The gas turbines that power electrical generators discharge exhaust gases at extremely high temperatures. Heat recovery steam generators (HRSGs) extract the heat from the gases to produce steam that powers steam turbines that in turn drive more electrical generators.
The typical HRSG includes multiple heat exchangers located one after the other in the flow of a hot exhaust gas from a gas turbine. Among heat exchangers are an economizer for elevating the temperature of feed water, an evaporator for converting the higher temperature feedwater discharged by the economizer into saturated steam, and a superheater for converting the saturated steam into superheated steam. Many HRSGs have more than one economizer, evaporator, and superheater operating at different pressures.
Some HRSGs utilize circulation-type evaporators. The typical circulation-type evaporator, which relies on density differences to circulate water through it, includes an overhead steam drum and a coil composed of tubes located in the flow of the hot gas, with the lower ends of the tubes being connected to the drum through a downcomer and the upper ends being in communication with the drum through risers. Heated water delivered by a pump through an economizer flows into the steam drum where it mixes with steam and water already in the drum. The water from the drum flows downwardly through the downcomer into lower ends of the tubes. The water thereupon rises upwardly in the tubes and absorbs enough heat from the gas flowing through the coil to become saturated. A portion of the saturated water converts to saturated steam. Both the saturated water and saturated steam flow upwardly into the steam drum. The saturated steam separates from the water in the steam drum and flows on to a superheater. Where a circulation-type evaporator has the tubes of its coil oriented horizontally, a pump may be needed to circulate the water through the coil.
Some HRSGs have large natural circulation-type evaporators of high capacity that operate at high pressures. These evaporators have large steam drums to accommodate the high capacity and thick walls to withstand the pressure. Indeed, a steam drum for a large capacity, high pressure, evaporator may have an external diameter of 80 inches and walls that are six or seven inches thick. The large capacity of the steam drum translates into a large volume and provides the drum with retention time, that is to say, it enables the drum to supply water to the coil in the absence of the delivery of water to the drum. This protects the coil from damage should the supply of water to the evaporator fail.
In order to avoid overstressing components of the HRSG, particularly the steam drums of its evaporators, the HRSG must undergo an extended start-up during which the heat-up rate is controlled, often by introducing hold points into the start-up procedure. The extended start-up delays operating the HRSG at peak efficiency. Moreover, the delay lengthens the time required to bring the HRSG—and any gas turbine with which it may be coupled—into compliance with emissions requirements.