What procedure helps prevent a vacuum when taking a boiler offline?

Opening the air cock is essential in preventing a vacuum when taking a boiler offline. When a boiler operates, it is under a certain steam pressure. When you shut down the boiler and remove the steam pressure, it is crucial to allow air to replace the steam. If the steam is removed without allowing air to enter, a vacuum can form inside the boiler due to the condensation of steam and the cooling of the remaining water. This vacuum can lead to structural damage to the boiler as it creates negative pressure.

By opening the air cock, you enable outside air to enter the boiler, which equalizes the pressure and prevents the creation of a vacuum. This simple step ensures that the boiler remains safe and intact during the offline process.

In contrast, opening the steam valve would not prevent a vacuum; it would actually allow steam to escape, which may not be desirable in this context. Closing the feedwater valve may restrict water flow but does not address the pressure issue directly. Reducing the pressure slowly can be part of a safe shutdown, but without allowing air in, a vacuum can still develop. Thus, opening the air cock is the most effective procedure in this scenario.