The most common material used for gas piping is black steel pipe conforming to ASTM A53 or ASTM A106. Malleable-iron or steel fittings should be used, except for stopcocks and valves. Above 4-in nominal size, cast-iron fittings may be used.
Most plumbing codes require that black steel piping exposed to the elements be treated to prevent deterioration.
Some local codes permit the use of brass or copper pipe of the same sizes as iron pipe if the gas handled is not corrosive to the brass or copper. Brazed or threaded fittings are generally used with these two materials.
Polyethylene (PE) and polybutylene (PB) with heat fusion joints and polyvinyl chloride (PVC) with solvent cement joints are used for outdoor underground installations, since these materials do not corrode and deteriorate as does black steel piping. If black steel is to be used underground, it must be provided with an exterior protective coating or tape or a cathodic-protection system, to prevent failure of the piping due to corrosion.
The usual gas supplied for heating and domestic cooking generally contains some moisture. Hence, all piping should be installed so it pitches back to the supply main, or drips should be installed at suitable intervals. Generally, unions or bushings are not permitted in gas piping systems owing to the danger of gas leakage and moisture trapping. To permit moisture removal, drips are installed at the lowest point in the piping, at the bottom of vertical risers at appliance connections, and at any other location where moisture might accumulate. Figure 14.13 shows typical drips for gas piping.