Which of the following fittings are not allowed on a gas piping system?

The selection of street elbows and closed nipples as not permitted in a gas piping system is based on safety and operational standards established within gas installation codes. Street elbows are designed to connect two pipes at a 90-degree angle with a built-in male and female end, which can lead to issues such as increased potential for leaks due to the abrupt turn in flow path and vulnerability to stress. Closed nipples, being short sections of pipe that have male threads on both ends, can also present risks when used in gas applications as they don't allow for any expansion or contraction movement of the gas system, which may result in stress points and possible failures.

In gas systems, fittings must enhance system integrity and maintain safety, and using fittings like street elbows and closed nipples can compromise those standards due to their designs not being conducive to maintaining a leak-proof joint under the conditions present in gas systems.

Other fitting types mentioned such as couplings and elbows, unions and tees, or connectors and flanges are commonly permitted as they allow for proper installation, maintenance, or modification of gas piping systems while adhering to safety regulations. They facilitate the necessary alignment, connection, and alterations while ensuring the integrity and safety of the gas line installations.