Third-party inspection (TPI) is where a purchase order meets the physical product. An independent inspector, appointed by the buyer or end user, verifies at the manufacturer's works that the equipment matches the order, the standard and the project specification before it ships. Done well, TPI catches problems while they are still cheap to fix. Specified poorly, it adds cost and weeks of delay without adding much assurance.
What TPI covers and what it does not
A TPI agency acts on the buyer's behalf. It witnesses tests, reviews records and confirms that the manufacturer's quality plan is followed. It does not replace the manufacturer's own quality control, and it does not transfer responsibility for conformity away from the manufacturer. The inspector can only verify what the purchase order and its referenced documents require, which is why the scope must be written down before production starts.
The level of inspection should match the risk. A critical sour-service wellhead or a large-bore pipeline ball valve justifies full witness of the key tests. Standard carbon steel flanges from a qualified mill may only need a final visual check and a document review.
The inspection and test plan (ITP)
The ITP is the backbone of any inspection. It is a table, normally prepared by the manufacturer and approved by the buyer, that lists every inspection and test activity in sequence, the governing procedure and acceptance criteria, the records produced and the involvement of each party. Involvement is expressed through standard codes:
| Code | Meaning | Effect on production |
|---|---|---|
| H – Hold point | The activity must be witnessed by the inspector | Work cannot proceed past this point without the inspector or a written waiver |
| W – Witness point | The inspector is notified and may attend | If the inspector does not attend after proper notice, work continues |
| R – Review | Records are reviewed without attendance | Documents are checked before release or within the final dossier |
| M – Monitor / surveillance | Random spot checks during production | No stoppage; observations are reported |
Use hold points sparingly. Every hold point is a potential stoppage, so reserve them for activities that cannot be repeated or inspected later, such as the pressure test, the final dimensional check before painting, or the inspection of a critical weld overlay. Agree the notification period for hold and witness points in the ITP; a few working days is common, and longer when inspectors must travel.
The pre-inspection meeting
For engineered or high-value orders, hold a pre-inspection meeting (PIM) shortly after the order is placed, with the buyer, the manufacturer's QA/QC team and the TPI agency. The aim is to remove ambiguity before any material is cut. Typical agenda items:
- Confirm the approved revision of the ITP, drawings and datasheets
- Review the applicable codes, project specifications and any deviations already agreed
- Confirm the production schedule and the expected dates of hold and witness points
- Agree the procedures: NDE, welding (WPS/PQR), hydrotest, painting and marking
- Agree the certificate type, the document format and the contents of the final data book
- Name contact persons and the route for non-conformance reports and concession requests
Typical inspection stages for valves, wellheads and line pipe
Valves
For API 6D pipeline ball valves and API 600 or API 602 gate and globe valves, inspection usually starts with material verification: checking that heat numbers on bodies, bonnets and closures match the mill certificates. Positive material identification (PMI) with an XRF or OES analyser confirms alloy content on stainless, duplex and alloy steel parts and on weld overlays. Castings may require radiographic or ultrasonic testing to ASME B16.34 or project requirements, with surface examination by magnetic particle or liquid penetrant. The pressure test is normally the key hold point: shell and seat tests to API 6D or API 598 as applicable, with calibrated gauges, recorded durations and no visible leakage through the shell. Valves for sour service also get hardness checks against the limits in NACE MR0175 / ISO 15156. Where fugitive emission type testing to ISO 15848-1 or API 641 is specified, it is usually verified through documentation rather than repeated for every order.
Wellhead and christmas tree equipment
For wellheads and christmas trees, API 6A defines much of the inspection content through the product specification level (PSL). Higher PSLs add requirements for material testing, NDE, hardness, traceability and longer hydrostatic holding periods, and PSL 3G adds a gas test. The inspector should confirm that the PSL, performance requirement, temperature class and material class on the nameplate and in the records match the order. See our guide to API 6A PSL, PR, temperature and material classes. Drift tests, assembly checks and function tests of valves and chokes are common witness points.
Line pipe
For API 5L line pipe, inspection takes place at the mill and focuses on mechanical tests (tensile, Charpy impact, hardness), chemical analysis, the mill hydrostatic test, NDE of the weld seam and pipe body, and dimensional checks of diameter, wall thickness, out-of-roundness, straightness and bevel ends. For PSL 2 pipe in sour service, HIC testing to NACE TM0284 and SSC testing to NACE TM0177 are usually required, either at qualification or per heat, as the specification states. Marking, end protectors, coating and stacking are checked before release.
Common to all items
Final inspection normally covers visual and dimensional checks against the approved drawing, marking and nameplate data, coating inspection (surface preparation grade to ISO 8501-1, dry film thickness to ISO 19840 or the project specification, adhesion where required) and packing: flange protectors, preservation, desiccant, crate marking and suitability for sea freight and for storage in the hot, humid and saline conditions common in the Gulf.
Certificates and the release note
EN 10204 defines the types of inspection document. A 3.1 certificate is issued by the manufacturer's authorised inspection representative, who is independent of the production department. A 3.2 certificate is additionally validated by the purchaser's authorised inspector or by an inspector designated by official regulations; in practice this is usually the TPI agency endorsing the certificate. If you need 3.2, state it in the purchase order and say whether it applies to the raw material, the finished product or both, because adding it after the fact usually means re-testing.
At the end of a successful final inspection, the TPI agency issues an inspection release note (IRN), sometimes called an inspection release certificate. Many buyers make the release note a condition of shipment and of payment. It should reference the purchase order, the items and quantities, and any open points or concessions.
Non-conformances and inspection delays
Common non-conformances
- Heat numbers on parts that cannot be traced to the supplied mill certificates
- Hardness readings above the NACE MR0175 / ISO 15156 limit on welds or heat-affected zones
- Test gauges with expired calibration, or test durations shorter than the standard requires
- Wrong flange face finish, ring groove dimensions or end connection rating
- Dry film thickness outside the specified range, or coating applied over poorly prepared surfaces
- Incomplete nameplates, or marking that does not follow the standard (for example MSS SP-25 for valves)
- Documentation submitted late, in the wrong format or with uncontrolled revisions
How to avoid inspection delays
- Define the inspection scope, certificate type and ITP requirement in the RFQ, not after the order.
- Approve the ITP and procedures early so that production is not waiting on paperwork.
- Appoint the TPI agency before the PIM and give it the full purchase order package.
- Ask the manufacturer to review its own records before calling the inspector for final inspection.
- Group witness activities where possible so that one visit covers several items.
- Agree how minor non-conformances will be handled (concession, rework or rejection) before they happen.
For sour-service items, read our guide to NACE MR0175 / ISO 15156 so that the hardness and material requirements in your ITP are realistic and complete.
Sending an RFQ with inspection requirements
When you request a quotation, list the applicable standards, the PSL or class, the inspection level, the required hold and witness points, the EN 10204 certificate type and the name of the TPI agency if one has already been selected. Send your requirements through our contact page and we will respond with a quotation that addresses inspection and documentation from the start.
Frequently asked questions
Who pays for third party inspection?
Usually the buyer appoints and pays the TPI agency, either directly or as a line item in the supplier's quotation. Costs of re-inspection caused by the manufacturer's own failures are commonly charged back under the purchase order terms, so agree this at RFQ stage.
Is an EN 10204 3.1 certificate enough?
For many standard items, yes, provided the manufacturer's quality system is acceptable to you. Specify 3.2 where the project specification, the end user or the criticality of the item requires independent validation, typically for pressure-containing parts in critical or sour service.
Can the inspector reject equipment?
The inspector reports non-conformances against the purchase order and the referenced standards. The decision to accept, request rework, grant a concession or reject normally rests with the buyer, based on the inspector's report and an engineering review.
How much notice should a manufacturer give for a witness test?
The notice period is agreed in the ITP or purchase order. A few working days is common for local inspectors; allow more when the inspector has to travel or obtain visas, which is often the case for factories outside the GCC.