Which type of safeguard is designed to re-close when upstream supply pressure subsides?

The type of safeguard designed to re-close when upstream supply pressure subsides is the check valve. Check valves are specifically designed to allow fluid to flow in one direction while preventing backflow. When the pressure on the upstream side of the valve is greater than the downstream pressure, the valve remains open, allowing fluid to pass through. However, when the upstream pressure decreases and falls below the downstream pressure, the valve automatically closes, preventing any reverse flow.

This functionality is crucial in various systems where maintaining flow in the intended direction is necessary for safety and operational integrity. For instance, in pump systems, check valves ensure that the pump remains primed and fluid does not flow back into the source when the pump is off.

The other choices serve different purposes: emergency shutoff valves are designed to isolate parts of a system in case of emergencies and do not re-close automatically, pressure relief valves are designed to relieve excess pressure in a system and may not re-close under normal operating conditions, and flow restrictors are used to limit flow rate rather than managing directional flow. Hence, the characteristics of the check valve align perfectly with the requirement of re-closing when upstream supply pressure falls, making it the correct answer.