CLADDING TECHNOLOGY SHANXI CO., LTDFactory Acceptance Test For Bimetal Pressure Vessels
Definition and Scope
The Factory Acceptance Test (FAT) is a comprehensive verification activity performed at the fabricator's facility before the pressure vessel is dispatched to the project site. It represents the final quality gate before shipment and provides the customer or owner with the opportunity to verify that the vessel meets all design, fabrication, and testing requirements. For clad and bimetallic pressure vessels, the FAT encompasses dimensional verification, non-destructive examination review, hydrostatic testing, and inspection of the overlay or cladding surface.
The FAT is typically conducted in conjunction with the Inspection and Test Plan (ITP), with the FAT activities corresponding to the final hold points and witness points in the ITP. The FAT is a contractual requirement in most petrochemical, nuclear, and energy sector projects and is often specified in the purchase order or technical specification.
FAT Preparation and Pre-Requirements
A successful FAT requires thorough preparation by both the fabricator and the customer. The following pre-requisites must be satisfied before the FAT can be scheduled:
- Completion of All Fabrication Activities: All welding, forming, machining, and heat treatment operations must be complete. Any outstanding repairs must be closed out with documented NDE results.
- Completion of All NDE Activities: All required non-destructive examinations (RT, UT, MT, PT) must be completed and the reports reviewed and accepted. The ITP typically specifies which NDE reports must be available for review during the FAT.
- Overlay Surface Preparation: For clad vessels, the overlay surface must be prepared for inspection—typically polished to a specified finish (e.g., 320-grit equivalent) to allow visual examination for surface defects, porosity, or lack of bond indications.
- Documentation Package: A complete documentation package must be compiled, including material test reports, welding records, NDE reports, heat treatment charts, dimensional inspection reports, and hydrostatic test records.
- Site Preparation: The vessel must be in a clean, accessible condition with adequate lighting, safe access platforms, and test equipment calibrated and available.
FAT Activities and Verification Checklist
The FAT typically follows a structured sequence of verification activities. The following table summarizes the key activities and their verification methods:
| FAT Activity | Verification Method | Acceptance Criteria |
|---|---|---|
| Visual Inspection of Overlay Surface | Visual examination with magnification if required | Free from cracks, lack of fusion, undercut exceeding 0.2 mm, porosity clusters |
| Dimensional Verification | Tape measure, dial gauge, coordinate measurement | Within drawing tolerances, typically ±1.5 mm for shell diameter, ±1 mm for flange face |
| Flange Face Inspection | Visual and gauge check | Flatness within 0.15 mm per 300 mm, free from gouging or welding on sealing surface |
| Nozzle Alignment | Go/no-go gauge or coordinate measurement | Within specified tolerance, typically ±1.5 mm for centerline |
| Hydrostatic Test Witness | Observation of test procedure, pressure gauge reading | Test pressure reached, held for specified duration, no visible leakage or pressure drop |
| Air Tightness Test (if applicable) | Soap bubble test or pressure decay method | No bubbles or pressure decay exceeding specified rate |
| Nameplate Verification | Visual check against data sheet | Correct material specification, design pressure, design temperature, serial number |
| Documentation Review | Review of documentation package | All required documents present, complete, and signed |
| Overlay Thickness Verification | Magnetic thickness gauge measurement at specified points | Minimum thickness per specification, typically 3 mm for 304 overlay, 6 mm for Inconel 625 |
| Corrosion Resistance Verification (if required) | Review of intergranular corrosion test report | Pass per ASTM A263 Practice A |
Hydrostatic Test Witnessing During FAT
The hydrostatic test is often the most critical activity witnessed during the FAT. For clad vessels, the hydrostatic test serves a dual purpose: it verifies the structural integrity of the vessel and provides a check for any leaks through the overlay layer or at the overlay/base metal interface.
Key considerations for hydrostatic testing of clad vessels include:
- Test Medium: Water is the standard test medium. For vessels with nickel-based or titanium overlays, the water quality must be controlled to prevent contamination—chloride content should be limited to below 25 ppm for stainless steel overlays and below 1 ppm for titanium overlays.
- Test Pressure: Typically 1.3 times the maximum allowable working pressure (MAWP) at the test temperature, unless otherwise specified in the design document.
- Soak Time: Minimum 30 minutes at test pressure, during which the vessel is visually inspected for any signs of leakage, bulging, or distress.
- Pressure Decay Monitoring: The test pressure should be maintained using a calibrated pump or by monitoring pressure decay. A pressure decay exceeding 0.5% per hour may indicate a leak.
- Drainage and Drying: After the test, the vessel must be drained and dried. For nickel-based overlays, compressed air drying is preferred over natural evaporation to prevent water staining or corrosion.
Lessons Learned and Practical Observations
Having participated in numerous FATs over the course of my career, I have observed several recurring patterns that impact FAT outcomes. The most common cause of FAT non-conformances is incomplete documentation rather than actual product defects. Fabricators often complete the physical fabrication and testing activities correctly but fail to compile the documentation package in a timely manner, leading to FAT delays.
Another common issue is the condition of the overlay surface at the time of FAT. Overlay surfaces that have been exposed to the shop environment for extended periods after welding may show surface contamination, oxidation, or minor damage from handling. This can lead to visual inspection non-conformances that require surface rework before the FAT can proceed. I recommend that overlay surfaces be protected with a temporary protective coating or wrap after the final overlay pass and before the FAT, with removal performed on-site under inspector observation.
The FAT is also an opportunity to verify the vessel's readiness for transport and installation. This includes checking that nozzles are properly capped and protected, that lifting lugs are in place and properly rated, and that the vessel is adequately braced for transport. Including these checks in the FAT scope prevents costly remedial actions at the project site.
In conclusion, the FAT is a critical quality assurance activity that should be planned with the same rigor as the fabrication process itself. A well-prepared FAT enhances customer confidence, reduces the risk of rejection, and provides a structured handover from the fabricator to the customer. Fabricators should treat the FAT as a project milestone and allocate adequate resources—both personnel and time—to ensure a successful outcome.