From the IRC Commentary:
(From the IRC)
G2419.4 (408.4) Sediment trap. Where a sediment trap is not incorporated as a part of the gas utilization equipment, a sediment trap shall be installed downstream of the equipment shut-off valve as close to the inlet of the equipment as practical. The sediment trap shall be either a tee fitting with a capped nipple in the bottom opening of the run of the tee or other device approved as an effective sediment trap. Illuminating appliances, ranges, clothes dryers, and outdoor grills need not be so equipped.
(Commentary)
❖ In addition to the code requirement, most appliance manufacturers require the installation of a sediment trap (dirt leg) to protect the appliance from debris in the gas. Sediment traps are necessary to protect appliance gas controls from the dirt, soil, pipe chips, pipe joint tapes and compounds, and construction site debris that enters the piping during installation and repairs. Hazardous appliance operation could result from debris entering gas controls and burners. Despite the fact that utilities supply clean gas, debris can enter the piping prior to and during installation on the utility side of the system and on the customer side.
Sediment traps are designed to cause the gas flow to change direction 90 degrees at the sediment collection point, thus causing the solid or liquid contaminants to drop out of the gas flow. See Commentary Figure G2419.4(1).
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The nipple and cap must not be placed in the branch opening of a tee fitting, because this would not create a change in direction of flow and would allow debris to pass over the collection point. Commentary Figure G2419.4(2)
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illustrates an improper sediment trap that is prohibited by this section. The code does not specify a minimum length for the capped nipple. Three inches minimum is customary. The sediment trap must be downstream of the appliance shutoff valve and as close to the appliance inlet as practical. Requiring the sediment trap to be downstream of the appliance shutoff valve makes sure that the sediment trap is within 6 feet of the appliance inlet. If there is 6 feet or less of piping between a sediment trap and the appliance inlet served, the intent of the code has been met, regardless of the shutoff valve location. Manufactured sediment traps are available that have the configuration of a straight section of pipe and are equipped with cleanout openings. Although it would be wise to install sediment traps at all appliance connections, they are not mandated by code for gas lights, ranges, clothes dryers, and outdoor grills. These appliances are susceptible to harm from debris in gas, especially ranges and clothes dryers, and the appliance manufacturer may require sediment traps where the code does not. The code.s logic is that these appliances are attended while in use therefore, the user would be aware of a problem.