a full list of heater interlock setpoints.

Inspection technique using an infrared camera to image temperature distribution on heater tube surfaces, refractory, and casing during operation. Hot spots on tube surfaces indicate coke buildup, localised overheating, or flame impingement. Cool spots on casing indicate refractory void or delamination. A standard tool for heater monitoring between turnarounds.

A heater configuration where a fan is located at the flue gas outlet (stack base) to actively pull combustion gases through the heater, creating draft. Contrasts with natural draft (buoyancy-driven) and forced draft (fan at air inlet). ID fans allow better draft control and can handle higher heat duties, but a fan trip creates an immediate operational emergency requiring rapid response.

The minimum concentration of a flammable gas in air below which ignition cannot be sustained. For methane (typical fuel gas component): ~5% v/v. For hydrogen: ~4% v/v. The LFL is the critical threshold for firebox purge certification — the firebox must be purged until hydrocarbon concentration is confirmed below 25% of LFL before reignition is permitted after a flameout or shutdown. Also called Lower Explosive Limit (LEL).

The heat released by complete combustion of a fuel, excluding the latent heat of the water vapour in the flue gas (i.e., assuming the water leaves as steam, not liquid). The LHV basis is more relevant to actual heater performance since stack gases exit well above the water dew point. Thermal efficiency calculated on a LHV basis will always be higher than the same calculation on a HHV basis.

A burner design that reduces nitrogen oxide (NOx) formation in the flue gas by controlling peak flame temperatures and staging the combustion air or fuel. Techniques include: fuel staging (ultra-low-NOx), air staging, flue gas recirculation (FGR), and internal flue gas recirculation. Low-NOx burners typically have a longer, softer flame profile and narrower stable operating range than conventional burners, requiring more careful adjustment.

Unequal distribution of process fluid flow between parallel tube passes in a heater coil. Causes one or more passes to receive less cooling flow, resulting in higher tube metal temperatures and accelerated coking in the under-flowing passes. Detected by unequal pass outlet temperatures. Caused by plugged pass inlet valves, coking, or incorrect balancing. Must be corrected promptly to prevent tube failure.

The maximum permissible pressure at the most restrictive component in the heater pressure envelope (typically the tube-to-header connection), as determined by design and confirmed by pressure testing. Operating above MAWP is a serious violation. MAWP is stamped on the heater nameplate and must be verified before any change in operating pressure or fluid service.

The lowest temperature at which the pressure-containing components of the heater can be operated at full design pressure without risk of brittle fracture. Operating below MDMT under pressure (e.g., during cold startup or after a freeze) can cause sudden catastrophic failure with no prior deformation warning. MDMT must be checked against ambient conditions before pressurising a cold heater.

Draft created solely by the buoyancy difference between the hot flue gas in the stack and the cooler ambient air — no fans required. The magnitude of natural draft is proportional to stack height and the temperature difference between flue gas and ambient. Natural draft heaters are simpler but more sensitive to ambient temperature changes and wind conditions. The majority of refinery process heaters operate on natural draft.

A collective term for nitric oxide (NO) and nitrogen dioxide (NO₂) formed during high-temperature combustion. NOx is a regulated air emission and a health hazard. Formation rate increases sharply with flame temperature (thermal NOx) and is also influenced by fuel nitrogen content (fuel NOx). Heater stack NOx is measured in ppmvd (parts per million by volume, dry basis). See low-NOx burner.

An automatic control loop that adjusts the total combustion air supply (via stack damper or forced-draft fan) to maintain a target flue gas oxygen percentage. O₂ trim maintains optimum excess air as fuel composition and firing rate vary, improving efficiency and reducing emissions. Requires reliable continuous flue gas O₂ analysis. O₂ analyser failures must be identified promptly to prevent the control loop operating on a bad signal.

The fraction of total calendar time that a heater is operating at design conditions, expressed as a percentage. A target on-stream factor of 95%+ is typical for refinery heaters. Unplanned shutdowns (tube failures, BMS trips, refractory collapses) reduce on-stream factor and drive the cost justification for preventive maintenance and condition monitoring programmes.

An instrument that continuously measures the oxygen content of flue gas, expressed as % v/v (dry basis). The primary tool for monitoring combustion efficiency and excess air. Zirconia-type analysers are the most common in refinery service. Requires regular calibration and probe maintenance to remain reliable. A reading of 0% O₂ with visible smoke indicates sub-stoichiometric combustion — a hazardous condition requiring immediate corrective action.

One complete flow path through the heater coil from inlet header to outlet header. Most heaters have multiple parallel passes to reduce the flow velocity and pressure drop required for a single tube circuit. Each pass carries an equal fraction of total process flow (when balanced). Pass outlet temperatures are monitored individually to detect maldistribution.

A small, continuously-burning burner used to ignite and stabilise the main burner flame. BMS logic requires the pilot to be proven lit before the main fuel valve is opened. Pilots may be continuously-lit (remain on during normal operation) or interrupted (extinguished after main flame is established). Pilot proving is a critical BMS safety function — an unproven pilot prevents main burner ignition.

A formal review conducted before first startup or after a significant modification to verify that the heater is mechanically complete, instrumentation is functional, safety systems are tested, and operating procedures are in place. Required by OSHA PSM regulations (29 CFR 1910.119) for covered processes. A PSSR checklist sign-off is required before fuel admission is permitted after a major turnaround.

The mandatory process of passing sufficient air through the firebox before ignition (or after a flameout) to dilute any accumulated flammable gas to below 25% of the LFL. Minimum purge requirement is typically 5 firebox volume changes at sufficient airflow, timed and proven by the BMS. Bypassing or shortcutting the purge is the most common cause of firebox explosions and is strictly prohibited.

The liquid or vapour flowing through the heater tubes that is being heated. In refinery service this is typically crude oil, vacuum residue, gas oil, naphtha, or reactor feed. The process fluid properties (flow rate, composition, phase, viscosity) determine the tube-side heat transfer coefficient, the tendency to coke, and the required operating pressure. Changes in process fluid service require engineering review before implementation.

The lower section of a fired heater where burners are located and heat transfer to process tubes occurs primarily by radiation from the flame and refractory walls. Also called the firebox. Operates at the highest temperatures in the heater system. The process tubes in the radiant section are the most thermally stressed components and are subject to the most rigorous inspection requirements.

High-temperature-resistant ceramic lining applied to the firebox walls, floor, and roof to contain heat, protect the steel casing, and re-radiate heat to the process tubes. Types include castable, brick, and ceramic fibre (blanket). Refractory condition directly affects thermal efficiency (via heat loss through the casing) and firebox temperatures. Spalled or cracked refractory is a significant hazard and must be repaired promptly. See fh-refractory.html.

The calendar time between planned shutdowns (turnarounds) for a heater, typically 2–5 years for refinery process heaters. Run length is limited by the rate of tube coking, refractory degradation, tube creep, and equipment corrosion. Operating at higher temperatures or duty accelerates these degradation mechanisms and reduces achievable run length. Predicting and extending run length is a key goal of heater condition monitoring.

A burst disc pressure-relief device installed in the process piping to protect the heater coil and downstream equipment against overpressure. Unlike a relief valve, a rupture disc is non-reclosing — once it bursts, it must be replaced before restarting. Some heaters use rupture discs in series with relief valves. Rupture disc integrity must be verified during PSSR and after any overpressure event.

In the context of fired heater operations, reliability refers to the probability that the heater will operate at design conditions for the planned run length without unplanned shutdown. Key reliability drivers: tube condition, refractory integrity, BMS function, burner condition, and instrumentation availability. Reliability is improved by structured preventive maintenance, operator rounds discipline, and condition monitoring programmes.

A discrete level (SIL 1–4) that quantifies the reliability requirement for a Safety Instrumented Function (SIF). Heater BMS trips are typically SIL 1 or SIL 2. SIL 2 requires a probability of failure on demand (PFD) of 0.01–0.001. SIL assignments drive the testing frequency, redundancy, and diagnostic requirements for the associated instrumentation. Determined by a formal Safety Integrity Level assessment (LOPA or fault tree).

The first one or two rows of process tubes at the entry to the convection section, positioned directly above the radiant section and exposed to both radiation from the firebox below and convection from the flue gas. They receive the highest heat flux in the convection section and are subject to the most rapid fouling. Also called shield tubes or shock rows. Their condition is a key indicator of convection section health.

An instrumented system designed to bring a process to a safe state when predetermined conditions are violated. The BMS is a type of SIS. SIS components (sensors, logic solvers, final elements) must be independent of the basic process control system (BPCS/DCS). Governed by IEC 61511. SIS demands proof testing at intervals defined by the SIL assessment to maintain the required reliability.

The breaking away of pieces from the refractory surface caused by thermal cycling, excessive temperature, or moisture. Spalled refractory exposes the steel casing to direct radiation and conduction, leading to casing overheating, and in severe cases, casing burn-through or structural failure. Spalling debris on the firebox floor can also block burner air registers. Requires immediate reporting and assessment for repair priority.

The theoretical condition where exactly the right amount of oxygen is supplied to completely combust all fuel, producing only CO₂ and H₂O with zero remaining O₂ and zero unburned fuel. In practice, heaters always operate with some excess air above stoichiometric to ensure complete combustion — the stoichiometric point is unstable and any deviation produces either CO (too little air) or excessive stack loss (too much air).

The temperature of flue gases exiting the top of the convection section and entering the stack. A primary indicator of heater thermal efficiency — the lower the stack temperature relative to the process inlet temperature, the more heat has been extracted from the flue gas. Rising stack temperature at constant duty indicates convection section fouling or increased excess air. Target stack temperature: 150–250°C depending on fuel sulphur content and heater design.

The ratio of heat absorbed by the process fluid to the total heat released by combustion of the fuel, expressed as a percentage. Typically 85–92% for well-operated refinery heaters. The main losses are stack heat loss (largest) and surface heat loss (relatively small). Efficiency = 100% − stack loss% − surface loss%. Improving thermal efficiency is the primary lever for reducing fuel consumption and CO₂ emissions from a fired heater.

Unwanted air infiltrating into the firebox through cracks in the refractory, degraded burner tile seals, observation door gaps, or around soot-blower penetrations. Tramp air contributes to total excess air without passing through the burner registers, making it impossible to control by register adjustment. Excessive tramp air raises excess air, reduces flame temperature, and makes combustion control difficult. Detected by high O₂ at low register settings or positive firebox pressure.

The temperature of the tube wall itself (as opposed to the process fluid inside or the flue gas outside). TMT is the critical parameter for tube life and safety — it determines the rate of creep, oxidation, and carburisation. TMT is always higher than the process fluid temperature by an amount determined by the heat flux and tube-side film coefficient. TMT is monitored by thermocouples welded to tube outer surfaces and by periodic infrared surveys.

The ratio of maximum to minimum stable firing rate for a burner, expressed as a ratio (e.g., 5:1) or percentage of design capacity. A burner with 5:1 turndown can fire stably between 20% and 100% of its maximum heat release. Below minimum turndown, flames become unstable and prone to flameout. Low-NOx burners typically have a narrower stable range (lower turndown ratio) than conventional burners.

The slow, permanent plastic deformation of tube material under stress at elevated temperature. Creep rate increases exponentially with temperature — even short-duration temperature excursions significantly accelerate life consumption. Visible as increased tube diameter (bulging) or sagging. Tubes approaching end-of-life from creep show measurable diameter increase and reduced wall thickness. Creep is the primary life-limiting failure mechanism for radiant section tubes.

The maximum concentration of flammable gas in air above which combustion cannot be sustained (mixture too rich). For methane: ~15% v/v. Above the UFL, the mixture is too fuel-rich for ignition. Relevant in confined spaces during purging — a space above the UFL is still hazardous because dilution with air will pass through the flammable range before becoming safe. Also called Upper Explosive Limit (UEL).

Any deviation from normal operating conditions that requires operator intervention beyond routine adjustment. Upsets range from minor (single burner instability, slight COT deviation) to severe (loss of process flow, tube leak, firebox explosion). Early upset recognition and correct response are the highest-value operator skills in heater operations. See fh-upsets.html for a full classification and response guide.

Steam injected into the process coil during steam-air decoking to maintain minimum flow velocity through the tubes, carry combustion products out of the coil, and prevent local hot-spot formation. Velocity steam flow is carefully controlled during decoking to balance coke burn rate against tube temperature limits. Insufficient velocity steam can allow coke burn to overheat tube sections; excessive flow quenches the burn and extends decoking time.

Direct observation of the firebox interior through observation ports (peepholes) using appropriate eye protection to assess burner flame condition, tube colour, refractory condition, and draft indicators. A critical routine operator task — most incipient problems give visual warning before instrumented alarm. Firebox visual checks should be performed at every operator rounds. Observation door procedures must be followed strictly to prevent blow-back injury.

The enclosed plenum chamber behind the burner front from which combustion air is distributed to individual burner air registers. In forced-draft heaters, the windbox is pressurised by the FD fan; in natural-draft heaters, it operates at near-atmospheric pressure. Positive windbox pressure relative to the firebox indicates loss of draft — a condition that will cause blow-back when burner doors are opened.

A light hydrocarbon oil (typically gas oil or naphtha) circulated through the process coil before and after steam-air decoking to soften coke deposits, cool the coil, and verify tube integrity after decoking. The wash oil circuit allows the operator to confirm that all tube sections are free-flowing before returning the heater to process service. Wash oil samples are checked for contamination that would indicate tube damage.