Why is the use of flexible-hose burner leads for propulsion boilers considered a fire hazard?

The use of flexible-hose burner leads for propulsion boilers is considered a fire hazard primarily due to their susceptibility to failure at the coupling joint. These couplings are critical points where the hose connects to other components and can experience significant stress due to temperature changes, pressure fluctuations, and vibration during operation. If the hose fails at these joints, it can lead to leaks of flammable materials, which might ignite if they come into contact with hot surfaces or flames, potentially resulting in a fire.

This risk emphasizes the need for robust installation practices, regular inspection, and maintenance of the flexible hoses to assure their integrity. The materials and construction of the hose play a crucial role in determining their reliability, and ensuring that the couplings are secure is vital to preventing hazardous conditions that could lead to fire incidents in the machinery dealt with by Machinist's Mates.