Extent of shop fabrication is often written too generally in equipment datasheets, although it directly affects execution cost.
In HRSGs, modularization defines how large the coil bundles are, how much adjacent steel is attached in the shop, and how much work is left for site erection.
Harp Construction
This is the lowest level of modularization in modern HRSGs. A harp includes upper and lower headers and the tubes connecting them. It is usually selected when large cranes are not available, field labor is inexpensive, or logistics are difficult. However, it leaves the most work for the site erector and is usually more about project limitations than efficiency.
Modular or Bundle Construction
Several harps are combined into a larger coil bundle or coil module. The module size is normally limited by transportation clearance, weight, or crane capacity. A key advantage is that modern HRSG coil modules are usually top-supported and delivered with top support steel and roof casing attached.
Goalpost-Style Modularization
In goalpost-style construction, the casing frame is erected first without casing panels, and the coil module is installed into this frame. The module includes a partial steel box that supports the coil bundle and acts as its own uprighting device. This can simplify lifting, but each harp still needs to be hung from roof beams after installation.
C-Frame Modularization
C-frame modularization is a higher level of shop fabrication. The coil module is delivered with floor, roof, sidewall casing, and main structural frame members already attached. C-frames are already top-supported, so there is no need to hang each harp separately after erection. They may cost more, but can reduce field construction time.
O-Frame / Shop Modular Construction
O-frame construction goes one step beyond the C-frame. It includes the coil bundle, roof panels, floor panels, both sidewall panels, structural moment frame beams, and internal baffling in one module. It is a high level of shop modularization, usually limited to smaller single-wide HRSG units.
Super Modules and Offsite Erection
Super modules are used when site access is favorable and local field labor is difficult or expensive. Large HRSG sections are fabricated offsite, often in a shipyard or fabrication facility with barge access, and shipped as very large modules. They may include multiple coil sections, drums, piping, and platforms.
Shop fabrication affects transportation, cranes, erection sequence, field labor, schedule, cost, and risk.
Economically, modularization shifts cost between shop and site. That is why it should be finalized during the bid stage, so bidders price the same execution basis and the purchaser compares total installed cost, not only equipment price.
Ref: Nooter Eriksen Handbook of Heat Recovery
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