Pressure test pressure and hold time

A pressure test proves strength and tightness at a pressure above the design pressure, and every construction code sets its own multiplier. The differences are not cosmetic. ASME BPVC Section VIII Division 1 requires 1.3 × MAWP for a hydrostatic test (it was 1.5 before the 1999 Addenda — always check the edition the item was built to), Division 2 requires 1.43 × MAWP, ASME B31.3 and B31.1 require 1.5 × design pressure for piping, and pneumatic tests sit much lower at 1.1 × MAWP for VIII-1 and B31.3 because of the stored energy involved.

The second factor is temperature. Allowable stress falls as design temperature rises, so a vessel designed for a hot service is tested cold with extra pressure to reach the same fraction of allowable stress: the multiplier is scaled by the stress ratio S_test/S_design, the ratio of allowable stress at test temperature to allowable stress at design temperature, both read from the code stress tables for the actual material. ASME VIII-1 caps that ratio at 6.5. The ratio never reduces the test pressure below the basic multiplier — where S_test/S_design is less than 1, use 1.

Hold time is where technicians are most often let down by memory. ASME B31.3 and B31.1 both require the test pressure to be held at least 10 minutes before examination; ASME VIII-1 UG-99 sets no numeric hold, leaving it to the design or the job specification, and common practice is 30 minutes. Examination pressure differs too: VIII-1 UG-99(g) allows the visual examination to be made after reducing to the test pressure divided by 1.3, while B31.3 345.2.2(b) requires the pressure to be reduced to the design pressure before the joints are examined.

Two things this arithmetic will not do for you. It will not confirm that the item, in its current corroded condition, can take the test pressure — that is a fitness-for-service calculation on measured thickness. And it will not make a pneumatic test safe: stored energy in compressed gas is orders of magnitude higher than in water, and pneumatic testing needs an exclusion zone, a documented risk assessment and a code-recognised justification for not using liquid.