POWER May 2016 - 54

FUNDAMENTALS
The Importance of Including
Balance-of-Plant Systems in
Condenser Maintenance
Keeping a power plant's surface condenser in proper working order requires
paying attention to balance-of-plant systems as well. Failure to monitor
and maintain cooling towers and vacuum pumps in particular can lead to
performance penalties or worse.
Kevin Boudreaux
T
he surface condenser at a power plant
has a significant effect on power generation-specifically,
the efficiency with
which power is generated. Any degradation in
condenser performance leads to elevated operating
back pressures (BP), heat rate penalties,
and-in some cases-a total plant trip.
Possible culprits for performance degradation
vary greatly; however, numerous condenser
performance issues are not rooted in
the condenser at all.
Experience has shown efficiency losses
that manifest in the condenser actually start
in balance-of-plant (BOP) systems such as
vacuum pumps, circulating water pumps, and
cooling towers. And yet, the attention these
systems receive pales in comparison to the
time and resources spent on condenser monitoring,
inspecting, and maintenance.
The intent of this article is not to repeat the
excellent work already done by the American
Society of Mechanical Engineers, the Electric
Power Research Institute, and the Heat
Exchanger Institute with regard to monitoring,
inspection, and maintenance checklists.
Instead, it examines two particular BOP components
that are notorious for causing condenser
performance issues: cooling towers
and vacuum pumps. It specifically explains
why they should be included in all online
monitoring and offline inspection and maintenance
programs related to the condenser.
Cooling Tower Operation Effects
on Condenser Operation
Most plant personnel understand in general
terms that the cooling tower operation directly
affects overall plant operation. The depth
of this understanding varies greatly throughout
the industry, and detailed knowledge of
the multitude of ways a cooling tower can
affect overall condenser performance is rare.
It has been my experience that there are two
specific areas of concern that, when left un54
addressed,
lead to significant losses in plant
efficiency and production: proper flow distribution
across cooling tower fill and recirculating
cooling water flow rate.
How cooling tower efficiency directly affects
a plant's heat rate may seem a bit convoluted.
For instance, for every 5-degree F
rise in basin water temperature, there is a
corresponding rise in condenser BP of 0.25
inches mercury (inHg); equating to a heat
rate efficiency loss of 0.5% or more, depending
on the type of plant.
How does this 5-degree change equate to
0.25 inHg? The rise in operating steam temperature,
and subsequently BP, is directly proportional
to the rise in condenser inlet water
temperature, with the steam tables showing
an increase of approximately 0.25 inHg for
every 5-degree F increase in steam temperature.
So it stands to reason that degradation
in cooling tower performance, resulting in a
higher basin water temperature, manifests as
an elevated condenser BP (Figure 1). This in
turn means lower turbine efficiencies, hence
the cooling tower's direct effect on overall
plant heat rate.
The process of evaporation (for this discussion
the process takes place over a cooling
tower) is used to remove heat, picked up
in the condenser, from cooling water. To optimize
evaporation rates, and thus heat removal,
cooling towers are designed so that the
proper amount of hot condenser outlet water
is sprayed across the entirety of the cooling
tower fill using a lateral and nozzle system
(Figure 2). Falling down the large surface
area of fill, cooling water comes in contact
with the proper amount of counter-flowing,
high-velocity air. This, in turn, causes evaporation-heat
removal via phase change-to
occur. The ratio of circulating water to airflow
is expressed as the liquid-to-gas ratio
(L:G ratio) and is a critical design point.
If laterals or nozzles should become obstructed,
dead zones can form. These dead
zones are areas in the fill where there is little
to no water, and thus no water-air contact;
moreover, these are areas where scaling and
1. Temperature rising. This chart shows the effect of inlet water temperature on operating
back pressure. Note that it assumes a 20-degree F temperature rise and a 6-degree F
terminal temperature difference. Source: Kevin Boudreaux, Nalco Co.
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
50
60
70
80
Cooling water inlet temp (F)
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POWER | May 2016
90
100
110
Back pressure (inHg)

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POWER May 2016

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