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PSI vs DUPRO vs pressure test witnessing vs loading supervision vs factory audit

Each inspection stage sees a different state of the valve. This guide compares what is checked, when to book it, and how to write hold points into the purchase order.

General Inspection2026-11-147 min read

PSIDUPROtest witnessingloadingfactory audit

Short answer: A pre-shipment inspection looks at a finished (often painted and packed) sample. A during-production inspection looks earlier, while a fault can still be corrected. Pressure-test witnessing is a hold point at the bench, not a paper review of a “PASS” certificate. Loading supervision confirms that the accepted goods are the ones that go in the container. A factory audit is a capability check before you buy. They complement each other; they are not substitutes.

By Kevin Lyu (吕中国), ValveWitness Inspection, Ningbo · Published 2026-11-14

In our sample report: the document is a pre-shipment inspection with a pressure-test witness log. Even with workmanship inside AQL and tests accepted, the overall result is PENDING because a certificate was missing and one coating sample was thin. That is a finished-lot visit. A DUPRO a few weeks earlier could have caught a flange-face scratch or a missing heat mark before paint and crates went on. See the sample report and the standards map.

The five stages compared

StageWhen it happensWhat is typically inspectedWhat it cannot see
Factory auditBefore you place (or substantially enlarge) an orderProduction capability, basic QC, test benches, gauge calibration, how certificates and heat numbers are handledThe quality of a lot that has not been made yet. It is not an ISO certification audit, and it is not a substitute for product inspection
During-production inspection (DUPRO)When a agreed portion of the order is in process — often while bodies are still unpaintedWorkmanship trends, dimensions, weld or overlay quality, heat marks, in-process pressure tests if they are on the ITPFinal paint, packing, the complete packed quantity, and container loading
Pressure-test witnessingAt the test bench, usually after assembly and before or during final painting, as a hold pointShell, seat and backseat (or API 6D cavity/DBB) tests: procedure, gauges, pressures, timed holds, leakage, serial numbers — see the test-standard guidePacking, coating DFT after painting, and whether the correct number of valves actually goes into the container
Pre-shipment inspection (PSI)When the lot is finished, and preferably packed or ready to packQuantity, packing and marks, AQL workmanship, dimensions, nameplates, documents, agreed tests and add-ons (DFT, PMI, operation)Defects already hidden under finish paint or inside closed crates, and tests that were never made a hold point
Loading supervisionAt container or truck stuffing, often at the factory or a forwarder yardCount, case condition, container condition, loading order, lashing, seal number and photosInternal workmanship or a pressure test. This is a shipping check, not a second PSI

Why painted, crated valves hide faults if you only book PSI

By the time a typical valve lot is offered for pre-shipment inspection, the bodies are painted, the flanges are capped, and many of the valves are already in wooden cases. The inspector can still do a great deal — open agreed cases, check marks, measure a sample, review documents, and witness remaining tests — but some conditions are already out of reach:

  • As-cast surface defects that are now under a full paint film.
  • Heat or melt marks that were never raised or have been filled and painted over.
  • Flange-face damage that occurred before the plastic or wooden caps went on, if the caps are not removed on a large enough sample.
  • A pressure test that was completed last week, with only a certificate left to read, if witnessing was not a hold point.

DUPRO is the usual way to see those items earlier. Pressure-test witnessing is the usual way to see the test itself. PSI then confirms the finished, packed condition and the documents. Loading supervision confirms the count as the doors close.

How to write inspection hold points into the purchase order

A hold point means the factory does not pass that step until the inspection is done, or until you have released it in writing. Witness points allow work to continue, but the inspector must be present for the stated activity. Typical wording on a valve order:

  1. Document and material review — 3.1 (or 3.2) certificates and heat marks available before assembly, or at latest before shipment; missing certificates hold release.
  2. DUPRO — at a stated production stage, for example when 30–50% of bodies are machined and unpainted, including a dimensional sample and a visual check of flange faces and heat marks.
  3. Pressure-test witness — the standard and edition, the leakage rate, the witness quantity, and that testing is a hold point until the inspector has seen the stated sample.
  4. PSI — when 100% are finished and an agreed percentage are packed; sampling plan as in ISO 2859-1 / Z1.4; DFT, PMI and operation tests if required.
  5. Loading supervision — if the risk is substitution, short shipment, or rough handling into the container.

A factory audit sits earlier than all of this. Use it when you have not bought from the plant before, or when the product or the test scope is new to them. It does not approve a lot.

A practical combination on a first or critical order

  • Audit or a short capability visit before you place the PO, if the factory is new to you.
  • One DUPRO while valves are still unpainted, plus a pressure-test witness hold point.
  • A PSI of the finished, packed lot with a written AQL plan and document checklist.
  • Loading supervision when the cargo is high value, or when the stuffing location is not the factory you already inspected.

See also: PSI, DUPRO, pressure-test witnessing, loading supervision and factory audit.

Sources

We report facts from the samples inspected. Standards named in these guides are public standards we work to, not certifications or accreditations we hold. We do not issue product certificates or accreditation marks. Test pressures, durations, leakage rates and acceptance numbers must be read from the licensed edition named in your purchase order; we do not reproduce copyrighted tables from the standards.

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