توربینهای گاز با کاهش بار در معرض افزایش دمای گاز خروجی از اگزوز ناشی از کاهش دبی جریان هوای گرم خروجی می باشند. بدین منظور استفاده از بار مصنوعی برای اجتناب از خاموش و روشن کردن پر هزینه واحد گاز در بعضی طرح ها مورد استفاده قرار می گیرد.
Keeping Metals Cool Enough. At low-load conditions it is often difficult to keep heat exchange surfaces below design temperatures or operationally limited temperature. The finned tube designs of pressure parts in HRSGs are very effective in moving heat from the exhaust gas to the tube wall. At part loads, several things happen to make this problem worse. The total mass flow of the GT exhaust is reduced, but often the temperature is increased. This results in lower steam flow from evaporators that is available to cool superheater and reheater tubes. Maintaining the required outlet steam temperatures while keeping intermediate metal temperatures below limits can be a challenge.
As an example, consider a large (170-MW) GT in combined cycle service. At design full-power conditions, exhaust gas flow is around 3,400,000 lb/hr at 1,150F to the HRSG. At low load (85 MW) flow is 2,456,000 lb/hr and 1,210F–1,215F.
Recently, a large GTCC plant in the U.S. implemented an extended turndown with a GT performance upgrade. The increased turbine exhaust temperature was around 1,208F–1,215F at about a 50% output level. Problems were observed with the existing desuperheater spray valves, which prevented raising spray flows, so steam temperatures rose from 1,048F to 1,058F–1,060F. This raised owner concerns about exceeding design tube metal temperatures in the superheaters and reheaters.
The maximum tube temperatures set for ASME Boiler and Pressure Vessel Code calculation is the design midwall (average) tube temperature allowed. The design allowance for spread in tubes temperatures is typically around 25F. Thus, the average tube temperatures should be 25F below the design temperature. The tube temperatures at the actual operating conditions were checked at the higher steam temperature for acceptability, as shown in Table 1. The values were acceptable but close to limits. Operation was not feasible until the desuperheater spray valves were modified to allow greater spray amounts.