A sediment trap, also called a drip leg or dirt leg, is a short capped section of pipe that hangs down from a gas line just before an appliance. It catches rust, scale, and debris carried in the gas so that grit cannot reach and clog the appliance's gas valve or burner. Fuel gas code requires one at most appliances.
A sediment trap is a small safety fitting on a gas line, and plumbers know it by several names: drip leg, dirt leg, or drip tee. Whatever it is called, it does one job. It catches solid junk traveling through the gas pipe before that junk can reach the appliance and cause trouble.
Gas pipe is not perfectly clean inside. Over the years a steel gas line can shed rust and scale, and any pipe can carry leftover metal shavings, thread cuttings, or dirt from when it was installed or repaired. Gas flowing through the pipe can push those small particles along toward the appliance. If that grit reaches a modern furnace, water heater, or range, it can foul the gas valve or clog the small orifices at the burner, causing the appliance to misfire or fail. The sediment trap stops that from happening.
The design is simple and clever. Just before the gas reaches the appliance, downstream of the shutoff valve, the pipe runs into a tee fitting. Gas turns and continues on to the appliance, but any heavier debris keeps going straight down into a short capped nipple that hangs below the line. That capped section is a dead end, so grit falls in and stays there, out of the gas stream. For example, when an old furnace is replaced, any rust the work knocks loose in the line drops into the drip leg instead of riding into the new gas valve.
Fuel gas code requires a sediment trap at most appliances. The rule, in Section 408.4 of the fuel gas code, calls for one to be installed downstream of the appliance shutoff valve and as close to the appliance inlet as practical, unless the appliance already has one built in. The vertical leg has to be a real reservoir, generally at least 3 inches long, so it can actually hold what it catches. An inspector will look for one, and a missing drip leg is a common correction on a gas appliance install.