Test Analysis
Reading the pressure–volume plot during the climb
The most useful diagnostic on a hydrostatic test is compiled while you are still pressurising, and it is thrown away by crews who only record the final numbers.
Plot pressure against cumulative volume pumped
Record the volume pumped and the pressure reached at intervals during the climb, and plot one against the other as you go. The theoretical elastic line can be calculated in advance from:
dP/dV = 1 ÷ ( V [ (D/(Et))(1 − ν²) + C ] )
That slope is what a sound, air-free, elastic system should follow.
Three signatures
- Tracking the theoretical line. Healthy elastic pipe, full of liquid, behaving as predicted.
- Shallow early, improving as pressure rises. Trapped air. Gas compresses far more readily than water, so early strokes buy little pressure; as the air compresses, the curve steepens towards the elastic line.
- Progressive flattening at the top. The onset of yielding. The pipe is taking permanent deformation and each additional volume buys less pressure than the last.
The first is what you want. The second is a fill problem you can still fix. The third is a reason to stop.
Why it matters more than the final reading
Trapped air discovered on the plot during the climb costs you a re-fill. Trapped air discovered from an ambiguous decay after an eight-hour hold costs you the test.
The plot is also the only evidence that distinguishes those two cases after the fact. Without it, a shallow decay and a small leak look alike.
Practical points
Record enough points to see curvature — a handful of readings will not show a knee. Meter the volume properly rather than counting pump strokes if the pump is not positive-displacement.
Plot it as you go. A plot compiled afterwards from a log is better than nothing, but by then the decision it should have informed has already been made.
From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.