9.2 Structural Components, Internal Trays, Linings & Skirts
Key Takeaways
- Inspect trays, packing, distributors, demisters, baffles, and supports for corrosion, erosion, fouling, distortion, loose or missing hardware, and evidence of abnormal hydraulic loading.
- Metallic strip linings, clad plate, and weld overlay have different failure modes; inspect both barrier integrity and the possibility of concealed base-metal damage.
- Nonmetallic linings require visual assessment plus a procedure appropriate to material, thickness, substrate, access, and expected defects.
- Holiday-test voltage and method are selected and calibrated from the governing coating procedure and manufacturer limits, not from one universal thickness threshold.
- Low-pressure leak checks at telltale or weep openings require an approved pressure, medium, isolation, and safety plan; never apply unrestricted plant air.
Structural Components, Internal Trays, Linings & Skirts
Pressure vessels utilized in distillation, absorption, reaction, and separation processes contain complex internal structural components and protective barrier linings. While internal non-pressure components are generally not designed to contain pressure, their mechanical failure, corrosion, fouling, or dislodgement can cause severe process disruptions, catastrophic column flooding, accelerated pressure shell corrosion, and loss of containment.
API Recommended Practice 572 provides detailed guidance for inspecting internal trays, packing assemblies, structural supports, metallic claddings, weld overlays, refractory linings, and non-metallic coatings. An Authorized API 510 Inspector must be proficient in identifying the root causes of internal component failure, evaluating lining integrity, and executing specialized nondestructive examination (NDE) protocols.
1. Distillation Column Internals & Fractionation Trays
Fractionation towers rely on horizontal tray decks to achieve intimate vapor-liquid contact. Tray designs fall into three primary categories:
+-----------------------------------------------------------------------------+
| DISTILLATION COLUMN TRAY CONFIGURATIONS |
| |
| [BUBBLE CAP TRAYS] |
| - Bell-shaped caps with slotted skirts over vertical vapor risers |
| - Cap held down by center bolts/frogs; fixed vapor flow path |
| - Highest turndown ratio; heavy, expensive, prone to fouling/corrosion |
| |
| [SIEVE TRAYS] |
| - Flat sheet metal decks perforated with round holes (1/8 in. to 1/2 in.) |
| - Vapor velocity prevents liquid from weeping through perforations |
| - Simple, low cost, susceptible to weeping at low rates and hole fouling |
| |
| [VALVE TRAYS] |
| - Perforated decks fitted with movable metal disks (valves) in cages |
| - Valves lift as vapor flow increases; close as vapor flow drops |
| - Excellent turndown and efficiency; valves can stick, wear, or blow out |
+-----------------------------------------------------------------------------+
Critical Tray Components & Inspection Parameters
- Tray Decks & Levelness:
- Levelness Tolerance: Trays must be level within strict engineering tolerances (typically $\pm 1/8\text{ in.} [\pm 3\text{ mm}]$ across the entire diameter). Out-of-level trays cause liquid maldistribution, channeling, vapor bypassing, and severe loss of separation efficiency.
- Weep Holes: Small drainage weep holes in tray decks must be verified clear to permit complete drainage during shutdown and prevent hydrostatic overloading during startups.
- Downcomers, Weirs, and Seal Pans:
- Inlet and Outlet Weirs: Inspected for corrosion, warping, and correct weir height.
- Downcomer Clearance: The vertical gap between the lower edge of the downcomer apron and the tray deck (or seal pan) must be measured. If the gap is too small, liquid backs up the column (flooding); if too large, vapor blows up the downcomer, destroying tray hydraulics.
- Seal Pans: Must be checked for corrosion thinning, sediment accumulation, and weld integrity.
- Support Rings, Trusses, and Hold-Down Hardware:
- Tray Support Rings: Heavy circumferential angle or bar rings welded to the vessel shell. The fillet weld attaching the ring to the shell must be inspected for cracking and undercut.
- Support Beams & Trusses: Major structural beams spanning large-diameter towers must be inspected for bending deflection, warping, and corrosion.
- Hold-Down Clamps and Fasteners: Bolted clamps secure tray panels to support rings, with slotted holes to accommodate differential thermal expansion between the alloy tray and the carbon steel shell. The inspector must check for loose, missing, or corroded bolts, missing washers, and cracked alloy clips.
2. Auxiliary Structural Internals: Packings, Demisters & Baffles
Beyond cross-flow trays, modern columns and separation drums incorporate packed beds, mist eliminators, and fluid flow conditioners.
+-----------------------------------------------------------------------------+
| AUXILIARY VESSEL INTERNALS & DEGRADATION MODES |
| |
| [PACKED BEDS (RANDOM & STRUCTURED)] |
| - Random Packing: Pall rings, Raschig rings, Intalox saddles |
| - Structured Packing: Corrugated, perforated embossed metal sheets |
| - Failures: Crushed elements, fouling/coking, dislodged bed limiters |
| |
| [DEMISTER PADS / MIST ELIMINATORS] |
| - Knitted wire mesh or chevron/vane packs in knock-out drums and towers |
| - Failures: Heavy particulate plugging, corrosion of tie-wires/grids, |
| pad dislodgement allowing liquid carryover into compressors/downstream |
| |
| [VORTEX BREAKERS & FEED SPARGERS] |
| - Cross-baffle, cruciform, or grated grilles over bottom outlet nozzles |
| - Feed spargers/dip tubes: perforated distributor pipes delivering feed |
| - Failures: Flow-induced vibration fatigue, nozzle throat erosion-corrosion|
+-----------------------------------------------------------------------------+
Demister Pads (Mist Eliminators)
Demister pads coalesce entrained liquid droplets from vapor streams. The inspector must check:
- Plugging & Fouling: Polymer, salt, or carbonaceous scale plugging the mesh, which increases pressure drop ($\Delta P$) and can collapse the support grid.
- Grid & Wire Integrity: Wire mesh corrosion, broken tie wires, and corrosion thinning of top and bottom support grids.
- Bypass Gaps: Gaps between pad sections or between the pad perimeter and the vessel shell that allow vapor to bypass the mesh without demisting.
Vortex Breakers & Internal Baffles
Vortex breakers installed over bottom liquid outlet nozzles prevent the formation of a liquid vortex that draws vapor into pump suction lines. Inspectors must verify:
- Attachment Weld Integrity: Heavy fluid turbulence can cause high-cycle fatigue cracking of the attachment welds.
- Corrosion & Erosion: High fluid velocity accelerates localized wall loss on the breaker vanes and adjacent bottom head shell plate.
3. Root Causes of Internal Component Failures
Internal vessel components fail primarily due to dynamic process upsets, extreme hydraulic loads, or chemical degradation:
| Failure Mechanism | Operational Root Cause | Observable Physical Damage | Preventative / Remedial Actions |
|---|---|---|---|
| Pressure Surges / Liquid Slugs | Rapid unit depressurization, boiler feed water trips, or water entering hot hydrocarbon tower causing sudden steam flashing (1 volume water expands to $\sim 1600$ volumes steam). | Trays ripped off support rings, deformed/crumpled decks, bent beams, dislodged packing. | Install robust hold-down clamps, explosion-hatch trays, strict startup drying procedures. |
| Flow-Induced Vibration | High-velocity two-phase inlet feed streams or acoustic pulsation from compressors. | Fatigue cracking at sparger weld necks, loosened tray clamp bolts, fretting wear at beam seats. | Install inlet impingement baffles, stiffen structural members, use self-locking nuts. |
| Fouling, Coking & Scaling | Thermal cracking of heavy hydrocarbons, salt crystallization, iron sulfide accumulation. | Blocked tray perforations, stuck valves, plugged demisters, extreme differential pressure ($\Delta P$). | Scheduled chemical solvent washing, anti-foulant injection, optimized temperature control. |
| Localized Erosion-Corrosion | High-velocity fluid jetting from nozzles without adequate impingement protection. | Grooving, metal thinning opposite inlet nozzles, perforated sparger walls. | Install 300-series SS or alloy impingement plates, upgrade sparger metallurgy. |
4. Metallic Linings: Strip Linings, Clad Plate & Weld Overlay
To protect inexpensive carbon or low-alloy steel pressure shells from severe corrosion, vessels are frequently constructed with internal metallic barrier linings.
+-----------------------------------------------------------------------------+
| METALLIC VESSEL LINING CLASSIFICATIONS |
| |
| [STRIP LININGS] |
| - Thin alloy sheets (12-16 gauge / 0.05-0.10 in.) fillet-welded to shell |
| - Narrow strips (3 to 6 in. wide) welded with alloy filler (e.g., 309L) |
| - Prone to thermal expansion buckling, tearing, and hidden shell leaks |
| |
| [INTEGRALLY CLAD PLATE] |
| - Metallurgically bonded alloy sheet (roll-bonded or explosion-bonded) |
| - 100% surface bond to base carbon steel (ASME SA-263, SA-264, SA-265) |
| - High mechanical integrity; inspect for disbonding, thinning, cracking |
| |
| [WELD OVERLAY (CRA)] |
| - Fusion-deposited corrosion-resistant alloy (GMAW, SAW, electroslag) |
| - Minimum two layers typical (e.g., 309L buffer + 347 or 316L top layer) |
| - Susceptible to dilution cracking, hot tears, and hydrogen disbonding |
+-----------------------------------------------------------------------------+
Strip Linings vs. Clad Plate vs. Weld Overlay
- Strip Linings: Constructed by attaching narrow alloy strips (such as Monel, 316L, or 410S) directly to the carbon steel shell using fillet welds. Because the thermal expansion coefficient of austenitic stainless steel is $\sim 30%$ greater than carbon steel, thermal cycling creates severe compressive stresses that cause strip buckling, bulging, and cracking at fillet welds. If process fluid leaks behind a strip, accelerated corrosion of the carbon steel shell occurs unseen.
- Clad Plate: The alloy layer (typically $1/16\text{ in.} \text{ to } 1/8\text{ in.}$ thick) is integrally bonded to the base plate at the steel mill during hot rolling or via explosive bonding. It behaves as a monolithic plate and cannot buckle under thermal cycles.
- Weld Overlay: Deposited directly onto the base metal using automated welding heads. Inspectors must verify the chemical dilution of the top layer and scan for hydrogen disbonding at the cladding-to-base-metal interface in high-pressure hydrogen service (e.g., hydrocrackers).
5. Non-Metallic Linings & Protective Coatings
Non-metallic linings provide chemical resistance and thermal insulation for severe services:
+-----------------------------------------------------------------------------+
| NON-METALLIC LININGS & COATING SYSTEMS |
| |
| [REFRACTORY LININGS (GUNITE, CASTABLE, BRICK)] |
| - Thermal barrier and erosion resistance (FCCUs, sulfur plants, cokers) |
| - Anchored to shell by welded hex-mesh or V-anchors |
| - Failures: Spalling, thermal cracking (>1/8 in.), anchor burn-off, coke |
| behind refractory causing shell overheating (detected by thermography) |
| |
| [GLASS-LINED STEEL] |
| - Vitreous enamel fused to steel at > 1500°F (acid synthesis reactors) |
| - Highly corrosion resistant to mineral acids; zero tolerance for flexure |
| - Failures: Mechanical impact chipping, thermal shock (>100°F delta T) |
| |
| [POLYMERIC & ELASTOMERIC COATINGS] |
| - Rubber lining (HCl / phosphoric acid), epoxy, vinyl ester, novolacs |
| - Applied to grit-blasted steel (NACE No. 1 / SSPC-SP 5 white metal) |
| - Failures: Blistering (osmotic/gas), holiday pinholes, hardening/gouging |
+-----------------------------------------------------------------------------+
6. Lining Inspection and Leak Checks
Select the method from the lining type and expected damage:
| System | Useful examination focus |
|---|---|
| Metallic strip lining | Visual weld examination, bulge/disbondment checks, leak-path investigation, targeted surface or volumetric NDE |
| Integrally clad plate or weld overlay | Thickness, surface cracking, chemistry where relevant, and interface disbondment methods suited to the construction |
| Refractory | Visual cracking/spall survey, anchor condition, sounding where appropriate, and external temperature evidence |
| Polymer/rubber/glass coating | Visual survey and a calibrated holiday method compatible with coating type and thickness |
Wet-sponge and higher-voltage holiday methods have different uses. The approved coating specification, material thickness, substrate, manufacturer guidance, and qualified procedure establish the method and voltage. Excess voltage can damage sound coating; insufficient sensitivity can miss a holiday.
Acoustic sounding can help identify a change in bond response, but it is comparative and technique dependent. Confirm suspicious areas by an appropriate follow-up method rather than declaring acceptance from sound alone.
Telltale or weep-opening leak checks use controlled low pressure and a suitable leak-detection medium under an approved procedure. Verify the pressure limit for the actual detail and lining. Isolate the test, use a regulator and relief protection, and never connect unrestricted plant air to a small trapped cavity or thin lining.
7. Reporting
Document component identity, location, lining type, surface preparation, method and equipment, calibration or sensitivity checks, test conditions, indication locations, photographs, measurements, and disposition. Compare the observed damage with process history and the base-metal inspection plan; a sound-looking barrier does not establish that concealed shell damage is absent.
During an internal inspection of a crude distillation tower, an inspector discovers that several sections of 316L stainless steel strip lining welded to the carbon steel shell have buckled and cracked at the fillet welds. What is the PRIMARY metallurgical and thermal root cause of this failure?
When performing a pneumatic leak test of reinforcing pad (repad) weep holes or metallic strip liner weep holes to detect weld breaches, what is the MAXIMUM allowable test pressure permitted by API RP 572?
An inspector uses a ball-peen inspection hammer to tap across a metallic strip-lined vessel head and an internal refractory lining. Which acoustic response indicates sound bonding versus disbonding?
What determines the appropriate holiday-detection method for a vessel coating or lining?