Field Reference / Inspection & Maintenance / Tube Assessment
Module 06
06 · Inspection & Maintenance

Tube Assessment

What operators need to know about tube condition — what to look for during rounds, what the inspection methods mean, how remaining life is evaluated, and when to escalate. Decision-making on tube fitness is the inspection engineer's role; getting the right information to them quickly is yours.

Your role vs. the inspector's role

Tube assessment is a shared responsibility. Operators and inspection engineers each own a distinct part of it.

Operator — Your responsibilities
  • Observe and record TMT trends and pass flow deviations every round
  • Report visible hot spots, discolouration, or sagging through the peephole
  • Record the date, pass number, and approximate location of any abnormal tube appearance
  • Flag TMT excursions immediately — even brief ones — to your supervisor
  • Document any period where a tube ran above its design limit, including duration
  • Know when to stop the heater before inspection is possible
Inspection Engineer — Their responsibilities
  • Select NDT method and set inspection scope
  • Interpret UT, RT and creep life data against API 530 / API 579 criteria
  • Determine minimum remaining wall (tmin) and remaining service life
  • Make the fitness-for-service decision — continue, monitor, or replace
  • Raise MOC documentation for any change to operating limits
  • Sign off pre-restart tube integrity acceptance
ℹ
Why this matters for operators
An inspection engineer working from good operator records — accurate TMT trends, excursion durations, hot spot locations — can make a far better remaining life assessment. Vague records lead to conservative decisions, more downtime, and unnecessary tube replacements. Your field data is the input to their calculation.

Visual inspection — what operators look for

Visual inspection through the peephole is the only real-time tube assessment tool available to an operator. It is not precise, but it is immediate. Know what you are seeing — and what it means.

⚠
Approach peepholes from the side — never straight on
If firebox pressure has gone positive due to a tube failure or combustion upset, opening a peephole can eject flame or hot gas directly toward you. Approach at an angle, check for positive draft at the gap before fully opening, and keep your face out of the line.
🔴 Glowing / incandescent tube Stop heater
  • Bright orange or white glow on a tube section — metal temperature is above ~700 °C
  • Indicates severe overheating — creep damage is occurring in real time
  • Often accompanied by a rising TMT alarm or one pass diverging sharply
  • Do not attempt to manage by reducing firing alone — shutdown and investigate
  • Log time, pass, and tube position before leaving the heater
🟠 Tube bulge or bow Stop heater
  • Visible deformation of the tube profile — it is no longer straight or circular
  • Bulge = localised creep deformation, failure is imminent or has already started
  • Bow = sustained thermal overload has relaxed the tube — reduced remaining life
  • Both conditions require immediate heater shutdown and inspection authority notification
  • A bulging tube must not be returned to service without engineering disposition
🟡 Discolouration / hot spot Escalate now
  • A pale, lighter section on one tube face — oxide scale colour shifts with temperature
  • Blue/grey tint (new oxidation) or bright patch against surrounding scale
  • Most common cause: localised coke deposition insulating that tube section
  • Record pass number, tube position (counting from the floor), approximate size
  • Inform supervisor; increase peephole inspection frequency to every round
🟡 Tube sag / misalignment Escalate now
  • Tube sitting lower in its support guide than adjacent tubes
  • Caused by creep elongation, support failure, or refractory displacement
  • Risk: tube contacts refractory or adjacent tube, creating a local hot spot
  • Not always linked to a TMT alarm — tube temperature may be within range
  • Document and notify inspection — physical measurement needed at next opportunity
🔵 Scale build-up / unusual deposits Monitor
  • Irregular, rough, or flaking external scale on one or more tubes
  • Heavy scale can indicate prior overheating or aggressive sulfidation
  • Scale spalling during a run can land on burner tiles — watch for flame impingement
  • Note whether scale is uniform or localised — localised is more significant
  • Record and include in inspection work scope for next shutdown
🔵 Uniform dark scale, even profile Record
  • Dark, uniform oxide scale on all tubes — normal for a heater that has been running
  • Even profile across the pass indicates balanced heat input and flow distribution
  • No action required during normal operation
  • Compare against the same view from the last inspection to track any change
  • Photograph at each planned peephole check if camera is available

NDT methods — what the inspector uses

Non-destructive testing is performed by certified inspection personnel, typically during planned shutdowns or following an abnormal event. As an operator, you need to understand what each method finds and what limits trigger action — so you can report the right things and understand inspection findings.

UT
Ultrasonic Thickness Testing
Finds
Wall thinning from internal corrosion, erosion, or general metal loss. Gives actual remaining wall thickness at the measurement point.
When used
Every planned shutdown; immediately after any TMT excursion or suspected tube event.
Limit
Tube with wall < 80% of tmin (minimum design wall) requires engineering disposition before restart.
Operator trigger: Report any TMT excursion, including its duration — this tells the inspector where to focus UT readings.
RT
Radiographic Testing
Finds
Internal corrosion, coke deposits, erosion, and weld defects — as a 2D image through the full tube wall. Better than UT for pitting and coke distribution.
When used
Return bends, welds, areas where UT access is restricted. Also used to confirm internal coke extent before decoking.
Limit
Interpreted by certified radiographer — wall shadow indicates wall thickness at that location. Defect sizing varies by technique.
Operator trigger: Unexplained dP rise or pass flow restriction — suggests internal coke or corrosion product. RT scope should include that pass.
IR
Infrared Thermography
Finds
Hot spots, coke insulation patterns, flame impingement zones — surface temperature mapped across the entire visible tube array during operation.
When used
Online — through sight glasses while heater is in service. Used to confirm operator-reported hot spots and to baseline tube condition.
Limit
Surface temperature only — does not measure wall thickness. Hot spot > 50 °C above adjacent tube is considered significant and warrants investigation.
Operator trigger: Any visually observed hot spot or discolouration — IR scan should be requested to quantify and map it.
PT / MT
Penetrant & Magnetic Particle Testing
Finds
Surface-breaking cracks at welds, return bends, and tube fittings. PT works on all metals; MT on ferromagnetic alloys only.
When used
After TMT excursion events; at return bends and weld seams whenever access is available during shutdown. Mandatory following any thermal fatigue event.
Limit
Any linear indication >3 mm at a weld or bend requires sizing and engineering disposition. Zero tolerance at pressure-containing welds.
Operator trigger: Report units that have had frequent startups, long creep exposure, or any prior tube event — these locations get PT/MT coverage.

Remaining life — the API 530 basis

API 530 is the standard used by inspection engineers to design and assess fired heater tubes. It combines two degradation mechanisms — corrosion/erosion (wall thinning) and creep (high-temperature plastic deformation) — into a remaining life assessment. You will not perform this calculation, but you need to understand what feeds it and what the outcome means.

API 530 Assessment Components — Operator Data Required
01
Corrosion allowance remaining The inspector measures current wall thickness (UT/RT) and subtracts the minimum allowable wall (tmin). The result, divided by the corrosion rate, gives years of life. Corrosion rate is estimated from historical UT records — every shutdown measurement improves the accuracy. Your role: report any upset conditions that may have accelerated internal corrosion (process chemistry changes, high-velocity slugs, acid excursions).
Routine
02
Creep life consumed (Larson-Miller) Each hour a tube runs above its design TMT consumes a fraction of its creep life. The Larson-Miller parameter relates temperature and time to material rupture. Even a single 30-minute excursion 50 °C above the design limit can consume a significant portion of remaining life. Your role: record every TMT excursion with time, temperature, and duration — this data directly enters the creep damage calculation.
Data critical
03
Design TMT limits by material Typical limits the inspection engineer uses as the creep threshold:

Carbon steel (CS): 450 °C  |  5Cr–0.5Mo: 600–620 °C  |  9Cr–1Mo: 650–700 °C  |  347 SS: 750–800 °C

Check your unit's data sheet for the exact limit — alloy selection varies. Operating even 10–15 °C above these limits matters over a full run.
Know your limit
04
The 80% wall rule A tube with measured wall thickness less than 80% of its design minimum wall is considered unacceptable for continued service under most site procedures (based on API 530 / API 579 FFS criteria). Below 80%, the tube requires engineering disposition — an explicit accept/reject decision with documented justification — before restart. This is not a guideline. It is a hard stop until an engineer has reviewed and signed off.
Hard limit
05
Remaining life <2 years When the combined assessment (corrosion + creep) shows less than 2 years of remaining life, most sites require an immediate decision: reduce operating severity, plan an early turnaround, or replace the tube. Running a heater with known sub-2-year remaining life without a documented plan is a mechanical integrity violation.
Escalate
⚠
Undocumented excursions destroy the assessment
If a TMT excursion is not recorded, the creep life calculation assumes it didn't happen. The remaining life figure will be optimistic. A tube that fails unexpectedly after a "good" assessment is often traceable to undocumented excursions.

Fitness-for-service — the decision framework

The inspection engineer applies API 579 (Fitness-for-Service) to reach one of three outcomes: continue as-is, continue with revised limits, or remove from service. Understanding the logic helps you frame your reports and escalations correctly.

?
Is there a confirmed tube bulge, visible crack, or confirmed pinhole leak?
YES → Remove from service immediately. No FFS assessment is valid for a tube that has already reached its failure mode. Replace the affected section and assess adjacent tubes.
NO ↓ Continue to next check.
?
Is measured wall thickness below 80% of tmin, or is remaining corrosion life <1 year?
YES → Tube must be replaced or re-rated before restart. Inspection engineer documents the disposition. No operational workaround is acceptable at this stage.
NO ↓ Continue to next check.
?
Has calculated creep life consumed exceeded 80% of design life?
YES → Remaining life assessment required. Options: replace tube, reduce operating TMT to extend remaining life, or set a hard retirement date. All options require engineering sign-off and MOC documentation.
NO ↓ Continue to next check.
?
Is remaining service life (corrosion + creep combined) less than the next planned turnaround interval?
YES → Inspection engineer sets a revised operating limit (lower TMT, reduced duty) or recommends early replacement. Heater may continue with documented restrictions. Operations must respect the revised limits — log any deviations.
NO ↓ Continue to next check.
✓
Tube is fit for continued service to the next planned turnaround
Inspection engineer documents the acceptance with a defined next inspection date. Operations continue normal monitoring. Any new TMT excursion or visual finding triggers a reassessment.

What to report — operator quick reference

All of the below should be recorded in the shift log and communicated directly to your supervisor. Time, pass number, and observation specifics are what make reports usable.

Tube Condition — Operator Reporting Guide
What you see or record Action required Who to tell Priority
Glowing tube, bulge, or visible deformation Shut down heater immediately. Do not wait for authorisation — use your emergency shutdown authority Supervisor, Inspection, Control room Immediate
TMT alarm: reading above design limit Record time, temperature, affected pass. Reduce firing rate. Inform supervisor. Log duration carefully Supervisor, Shift lead Immediate
Visible hot spot or discolouration through peephole Record pass, tube position (from floor), approx size. Notify supervisor. Request IR scan if available Supervisor, Inspection Urgent
One pass diverging — COT rising, flow dropping vs. others Check for coke indicators (dP rise). Document trend. Notify supervisor. Do not compensate by increasing fuel Supervisor, Engineer Urgent
Tube sag or misalignment observed Record location and observation. Continue monitoring for TMT rise at that pass. Report to inspection for next shutdown scope Supervisor, Inspection Urgent
Rising pass dP with no flow change Log trend — likely internal coke. Report for decoking assessment. Do not increase firing rate to compensate Supervisor Log & monitor
Unusual external scale pattern observed Photograph if possible. Document location and appearance. Add to next shutdown inspection scope Inspection (at next planned contact) Log & monitor
Normal, uniform scale on all tubes, all TMT within range Record in shift log. No action Shift log only Routine
📋
When in doubt — escalate and document
The threshold for raising a tube concern should be low. A phone call to the inspection engineer costs nothing. A missed excursion or unrecorded hot spot costs tube life — or worse. If you are unsure whether something is significant: record it, report it, and let the engineer decide. That is the correct division of responsibility.