What is the primary purpose of having 5 psi in the lateral piping on the 51 AF-4 rapid defuel valve?

The primary purpose of maintaining 5 psi in the lateral piping on the 51 AF-4 rapid defuel valve is to ensure that the hydrant outlet line stays full of fuel. This pressure helps maintain the integrity and flow of the fuel system during operation. By keeping the hydrant outlet line pressurized, it prevents the fuel from vaporizing or becoming stagnant, which could lead to operational inefficiencies or hazards during rapid defueling operations.

A full hydrant outlet line is crucial for effective defueling, as it allows for a continuous, steady flow of fuel. This is particularly important in rapid defuel scenarios where the speed of operation is critical, and any interruption in flow could cause complications. The 5 psi pressure acts as a safeguard to support this requirement, ensuring that the system operates reliably under the intended conditions.

Other options may relate to different aspects of fuel management or operational safety, but they do not align with the specific function of the 5 psi pressure in the context of maintaining the flow and integrity of the hydrant outlet line.