What operational procedure should continue until a liquid propane cylinder is cool?

The operational procedure of flowing water directly on the liquid propane cylinder should continue until the cylinder is cool due to the nature of propane and the risk associated with it. When a propane cylinder is exposed to heat, the pressure inside the cylinder increases, potentially leading to a catastrophic failure or explosion if the cylinder ruptures or releases its contents due to overpressure.

Applying water to the cylinder serves two critical purposes: it cools the metal of the cylinder and mitigates the risk of ignition or explosion by preventing the release of high-pressure vapor. Water helps to absorb heat, thereby lowering the temperature of the cylinder and reducing the internal pressure. This application of water is especially crucial in incidents where the cylinder is involved in a fire, as it can provide a protective barrier and reduce the intensity of the fire surrounding it.

Other options, such as stopping all water application or switching to foam application only, would not be sufficient or appropriate in situations where cooling the cylinder is essential to prevent a dangerous failure. Utilizing wet chemical agents may not be effective either since they are not designed specifically for cooling high-pressure gas cylinders and are more suitable for specific fire types. Therefore, continuing to flow water directly on the cylinder is the most critical procedure until it reaches a safe temperature.