A pump that passes every factory test can still fail on site, and a pump that was never properly tested at the factory can hide a problem until it is far more expensive to fix. Factory acceptance testing (FAT) and site commissioning are two separate checkpoints that catch different classes of problems, and a buyer who treats them as one continuous approval usually finds out the difference the hard way. Here is what each stage should actually cover.
FAT and commissioning: two different checkpoints
FAT happens at the manufacturer’s works, before the pump ships, on a test stand built and calibrated by the vendor. It confirms the pump itself — as built — meets its hydraulic performance, mechanical integrity and material requirements under controlled conditions. Commissioning happens afterward, on site, once the pump has been installed on its actual foundation with its actual piping, coupling alignment, instrumentation and control system. Commissioning confirms the installation as a whole, not just the pump, is ready to run and then actually reaches stable operation. A pump can pass FAT without a single deviation and still run into trouble at commissioning if the piping was not properly supported, the foundation grouting settled, or the alignment shifted during transport — which is precisely why relying on FAT results alone and skipping a disciplined commissioning checklist is a false economy.
Witness levels: what the buyer actually attends
Not every test needs the buyer standing at the test stand. Most specifications define a small set of witness levels, similar to the logic covered in our guide to third-party inspection: witnessed, where the buyer or its inspector is physically present for the full test; observed or monitored, where the inspector reviews the setup and checks in without necessarily staying for the entire duration; and documented (unwitnessed) review, where the manufacturer runs the test alone and reports results afterward in a certified report. Which level applies to which test — hydrostatic, performance, mechanical run, NPSH — needs to be agreed and written into the purchase order before the pump is built, not requested after the manufacturer has already scheduled its own internal test date. A vendor is under no obligation to delay or repeat a test because a witness request arrived late.
The core factory tests
A typical centrifugal pump FAT package for oil and gas service draws on several distinct tests, each checking a different failure mode rather than repeating the same check:
| Test | What it confirms |
|---|---|
| Hydrostatic test | Pressure-containing parts (casing, cover) hold rated test pressure without leakage or structural failure |
| Performance test | Head, flow and efficiency match the quoted curve, per HI 14.6 or ISO 9906, across multiple points across the operating range rather than one design point |
| NPSH test | The pump's actual NPSH-required curve, not just a calculated estimate, confirmed at the flow points that matter most to the buyer's operating range |
| Mechanical run test | Bearing temperature, vibration and seal or packing leakage stay within limits at rated conditions over a sustained run once temperatures stabilize |
| String test (optional) | The pump running coupled to its actual driver, gearbox and other train components, revealing coupled-train vibration or torsional issues a bare pump test cannot show |
Not every service warrants all five. Hydrostatic and performance tests are close to universal; NPSH testing, a full mechanical run test and a string test are more commonly reserved for critical, continuous or high-energy services, and should be called out explicitly on the datasheet or inspection and test plan rather than assumed.
What a FAT checklist should actually cover
- Calibration certificates for the test instruments (pressure gauges, flow meters, vibration probes) are current and traceable, not just present
- Material and NDE reports for pressure-containing parts have been reviewed and closed out before the test starts, not left as an open item
- The performance test covers multiple points across the operating range, not only the single rated point
- Vibration and bearing temperature readings are compared against a stated acceptance limit, not just recorded for the file
- Any punch-list items raised during the test are documented in writing with an agreed close-out date, not verbally waived on the shop floor
- The manufacturer's data book draft, or at minimum the test report itself, is issued before the pump ships, not weeks afterward
Site commissioning: what changes after FAT
Once the pump arrives on site, commissioning re-checks several things FAT could not test, because they depend on the actual installation rather than the pump alone: coupling alignment is rechecked after transport and after the piping is fully bolted up, since piping strain on the pump nozzles can pull a perfectly aligned pump out of tolerance; suction and discharge piping is flushed and checked for cleanliness, since factory testing runs on clean water while site piping can carry mill scale or construction debris that damages a mechanical seal within minutes of startup; motor insulation resistance is tested after any period of storage or transport, since moisture ingress during shipping is common; and the seal support system, if fitted, is functionally checked — flush, buffer or barrier fluid circulating and at the correct pressure — before the pump is started, not confirmed after the fact from a leaking seal. A brief, low-load initial run with close monitoring of vibration, bearing temperature and seal condition normally precedes handing the pump over to full continuous operation.
Spares and consumables to have ready at commissioning
A pump that passes FAT and arrives on site cannot actually be commissioned without a handful of items that are easy to leave off a purchase order because they seem minor compared to the pump itself. Gaskets and fasteners for the suction and discharge flanges are frequently assumed to be supplied locally, and just as frequently turn out not to match the pump’s flange rating or bolt pattern when the installation crew actually needs them. The correct grade and quantity of bearing lubricant, and flush or barrier fluid for the seal support system if one is fitted, need to be on site and confirmed compatible with the process fluid and ambient conditions before first start, not sourced from whatever the site warehouse happens to stock. A basic commissioning spares set — typically the gasket, O-ring and consumable items likely to be disturbed during installation and initial alignment checks — is worth ordering alongside the pump rather than treated as a separate purchase raised only once the installation crew discovers something is missing. None of this replaces a proper capital and insurance spares strategy for the life of the pump; it is simply the short list that determines whether commissioning can start on the date the schedule assumes it will.
Common FAT pitfalls buyers should watch for
A few recurring issues account for most of the disputes that surface after a pump has already shipped. A witness request that arrives after the manufacturer has already scheduled and run the test internally leaves the buyer with only a documented report and no leverage to request a repeat. A performance test limited to the single rated point, rather than several points spanning the operating range, can hide a curve shape problem that only shows up away from that one point. Vibration and bearing temperature readings recorded without a stated acceptance limit to compare them against are effectively unverifiable after the fact. And punch-list items agreed verbally on the shop floor, without a written close-out date, have a habit of resurfacing as unresolved only once the pump is already on a ship. None of these require expensive fixes — they require agreeing the inspection and test plan, in writing, before the pump goes into production rather than after.
Sourcing a pump and want the inspection and test plan set correctly from the start? Send us your pump datasheet and Oillinko will agree the witness level, test scope and documentation requirements with the manufacturer before the order is placed, and coordinate the FAT itself so nothing is scheduled without your knowledge. Browse our pumps & rotating equipment category, or see our guide to reading a pump curve to know what the performance test result should actually look like before you sign off on it.
Frequently asked questions
What is the difference between a witnessed and an unwitnessed factory test?
A witnessed test is performed with the buyer or its third-party inspector physically present, able to verify the test setup, instrument calibration and results in real time. An unwitnessed (documented) test is run by the manufacturer alone, with results reported afterward in a certified test report. Between the two sits an observed or monitored level, where the inspector reviews the setup and may check in on the test without being present for the full duration. The level required is a purchasing decision, stated in the RFQ and purchase order, not something left to the vendor's discretion once the order is placed.
Do all pumps need a mechanical run test?
No. A mechanical run test is typically specified for pumps in critical or continuous services, or where the buyer's specification or API 610 datasheet calls for it explicitly, rather than for every pump regardless of service. It is common practice to run it for a period at rated conditions once bearing temperatures have stabilized, checking vibration, bearing temperature and seal or packing leakage against acceptance limits, but the requirement itself is a datasheet decision agreed before the order is placed, not an automatic default on every pump.
What is a string test and when is it worth requesting?
A string test (also called a complete unit test) runs the pump coupled to its actual job driver, gearbox and other train components together, rather than testing the pump alone against a test-stand motor. It reveals coupled-train issues — torsional behavior, coupling spacer dynamics, combined vibration signatures — that a bare pump test cannot show. It is optional under API 610 and adds cost and schedule, so it is usually reserved for large or critical trains (compressor-class drivers, gearboxes, multistage high-speed pumps) rather than requested as standard practice on every order.
What is the actual difference between FAT and commissioning?
Factory acceptance testing (FAT) happens at the manufacturer's works before shipment, confirming the pump meets its hydraulic, mechanical and material requirements in a controlled test-stand environment. Commissioning happens on site after installation, confirming the pump as actually installed — with its real piping, alignment, instrumentation and control system — is ready for and then successfully reaches normal operation. A pump can pass FAT cleanly and still fail commissioning if site alignment, piping strain, or auxiliary systems were not handled correctly, which is why the two are separate checkpoints rather than one continuous approval.
