Sootblower Erosion Inspection: Visual Triage and Boiler-Tube UT

Why one reading does not bound erosion

One valid UT spot answers one location. It does not establish the length, width or minimum thickness of a directional wear zone, and it says nothing about a neighboring face or row that the probe did not reach. If the report cannot reconstruct the tube, face and acquisition point, even that local answer is hard to repeat.

Start with the physical hypothesis. A cleaning jet or abrasive gas stream can strike one side of a tube, continue into aligned rows and change direction around nearby geometry. A convenient aisle route may show a polished flat on the first row while deposits or occlusion hide the next. Build the coverage plan around the suspected impact direction, not around the clearest video lane.

Keep four decisions separate. The camera records an observation. A procedure-accepted UT response produces a measurement. Qualified reviewers test a mechanism hypothesis against wider evidence. The owner then decides further examination, repair, operating action or return to service. Do not collapse those steps into a single ‘defect detected’ result.

Where does sootblower and ash erosion appear?

Begin near the sootblower and follow the credible jet path through the bank. Babcock & Wilcox describes boiler-tube erosion as outside-diameter metal loss from impingement and identifies common locations near sootblowers, on leading edges of economizers, superheaters and reheaters, and around gas-flow velocity or direction changes. Those are priority zones, not a universal damage map.

Convection-pass geometry can hide the very faces that matter. B&W notes that ash can collect on leading edges and between aligned tubes in narrow tube-bank passages. Record the deposit state and line of sight for each planned face. ‘Not visible’ is a useful result; ‘looks acceptable’ from the wrong face is not.

Build a tube-bank location system

Name the boiler, elevation, bank, row, tube and face before recording condition. Add the viewing direction or clock convention, sootblower ID, route start, travel direction, image or video time and the approved drawing revision. ‘Tube 18’ is incomplete if two crews can count from opposite ends.

Separate the planned route from the achieved route. An annotated tube-bank map should show where the camera travelled, which faces were adequately viewed, where UT was attempted and where deposits, supports, bends or unstable travel stopped the work. Video time alone is not a durable asset location.

Encoder-correlated video can help connect evidence to travel, but the project must define its reference checks and uncertainty treatment. If the reference is interrupted or confidence is lost, close that segment, record the gap and start a new controlled segment. Never stretch a position trace across an unverified interruption.

Remote visual triage

Remote visual testing extends the view into difficult-to-access areas with optical equipment; ASNT distinguishes it from direct visual examination. In a tube bank, use it to document deposits, flattened or polished zones, grooves, abnormal texture, shields, supports, earlier repairs and route obstructions. It is a surface-observation method, not a thickness measurement.

Use controlled terms in the report: observed, not observed in an adequate view, not visible and not inspected. A polished band or flat may support an erosion hypothesis, but appearance alone does not prove sootblower alignment, wet steam, fly-ash loading, thermal fatigue or remaining wall. Photograph the wider row relationship before taking the close view so another reviewer can place the indication.

Cleaning and surface condition

Record the as-found deposit before any local preparation. Show its extent, texture and relationship to the tube face, then identify who authorized cleaning, which area changed and how removed material was controlled. Once the deposit is gone, the original condition cannot be reconstructed from memory.

An obscured surface is not a clean result, and a visually clean patch is not automatically ready for UT. Surface roughness, adherent scale, curvature and access may still prevent stable seating or coupling. Cleaning and UT preparation remain owner-approved work scopes outside the MicroRover inspection workflow.

Targeted UT confirmation

Visual triage selects a candidate location. The approved UT procedure decides whether it yields a valid thickness. Confirm the material and sound-velocity basis, tube curvature, surface condition, local preparation, couplant, probe seating, temperature, equipment checks, accepted back-wall response, repeatability treatment and units required by that procedure.

ISO 16809:2025 covers ultrasonic thickness-determination principles, while ISO 16831:2025 separately addresses characterization and verification of thickness equipment. The distinction matters: verified equipment does not rescue an unstable field response caused by geometry, roughness, scale, temperature, material behavior or poor coupling.

ASTM E797/E797M-21 addresses manual contact pulse-echo thickness measurement. Do not assume that a manual practice, probe limit or acceptance rule automatically applies to robotic acquisition. The project NDT authority must approve the actual technique, reference blocks, checks, response rules and retained evidence for the configured application.

Reject an unstable echo. Label the location invalid or no data, preserve the context required by the procedure and route it to preparation, repeat acquisition, manual access or another qualified method. A coerced number fills a table and weakens the decision.

Bound the wear zone

Expand from the indication under the owner’s procedure until the required boundary is established or the method reaches a declared limit. The expansion may include adjacent axial positions, other clock positions, the opposite exposed face, neighboring tubes or deeper rows. Let the impact direction, bank geometry and damage hypothesis set the pattern; there is no defensible universal grid pitch.

Keep every expansion point tied to the same location system and evidence state. If a bend changes probe seating, a support masks the face, deposits remain, or the route cannot reach the next row, stop crediting coverage there. Record the boundary and hand the unresolved zone to the role that can change access or method.

Where MicroRover fits

MicroRover, also called MicroBot, is designed for constrained second-pass tube-bank access. It carries seven Full HD cameras and supports UT spot readings where applicable, with encoder-correlated video or notes. In this workflow, the cameras locate and document visible conditions; targeted UT adds local thickness only where the configured technique returns an acceptable response.

PetroBot uses 35 mm minimum coil spacing as an initial screen, not a route-clearance promise. Review openings, actual spacing, bends, row access, supports, loose deposits, temperature, platform stability, power, clean couplant water, tether path, exclusion zones and recovery before mobilization. A nominal gap on a drawing does not prove that the complete route is traversable or measurable.

Close the report on the coverage actually achieved, not the route originally planned. Every unreachable tube, unseen face, interrupted segment and rejected UT response belongs in the exception list with its location and next-method decision.

Join inspection with sootblower history

Match the mapped condition to the sootblower identity and the records that describe its operation. Where available, review maintenance and alignment checks, travel or sequence, pressure and temperature, moisture-trap or drain concerns, ash and deposit history, recent leaks and prior tube or shield work. Location correlation can strengthen or weaken a mechanism hypothesis; it does not automate diagnosis.

EPRI’s sootblower-erosion guidance links visual examination and UT wall-thinning evaluation with checks on sootblower condition and operation. That loop matters after a repair. Replacing wall without addressing a credible blower or operating contributor can leave the same exposure in place. The owner and OEM determine which operating, hardware or protection changes are justified.

Preserve alternative explanations. Erosion-corrosion, thermal fatigue, fuel-ash corrosion, overheating and other mechanisms can overlap in appearance or location. A qualified failure analysis may need operating history, metallography, deposit analysis or another examination method before the plant assigns cause.

When does the job need another method?

Change method when the unanswered zone matters more than the convenience of the current route. Manual access, different remote visual access or another qualified NDT method may be needed when deposits remain, the required face cannot be seen, coupling fails, geometry produces an unstable response, or the suspected mechanism is not addressed by visual and spot-thickness evidence.

MicroRover’s main role here is suitable second-pass access. MagRover is a separate first-pass option where geometry, ferromagnetic surface condition and site controls support magnetic adhesion. Do not substitute one product’s stated capability for the other. Screen the actual route and preserve the handoff boundary.

A changed method still needs the same location language. The follow-up team should receive the bank, row, tube, face, reference direction, indication record and exact no-data reason. Otherwise the handoff creates a second set of evidence that cannot be joined to the first.

What makes the deliverable decision-ready?

Build the deliverable so an independent qualified reviewer can reconstruct the work. Include the approved drawing and location convention, planned and achieved routes, sootblower and gas-path references, as-found imagery, observation terms, targeted-UT setup and validity records, accepted values, rejected responses, inaccessible faces, preparation history and links to the required raw evidence.

Keep observation, measurement, hypothesis and disposition in separate fields. A video note may say ‘smooth flattened area observed on the sootblower-facing side.’ The measurement record may say ‘valid UT spot at the named position’ or ‘unstable response—no data.’ A reviewer may propose a mechanism. The owner’s authorized roles issue the action.

Name the code edition, inspection program and acceptance rules that govern the work. ASME BPVC Sections I, V and VII, the 2025 NBIC and API RP 573 cover different scopes, so apply each reference only where the owner or jurisdiction has adopted it. ISO 9712:2021 sets out an NDT personnel qualification and certification framework; project records establish who is qualified and which deployed configuration is approved.

Pre-quotation checklist

Send enough information to test route, method and decision fit before the outage starts. The package should identify the exact boiler and bank, the governing inspection question and the roles that will accept data and issue disposition. Unknowns should remain visible in the proposal instead of turning into assumed coverage.

Frequently asked questions

Technical references

• Babcock & Wilcox — Finding the Root Cause of Boiler Tube Failure
• Babcock & Wilcox — Sootblower and Boiler Cleaning Terminology, Principles and Applications
• EPRI 1008037 — Tube Repair and Protection for Damage Caused by Sootblower Erosion
• ASNT — Visual Testing
• ISO 16809:2025 — Non-destructive testing: ultrasonic thickness determination
• ISO 16831:2025 — Verification of ultrasonic equipment for thickness determination
• ASTM E797/E797M-21 — Manual ultrasonic pulse-echo contact thickness measurement
• ISO 9712:2021 — Qualification and certification of NDT personnel
• ASME — 2025 Boiler and Pressure Vessel Code
• National Board — 2025 National Board Inspection Code
• API — Standards Plan listing for API RP 573