What mechanism must remain closed until manually reset in overpressure protection systems?

The correct choice refers to an automatic shutoff device, which is designed to prevent the flow of materials when a predetermined pressure threshold is exceeded. In overpressure protection systems, this device remains closed until it is manually reset, ensuring that the system cannot inadvertently resume operation until it has been confirmed safe to do so.

This mechanism is crucial for maintaining safety, especially in environments where high pressure can lead to catastrophic failures or hazards. The manual reset feature adds an extra layer of control, requiring human intervention before the system can be reactivated, thereby ensuring thorough checks and assessments are performed to address any underlying issues that caused the overpressure condition.

In contrast, pressure relief valves are designed to open automatically when a certain pressure is reached, allowing for safe venting without requiring manual intervention. Check valves, typically used to prevent backflow in a system, do not have a closing mechanism related to overpressure conditions specifically. Bypass valves are designed to divert flow under certain conditions and can operate continuously or automatically without the need for a manual reset. Each of these components plays a role in maintaining system integrity and safety but does not require a manual reset after an overpressure event.