What device must be installed in a liquid transfer hose equipped with a shutoff valve at the discharge end?

In a liquid transfer hose that has a shutoff valve at the discharge end, it is essential to install a hydrostatic relief valve. This device plays a critical role in maintaining safety during the transfer of liquid propane or similar gases. When liquid is transferred from one container to another, especially in situations where a sudden blockage occurs at the discharge end, pressure can build up in the hose.

A hydrostatic relief valve is designed to open when the pressure reaches a certain level, allowing excess liquid to escape and preventing dangerous overpressure conditions that could lead to hose failure or leaks. This is particularly important because it ensures that any excess pressure is safely released rather than allowing it to cause a potentially hazardous situation.

The other devices mentioned in the options serve different purposes. A flow control valve regulates the rate of flow but does not address pressure build-up. A backflow preventer is focused on stopping the reverse flow of liquid, which is crucial for preventing contamination but doesn't mitigate pressure risks during transfer. A pressure gauge simply measures the pressure in the system without actively controlling or mitigating pressure conditions. Hence, the hydrostatic relief valve is the appropriate device in this context for safety and compliance in liquid transfer operations.