Final answer:
The component with two sensing lines in an AFFF system is the proportioner; it mixes the AFFF concentrate with water from the firemain to create the foam solution that extinguishes fires.
Step-by-step explanation:
The component in an Aqueous Film Forming Foam (AFFF) system that has two sensing lines, one connected to the firemain and another connected to the AFFF concentrate, is known as the proportioner. The proportioner is key to mixing the AFFF concentrate with water to create the foam solution. Its design permits accurate mixing ratios, which are critical for the effectiveness of the AFFF in extinguishing fires, particularly Class B fires involving flammable liquids. The sensing lines are used to ensure the correct balance of concentrate and firemain water pressure, enabling the formation of a high-quality foam that can spread over the surface of the flammable liquid and smother the fire by separating the fuel from the air.