What must be ensured about the pressure relief valve of a gas container?

The correct choice highlights the critical requirement that the pressure relief valve of a gas container must be in direct communication with the vapor space. This is essential because the valve’s primary function is to release excess pressure that may build up within the container due to various factors such as thermal expansion or vaporization of the gas. By being in direct communication with the vapor space, the relief valve can effectively sense the pressure and respond correctly by venting to prevent over-pressurization. If the valve were not situated in the vapor space, it might not function properly, potentially leading to dangerous situations such as container rupture.

In contrast, positioning the valve to face downwards would impede gas flow, reducing its effectiveness. A requirement for the valve to be 3 ft above the container is not standard and does not align with safety protocols regarding pressure relief valves. Additionally, securing the valve with a metal plate may not be necessary and might even interfere with its ability to function properly in the event of a pressure increase. Thus, ensuring the direct communication with the vapor space is paramount for safety and operational efficiency of a gas container's pressure relief system.