Design of Integrated Bleed System for Aeroderivative Gas Turbine

Abstract In some multi-shaft gas turbine (GT), low pressure compressor (LPC) bleed air vents via the LPC bleed air collector to ambient in order to adjust airflow to the high pressure compressor (HPC) during part-load operation and transients. This bleed air is controlled by a variable bypass valve (VBV) and must be ducted outside the enclosure through a dedicated duct. Traditionally, this implies the design of an external duct mounted on the top of the enclosure, which needs to withstand the thermal loads, to respect the acoustic requirements and to allow a proper operation of the engine in all conditions. This work describes an innovative design of the VBV discharge system, by integrating VBV duct with the GT exhaust duct. In such configuration the LPC bleed air is routed in a duct placed inside the enclosure and connected to the GT exhaust stack, helping to reduce weight, volume and cost of the entire unit. However, this solution requires to optimize and solve several technical challenges such as the coupling of the bleed airflow with the GT exhaust flow guaranteeing a proper uniformity of temperature and velocity, in order to reduce thermal stresses to the silencer panels and eventually also waste heat recovery units. On the other hand, when the VBV is closed, there is a possible backflow of hot exhaust gases from the GT stack towards the VBV collector, which may damage the components installed in that area. The proposed solution has been designed carrying out an extensive CFD simulation campaign and validated by experimental tests.