Why is it important for a vapor service valve to have a plug or cap when not in use?

A vapor service valve must have a plug or cap when not in use to prevent gas leakage and ensure safety. By sealing the valve, you eliminate the risk of gas escaping, which can pose a significant hazard. Leaking gas can lead to fire or explosion risks, as well as environmental concerns from gas emissions.

Moreover, capped valves contribute to the overall integrity of the LP gas system. They help maintain pressure and prevent contamination or debris from entering the valve, ensuring that it operates safely when reactivated. Therefore, having a proper cover for the valve is crucial for essential safety standards and compliance with regulations.

The other options, while potentially beneficial in specific contexts, do not address the primary function of a plug or cap in ensuring safety and preventing gas leaks.