Pressure Vessel Nozzle Inspection: Separate Shell UT From Nozzle Gaps

What does a shell reading near a nozzle tell you?

A valid shell reading describes the prepared point where the probe made contact. At the pad edge, weld crown or neck, both geometry and contact change. Mark an attempted point rejected or a blocked position unexamined; never fill either with the nearby shell value.

Start the work pack with the inspection question. If it concerns wall loss on the shell, lay out a shell band with identifiable points. If it concerns the neck, weld or a suspected leak, give that feature its own examination scope. API 510 addresses in-service vessel inspection and evaluation; API RP 572 covers vessel-inspection planning, methods and records.

Which nozzle features belong in the scope?

Walk the drawing and the asset together. Identify the accessible shell band, nozzle neck, shell-to-nozzle weld region, reinforcing pad if fitted, flange and bolting, connected piping and each quadrant hidden by permanent steel. Note insulation, coating and repairs where present. A pad is not assumed on every nozzle; use the as-built detail.

For each feature, write the credible condition from service and prior records: external corrosion, localized pitting, coating breakdown, staining, deformation or a reported leak. Record what is visible without assigning a damage mechanism from color alone. A suspected crack or weld flaw calls for a technique selected for that flaw and geometry; ordinary thickness spots cannot establish crack freedom.

How should the vessel and nozzle be located?

Fix a reproducible vessel datum before collecting data: vessel ID, drawing revision, axial origin, circumferential zero, direction and viewing end. Tie the nozzle tag and centerline to that system. Around the nozzle, use clock positions viewed from a named direction, distance from the pad or weld edge, and a photograph that shows the fixed reference.

Put the convention on the route sketch and in the image index. A point marked only ‘near nozzle’ cannot be revisited; a clock position without its viewing direction can send the next crew to the opposite side. Keep the attempted location even when its measurement is rejected.

Where can visual inspection help?

Photograph the nozzle from an orientation view, then capture each accessible neck, weld, pad edge and flange quadrant. Record coating loss, corrosion products, staining, deformation, leakage clues and the exact edge of an obstruction. A close view needs a location and direction; otherwise it may be impossible to place against the nozzle drawing.

Visual inspection describes exposed surfaces. It does not measure remaining wall or establish the condition behind a pad, under a flange or inside the bore. OSHA's pressure-vessel inspection manual treats visual and ultrasonic examinations as different methods with different detection reach. Send a suspicious feature to the qualified inspector with its image and location, rather than naming a hidden defect from appearance.

When is a contact-UT point valid?

Accept a thickness point only under the approved procedure for this material, geometry and equipment. Check the prepared surface, seating, couplant, reference setup, material velocity and a stable, repeatable backwall response. Preserve the response and setup with the coordinate. An instrument display alone is not an accepted thickness result.

Curvature, a weld crown or the pad edge can change contact and the echo. ASTM E797/E797M-21 is a manual pulse-echo contact practice whose published scope is primarily flat, parallel surfaces and has limited applicability to nonparallel or concentric ones. ISO 16809:2025 describes time-of-flight thickness determination, while ISO 16831:2025 addresses equipment verification. Apply the job's qualified procedure and reject a position when its validity criteria fail; a nearby good point does not replace it.

Where does MagRover fit?

Use MagRover for camera-supported visual work and UT spot or scan acquisition on a suitable accessible ferromagnetic shell route. PetroBot lists vessel shells among its applications and bounds the pressure-vessel service to surfaces suitable for magnetic adhesion and UT preparation. The assignment is an adjacent shell route, not an automatic examination of the nozzle neck or weld.

Trial representative material, coating, curvature and clearance before setting the route. Confirm adhesion, stable travel, probe seating, response, location control, tether path and recovery. If the trial cannot demonstrate one segment, draw its boundary and give that segment another access or method plan.

What stops robotic coverage?

A neck radius, weld cap, raised pad, pipe, support or coating transition can interrupt a route beside a valid shell point. Temperature, contamination and limited clearance can also affect adhesion or contact. Plan a stop point, safe retrieval and an examination handoff before the first pass.

Operations controls the site state, permits, isolation, exclusion area and recovery arrangements. Review the actual equipment configuration and area classification with the responsible team. Stop when adhesion, contact, response or retrieval no longer meets the agreed criteria; document the boundary rather than smoothing the route line across the gap.

How do you report accepted, rejected and unexamined positions?

Use three explicit states. An accepted point has a procedure-valid reading and a reconstructable coordinate. A rejected attempt retains its intended coordinate, attempted method and reason: poor seating, unstable echo, failed setup or lost location. An unexamined position had no attempt because access, geometry, insulation or safety prevented one.

Attach the date, performer, method and setup to the record; link the image or response to the vessel ID and nozzle tag. For US facilities covered by OSHA's process-safety-management rule, 29 CFR 1910.119(j)(4) specifies inspection and test documentation including equipment identification, performer, date, description and result. Elsewhere, follow the owner's applicable record procedure while retaining the same traceable locations and outcomes.

When should another NDT method or access plan take over?

Hand off when the question changes or the route fails. A thickness spot addresses local wall thickness; it cannot clear a suspected crack, assess weld quality or measure steel hidden beneath a pad. The qualified inspector selects an examination technique for the suspected condition, material and geometry, then defines its own coverage and acceptance criteria.

If insulation covers the shell, plan exposure or a separately validated screening method with its own limits. If piping blocks the lower quadrant, arrange controlled access or record why it must wait. Internal bore condition belongs to an internal examination plan. Keep these tasks tied to the same nozzle tag so the final report can show where one method ended and another began.

What does the owner decide after the survey?

Give the owner-authorized inspector one reconciled map: accepted readings and their actual footprint, visible observations, rejected attempts, blocked features and assigned follow-up. Compare that map with the planned component list. Resolve each remaining gap by a named method, access change, later examination or engineering review under the vessel program.

The inspector and engineering team apply design, service history, prior results and the applicable assessment basis to decide whether more examination, repair or another action is needed. The inspection deliverable should let them find the evidence and its boundary quickly. A list of shell numbers without the unresolved nozzle locations cannot do that job.

Frequently asked questions

Technical references

• API 510 — Pressure Vessel Inspection Code
• API RP 572 — Inspection Practices for Pressure Vessels
• ASTM E797/E797M-21 — manual ultrasonic contact thickness
• ISO 16809:2025 — ultrasonic thickness determination
• ISO 16831:2025 — verification of ultrasonic thickness equipment
• OSHA — Pressure Vessel Guidelines
• OSHA — 29 CFR 1910.119(j)
• PetroBot — MagRover
• PetroBot — Pressure Vessel Inspection