Question 194Steam Plants - Assistant Engineer

The correct answer is D) With the air cock open, the boiler drains without producing a vacuum. When draining a water-tube boiler, it is important to open the air cock to prevent the formation of a vacuum inside the boiler. If the air cock is left closed, the rapid draining of water can create a vacuum, which could potentially damage the boiler's structure. By opening the air cock, air is allowed to enter the boiler, equalizing the pressure and allowing the water to drain smoothly without creating a harmful vacuum. The other options are incorrect because: A) Rapidly draining the boiler with the air cock closed is the problem that needs to be avoided. B) Air mixing with the water does not provide a "cleansing effect" in the tubes. C) Drying out the boiler's surface is not the primary reason for opening the air cock during draining.