In special cases, the Commandant of the Coast Guard may permit cargo piping to pass through machinery spaces, provided that the only cargo carried is which of the following?

The correct answer highlights that the Commandant of the Coast Guard may allow cargo piping to pass through machinery spaces specifically when the cargo being transported is Grade E. This classification is significant because Grade E is defined as a cargo that has a relatively low flash point and presents a lesser risk compared to other grades, especially concerning fire hazards.

In this context, the authorization to run piping through machinery spaces recognizes the conditions under which the associated risks can be effectively managed. Grade E cargoes are generally associated with less volatile and less flammable properties, thus making it more acceptable to have them in proximity to machinery spaces, which are typically considered higher-risk areas due to their operation and potential ignition sources.

The other grades mentioned, such as A, B, or D, include more hazardous materials that possess higher flash points or flammability risks, and allowing their piping through machinery spaces is not considered safe or prudent. Therefore, this regulation strikes a balance between operational flexibility and safety, ensuring that risks are minimized while allowing for efficient transfer of safer cargo.