Heat recovery steam generators |
What do HRSG tube failure statistics tell us?
HRSG (heat recovery steam generator) tube failures remain a significant threat to the reliable and cost- effective operation of combined cycle power plants, by causing costly forced outages for repairs. Tetra Engineering has been delivering analyses and inspections of boilers, HRSGs and steam plant to operators of combined cycle and conventional thermal power plants for over 30 years. By inspecting and performing root cause of failure analyses on hundreds of HRSGs over that time, they have accumulated a large data base on the cause and frequency of the various types of HRSG tube failures observed
Misha Gadher and Mark Taylor Tetra Engineering Europe, Sophia-Antipolis, France*
The typical large HRSG used for utility power generation is a counter-flow heat exchanger with thousands of boiler tubes. These are spread between the various modules that form the interface at which the waste heat from the gas turbine exhaust is recovered and converted into steam for power generation. Cold feedwater enters the back (stack) end of the HRSG and flows against the exhaust flow direction through economiser, evaporator, and superheater modules to exit as superheated
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steam. The HRSG can have from one to three pressure levels depending on size and specific plant requirements. Each pressure level requires separate economiser, evaporator, and superheater modules. Typically, triple pressure units will include reheat modules as this further improves cycle efficiency. Supplementary firing using burners installed in the gas path are also often included in the design. These allow the amount of heat in the exhaust gas to be increased, thereby raising steam production.
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The tubes in the different modules are made of various carbon and alloy steel grades. The choice of material is driven principally by design temperature and pressures, although cost is also a consideration if more than one steel grade would meet the design service requirements.
HRSGs have undergone substantial changes in design over the past 30 years as the technology has matured, starting from small single-pressure units driven by aeroderivative gas turbines to today’s large triple-pressure units with reheat running steam pressures in the high-subcritical region[1]
, not to mention
the various once-through designs. Besides fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications. Baseload operation was normal for earlier HRSGs, but today’s deregulated electric power market requires
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Examples of HRSG tube failures 1 - tensile overload in reheater tube–header weld; 2 - creep failure in superheater; 3 - underdeposit corrosion, leading to wall thinning and crack failure (surface deposit was removed during metallurgical analysis to show failure location clearly); 4 - corrosion fatigue, cracks originating from pits in preheater tube; 5 - fatigue cracks in HP economiser tubes; 6 - flow accelerated corrosion, wall thinning and failure in LP evaporator tube
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* Email addresses:
misha.gadher@
tetra-eng.com;
mark.taylor@tetra-eng.com 12 | March 2023|
www.modernpowersystems.com
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