What device is essential to prevent flow back into the fuel system for LP-gas?

The essential device to prevent flow back into the fuel system for LP-gas is the check valve. This device is designed to allow the flow of gas in one direction while preventing it from flowing back in the opposite direction. This is particularly important in liquefied petroleum gas (LP-gas) systems to ensure safety and maintain system integrity.

In an LP-gas system, if flow back were to occur, it could lead to dangerous situations such as pressure imbalances, potential leaks, or even hazardous conditions arising from gas escaping into areas where it could become a risk to health or safety. The check valve effectively mitigates these risks by serving as a one-way gate, ensuring that the gas flows only as intended and preventing any unintended backflow into the fuel system.

Other devices listed, such as a flow regulator, manual shutoff, and pressure gauge, serve different but important functions within an LP-gas system, such as controlling gas flow rates, stopping flow in emergencies, and monitoring pressure levels, respectively. However, it is the check valve that specifically addresses the critical need to prevent flow back into the fuel system.