If no one can be near the piping during the pressure test, how does one soap each joint in the piping?
Of course, I make it clear that there are actually TWO (2) different phases of the “leak test” (the phrase the code uses):
Pressure Test and then the Leak Test
From a safety standpoint, pneumatic testing introduces a severe stored energy hazard. A rupture at full test pressure can generate a lethal blast wave and dangerous projectile fragmentation, which is why strict exclusion zones are required.
ASME B31.3 resolves this conflict between personnel safety and the mandatory joint examination by separating the test into a structural proving phase and a leak examination phase. We do NOT “soap” the joints while the system is at its maximum test pressure! Here is how the sequence is dictated under Paragraph 345.5.5 (Pneumatic Leak Test Procedure):
- The Preliminary Check
Before ramping up to dangerous pressures, the system is pressurized to a low baseline—typically the lesser of 25 psig or 50% of the test pressure. A preliminary check for massive leaks is conducted here while the stored energy is still relatively low.
- Ramping to Test Pressure
Personnel are evacuated from the exclusion zone. The pressure is then gradually increased in steps (typically increments of 10% of the test pressure), allowing time for the piping to equalize strains. This continues until the system reaches the full Test Pressure (which is 110% of the design pressure).
- The Structural Hold
The system is held at the full Test Pressure for at least 10 minutes. No personnel are allowed near the piping during this phase. This step is strictly to prove the mechanical and structural integrity of the system.
- The Pressure Drop (The Key Step)
Once the structural integrity is proven, the pressure is reduced down to the Design Pressure.
- The Examination Phase
Only after the pressure has been reduced to the Design Pressure is the exclusion zone relaxed. At this lower, intended operating pressure, the risk of a catastrophic, explosive rupture is mitigated. Inspectors can now safely approach the piping system and apply the leak-detecting fluid (soap) to each joint to verify tightness.
By stepping the pressure back down to the design limit before anyone approaches the pipe, the code ensures that the system has already survived a pressure 10% higher than what the inspectors are currently being exposed to.