Which type of piping is designed to primarily handle vapor service according to pressure limits?

Polyethylene piping is specifically designed to handle vapor service effectively due to its intrinsic properties, particularly its ability to resist the permeation of gases. This makes it ideal for applications involving propane and other gases, ensuring that the vapor is contained within the piping system without significant leakage.

Moreover, polyethylene's flexibility and resistance to corrosion contribute to its ability to operate under the varying pressure conditions found in vapor service. This material is lightweight and easier to install, which is advantageous in both residential and commercial applications.

The other types of piping, such as steel, copper, and cast iron, while they may be used in some gas systems, are generally more suited to liquid or high-pressure gas transport and can have issues such as corrosion (in the case of steel and cast iron) or restrictions on bending (in the case of copper). Therefore, for primarily vapor service applications, polyethylene stands out as the best choice due to its designed characteristics and pressure handling capabilities.