API 598 is the inspection and pressure-testing standard for valves built to API design codes. It requires a shell test at 1.5 times the 38°C rating, a high-pressure closure test at 1.1 times rating, and for most valves a low-pressure air seat test — with pass criteria that depend on seat type and valve size.
API 598 is the document your test certificates answer to when valves are designed to API 600, API 602, API 608, API 623 or API 594. Three tests form its core. The shell test pressurises the assembled body-bonnet envelope, valve part-open, to 1.5× the ambient rating: no visible leakage through walls, joints or packing (a packing allowance exists but adjustment is permitted). The high-pressure closure test loads each seat at 1.1× rating: resilient seats must be drop-tight; metal seats earn a small allowance in drops or bubbles per minute that scales with size. The low-pressure air seat test at 4–7 bar catches the fine defects water hides.
The standard also fixes minimum test durations by size (15 seconds for a 2" valve, minutes for large bores), backseat testing for rising-stem valves, and the sampling rules — though reputable manufacturers test 100% of production, not samples. Note that API 6D pipeline valves follow their own annex tests rather than API 598.
On the PO, cite "inspection and testing per API 598, latest edition" and request the recorded results. VTXFLOW tests every valve on computerised benches with certificates traceable to the body heat number.