pwr_july-2024 - 11

O&M
Tips for HRSG Maintenance
and Upgrade Success
The heyday of combined cycle plant construction
(especially in the U.S.) was at
the start of the millennia. Between 2000
and 2005, about 130 GW of natural gas-
fired combined cycle plants opened in
the U.S., according to the U.S. Energy
Information Administration (EIA). Since
then, the annual totals have been more
modest. But the average per year has
remained steady in the 4 GW to 8 GW
range, with some years surpassing 15
GW. In 2023, for example, 16 U.S. natural
gas-fired power plants came online totaling
8.6 GW. That's up from 11 in 2022,
amounting to 5.6 GW. The EIA expects
another 20 new natural gas-fired power
plants to come online in 2024 and 2025.
A steady built-out of new facilities is
good news for heat recovery steam generator
(HRSG) manufacturers. And with the
bulk of the combined cycle fleet now aged
20 years or more, there are plenty of plants
around that are in urgent need of maintenance,
repair, and upgrade work. Here are
some key points to consider in the maintenance
and upgrading of HRSGs.
HRSG Logistics Are Challenging
HRSGs are big. Transporting them to
the site requires careful consideration
to ensure that they can be taken to the
facility and moved to where they will be
installed in the plant. To reduce transportation
difficulties and make it easier
to get the HRSG where it needs to be,
some manufacturers deliver the different
HRSG modules and components in
stages to reduce clutter and avoid taking
up too much space on the ground. After
all, space is almost always an issue in
any plant. Designs are typically compact.
There is limited space to integrate the
HRSG into the building or site layout. Advance
planning and plenty of homework
is needed to scope out the layout of the
facility and determine how much room
is available for the HRSGs. The design
must be tailored to fit the available space
and minimize plant modifications.
Unforeseen problems often crop up
once the equipment arrives onsite if planning
is perfunctory. Large parts may not
clear entrances, tight turns, or other headway
restrictions. When the equipment arrives
at the combined cycle location, there
needs to be enough space (Figure 1) to
stage and assemble the HRSG. Those
July 2024 | POWER
vents, and fittings. Components will
degrade over time. Even if well maintained,
they will eventually need repair
or replacement. Look out for rust, leaks,
and other damage. Poorly operated units
sometimes need tube replacements in
as little as a couple of years. Properly
maintained units, on the other hand, can
run well for decades.
Once inside the unit, those main1.
Space can be tight at plants. Plan appropriately
for staging and movement of components
on job sites. Courtesy: Rentech Boiler Systems
working on HRSG installations, upgrades,
or repairs need to have enough room allocated
to them to do the work and sufficient
space for components to be stored
and workers to use as an onsite office.
Other Items to Stay Abreast Of
Having a solid plan is not only important
during HRSG installation, upgrade, and
repair projects, it's also essential throughout
the life of a unit. Consider placing a
keen focus in the following areas.
Maintenance. HRSGs must deal with
high temperatures and high steam pressures.
They are pounded by hot, pressurized
water and steam constantly.
Components must be able to withstand
extremes between hot and cold conditions.
Not surprisingly, the most poorly
maintained part of any HRSG tends to
be the pressure vessel. The pressure
vessel needs to be well maintained to
preserve performance and longevity.
Failure to do so could lead to unscheduled
plant shutdowns.
Testing and Chemistry. Go to any
HRSG user event and a lot of the sessions
concern HRSG chemistry. Water
quality and water chemistry play an essential
role in HRSG health and output.
Operators need to take great care to
maintain their HRSGs within the parameters
laid out in manufacturer guidelines.
That means regular testing, analysis, and
tuning to counter any degradation issues
detected. Corrosion, deposition, and other
problems can be found early if regular
inspections are being done and the right
water treatment steps are being taken
as part of HRSG maintenance schedules.
Hardware Inspections. As well as
maintaining water chemistry, plant operators
also need to keep a close eye
on all HRSG components, piping, valves,
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taining HRSGs should ensure that heat
transfer surfaces are cleaned to remove
calcium, magnesium, silica, rust, and any
other material coating the water sides
of the heat transfer tubes. Failure to do
so lowers thermal conductivity, retards
heat transfer, produces overheating, and
eventually leads to tube failures. All it
takes is a 1/64-inch layer of iron and silica
scale to produce a 3.5% fuel loss in an
HRSG. Yet, a poorly maintained HRSG
with poor water chemistry can build up
that amount of scale quickly.
Complying with Emissions Regulations.
HRSGs are often upgraded as part
of a plant's efforts to adhere to stricter
environmental regulations. HRSG maintenance
actions should pay attention
to how well the facility is dealing with
the emissions limitations imposed upon
it by local air quality districts. Those
HRSGs that already utilize supplemental
firing or wish to add this feature need to
carefully monitor emissions levels and
understand how changes may impact
NOx
and
and CO. In some cases, selective
catalytic reduction (SCR) for NOx
oxidation catalysts to reduce CO may be
needed to fulfill permitted emissions requirements.
For other HRSGs, burners
should be considered that are designed
to lower NOx
output. The right burners
can sometimes mean that not as much
reagent is needed for SCR. When space
is too tight for a full-sized SCR, a burner
retrofit may make it possible to use a
more compact SCR because it doesn't
need to deal with the higher mass flow
or higher NOx
levels.
These are just a few pointers that can
help an HRSG to run more efficiently
and that can avoid unforeseen difficulties
during HRSG maintenance and upgrade
projects. By adhering to them and other
maintenance best practices, operators can
look forward to long life for their HRSGs.
-Kevin Slepicka is vice president
of Heat Recovery Boilers at
Rentech Boiler Systems.
11
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