When is it necessary for the vent between the Main Stop valve and the Automatic Non-return valve to remain open?

The requirement for the vent between the Main Stop valve and the Automatic Non-return valve to remain open during start-up and shut down procedures is crucial for safe and effective operation of a boiler system. During these phases, there are changes in pressure and temperature, which can lead to the buildup of steam or other gases. By keeping the vent open, it allows for the safe release of any pressure buildup that may occur. This ensures that no steam or liquid can become trapped behind the non-return valve, which could potentially lead to damage to the system or create hazardous conditions.

In regular operations, the system is designed to function with the automatic non-return valve in place, and maintaining the vent open would not be necessary or appropriate, as it could disrupt normal operation. Maintenance procedures typically involve isolating portions of the system for safety, but they do not require constant venting like start-up and shutdown do. Emergencies are generally unpredictable situations, and while contingencies should be in place, keeping the vent open specifically for emergencies isn't a standard operational procedure. Thus, the necessity of having the vent open is primarily aligned with the dynamic pressure changes that occur during start-up and shut down.