Gas Piping, Valves, and Burner Components
Domain 3, Gas Piping, Valves, and Burner Components, tests pipe sizing, sediment traps, combination gas valves, orifices, burners, and LP conversion.
Piping and leak testing
Size gas pipe from the connected load, the longest run, and the allowable pressure drop, including fittings and future appliances. Install a shutoff valve, union, and sediment trap/drip leg at the appliance. Leak-test new piping with air or nitrogen per code; never use a flame. Use soap bubbles for operating joints and check the valve, manifold, and pilot tubing.
Gas valve and burner
A combination gas valve contains a manual valve, pressure regulator, redundant safety shutoff, and operator. Manifold pressure is measured with burners firing: about 3.5 in. w.c. for natural gas and 10–11 in. w.c. for propane. The orifice meters gas; propane needs a smaller orifice because its heating value per cubic foot is much higher. Primary air is adjusted at the shutter; secondary air enters around the burner. Carryover slots light burners in sequence.
Worked example
An 80,000 BTU/hr natural gas furnace is converted to propane. Propane has roughly 2.5 times the BTU/ft3 of natural gas, so the orifice area must be reduced accordingly. Raising manifold pressure without changing orifices overfires the burner and can produce CO. A correct conversion changes orifices, the regulator spring, and the nameplate marking.
Traps
Do not apply pipe dope to flare fittings or allow it inside the gas valve. Do not set manifold pressure before the burners light or with only the pilot on. Do not open the air shutter to cure a yellow flame before checking orifice size and gas pressure. Never omit the sediment trap.