POWER June 2010 - 32

combined cycles
Table 2. HRSG damage factors and the effect of cycling. Source: Vogt Power International Inc.
Mechanism
Damage factors
Low-cycle fatigue
Creep
Thermal shock
Oxidation
Differential expansion
Corrosion fatigue
Corrosion in tubes
Flow-accelerated corrosion
Corrosion product
migration
Scale and deposits
Erosion
Corrosion outside the tubes
Erosion outside the tubes
Damage occurs at low cycles when the strain is high. This is the
prevailing damage mechanism in HRSGs.
Damage due to material being at high temperature for considerable
periods of time.
Impingement of cold water/steam on hot surfaces can damage
the material.
Oxidation and exfoliation due to high temperature.
If adjacent tubes or pipes are at different temperatures, uneven expansion
can stress both tubes. Piping supports can also affect stresses.
The crack initiation occurs due to fatigue cracks in the magnetite
layer, and further damage is caused by fatigue and corrosion. Damage
typically occurs between 300F and 500F.
Due to chemical imbalance.
Corrosion accelerated due to chemistry and flow.
Migration of corrosion product may cause further corrosion or other
damage.
Temperature fluctuations may result in deposits.
Transient high velocities may initiate or perpetuate erosion.
Due to various gas constituents.
Due to particles in gas and high velocities.
The liner plates, structural supports, and other components are damaged
due to uneven heating and expansion.
Effect of cycling
Increases due to higher number of cycles.
Although cyclic plants do not operate as long, decrease in life is
experienced due to creep/fatigue interaction.
Decreases life.
Not affected.
Transient spikes may get metal into plastic region.
Accelerates the effect.
Common chemistry imbalances at start-ups can exacerbate the effect.
Reduced water chemistry controls increase the possibility.
Intermittent heating and cooling can dislodge the deposits.
Due to effect in chemistry.
Low steam pressures at start-up cause high velocities.
Frequent shutdowns result in more frequent condensation.
No effect.
Corrosion in non-pressure parts Condensation of acid and/or water on cooler parts causes corrosion. Increases.
Differential expansion in nonpressure
parts
HRSG, not merely the theoretical operating
profile. Deviate from those design assumptions,
and the actual life of critical components
may be severely compromised.
A rational approach for determining remaining
equipment life is to first develop a
methodology that will identify where damage
will likely occur in the plant, quantify
the impact of that damage on equipment life,
determine new operational limits to minimize
equipment damage, and then estimate
the economic impact of those measures. The
purpose of this article is to demonstrate this
analysis approach on the HRSG portion of
the combined-cycle plant and show that this
approach produces data that can be used to
optimize operation and maintenance (O&M)
costs on a life-cycle basis.
Cycling and Fast Starts
Cycling occurs when units are required to be
brought online and shut down to meet grid
demand and to provide the owner/operator
with the most cost-effective operating profile.
When cycling, units are generally kept online
for short durations, usually a few hours or
a few days. For example, the units may be
brought online to meet excess demand during
peak hours and then shut down overnight.
This is regular cycling in the sense that the
time and duration of operation is well defined
and the schedule is predictable.
32
Sometimes the units meet electricity
demand when a baseload unit is down for
maintenance or repairs. If the maintenance
is regularly scheduled, then the cycling unit's
operation is also predictable. However, there
may be times when the regular baseload units
are shut down for a forced outage. Forced
outages, by definition, are unpredictable in
both occurrence and duration.
Another type of cycling occurs when owners
want to take advantage of a market price
opportunity. Power grids buy power daily,
based on the market price. Power generators
may want to start a unit on very short notice
if the market price will result in what they
believe will return a quick profit.
Start-up time is the most important, and
most controllable, statistic for cycling units.
If the start-up time is predictable, then units
can be started with well-defined and optimized
operating procedures in advance of
the demand. However, unscheduled startups
may be tied to a particular power market
opportunity offering potential extra revenue.
If the unit starts up more quickly, then more
revenue can be generated by supplying the
power sooner. Thus, faster starts are desirable
to reduce start-up costs or maximize
revenue potential.
Cycling and faster starting may generate
additional revenue, but doing so also increases
the life consumption of an HRSG. Any
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Increases.
baseload-designed unit can last the predicted
life in cycling mode if it is started slowly
enough. The pressure in the HRSG can be
ramped up at a specified rate that ensures all
components are not adversely affected. However,
these specified ramp rates may be too
slow and thus take too much time for starting,
or they may simply be too slow to be
practical for today's fast-moving electricity
markets. Fast starting damages the unit because
rapid ramping produces much higher
stresses, causing faster unit deterioration.
It is not practical to operate baseload-designed
units under cyclic conditions without
a greater, and unknown, level of deterioration
of HRSG components. Few owners know
how much additional life is consumed during
such an event, and fewer still add into
their market bid price an amount of money
that represents the amortized value of the
equipment loss of life. The question becomes
not whether there is an unknown amount
of accelerated HRSG damage occurring by
cycling any combined-cycle plant but how
much and where the damage occurs.
HRSG Damage Mechanisms and
the Effect of Cycling
A number of common damage mechanisms
are experienced by HRSGs during start-ups
and operations. A list of the various damage
mechanisms affecting the HRSG life cycle
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POWER June 2010

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