Where must a drain valve be installed in relation to the main steam line?

The correct positioning of a drain valve in relation to the main steam line is critical for the proper operation and maintenance of a boiler system. A drain valve should be installed between the main steam line and the turbine throttle valve to allow for the removal of any condensate or unintentional buildup of liquid that could adversely affect the performance of the turbine.

By placing the drain valve in this location, it ensures that any necessary maintenance can be performed without opening the entire system, which maintains the integrity and safety of the steam line. It eliminates any potential risks of having water or condensate in a steam line, which can lead to water hammer or other operational dangers.

The other potential options, such as placing the valve before or after the turbine throttle valve or at the condenser outlet, would not effectively allow for the management of condensate in a manner that protects both the boiler and turbine system from damage and ensures efficient operation.