The steam drum is the primary interface between water and steam. In a coalfired plant, boiler feedwater passes through the economizer and into the drum where the steam separates from the feedwater and is drawn off to the superheater. In combined-cycle operations, a Heat Recovery Steam Generator (HRSG) serves the same purpose as a boiler. It is a gas-to-water heat exchanger that extracts energy from the gas turbine exhaust gases and uses it to create steam for the steam generator. HRSG Drums can be high or low pressure varieties.
Maintaining constant liquid level in the upper part of the drum is necessary to provide the proper quality of steam. Instrumentation must withstand high temperatures and pressures.