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chromium content; 100% sealing efficiency is provided with
• For coil bundle sections at highest temperatu- air sealing fans 2x100%;
res, like superheater section (even if in OTSG • Faster start-up with reduced water inven-
there is not a physical separation among each tory level: OTSG tube bundle usually does
section) oxide layers that grow above about not need to be filled of water, because,
1-2 mils (25-50 µm) in thickness for auste- during start-up, the first water entering
nitic steels typically used in boilers, have a hot coils is suddenly vaporized, so contri-
tendency to delaminate, and to blister and/or buting, with the controlled vent/steam by-
exfoliate. The impacts on heat transfer and on pass, to progressively raise-up pressure
the integrity of the system can be dramatic, and temperature till to the set levels. This
exposure to rapid temperature changes is a reduces the start-up time and the size
key factor in causing exfoliation of the oxide and weight of Demi water tank, typically
[7]; this is the reason why for this critical high located in the hull of FPSO.
temperature sections Ni-Alloys metallurgy is
selected, since they enable a more stable oxi- Another further advantage of OTSG is in exchan-
de layer formation than Austenitic steels; ging tubes arrangement: differently from Drum-
• excellent mechanical properties even at high type HRSG where the tubes are vertical, because
temperatures (approximately up to 600°C) as- of the natural circulation needs to be established
sured by the high Nickel-Chromium content of by steam-water mixed
these Alloys, without suffering of detrimental phase density gradient Baker Hughes developed several
phenomena (like creep) and so achieving dry- in the tubes, OTSG coils
run operation capabilities; this possibility leads are horizontally arran- improvements which can help
to a series of advantages: ged instead, so they can “find the proper trade-off in
• By-pass stack may not be more required, be crossed by exhaust performances and flexibility
OTSG may operate in dry-run: weight gas in a vertical pattern,
and footprint impacts can still be reduced from bottom to top, so while reducing weight, footprint,
compared to traditional HRSG. In some allowing to locate the CAPEX and OPEX
cases, by-pass duct is still installed, to Equipment vertically,
improve OTSG operational flexibility, sin- upon the GT Exhaust di-
ce Equipment regulation can be done scharge plenum. This ar-
both on water flowrate and on exhaust rangement improves the CCGT module layout, so
gases. In configuration without by-pass, that the outlet stack typical of HRSG is limited in
also diverter damper is not needed, with OTSG to few meters of exhaust outlet duct length,
the advantage of less weight, costs, and as necessary to ensure a good dispersion.
higher reliability, with less moving parts As cons of OTSG adoption, it should be mentio-
applied. Usually, diverter damper to insure ned the impossibility to remove water impurities or

Figure 7 - OTSG with partial section view on tube bundle (Courtesy of Figure 8 - Vertical HRSG (Courtesy of Boustead International Heaters - BIH)
Boustead International Heaters - BIH)

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