9.1 Internal & External Vessel Inspection Practices (API 572)

Internal & External Vessel Inspection Practices (API RP 572)

For the current exam cycle, API RP 572 is included with Annex A and Annex B; Annex C is excluded. Use the effectivity sheet for the exact edition and annex boundary.

For the current exam cycle, API RP 572 is included with Annex A and Annex B; Annex C is excluded. Use the effectivity sheet for the exact edition and annex boundary.

For the current exam cycle, API RP 572 is included with Annex A and Annex B; Annex C is excluded. Use the effectivity sheet for the exact edition and annex boundary.

In the refining and petrochemical industries, pressure vessels operate under extreme thermodynamic conditions, handling volatile hydrocarbons, hydrogen, acid gases, and corrosive chemicals. Ensuring the structural integrity and mechanical reliability of these assets throughout their operating lifecycle requires rigorous, standardized inspection methodologies. API Recommended Practice 572 (Inspection Practices for Pressure Vessels: Towers, Drums, Reactors, Heat Exchangers, and Condensers) provides the foundational inspection guidelines referenced directly by API 510 (Pressure Vessel Inspection Code).

An Authorized API 510 Inspector must possess an exhaustive understanding of both external on-stream evaluations and internal out-of-service examinations. This includes the mandatory safety protocols governing confined space entry, the systematic execution of visual inspections (VT), the identification of degradation morphology, and the deployment of specialized measurement tooling.

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1. Entry Readiness and Isolation

Internal vessel entry is controlled by the owner-user's approved confined-space and hazardous-energy program. Before the inspector enters, the responsible facility personnel must verify the vessel is isolated from process material and energy, depressured, drained, cleaned as required, and covered by an authorized permit and rescue plan.

Isolation must address every connected process path and energy source. Blinds or blanks, spool removal, locked valves with bleed provisions, disconnections, or other methods may be used only when they satisfy the facility procedure and applicable requirements for the actual system. The inspector confirms documented status and does not treat one unverified closed valve—or personal protective equipment—as a substitute for isolation.

Atmospheric testing addresses oxygen, flammability, and each toxic contaminant identified by the hazard assessment. Acceptance limits, test locations, ventilation, and continuous or periodic monitoring frequency come from the entry permit and applicable requirements. Conditions can change as deposits are disturbed, so an initially acceptable reading does not end monitoring responsibility.

Cleaning should expose the surfaces needed for inspection without creating a new chemical, pyrophoric, thermal, or pressure hazard. Iron sulfide and process residues require a service-specific plan. Lighting, access equipment, communications, attendants, and inspection tools must be compatible with the classified location and permit.

2. External Vessel Inspection Protocols (API RP 572 Section 8)

An external visual inspection per API 510 (Section 5.5) and API RP 572 (Section 8) is performed while the vessel is in-service or out-of-service to verify the mechanical integrity of structural supports, foundations, exterior pressure boundaries, attachments, and weatherproofing.

+-----------------------------------------------------------------------------+
|               EXTERNAL PRESSURE VESSEL INSPECTION CHECKLIST                 |
|                                                                             |
|   [FOUNDATIONS & CONCRETE]                                                  |
|   - Check for differential settlement, cracking, spalling, chemical attack  |
|   - Verify vessel plumbness (vertical columns) and levelness (horizontals)  |
|   - Inspect concrete-to-steel interface for water pooling and crevice rust  |
|                                                                             |
|   [ANCHOR BOLTS & BASE RINGS]                                               |
|   - Check for heavy corrosion/necking at base plate contact zone            |
|   - Verify full thread engagement, tight double nuts, and lock washers      |
|   - Ensure anchor bolt holes allow slotted movement on sliding saddles      |
|                                                                             |
|   [SUPPORT SKIRTS & SADDLES]                                                |
|   - Inspect skirt fireproofing for cracks, spalling, and water ingress      |
|   - Verify skirt WEEP HOLES are open, clear, and unplugged                  |
|   - Examine skirt-to-head weld for fatigue cracking and thermal distortion  |
|   - Inspect horizontal vessel saddles, slide plates, and anchor bolt slots  |
|                                                                             |
|   [INSULATION, CLADDING & CUI]                                              |
|   - Look for punctured jackets, missing caulking, sagging insulation        |
|   - Target high-risk CUI zones: nozzle penetrations, rings, support clips   |
|   - Perform thermal imaging (thermography) or UT through insulation plugs   |
|                                                                             |
|   [GROUNDING LUGS, NAMEPLATE & APPURTENANCES]                               |
|   - Verify grounding cable bonding continuity and corrosion-free lugs       |
|   - Confirm ASME/API nameplate is securely attached, clean, and legible     |
|   - Inspect ladders, platforms, davits, and structural attachment welds     |
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Foundations, Anchor Bolts & Structural Alignment

• Concrete Foundations: Inspected for differential settlement that introduces severe bending stresses into attached piping and nozzles. Concrete is evaluated for spalling, freeze-thaw cracking, chemical degradation (acid or amine erosion), and rust staining indicating corroded rebar.
• Anchor Bolts: Particularly vulnerable to crevice corrosion at the junction where the bolt enters the concrete or passes through the base plate. Bolts should be tapped with a light hammer to detect subsurface fracture or necking. Ensure nuts are fully engaged (at least flush with the bolt end, preferably showing 1-2 exposed threads) and not severely corroded.
• Sliding Saddles on Horizontal Vessels: Horizontal vessels undergo thermal expansion and contraction. One saddle is fixed (bolted tight in round holes), while the opposite saddle is sliding (slotted holes with loose nuts or Teflon/bronze slide plates). The inspector must verify that the slotted holes are clean, lubricated, unpainted, and free of debris to prevent thermal binding that could buckle the shell or shear the foundation.

Support Skirts, Fireproofing & Weep Hole Diagnostics

Vertical towers and reactors are typically supported by cylindrical steel skirts welded directly to the bottom formed head. Key inspection focus areas include:

1. Fireproofing Integrity: Concrete, gunite, or lightweight fireproofing applied to the skirt exterior must be examined for cracks, bulge deformation, and delamination. Moisture entering cracked fireproofing causes aggressive, hidden exterior corrosion of the steel skirt.
2. Skirt-to-Bottom-Head Attachment Weld: This weld experiences high cyclic thermal and mechanical bending stresses. It must be inspected visually and with magnetic particle (MT) or liquid penetrant (PT) testing for fatigue cracking.
3. Skirt Weep Holes: API RP 572 mandates that weep holes in skirts and bottom head support zones must remain open and unobstructed:
   • They prevent moisture/water buildup inside the enclosed top cavity of the skirt.
   • Critically, they provide an immediate visual leak indicator if the bottom head or attachment weld breaches, preventing pressurized process fluid from accumulating inside the skirt fireproofing.

Grounding Lugs, Nameplates & Insulation Weatherproofing

• Electrical Grounding Lugs: Must be inspected for tight, uncorroded bonding connections. Static electricity generated by hydrocarbon flow must safely dissipate to ground to prevent internal spark ignition.
• ASME / API Nameplate: The nameplate is a legal code artifact. The inspector must verify that it is legible, securely welded/bracketed to the vessel shell (never buried under insulation without an external duplicate or viewing cutout), and accurately lists the MAWP, Design Temperature, Minimum Design Metal Temperature (MDMT), Year Built, and ASME "U" or "U2" Stamp.
• Insulation & CUI: Weather-jacketing seams, bands, and nozzle seals are scrutinized for moisture ingress. Corrosion Under Insulation (CUI) occurs primarily between $10^\circ\text{F} (-12^\circ\text{C})$ and $350^\circ\text{F} (175^\circ\text{C})$ for carbon steel, peaking between $170^\circ\text{F} (77^\circ\text{C})$ and $250^\circ\text{F} (121^\circ\text{C})$.

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3. Internal Vessel Inspection Protocols (API RP 572 Section 9)

Out-of-service internal inspection provides direct, unobstructed access to the vessel's pressure-retaining boundary, weld seams, and internal structural components.

+-----------------------------------------------------------------------------+
|               INTERNAL PRESSURE VESSEL INSPECTION WORKFLOW                  |
|                                                                             |
|   [1. PREPARATION & INITIAL OBSERVATION]                                    |
|   - Provide 12V/24V explosion-proof lighting and thorough cleaning          |
|   - Record "as-found" condition: color/thickness of deposits, foulants, coke|
|   - Collect scale/deposit samples for chemical and metallurgical analysis   |
|                                                                             |
|   [2. SHELL & HEAD PRESSURE BOUNDARY SCANNING]                              |
|   - Perform 100% visual scan of shell courses, heads, and transition cones  |
|   - Identify uniform thinning, localized pitting, grooving, erosion         |
|   - Measure pit depths with mechanical dial/needle pit depth gauge          |
|   - Check for hydrogen blistering, step-wise cracking, and laminations      |
|                                                                             |
|   [3. WELD SEAM & NOZZLE REINFORCEMENT EVALUATION]                          |
|   - Inspect longitudinal (Cat A) and circumferential (Cat B) weld seams     |
|   - Inspect nozzle-to-shell attachment welds (Cat D) for corner cracking    |
|   - Target Heat-Affected Zones (HAZ) for preferential weld attack and SCC   |
|   - Check repad weep holes (ensure open, dry, and free of process leakage)  |
|                                                                             |
|   [4. STRUCTURAL DISTORTION & CRACKING ASSESSMENT]                          |
|   - Check for shell bulges, out-of-roundness, and vacuum buckling           |
|   - Measure thermal sag/distortion on internal support ledges and beams     |
|   - Perform surface NDE (WFMT, PT, ACFM) on high-stress weld junctions      |
+-----------------------------------------------------------------------------+

Visual Scanning & Degradation Topography

Upon entering the cleaned vessel under adequate lighting (minimum 100 foot-candles / 1000 lux recommended for detailed visual examination):

1. Initial "As-Found" Assessment: Before abrasive blasting or high-pressure washing, note the distribution of deposits, sludges, and corrosion product. The appearance of scale (e.g., green nickel salts, red ferric oxide, black iron sulfide) reveals the local chemical environment.
2. Pitting Morphology: Pitting is measured using a mechanical dial or needle pit depth gauge:
   • Record maximum pit depth, pit diameter, and pit cluster density (pitting frequency per unit area).
   • Evaluate pit clusters against the API 510 pitting criteria (Section 7.4.3: pitting depth cannot exceed the corrosion allowance, total area of pit clusters cannot exceed $2\text{ in.}^2 [13\text{ cm}^2]$ in any $8\text{ in.} [200\text{ mm}]$ diameter circle, and pit length along an inspection line cannot exceed $2\text{ in.} [50\text{ mm}]$ in any $8\text{ in.}$ length).
3. Grooving & Erosion-Corrosion: Common adjacent to inlet nozzles, downcomer exit points, reboiler return lines, and around liquid-vapor interfaces where turbulence and velocity strip protective passive films.
4. Hydrogen Blisters & Bulges:
   • Hydrogen Blistering: Occurs in sour ($H_2S$) service when atomic hydrogen diffuses into the steel and recombines into molecular hydrogen ($H_2$) at microscopic laminations or inclusion interfaces, creating extreme internal pressure that plastically deforms the surface into blisters.
   • Thermal Bulges & Creep: In high-temperature reactors or fired vessels, localized hotspots cause shell yielding and bulging.

Weld Seam Scanning & Heat-Affected Zone (HAZ) Examination

All weld seams—longitudinal, circumferential, head-to-shell, and nozzle attachment welds—must be visually scanned and inspected for:

• Preferential Weld Corrosion: Accelerated galvanic or microstructural corrosion targeting the weld deposit or HAZ.
• Environmental Cracking: Such as Caustic Embrittlement, Amine SCC, Chloride SCC, and Wet $H_2S$ cracking (HIC/SOHIC). Cracking predominantly initiates in the hardened HAZ adjacent to the fusion line.
• Nozzle Attachment Welds (Category D): The inside corner radius of nozzle-to-shell welds is a severe stress concentration zone susceptible to fatigue cracking and thermal shock cracking.

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4. Essential Inspection Tooling & Measurement Techniques

API RP 572 outlines the standard toolkit required for comprehensive vessel inspection:

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5. Inspection-Planning Checks

• Verify the entry/isolation documentation; do not invent a universal valve or blind rule from an inspection guide.
• Tie each inspection location and method to an expected damage mechanism.
• Treat supports, skirts, nozzles, insulation, and internals as part of the condition assessment even when they are not all pressure-retaining.
• Record location, extent, morphology, dimensions, photographs, and NDE identifiers so findings are reproducible.
• Escalate crack-like flaws, distortion, active leakage, or thickness below acceptance criteria for appropriate engineering evaluation.