NFPFA 99 specifies that the medical gas piping system must be protected against all EXCEPT which type of material?

The National Fire Protection Association (NFPA) 99 outlines safety standards for medical gas systems, emphasizing the importance of protecting the integrity and safety of these systems. The correct answer indicates that the medical gas piping system is not explicitly required to be protected against combustible liquids.

Combustible liquids, while potentially hazardous in certain circumstances, are not directly involved in the medical gas system's primary function, which centers around managing gases like oxygen, nitrous oxide, and medical air. The primary concern for medical gas piping systems is ensuring they are safeguarded against materials that could compromise the safety and functional integrity of the gases being transported.

In contrast, flammable gases are of particular concern because they can ignite and cause fires or explosions if they come into contact with an ignition source. Similarly, corrosive substances can damage piping and fittings, leading to leaks and failures. Explosive compounds pose a direct threat to safety and infrastructure, necessitating stringent protection measures.

Thus, while combustible liquids are a consideration for overall facility safety, they do not directly affect the medical gas piping system in the same urgent manner as the other materials listed. This distinction helps clarify why protection against combustible liquids is not mandated within the context of NFPA 99 regarding medical gas systems.