Threaded Gas Line Leak at a Coupling
During a 10-psig air test of a 1-1/2-inch Schedule 40 black steel gas line, a leak is found at a threaded coupling. The line was assembled with a power vise and a hand threader. The fitter used a new 1-1/2-inch NPT die, cut the threads, applied pipe dope to the female coupling threads, and made up the joint with a wrench until it felt tight. About four full threads remain exposed on the pipe. The leak bubbles at the coupling. The fitter wants to tighten the coupling another half turn and re-test. The line serves a 5-psig appliance and is in a basement with limited ventilation. The pipe run is 25 ft with six couplings and two unions.
1. What is the likely cause of the leak, and is tightening another half turn acceptable?
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Four exposed threads on a 1-1/2-inch NPT joint suggests the pipe threads may be cut too long or the coupling is not fully engaged. Thread engagement is controlled by the thread length and the fitting; exposed threads alone are not a reliable gauge, but four full threads is more than the usual 2–3. The pipe dope was applied to the female threads, which can push sealant away from the leak path and leave the male threads dry. Tightening another half turn is not acceptable as a first fix: it can split the malleable iron coupling, distort the threads, or over-stress the pipe, and it does not correct an overcut thread. Depressurize, disassemble, and inspect.
2. What is the correct repair and makeup procedure?
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Clean the old dope from both threads and inspect with an NPT thread gauge or a known-good fitting. If the pipe thread is overcut or damaged, cut off the nipple behind the threads and rethread a new end, or replace the nipple. Use a sharp die and the correct thread length. Apply a gas-rated pipe sealant to the male threads, leaving the first two threads clean, and make up the joint with a proper wrench and a backup wrench. Turn until snug, then about two to three turns for this size, leaving two to three threads exposed. Do not use excessive torque. If the coupling is cracked or the threads are flattened, replace it. Re-test with air or inert gas and soap solution.
3. What test precautions apply in this basement?
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Use a low-pressure test with air or inert gas only as permitted by the code; never use oxygen or a flammable gas. Keep the test pressure at the specified 10 psig or lower, and use a gauge and relief device on the test rig. Isolate the appliance and ensure the test gas cannot enter it. Ventilate the basement, keep ignition sources out, and use soap solution, not a flame, to find leaks. Barricade the area because a threaded joint can blow apart under pressure. After the test, release pressure slowly and remove the test equipment. If the leak persists, replace the joint rather than repeated tightening.