Pressure Test Drop on a 185-Foot Black Iron Gas Line
A crew is pressure-testing 185 ft of new 1-inch Schedule 40 black iron gas piping in a retail build-out. The system has 14 outlets, a 2-inch meter riser, and an operating-pressure regulator set at 7 in. w.c. The job specification requires a 10-psig air test held for 2 hours with no drop. At 7:00 a.m. the gauge reads 10.0 psi and the ambient temperature is 72 F. At 11:00 a.m. the gauge reads 9.6 psi and the ambient temperature is 48 F. The gauge is a 0-15 psi unit with 0.1 psi graduations. The helper wants to soap every joint and start breaking unions. The inspector is due at 1:00 p.m., and the general contractor is pushing the crew to sign off the test so the drywall can close up the walls.
1. Is the 0.4 psi drop evidence of a leak? Show the reasoning with numbers.
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No. Convert to absolute pressure and use the constant-volume relationship P1/T1 = P2/T2. Gauge pressure 10 psig is 24.7 psia. T1 = 72 + 460 = 532 R. T2 = 48 + 460 = 508 R. Predicted pressure at 48 F with no leak: 24.7 x (508/532) = 23.6 psia, or about 8.9 psig.
The gauge should have read near 8.9 psi if the only thing happening was a 24 F cooldown. It read 9.6 psi, which is 0.7 psi higher than the thermal prediction. In other words, the observed 0.4 psi drop is smaller than the temperature effect alone would produce, so the system is behaving like a tight system, possibly with a little solar gain or warm makeup air acting on part of the run.
Two cautions before signing anything. First, the 1-hour interval is not the specified 2-hour test, so the test is incomplete regardless of the interpretation. Second, a 0.1 psi graduation on a 0-15 psi gauge is poor resolution for a 10 psi reading; the numbers are directional, not precise. Do not break unions on thermal drift.
2. Describe a defensible retest procedure that satisfies the specification and protects the crew if the inspector questions the result.
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Bring the system back to 10 psig and let it sit 30 to 60 minutes before starting the clock so the air and pipe reach thermal equilibrium; 185 ft of steel and 14 outlets take time to stabilize. Then run the full 2-hour hold. Record start and end pressure plus pipe or ambient temperature at both points, on a signed test tag with gauge serial number, so the correction can be reproduced.
If the ambient is swinging more than about 10 F, either retest overnight when temperatures are stable, or correct the reading to a reference temperature using P2 = P1 x T2/T1 in absolute units and document the math. For tighter resolution, use a 0-5 psi gauge with 0.1 psi divisions on a lower-pressure test, or a calibrated recording gauge.
Only if the temperature-corrected drop still exceeds the specification do you soap-test joints, starting with threaded connections, unions, and the regulator and meter connections. Never use a flame. If a leak is found, repair and restart the entire test, not just the repaired section.
3. A soap test later reveals bubbles at a union. Outline the repair and the rules that apply to that fitting.
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Shut the test pressure down and bleed it off before touching the union; never tighten a gas joint under pressure. Disassemble, inspect both seats and the threads for galling, pipe dope residue, or a cross-threaded nipple, and replace damaged parts rather than forcing them.
Clean the male threads and apply a pipe dope listed for gas service, or a listed thread tape, not a thick stack of both. Assemble hand-tight, then snug with the proper wrenches, backing up both halves so the piping is not twisted. Do not over-torque a union to stop a leak; that distorts the brass seat and creates a bigger leak.
Check the repair with leak-detector solution, then re-pressurize and repeat the full 2-hour, temperature-documented test on the whole system. Confirm the union is in an accessible location. Unions and other threaded fittings that must be serviced cannot be buried in a wall or ceiling; if this union ends up concealed, replace that section with a welded or brazed joint or relocate the fitting before the drywall goes up.