Every HRSG requires an exhaust gas pressure boundary around the heating surfaces, and each type brings with it particular design and operational criteria. For example with a hot casing, the most significant design problem is the provision of adequate support, particularly where two-shift operation is being applied. Cold casing requires careful consideration of the internal insulation and liner plate attachments to maintain internal pressure part expansion clearance while minimising gas bypass of the heating surfaces.
Prior to examination of a duct casing much can be gained for an operator if a thermographic survey is undertaken just prior to outage (see Technical Services). The infra red camera will highlight areas of uneven heat loss. For all types of duct, complementary internal examination is invaluable. Most of an inspection can be carried out visually, while keeping a condition record assessment from the structural integrity discipline backed up by an indexed photographic record will greatly assist third party inspectors, insurers and the design reviewer.
A hot casing design poses particular challenges for arrangement of the supporting steelwork. Doosan Babcock has carried out extensive finite element work on both full casing and detail design features as shown in the figures. These analyses greatly assisted in the understanding of the behaviour of the casing globally, surpassing classical calculations by far because all the mechanism interactions could be modelled. The analysis of the detailed components could be subjected to transient loading including the effects of duct and local cavity radiation for fatigue stress assessment.
All designs of HRSG plant need expansion joints, and these are normally a proprietary item. Rectangular expansion joints, particularly for a hot casing design, are more subject to buckling deformation than round joints, but all types require careful consideration of the insulation location and flow plate clearances during operational cycles. Doosan Babcock has worked with major manufacturers of these joints to provide assistance in compatibility with casing design, and has undertaken significant Finite Element Analysis of joints to resolve cyclic high temperature issues.
Doosan Babcock has considerable experience in the design and manufacture of hot casings and expansion joints, with particularly demanding operational experience in Asia. Domestic experience on inspection and repair of all HRSG casing types has been growing, with considerable interest in being shown in the recording process.