FFS + FEA Validation of a Maleic Anhydride Refiner Still Pot
How an API 579 Level 3 Fitness-For-Service assessment cleared two as-built deviations on a critical pressure vessel, without rework, without schedule slip, with full code-defensible documentation.
API 579-1 / ASME FFS-1 (2021)
ASME VIII Div 2 Part 5
Petrochemical | Maleic Anhydride
Level 3 Assessment
Problem
Two as-built deviations on a critical vessel, weeks before commissioning. Action: API 579 Level 3 FFS + ASME VIII Div 2 Design-by-Analysis on the actual 3D-scanned geometry.
Outcome
Vessel cleared for 20-year service, no rework, no schedule slip, full code-defensible documentation.
Executive Summary
The Decision Problem
When a fabricator reports an as-built deviation on a critical vessel, the owner or EPCC contractor usually has three bad options: rebuild (months of slip, six- to seven-figure cost, 9–12 month stainless vessel lead time), accept the risk (a permanent unverified-deviation flag on the asset record), or negotiate (which licensees and operating authorities don’t accept). This case study documents the fourth option.
What We Did
On a Maleic Anhydride plant, two as-built deviations were flagged on the Refiner Still Pot D-16207, a 1 mm dish head thickness reduction and a Hi-Lo weld misalignment at the largest process nozzle (N11). Ideametrics performed a Level 3 Fitness-For-Service (FFS) assessment per API 579-1 / ASME FFS-1 (2021), backed by full Design-by-Analysis under ASME VIII Div 2 Part 5, using a 1.27M-element ANSYS model built from the actual 3D-scanned geometry, not idealised drawings.
Business Outcome
- Vessel went into service on schedule, zero rework, zero project slip
- 9–12 months of replacement lead time avoided (stainless vessel procurement cycle)
- Six- to seven-figure rebuild cost avoided
- Full documentation package delivered for regulator, licensee, and insurer
- No permanent “unverified deviation” flag on the operating record
Technical Outcome
- Every ASME and API 579 stress check passed across all 5 operating load cases
- Buckling safety margin at full vacuum: 2.4×
- Cumulative fatigue damage over 20 years: 0.027 (2.7% of fatigue life used)
- Both deviation locations: zero fatigue contribution
- Conservative posture, RSFa = 1.0 (no allowable-stress relaxation claimed)
Why This Is Repeatable for You
When a fabricator reports a deviation on your critical asset, the right engineering assessment does not just tell you whether the vessel is safe. It gives you documented, code-defensible evidence that replaces project risk with certainty, acceptable to licensees, regulators, and insurers without negotiation.
1. The Engineering Challenge
In an EPCC project for a Maleic Anhydride (MAN) refiner and briquetting plant, two as-built deviations were flagged on the Refiner Still Pot D-16207 just before commissioning:
- Deviation 1 – Dish head thinner than design. The South Side Dish Head was fabricated at 19.5 mm against a required minimum of 20.5 mm.
- Deviation 2 – Hi-Lo weld misalignment at Nozzle N11. 3D laser scanning of the as-built Q-lip nozzle joint revealed a measurable centreline offset, a known driver of secondary bending stress and fatigue initiation.
3D laser scan of Hi-Lo weld misalignment at Nozzle N11 on the Refiner Still Pot pressure vessel – measured offsets +10 mm and +4 mm above the vessel surface, −3 mm below the vessel surface, defining the Q-lip nozzle weld eccentricity assessed under API 579-1 Annex 2D Level 3 Fitness-For-Service.
The EPCC contractor faced a binary choice: reject and rebuild (six- to seven-figure cost, months of slip, 9–12 month replacement lead time for a stainless vessel of this size), or accept as-is with code-defensible documented proof. A standard ASME Section VIII Division 1 hand-calculation cannot evaluate either deviation. The only defensible path was a Level 3 Fitness-For-Service assessment per API 579-1 / ASME FFS-1 (2021), coupled with full Design-by-Analysis under ASME Section VIII Division 2, Part 5.
2. Why This Vessel Matters
Maleic Anhydride (MAN) is a foundational petrochemical intermediate, the precursor for unsaturated polyester resins, food/pharmaceutical additives, BDO, GBL, lubricant additives, and agrochemicals. The Refiner Still Pot sits at the heart of the refining train, where crude MAN is heated under controlled vacuum to drive off impurities.
Vacuum + Cyclic Thermal Duty
Full vacuum external pressure with a corrosive hot organic inside. Thermal swings between 93°C and 167°C, repeated 15,928 times over 20 years.
25+ Nozzles
Process inlets, outlets, instrumentation, manways. Every nozzle is a structural discontinuity, every one a potential stress concentration.
Single-Point Failure Risk
One-of-a-kind vessel. Loss of containment halts the entire refining train. Replacement lead time 9–12 months.
3. Detailed View of Nozzle N11 - the Deviation Focus
Nozzle N11 is the largest process nozzle on the vessel. It is also where the as-built Q-lip weld joint exhibited measurable Hi-Lo misalignment, the trigger for the API 579 Level 3 assessment. The images below document the as-built condition:
Figure 2a: As-built fabrication photo – chalk-marked clock positions where Hi-Lo offsets were measured
Photographic record of the as-built Nozzle N11 on the Refiner Still Pot pressure vessel during fabrication — clock-position chalk markings around the nozzle opening showing where Hi-Lo weld misalignment offsets were measured for the API 579 Annex 2D Level 3 Fitness-For-Service assessment, Q-lip joint, stainless steel SA 312 TP 316L
Figure 2b: Close-up of the Hi-Lo discontinuity at the N11 weld – red lines mark the offset profile
Close-up photo of the actual Hi-Lo weld misalignment at Nozzle N11 on the Refiner Still Pot, red lines mark the discontinuity profile across the Q-lip butt-weld joint, showing the eccentric load path that creates secondary bending stress under pressure cycling
Figure 2c: Full Hi-Lo levelness survey, radial offset (mm) at every clock position. This is the FFS input geometry.
Hi-Lo levelness survey of Nozzle N11 on the Refiner Still Pot, radial offset measurements in millimetres at every clock position from 0 to 360 degrees, ranging from +10 mm above vessel surface at 315 degrees to −3 mm below at 225 degrees, used as input geometry for the API 579 Level 3 stress analysis in ANSYS
Figure 2d: CAD side view, visible step at the N11/shell intersection where the Hi-Lo manifests
3D CAD side view of the Nozzle N11 to vessel shell intersection on the Refiner Still Pot showing the wall offset where the Hi-Lo weld misalignment manifests as a step at the butt-weld joint, visualising the eccentric load path geometry assessed under API 579-1 Annex 2D
4. Codes & Standards
This is not a one-code analysis. The work sits at the intersection of three governing frameworks:
Conservative posture: RSFa = 1.0
While API 579 Annex 2F permits a Remaining Strength Factor of 0.90, this assessment used RSFa = 1.0. No allowable-stress relaxation was claimed. The verdict had to be defensible to the client, licensee, and operating authority without further negotiation.
5. Why FEA + FFS Was the Only Defensible Path
Hand calculations under ASME Section VIII Div. 1 use code-formula stresses derived from idealised geometry. Every one of those assumptions was violated by the deviations:
What hand calculations cannot capture
- Non-linear effect of dish head thickness reduction on buckling capacity and combined stress fields
- Secondary bending stress from Hi-Lo weld misalignment, a stress component Div. 1 simply does not model
- Coupled vacuum + thermal cyclic loading at every nozzle reinforcement and saddle horn
- Whole-vessel stress field across 25+ nozzles, not the linear superposition of single-nozzle checks
- Buckling failure mode shift when one dish head is thinner than the other
- 20-year fatigue damage requires explicit cyclic-stress decomposition at every critical location
This is why API 579 requires a Level 3 assessment for nozzle-region weld misalignment, the discontinuity stresses make Level 1 and Level 2 screening assessments inappropriate.
6. Verdict at the Deviation Locations
The two as-built deviations, the Hi-Lo at Nozzle N11 and the 19.5 mm South Side dish head, are the focus of this entire FFS. The table below pulls every ASME + API 579 stress check at those locations into one view:
Bottom line: The worst stress at the Hi-Lo zone on N11 is 96.41 MPa under LC4 – 91% of allowable, with margin remaining. Both dish heads sit comfortably below allowables on every check. Neither deviation contributes any fatigue damage over the 20-year life.
7. Headline Results
ALL PASS
ASME + API 579 stress checks across 5 load cases
2.4×
Buckling safety margin under full vacuum
0.027
Fatigue damage factor over 20 years (2.7% of life)
< 0.8%
FEA equilibrium validation error
Figure 3: Equivalent stress under LC1 – peak at Nozzle N3 (170.79 MPa)
ANSYS equivalent von Mises stress contour plot for Load Case 1 on the Refiner Still Pot pressure vessel – peak stress 170.79 MPa at Nozzle N3, color legend in MPa, for ASME VIII Div 2 Part 5 plastic collapse screening
Stress Classification Line path at the Hi-Lo weld misalignment region on Nozzle N11 of the Refiner Still Pot under Load Case 3, used for API 579-1 Annex 2D Level 3 FFS stress linearization into membrane, bending and peak components
8. Lessons Learned
Finding 1 - Hi-Lo offset can pass FFS without rework when modelled from real scan data
Many Hi-Lo deviations are accepted under engineering judgement. A Level 3 FFS, built on actual 3D scan geometry, replaces judgement with documented stress numbers. At N11, the Hi-Lo contributed 96.7 MPa primary local membrane stress — well within the 117.2 MPa allowable.
Finding 2 - A 1 mm dish head reduction is not catastrophic, but it must be quantified
The South Side dish head at 19.5 mm passed every stress check across all five load cases. But the answer required the full vessel-system FEA, Div. 1 formulae alone could not have given a defensible verdict.
Finding 3 - The fatigue governor is often the saddle, not the deviation
The dominant fatigue location was the saddle horn (Df = 0.027), not the nozzles or dish heads. A valuable insight for in-service inspection planning, focus inspection effort where damage actually accumulates.
Request the Complete Technical Report
This public case study summarises the assessment. The full engineering report, sent direct to your inbox, contains:
- Complete stress integrity tables for all 5 load cases (Equivalent, PL, PL+Pb+Q, Pm, Pm+Pb, Triaxial Local Failure)
- Stress Classification Line plots for every critical location
- Eigenvalue buckling mode shapes with load multipliers
- Fatigue damage factor calculations for every nozzle (25+ locations)
- Mesh sensitivity (grid independence) study
- FEA software validation, hand-calculation cross-check + global equilibrium check
- Full API 579 Annex 2D Hi-Lo assessment workings
- What could have gone wrong risk analysis + fabrication/installation/inspection recommendations
- Limitations & assumptions register