Field Reference / Operations / Online / Offline Decoking
Module 02
Module 02 · Operations

Online / Offline Decoking

Coke deposits inside process tubes reduce heat transfer, raise tube metal temperatures, and restrict flow. This page covers how to recognise coking, when each decoking method applies, and the procedures for both online (steam-air) and offline (mechanical / pigging) methods.

Why Coke Forms

Coke is a carbonaceous deposit that builds on the inner tube wall when process fluid (typically hydrocarbons) is overheated or has insufficient velocity. Two mechanisms drive it:

Thermal / Pyrolytic Coking
  • Occurs when tube skin temperature exceeds fluid's thermal stability threshold
  • Hydrocarbon cracks, leaving a hard carbon layer bonded to the tube wall
  • Common in high-severity furnaces: ethylene crackers, visbreakers, delayed coker heaters
  • Progresses fastest near burner tips — the highest radiant flux zone
Catalytic / Velocity Coking
  • Occurs when flow velocity drops below the minimum threshold to sweep deposits away
  • Accelerated by iron or nickel on the tube surface acting as a coke-formation catalyst
  • Common in CDU/VDU preheat trains and any service with low-velocity slug flow
  • Soft, layered deposits — more amenable to online removal than hard pyrolytic coke

Recognising Coking

The DCS will show several trends before a pass becomes critically cooked. Monitor the following in combination — a single indicator can be misleading; two or more together demand action.

01
Rising tube metal temperature (TMT) — with no change in firing or feed rate. Coke insulates the tube wall, forcing the tube to get hotter to transfer the same heat.
Early
02
Increasing pass outlet temperature spread — coking distributes unevenly across passes. One pass will show higher COT with lower flow as its tubes restrict.
Early
03
Rising pressure drop across the pass — reduced tube bore diameter increases resistance. Compare current dP against design baseline and trend over time.
Action
04
Declining duty / efficiency — heat absorbed by the process falls while firebox temperature and flue gas losses increase. Stack temperature may also rise.
Action
05
TMT approaching design limit — typically 600–650 °C for CS/5Cr tubes; 700–750 °C for 9Cr. Once within 25 °C of the limit, decoking can no longer be deferred.
Critical
06
Visible hot spot on tube during inspection window — bright or discoloured section of tube visible through sight glass. Indicates local coke concentration.
Critical
⚠
Do not compensate for coking by increasing firing
Raising fuel to maintain outlet temperature when tubes are coking accelerates TMT rise. The correct response is to reduce severity and initiate the appropriate decoking procedure.

Choosing the Right Method

The choice of decoking method depends on coke type, deposit severity, unit criticality, and whether the heater can be taken offline. Use this summary to guide the decision:

Method Selection Guide
Factor Online (Steam–Air) Offline (Mechanical / Pigging)
Unit status Heater remains in partial service Heater fully shutdown & isolated
Coke type Soft, early-stage deposits Hard pyrolytic; heavy layered coke
Production impact Reduced throughput during decoking Full production loss for duration
Frequency Months between runs (some units weekly) Typically per-turnaround or as-needed
Risk level High — live firebox, steam-air chemistry High — confined space, tube integrity, CO
Typical duration 12–48 hr 24–72 hr
Post-check required TMT/dP trending post-restart Visual inspection + hydrostatic or PIG run

Procedures

Select a decoking method below to view the full procedure:

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Online Steam–Air Decoking
The heater remains lit. Feed is displaced by steam, then a controlled steam–air mixture burns the coke off the tube wall as CO and CO₂. The products vent to atmosphere or a designated burn-off stack. High risk — requires close monitoring of tube temperatures and combustion products throughout.
🔴
Critical — air injection rate governs tube integrity
Introducing air too quickly causes a runaway exothermic reaction inside the tube. TMTs can spike by hundreds of degrees in seconds. Always start at minimum air flow and step up slowly against TMT response. Never leave unattended during air injection phase.
Online Steam–Air Decoking Procedure
Type: Decoking — Online Steps: 14 Typical duration: 12–48 hr
01
Obtain permit and notify operations
Raise a hot work / decoking permit. Notify the shift superintendent, process engineer, and adjacent unit operators. Confirm the burn-off destination (stack, scrubber, or flare) is available and cleared.
02
Reduce heater duty
Cut feed rate to minimum stable flow. Reduce firing to lower TMT and COT toward the steam–air decoking target range — typically COT 400–450 °C. Allow temperatures to stabilise before proceeding.
03
Displace feed with steam
Introduce steam to the pass inlet — typically HP steam 10–15 bar — at a rate sufficient to purge feed from the tubes. Confirm clean steam is flowing at the outlet before advancing. Do not start air injection while any feed remains in the tubes.
04
Establish steam-only circulation and stabilise
Run steam through all passes for 30–60 min to purge remaining hydrocarbons. Check outlet for hydrocarbon content if sampling point exists. Verify TMTs are stable and within acceptable pre-decoking range.
05
Introduce air — start at minimum rate
Open air injection valve to ≤ 5% of design decoking air rate. Monitor TMT response for 5–10 min before each step increase. The coke combustion reaction is exothermic — TMTs will rise. Target a TMT rise rate of ≤ 10 °C / 10 min. If TMT rises faster, hold or reduce air.
06
Step up air flow incrementally
Increase air in 5–10% increments, allowing stabilisation between each step. Maintain firebox at reduced firing to support the reaction without overheating. Watch for CO breakthrough at outlet — confirms active coke combustion.
07
Monitor CO₂/CO ratio at outlet continuously
A rising CO₂ / falling CO ratio over time indicates the coke front is burning through. When CO₂ drops toward zero with no CO present, the coke in that zone is exhausted. Log readings every 15–30 min throughout.
08
Respond to TMT alarm — halt air injection immediately
If any TMT reaches the decoking alarm setpoint (typically design limit − 50 °C), isolate air injection without delay. Do not reduce gradually — shut air fully. Increase steam flow to cool. Hold until TMTs return to safe range before considering restart of air injection.
09
Maintain steam flow throughout air injection phase
Steam moderates the reaction and carries combustion products to the outlet. Do not reduce steam while air is flowing. A steam-to-air mass ratio of 3:1 to 5:1 is typical — confirm with the site-specific decoking procedure.
10
Confirm decoking complete
Decoking is complete when: CO₂ output has dropped to near-zero, all TMTs have stabilised at or below pre-decoking baseline, and outlet gas shows no further combustion products with air flowing at maximum decoking rate for ≥ 1 hr.
11
Isolate and purge air — steam-only purge
Close air injection valve fully. Continue steam flow for ≥ 30 min to purge all air from the tube system before reintroducing hydrocarbons. Failure to purge air creates an explosive mixture on feed return.
12
Restore firing and feed
Gradually restore firing to operating levels. Introduce feed slowly against steam, displacing steam from the outlet side. Confirm flow on all passes before closing steam inlet.
13
Return heater to normal operations
Ramp to design throughput and firing per normal operations procedure. Monitor TMT and pass dP closely over the next 4–8 hr to confirm improved heat transfer profile.
14
Close out permit and log decoking data
Record: start/end time, pre/post TMT readings per pass, pre/post dP, maximum TMT reached during decoking, air and steam rates used. This data informs the next decoking interval forecast.
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Offline Mechanical Decoking
The heater is fully shutdown, depressured, purged, and isolated before work begins. Coke is removed mechanically — by water-jet pigging, drill rods, or high-pressure hydraulic cleaning — once tubes are confirmed cool and gas-free. Confined space and residual CO exposure are primary hazards.
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CO hazard — confirm gas-free before tube entry or opening
Coke and scale can off-gas CO as the tube cools, and CO can accumulate in the firebox and in opened headers. Atmospheric testing for CO, H₂S, and O₂ is mandatory before any line opening or confined space entry. All personnel working near open tubes must carry personal gas monitors.
Offline Mechanical Decoking Procedure
Type: Decoking — Offline Steps: 13 Typical duration: 24–72 hr
01
Raise permit — confined space + line opening
Issue cold work, confined space entry, and line opening permits. Confirm the decoking contractor is safety inducted and briefed on site-specific gas hazards. Identify all drain and vent points required.
02
Shut down and isolate the heater
Execute the planned shutdown procedure. Isolate fuel, feed, and all process connections with physical spectacle blinds or spade plates — not valves alone. Depressurise to atmospheric. Lockout-tagout (LOTO) all energy sources including steam, BFW, and instrument air if applicable.
03
Steam-purge all tubes and headers
Purge with LP steam through the tube circuit, displacing all residual hydrocarbons. Purge until the outlet hydrocarbon reading is at background level on a LEL detector. Purge duration is typically 4–8 hr for large furnaces. Do not open tube headers until purge is confirmed complete.
04
Cool tubes — natural or forced cooling
Allow tubes to cool to ≤ 50 °C measured at headers and plug sheet before mechanical access. Forced cooling using nitrogen or air sweep through the firebox may be used once the firebox is confirmed hydrocarbon-free. Do not enter the firebox until TMTs and refractory surface temperatures confirm cool.
05
Atmospheric test — gas-free certificate required
A competent person tests for: O₂ 19.5–23.5%, LEL < 1%, CO < 10 ppm, H₂S < 1 ppm. Testing must be done at the header level and inside the firebox if entry is required. Gas-free certificate must be issued before tube opening can begin. Re-test every 4 hr or after any disturbance.
06
Open headers / plug sheets
Remove tube plugs or open header boxes to give access to tube bore ends. Inspect the visible coke profile. Sample for confirmation of deposit type. Photograph or video the tube ends before decoking to document initial condition.
07
Select and set up decoking method
Choose method based on coke hardness and tube geometry: High-pressure water jetting (typically 700–1500 bar) for softer deposits; hydraulic pigging for uniform tubes with accessible ends; mechanical rodding for very hard coke in straight runs. Establish water containment — coke slurry disposal must be managed to drain or skip.
08
Decoke tube by tube — log progress
Work systematically from one header end. Log each tube as complete. Pause periodically to re-test atmosphere inside the firebox and header boxes. Change decoking personnel per fatigue policy — water-jet operations are physically demanding.
09
Inspect tubes for damage
Once decoking is complete, visually inspect accessible tube ends. Look for pitting, wall thinning, blistering, or cracking — especially in hot-spot areas identified prior to shutdown. If internal damage is suspected, arrange for UT thickness measurement before returning to service.
10
Flush and blow-out tube circuit
Flush the full tube circuit with water to remove all coke fines before closure. Follow with a nitrogen blow through to dry. Confirm drain is clear at all low points. Water left in tubes can cause steam hammer on startup.
11
Close and pressure-test headers
Replace all tube plugs, gaskets, and header cover plates to the correct torque specification. Pressure-test the tube circuit with nitrogen or water to 1.1× design pressure and hold for 30 min minimum before removing blinds or reinstating process flow.
12
Remove isolation blinds — restore heater
Remove all spectacle blinds and spade plates as per the blind register. Remove LOTO tags and padlocks only by the person who installed them or their designated representative. Confirm all items on the blind register are accounted for before startup authority is given.
13
Close out permits — log and return to startup
Close all work permits. Document: tubes decoked, coke volume removed (estimated), inspection findings, any tube damage noted, and pressure test results. Return heater to startup procedure. Monitor TMT and dP closely for the first 24 hr post-restart.

Common Decoking Mistakes

Starting air injection before full hydrocarbon purge
Even trace hydrocarbons in the tube during air injection can ignite explosively. Steam purge must be confirmed — not assumed — before air flows.
Stepping up air too fast
The coke combustion reaction accelerates exponentially with temperature. Rushing air increases risks a runaway TMT excursion within minutes. Patience at each step is not optional.
Leaving the DCS unattended during air injection
TMT can spike faster than a scan interval. A dedicated operator at the DCS is required throughout the air injection phase — this is not a task to leave on autopilot.
Reintroducing feed without purging air
After online decoking, air must be fully displaced by steam before feed is introduced. Hydrocarbon + air in a hot tube = deflagration risk.
Opening tube headers without a gas-free certificate
Coke can hold and off-gas CO, H₂S, and hydrocarbons well after apparent purge completion. A gas-free certificate is mandatory — not a formality.
Not inspecting tubes after mechanical decoking
High-pressure water-jetting can erode tube walls, especially in areas already thinned by corrosion or hot-spot damage. Don't return to service without checking tube wall thickness in suspect areas.
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