Pressure testing and purging. Before gas ever flows, the system has to prove that it is tight. Episode ten of the Gas Service Animated series.
Chapter eight gives us two different jobs that are easy to confuse. A pressure test proves new or modified piping tight, using air at elevated pressure before the system ever carries fuel gas, with appliances kept out of the test. A leak check happens on an existing system at operating pressure, every time gas is turned on into new piping or service is restored after an interruption. One proves the construction. The other proves it is safe to turn the gas back on right now.
For the pressure test, the medium is air, carbon dioxide, or an inert gas such as nitrogen, and never oxygen. A typical low pressure residential system is tested at not less than three pounds per square inch, held for at least ten minutes, while systems operating at higher pressure test at not less than one and one half times their working pressure for longer. The gauge must be able to clearly show a pressure loss, and the pressure source is isolated before the clock starts. A needle that holds steady is a pass. Any unexplained drop means a leak.
Not everything on the line can survive a three pound test. Appliances and equipment rated below the test pressure are disconnected and the piping is capped, and regulators and meters not rated for the test are removed or isolated the same way. Equipment that is designed to take the pressure may simply be isolated by closing its shutoff valve. And a single closed valve is never trusted as the only wall between fuel gas and the test medium. Protect first, then pressurize.
If the needle falls, the leak has to be found with a listed combustible gas detector, a noncorrosive leak detection fluid such as a soap solution, or another approved method. A flame is never used to hunt for a leak. Brush every joint and watch for bubbles that grow. Then the failed section is repaired or replaced, and the affected piping is retested until it holds.
Restoring service on an existing system means turning the gas on with every appliance valve closed, after confirming there are no open fittings and that unused outlets are plugged or capped. Immediately after the gas is on, the code requires a leak check before anything runs. One common method your company may specify is watching the small test dial on the meter, the proving dial from the meter episode. With everything off, that dial must stand perfectly still, because movement means gas is escaping somewhere, and the supply stays off until repairs are made.
A tight pipe full of air still cannot feed a burner, so placing the system in operation means displacing that air with fuel gas. On small residential systems, the code permits purging to a burner where the gas is promptly ignited, or to a listed combustible gas detector, working outlet by outlet from the meter outward. Larger systems follow stricter written procedures, often displacing the air with inert gas first and discharging outdoors, away from ignition sources and building openings, continuously attended and monitored. Either way, keep waiting ignition sources away and ventilate the space.
Purging runs the other direction too. Before piping is cut, welded, or removed from service, the fuel gas inside must be displaced safely so it cannot mix with air near the work. The code calls for residual gas to be vented and for abandoned lines to be purged, with the exact steps coming from your company and the gas supplier. Taking a line out of service deserves the same respect as placing one in.
Before the job is signed off, the record has to stand on its own. The test pressure and duration are documented and meet the code minimums. Every joint was exposed during the test and proven tight. Appliances are reconnected, and each connection is leak checked at operating pressure. And the purge is complete, ending with a stable blue flame at the burner instead of a sputter of trapped air.
Pressure test to prove new pipe. Leak check to restore service safely. Purge in to light the burner, and purge out to make the line safe for work. Next episode, corrugated stainless steel tubing and bonding.