“Turbine HRSG Design and Performance” “Hot Topic Hour” at 1 p.m. on November 5, 2009 Included Discussions of Drum and Once-through Design as Well as the Role of Duct Burners

Bik Lakhanpal of, Innovative Systems Technology cited the advantages of Once-through Steam Generators (OTSG) in Fast Start and Cycling Applications. The OTSG is a drum-free HRSG using thin wall tube materials of Incolloy 800 and 825. The high material strength allows for thin tube walls which are quicker to heat, so this feature is well suited to cyclic operations. With no fixed economizer, evaporator and superheater sections, there is considerable startup flexibility.

Lew Douglas, Staff Consultant, Nooter/Eriksen, discussed the important features to consider in selecting HRSGs for Cycling Applications. Important mechanical features include coil flexibilities and proper piping layouts. Low cycle fatigue is caused by restrained thermal growth as well as water quenching. Coil flexibility eliminates the low cycle fatigue. As a general rule, allowing parts to move freely is always better than relying on internal coil flexibility. Spring- supported headers are a good way to provide the flexibility.

Lew also provided us with a paper providing considerable detail on the subject. 

Paper by Lew Douglas Nooter Erickson - Nov. 5 1 pm.pdf

Richard Brown, Director of Duct Burner Products Coen Company, discussed  performance and design issues with duct burners. The duct burners can be fired with lower quality fuels than utilized by the turbine. This has cost advantages but presents challenges in burner design. Insulated fuel gas manifolds help to prevent buildup. Water condensation can cause severe problems both inside and outside the HRSG. Corrosion is also a worry. Good CFGD modeling can make help to minimize these problems by indicating the flow distribution and burner criteria necessary to provide uniform temperature in the gas stream.