Failed Pressure Test on a 2 psig Rooftop Unit Gas Line
A contractor is installing a 2 psig natural gas line to a new 300,000 Btu/h rooftop unit. The run is 80 ft of 1-1/2 in. Schedule 40 black steel with 12 threaded joints, two 90° elbows and a 1-1/2 in. line pressure regulator at the unit. The installer used three wraps of PTFE tape on all male threads and a 24 in. pipe wrench; a backing wrench was used on only the second joint. After assembly the line was pressurized to 30 psig with air for 30 minutes and the gauge dropped from 30 to 28.5 psig. The installer tightened two joints and retested at 20 psig for 10 minutes; the gauge held. He wants to put the line in service. The gauge is a 0–60 psig dry gauge with 2 psig graduations, and the ambient temperature is stable.
1. Evaluate the validity of the two pressure tests.
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The initial test failed. A 1.5 psig drop over 30 minutes is a leak, not gauge error, because the gauge is readable to 2 psig and the ambient temperature is stable. The later 20 psig, 10-minute test does not redeem the line. It was conducted at a lower pressure and shorter duration, and the installer tightened two joints after the failure, so the retest proves only that those two joints changed. B149.1 requires a test pressure and duration appropriate to the system; for a 2 psig line the test must be at least the code-specified pressure for the full duration. A 30 psig air test is acceptable only if all components can withstand it, but the observed drop means failure. The line cannot be placed in service on the basis of a shortened retest.
2. What are the likely leak points and joining problems, and how would you isolate the leak?
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The likely leak points are the 12 threaded joints, especially the 10 made without a backing wrench. Over-tightening 1-1/2 in. malleable fittings can crack a fitting or stretch threads, and PTFE tape can shred and hold a joint open if it enters the pipe. The two joints tightened after the failure are suspect, but the leak could be elsewhere. Isolate the run in sections with blinds or caps and soap-test each joint at a safe test pressure. Use a calibrated gauge and allow temperature equalization before reading. Inspect all joints for thread engagement, cracking, and tape debris. Do not rely on smell or a non-corrosive leak detector alone on a new 2 psig line.
3. What must be completed before the line is put into service?
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Repair or remake every suspect joint, then retest the complete line at the code-required pressure for the full duration with no drop. Confirm that the line regulator is rated for 2 psig inlet and the correct outlet pressure, set the appliance regulator, and leak-test the regulator vent and all appliance connections. Purge the line to a safe location, verify manifold pressure at high and low fire, and check combustion. Complete the pressure-test record and the piping inspection before the unit is fired. The installer should also be corrected on backing-wrench use and tape application so the same defect is not repeated on the remaining joints.