Question 149Steam Plants - QMED

The correct answer is C) Economizer drain valve. Before lighting off a boiler, the economizer drain valve should be closed to prevent water from draining out of the system, which could lead to issues during the boiler startup process. The economizer is a heat exchanger that preheats the boiler feed water, and the drain valve is used to remove any accumulated sediment or water from the economizer. The other options are incorrect because the superheater vent valve, superheater drain valve, and air cock valve are not directly related to the initial boiler lighting process and do not need to be closed before this step.