Line under test
Test medium
Stored energy at this pressure — both media, same pipe
WATER — HYDROSTATIC
—
AIR — PNEUMATIC
—
—TIMES MORE ENERGY IN THE AIR
What the supports must carry during the test
| Case | kg / m | Total tonnes | × operating |
|---|---|---|---|
| Pipe steel (empty) | — | ||
| Operating (pipe + process) | 1.00 | ||
| Hydrotest (pipe + water) |
Grey = steel, blue = water. That blue fraction is the load case your rack,
your foundations and your spring hangers must be checked for — W + water, travel stops
pinned. It is the only genuine engineering argument for a pneumatic test, and it is solved
with temporary supports, not by changing medium.
Method
Water — E = p²·V / (2K) with K = 2.2 GPa. The pipe wall is also a spring; including it
(1/K_eff = 1/K + D/(E·t)) raises the water figure roughly 30–40 %, which never changes the verdict.
Entrained air does: 1 % trapped air drops K_eff by an order of magnitude — vent your high points.
Air — isentropic expansion to atmosphere, E = p₁V/(k−1)·[1 − (pₐ/p₁)^((k−1)/k)], k = 1.4, absolute pressures.
TNT-equivalent = E / 4.6 MJ per kg. Indicative radius uses Hopkinson–Cranz scaling R = 15·W^⅓ metres, which is about the 1 psi (6.9 kPa) injury/window-breakage threshold. A real exclusion zone is calculated by the method in ASME PCC-2 Article 5.1 and put on the permit.
Code test pressures — B31.3: hydrostatic 1.5 × design × (S_T/S); pneumatic 1.1 × design, with a preliminary check at the lesser of 1.7 barg or 50 % of test pressure and then 10 % steps with a hold and a leak check at each one.
Air — isentropic expansion to atmosphere, E = p₁V/(k−1)·[1 − (pₐ/p₁)^((k−1)/k)], k = 1.4, absolute pressures.
TNT-equivalent = E / 4.6 MJ per kg. Indicative radius uses Hopkinson–Cranz scaling R = 15·W^⅓ metres, which is about the 1 psi (6.9 kPa) injury/window-breakage threshold. A real exclusion zone is calculated by the method in ASME PCC-2 Article 5.1 and put on the permit.
Code test pressures — B31.3: hydrostatic 1.5 × design × (S_T/S); pneumatic 1.1 × design, with a preliminary check at the lesser of 1.7 barg or 50 % of test pressure and then 10 % steps with a hold and a leak check at each one.