POWER October 2021 - 48

WATER
Water Intake Reliability in the Age
of Environmental Uncertainty
Thermal power plants need a continuous supply of cooling water to operate, but as
the natural environment changes, more and more screen blockages are occurring
at cooling water intakes. Maintaining intake equipment in good order and having
a sound program to mitigate problems when events occur is important for intake
reliability.
Timothy Hogan, Maarten Bruijs,
and Jonathan Black
W
orldwide, power generation is
transitioning away from fossil fuels
and toward renewable energy
sources such as wind and solar. During
this transition, and potentially thereafter,
nuclear power remains critical for grid stability
until reliable and economical energy
storage options become available.
Nuclear and fossil-fueled thermal power
generation require a continuous supply
of large quantities of clean cooling water
from natural sources. Even facilities relying
on closed-cycle cooling systems require a
significant amount of clean makeup water.
Therefore, access to sufficient, clean cooling
water is a necessity for safe, reliable,
and efficient power generation.
In recent years, however, there has
been increased risk of intake blockages
due to environmental changes in natural
cooling water sources. Problems have
been caused by salp swarms in Korea,
tiny shrimp masses in China, jellyfish
blooms in Israel, and seaweed freed by
storm activity in Russia, among others.
Changes in environmental conditions
such as increasing water temperature and
carbon dioxide concentrations will not
only have the obvious effect on thermal
efficiency, but are also likely to increase
the frequency of harmful algal blooms,
invasive algae, and jellyfish blooms.
In addition to free-floating material
(anthropogenic or biological) that can
clog intakes, the nature and extent of
biofouling (such as barnacles and mussels)
on the submerged structures is
also affected by changing environmental
conditions. As Ali Ahmad noted in an article
published July 5, 2021, in the journal
Nature Energy, these sorts of intake
blockages can be considered an indirect
effect of increasing water temperatures-something
that warrants careful
consideration in the operation of existing
or design of new power plants.
48
Cooling Water Screening Systems
Cooling water is primarily used for cooling
in condensers, a critical component
in the steam power cycle. Cooling water
is just as important for emergency and
auxiliary service water systems, and it
must be screened to remove debris in all
cases. The screening assembly is determined
by the required cooling water flow,
and prevents waterborne debris and biota
from entering the cooling water system.
Most cooling water intakes apply several
screening stages to protect downstream
equipment, ensure continuous operation
of the intake, and maintain the safe, continuous
operation of the facility.
Intake screening typically consists of
two main screening stages: one or more
coarse stages (such as static trash racks,
which are usually equipped with raking
systems) and a fine stage (such as a
rotating/traveling water screen or drum
screen). The trash racks prevent large
debris from damaging the downstream
fine screens. The fine screens remove
smaller debris that would otherwise clog
the condensers and heat exchangers.
Some facilities may also incorporate a
diversion structure (such as a trash boom
or barrier net) upstream of the coarse
and fine stages to prevent debris from
reaching the intake. Environmental regulatory
requirements may require that fine
screens be fitted with fish protection features
to protect aquatic life.
Failures sometimes occur in intake
screening systems. The two primary
sources of failure are poor maintenance
of the screening systems and reliance
on manual or scheduled screen rotation
rather than equipment sensors (such as
head differential sensors). It is critical that
operational personnel are trained to manage
intake blockage events and maintain
intake screening equipment. Regardless
of the root cause, each source of failure
can result in screens not being operable
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or prematurely breaking down when
called to operate during high debris loading
conditions. Such events can lead to
unit derates and forced outages.
Power plant operators must continually
assess intake blockage risk and make
modifications to maintain design and
operation margins at the intake. However,
the unpredictability of long-term
(decadal) environmental changes makes
them challenging to manage. Just as
plant operators must monitor and assess
the condition of intake screening equipment,
owners/operators should also
monitor and prepare for these changing
environmental conditions.
Underpinning the civil, mechanical, and
hydraulic design of an intake screening
system are assumptions about the types
and amounts of debris to be expected.
Each debris type differs in terms of size,
shape, constitution, density, and durations
of events. These are strongly variable by
geographic location, intake position in the
source water (such as depth and onshore
versus offshore arrangements), seasons,
meteorological conditions, local biology,
and hydraulic conditions.
Depending on the debris type and
size, the material is either retained by the
trash rack or the fine screens. Relying on
either the coarse or fine screening stage
as the primary debris collection point is a
conscious design choice, though debris
can be balanced between stages to prevent
overtaxing one stage (Figure 1).
Intake Reliability
Intake reliability refers to the ability of an
intake structure to provide cooling water
under
all
environmental
conditions-
both routine and unexpected. Reliable
intake operation is a function of the suite
of technologies, operational modes,
personnel, training, and procedures employed
by a facility.
Such reliability is not ensured via a simple
checklist; it requires a culture of awarePOWER
| October 2021
http://www.powermag.com
POWER October 2021

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