Final answer:
When LP-gas liquid can be isolated between shutoff valves, a block valve must be installed in each closed section of piping.
Step-by-step explanation:
When LP-gas liquid can be isolated between shutoff valves, a valve called a block valve must be installed in each closed section of piping. Block valves are used to completely stop the flow of a liquid or gas in a pipeline. They are designed to have no leakage when closed, ensuring that the LP-gas liquid remains isolated between the shutoff valves.